document_title: Blair Waldorf

question_text: who's the dad of blair waldorf's baby

question_tokens: who 's the dad of blair waldorf 's baby

text: Blair Waldorf

text: Character in print[edit]

text: Gossip Girl is a series of novels about socially prominent young adults in New York City. The story primarily follows Blair Waldorf and her best friend Serena van der Woodsen during their years in high school and college. Due to her fame on the Upper East Side, Blair is featured on the website of "Gossip Girl," an anonymous gossip blogger whose posts appear occasionally throughout the story.

text: Novel series[edit]

text: In the first book, Blair is introduced as a privileged, comically vain overachiever.[6] She is described as an alluring brunette,[7] and occasionally models her appearance and demeanor after famous actresses, including Marilyn Monroe and, most often, Audrey Hepburn.[8] In early novels, the character is also written as bulimic.

text: Blair is largely motivated by matters surrounding family, romance, and ambition. However, her tendency to overachieve can lead to feelings of paranoia, with dramatic or comical results. In a review for The New Yorker, Janet Malcolm remarked that Blair's issues made her "both a broader caricature and a more real person" than the other Gossip Girl characters.[6] In a 2009 interview, Gossip Girl creator Cecily von Ziegesar claimed to identify with Blair the most, stating, "She is so unpredictable and dramatic. Such a bitch, but we understand why she is a bitch and we like her anyway".[9]

text: Story lines[edit]

text: Throughout most of the series' run, Blair grapples with a number of changes within her family. Two years prior to the opening novel, Blair's parents divorced after her father ran off with another man. When her mother remarries, and her father leaves the country, Blair has difficulty accepting her stepfather. In addition, her stress over these matters occasionally affects her other relationships.[10] However, Blair ultimately remains close to her father Harold, who she often turns to for comfort.[11] By contrast, she maintains a somewhat tenser relationship with her mother Eleanor. It is also revealed that Blair has struggled with bulimia.[12]

text: In the opening novel, Blair learns of an affair between her friend Serena and her boyfriend Nate. This marks the beginning of the story's primary love triangle, which recurs throughout the series.[13] Blair's romantic life has various effects on her character development. After Nate repeatedly hurts her, she eventually refuses to take him back until she believes in his ability to commit to her.[14] In pursuing Nate, however, Blair herself cheats on her new boyfriend Pete, which results in her losing both of them.[15] She eventually begins to acknowledge her mistakes, with her father's help.[16] She later grows closer to Chuck Bass (which initially occurred in the series' television adaptation), who she'd previously known for years, which leads them to briefly date one another.[16]

text: Blair is noted for an over-achieving nature, which often appears in humorous scenes. In a review for New York magazine, Emily Nussbaum lauded one of Blair's fantasies, which involves "joining the Peace Corps, getting a killer tan, winning the Nobel Peace Prize, and having dinner with the president, 'who would then write her a recommendation to Yale, and then Yale would fall all over themselves to accept her.' "[17] The New Yorker's Janet Malcolm remarked that unlike some of her forerunners in film and literature, "Blair already has all the money and position anyone could want. She is pure naked striving, restlessly seeking an object, any object, and never knowing when enough is enough."[6]

text: Blair encounters a setback during her interview at Yale by revealing the recent stress in her life, and then kissing her interviewer on the cheek upon dismissal. Her father then makes a donation to the school, though Blair is still wait-listed.[18] In the twelfth book, I Will Always Love You, it is revealed that she has been admitted to the university.[19]

text: In addition to her feelings for Nate, Blair is sometimes said to feel competitive with Serena in other areas, including matters of beauty and popularity.[20] This also leads to an occasional envy on Blair's part.[17] It is unclear how much of Blair's perception of Serena is in line with reality; the narrative describes both characters as "hands down the two hottest girls on the Upper East Side, and maybe all of Manhattan, or even the whole world."[7]

text: Manga series[edit]

text: In 2010, Yen Press began publishing a manga adaptation titled Gossip Girl: For Your Eyes Only, written and illustrated by HyeKyung Baek. This series adapts notable scenarios from the novel—including the triangle with Nate and Serena—but also features new material.

text: After losing her position as queen bee, Blair attempts to regain her former status while adjusting to a less privileged lifestyle.[21] In addition to this series, Blair also appears in a manga adaptation of the novel Gossip Girl: Psycho Killer, a parody of horror stories.[22]

text: Television series[edit]

text: In 2007, Gossip Girl was adapted for television. According to Cecily von Ziegesar, the television character is largely faithful to the original.[9] Among the aspects to be maintained are her admiration for Audrey Hepburn and her interest in Yale University. However, the series is also noted for its deviations from the source material,[6] including the exclusion of Blair's brother Tyler. The show also explores romances between Blair and multiple male leads, resulting in occasional love triangles. In the fifth season, Blair is revealed to be pregnant.[23]

text: Among fans and the media, Blair's bond with Chuck Bass was commonly known by the portmanteau "Chair", while her relationship with Dan Humphrey was referred to as "Dair". The nicknames and viewer interest in these relationships were recognized by the show's producers.[24]

text: Casting[edit]

text:
“
They were like, 'Be bitchy and nice, ugly and pretty, young and old, stupid and smart, innocent and slutty, blond and brunette. Can you be all those things?'
”

— Leighton Meester [25]

text: To prepare for the part of Blair, actress Leighton Meester, a natural blonde, dyed her hair brown before auditioning, and also studied the first novel.[26][27] She has described the character as multi-faceted, labeling her "a little bit of everything which is pretty amazing."[27] Like von Ziegesar, Meester has also claimed to relate to Blair on certain levels. Prior to the show's debut in 2007, the actress stated that, "The only way to play Blair, or any character, and make her human, is to find what she is inside me. And I know I have my insecurities, too." She went on to say that, "The way Blair and I are not alike when it comes to insecurities is: She pays so much attention to hers!"[28] Meester's casting was described by Yahoo! as a star-making role which moved her "into the pop culture vanguard,"[29] while Cecily von Ziegesar has called her a perfect choice.[30]

text: In December 2010, Meester revealed plans to leave the show in 2012. E! Online and other outlets speculated that her departure would possibly mark the end of the series.[31]

text: Season 1[edit]

text: In Season 1 of Gossip Girl, Blair is introduced as the Upper East Side's beautiful and popular Queen B. She is the daughter of Eleanor Waldorf, a famous fashion designer. She is dating Nate Archibald, and is best friends with Serena van der Woodsen. She also finds a close companion in Nate's best friend and her childhood friend Chuck Bass, who becomes a partner for her schemes. When Serena returns home from boarding school, Blair learns from Nate that he lost his virginity to a drunken Serena over a year ago.[32] Blair retaliates by publicly revealing Serena's connection to a rehab hospital. She then learns that the actual patient is Serena's younger brother, Eric, who had been committed after a suicide attempt. Afterward, a remorseful Blair reconciles with Serena.[33][34]

text: After learning that Nate no longer loves her, Blair sleeps with Chuck, eventually falling for him. This leads to a heated affair and an eventual love triangle. Her inability to choose creates much of the first season's story line.[35] There are brief mentions of Blair's past struggle with anorexia-bulimia that are never mentioned again past the first season.

text: She also begins a brief power struggle with freshman Jenny Humphrey. After she unites with Chuck and Nate in order to save Serena from the scheming Georgina Sparks, Chuck realizes that his feelings for Blair are real and suggests that they spend the summer together in Tuscany. However, he is discouraged by his father at the last minute, and stands Blair up.[36]

text: Season 2[edit]

text: At the launch of the second season, Blair was described by creators as the queen at the center of the Gossip Girl chess game.[37] A large portion of her story line in Season 2 revolves around her love-hate relationship with Chuck Bass, which was labeled "the heart of GG" by People magazine.[26] While competing with Serena, Blair forms an unexpected friendship with Jenny, who states that they each work for everything they achieve, while Serena often glides through life. During their interviews at Yale University, Blair and Serena apologize for their ill feelings and resume their friendship.[38]

text: In the episode "O Brother, Where Bart Thou?", Chuck is devastated by news of his father's death, prompting Blair to offer her support while telling Chuck that she loves him. He initially shuns her advances, but later turns to her for comfort.[39] However, the two stop seeing each other due to Chuck's uncle, Jack Bass, convincing him he has an inability to commit to a relationship.[40] After being rejected by Yale, Blair finds unexpected encouragement from Nate.[41] She is later accepted into New York University, and her competitive relationship with Georgina is eventually renewed.

text: As the season ends, Blair crowns Jenny the new queen of Constance Billard School. In the season finale, it is discovered that Blair slept with Chuck's uncle Jack and that Chuck had slept with Vanessa Abrams. Chuck then departs for Europe. He later returns to New York and reconciles with Blair while declaring his love for her, and the two finally begin a committed relationship.[42]

text: Season 3[edit]

text: In the third season, Blair joins Vanessa, Georgina, and Jenny's brother Dan at NYU.[43] Much of her story line concerns her inability to attain her previous status at her new school. She finds emotional support from her mother, as well as Chuck. However, she and Chuck separate once again when Blair feels that he manipulated her while competing with his uncle. ( Chuck made a deal with Jack that Blair could sleep with him and in exchange, Chuck gets his hotel back that Jack initially stole to 'destroy Chuck'. )

text: She later transfers to Columbia University, and learns that an emotionally reformed Chuck was responsible for her enrollment.[44] They later team up as part of a role-playing scheme to help Serena's mother and Chuck's adopted mother, Lily. In the season finale, Chuck attempts to propose to Blair, but is interrupted by Dan, who reveals that Chuck had slept with Jenny. Two weeks later, Blair and Serena depart for Paris intending to spend the summer together.[45]

text: Season 4[edit]

text: In Season 4, Blair and Chuck become competitive once again, but eventually resume their sexual relations before recognizing their love for one another. When the relationship interferes with their business interests, she and Chuck break up once more. Chuck promises he'll wait for her, and both affirm their belief that their love will reunite them in the end.[46]

text: Blair then teams up with Dan when the two share common goals. They also end up working together at W. magazine, where friction develops between the two.[47] On Valentine's Day, she discovers that Chuck has romantic feelings for Raina Thorpe, the daughter of his business rival. Later, she and Dan spend the evening talking on their cell phones while watching Rosemary's Baby.[48] Blair later quits W. and is shown asleep with Dan in his Brooklyn flat. Later, upon growing curious of their feelings for one another, Blair and Dan share a kiss before the mid-season hiatus.

text: Blair eventually decides that she wants to be with Chuck, but shuns him once again after he tries to humiliate Dan.[49] She is later courted by a prince from Monaco named Louis. During a private confrontation, a drunken Chuck punches a window after he finds out Prince Louis has proposed to Blair, which cuts Blair's face as it shatters. Afterward, Blair chooses to accept a proposal from Louis.[50] Blair later attempts to warn Chuck about potential trouble in his family. She is then abducted by an enemy of the Basses, Raina's father Russell Thorpe. Chuck later rescues Blair and apologizes for his violent actions. Following a night out together, the two have sex before Chuck advises Blair to return to Louis, believing that she will be happier with him. However, the season ends with the revelation that Blair may be pregnant.[51]

text: Reaction to triangle[edit]

text:
"Chuck and Blair are always going to be connected in their way, and Dan and Serena are always going to be connected in their way. But that doesn’t mean that there won’t be new roads to go down that might be different than what we initially anticipated."
—Joshua Safran, producer[52]

text: Amidst the fourth season, the romance between Dan and Blair became a polarizing topic among viewers[47] which also drew significant media interest. Jarett Wieselman of the New York Post applauded the development, feeling that Blair had "more chemistry" with Dan than with Chuck.[53] New York magazine's Chris Rovzar called Blair and Dan's story line "believable", citing their common ground in education and taste. Rovzar further stated, "Since they live in a world where both only seem to have half a dozen real friends (if that), is it so crazy they'd end up together?"[54] An article from The Huffington Post declared that "Dan and Blair together are like Harry Burns and Sally Albright reincarnated — the couple was obviously inspired by When Harry Met Sally." It further commented that the couple paralleled Pacey and Joey’s relationship from Dawson's Creek and concluded that Dan and Blair were "the most inspired storyline and couple of the show". [55] Entertainment Weekly stated that Dan and Blair were like stars in an "updated version of You've Got Mail".[56] Tierney Bricker of E! ranked all 25 Gossip Girl couples placing 'Dair' as the third best couple after Blair and Serena (#1) and 'Chair' (#2). Bricker stated that “[Dan and Blair], out of all the main characters, were the most well-suited for each other. In real life, they would've been 'endgame'."[57]

text: Dawn Fallik of The Wall Street Journal was less positive, asserting that "both characters have been so Blandified that there’s no fun left in the show."[58] A writer for E! Online's Team WWK labeled the Dan/Blair relationship "nomance nonsense".[59]

text: With regard to Chuck and Blair, Meester stated, "I can really relate to it—not necessarily because it's this dramatic, tumultuous relationship, but because the way they love each other is very real, and not for the sake of being dramatic. It's actual love. There's nobody for each other but them."[60] Meester also expressed fondness for Dan and Blair, however, stating, "I think they're good for each other in a lot of ways, in a way that Chuck and Blair aren't."[47] Badgley claimed that he thought "Blair [was] Dan’s soul mate"[61] and further stated that he thought the Blair and Dan storyline was "the most exciting for Dan as a person".[62] When Hollywoodlife.com asked Ed Westwick, who played Chuck Bass, which character he thought loved Blair more, Chuck or Dan, Westwick pointed to Badgley, saying "definitely him."[63]

text: Producers initially noticed chemistry between Blair and Dan in the Season 1 episode "Bad News Blair". According to producer Joshua Safran, the creators planned to revisit their relationship once the timing was right. Safran also stated that the outcome wasn't necessarily decided ahead of time. "One thing we are very conscious of—and I know some fans get upset about this—is we really try to treat the characters as living, breathing, well-rounded individuals. And we're often surprised by where their journeys take them; they open new doors for us all the time."[52]

text: Controversy[edit]

text: Following the twentieth episode of Season 4, Safran spoke on behalf of the series regarding the scene in which Chuck became violent with Blair.[64]

text:
“
The way we viewed it, I think it's very clear that Blair is not afraid in those moments, for herself. They have a volatile relationship, they always have, but I do not believe—or I should say we do not believe—that it is abuse when it's the two of them. Chuck does not try to hurt Blair. He punches the glass because he has rage, but he has never, and will never, hurt Blair. He knows it and she knows it, and I feel it's very important to know that she is not scared—if anything, she is scared for Chuck—and what he might do to himself, but she is never afraid of what he might do to her. Leighton and I were very clear about that.
”

text: In response to these comments, Carina MacKenzie of Zap2it stated, "We're left wondering if Safran missed the part where she went home bleeding because Chuck was using physical intimidation to release his own emotions."[65] While reviewing the episode, Tierney Bricker of Zap2it felt that there were "really no excuses for Chuck Bass anymore."[66] MacKenzie concluded that Chuck's behavior throughout Season 4 fit the signs of an abusive relationship, citing examples from HelpGuide.org, a non-profit health resource. She noted Chuck's public humiliation of Blair, his attempt to pawn her during a business deal, and his use of physical intimidation. MacKenzie also called the show's explanation "disturbing, particularly given the young, female target demographic of Gossip Girl and The CW."[65]

text: In a review for the Los Angeles Times, Judy Berman addressed Safran's description of Blair during the scene. "Considering how terrified Blair looked at the end of their encounter, and how quickly she got out of there, the show is sending a mixed message at best." She went on to state, "We have no right to expect Gossip Girl to be a paragon of morality, or even realism, but the idea that true love requires taking a shard of glass to the face is disturbing even in this alternate, soap-opera dimension."[67]

text: Season 5[edit]

text: In the fifth season premiere, Blair continues to plan her wedding, but begins to encounter problems in her relationship with Louis. It is later revealed that she is pregnant.[23] Blair tells Chuck that the child is Louis', and states that part of her wanted Chuck to be the father.[68] Dan becomes Blair's confidante and is shown to be in love with her, though she remains oblivious to his feelings and states that there is nothing more than friendship between them.

text: Though she insists that she is in love with Louis, Blair begins to seek Chuck out as the season progresses. The two eventually declare their love for each other prior to a car accident in her limousine while being chased by paparazzi. Though both recover, Blair suffers a miscarriage from the crash. After the crash Blair decides that she must commit herself to Louis, converting to Catholicism and cutting off any connection with Chuck. At the wedding, Gossip Girl releases a recording of Blair confessing her love for Chuck. Nevertheless, Louis and Blair get married, making Blair a Princess of Monaco, though Louis informs her that they will have a loveless marriage of convenience.[69] She then receives support from Dan, leading them to share a kiss on Valentine's Day. Amidst these developments, Blair grows conflicted between her feelings for Dan and Chuck.[70] After taking steps to end her marriage, she chooses to begin a romantic relationship with Dan.[71] By the end of the season, however, after a debate about which love is the best—with Dan she feels safe, with Chuck she feels vulnerable—Blair declares that she is still in love with Chuck, and chooses to pursue him.[72]

text: Season 6[edit]

text: In the final season, Blair resumes her romantic relationship with Chuck, while Chuck and his father Bart—who is revealed to be alive in the previous season—become bitter rivals. Blair pursues her career as head of Waldorf Designs, with several mishaps, before staging a successful line.[73] In the penultimate episode, Bart falls to his death while trying to attack Chuck atop a building. Afterward, Blair and Chuck depart together. In the series finale, Blair marries Chuck which results in her not having to testify against him in his father's murder case. Five years later, Blair is shown to be running her mother's successful fashion line and she works with Jenny in a line called "J for Waldorf", and she and Chuck are shown to have a son named Henry.[74]

text: Reception[edit]

text: In print[edit]

text: While covering the book series, Janet Malcolm of The New Yorker labeled the character "an antiheroine of the first rank," and asserted that "the series belongs to awful Blair, who inspires von Ziegesar's highest flights of comic fancy." She also compared Blair to vintage film and literary figures such as Becky Sharp of Vanity Fair, and Lizzie Eustace of The Eustace Diamonds.[6] In addition, the character has drawn comparisons to notable contemporaries, including Lila Fowler of the Sweet Valley High series.[75]

text: In the 2007 book Children's Literature and Culture, writer Harry Edwin Eiss chastised the depiction of Blair's bulimia. "If handled properly, the inclusion of her illness could have provided a powerful lesson for young adult readers who worry about their weight and food consumption. Unfortunately, Cecily von Ziegesar, the author of the series, presents a seriously flawed treatment of the problem. In a failed attempt at humor, the writer regards Blair's sickness as just another source of gossip," he said.[76] Emily Nussbaum of New York magazine had similar comments, calling the bulimia "more of an icky weakness than a full-fledged pathology." However, she went on to commend Blair as "hilariously self-centered".[17]

text: Julie Opipari of Manga Maniac Cafe gave the initial Gossip Girl manga a negative review, citing displeasure with the characters and plot. However, she acknowledged that this gave her a greater appreciation for Blair in the second volume, noting that Blair "really had to learn how to rough it" after losing her privileged lifestyle. She went on to state that "Blair is one character that is fun to hate on. So imagine my surprise when I actually started to like her by the end of the book".[77]

text: On screen[edit]

text: The show's breakout character,[78] Blair Waldorf has garnered much media recognition. Yahoo! proclaims Blair a member of "television's pantheon of razor-witted, solipsistic high school Alpha females."[29] While commenting on Meester, New York magazine's 2008 cover story of the series states, "Her villain-you-want-to-root-for is the most sophisticated performance on the show."[79] In another 2008 article, People magazine commented that "Meester has burst out of this ensemble to stardom."[80] Variety described her performance as similar to that of "a predatory junior Joan Collins who practically breathes fire out of her pinched, perfectly WASP-ish nostrils."[80] FHM Online ranked the actress the "Hottest TV Star" of autumn 2008, stating that as Blair Waldorf, "Leighton Meester has stolen the spotlight with her mind-blowing good looks and amazing performance."[81] OK! magazine likened her character to Audrey Hepburn's portrayal of Holly Golightly.[82] The physical similarity was also noted by USA Today.[83]

text: In its 2009 "Hot List", Rolling Stone cited Blair as "the reason we love the back-stabby soap most."[84] Regarding the fictional fame of Blair's friend Serena, Tim Stack of Entertainment Weekly asserted that "Serena may be the star of the media but Blair is quickly becoming the star of this show."[85] While citing Serena's long-revered allure, Glamour and its readers compared the two characters in 2008, with Blair being recognized as more beautiful than Serena.[86] In a review from The Atlantic, Blair and Serena's friendship was praised as "it offered a relationship whose depth and complexity approached Rory and Paris' [from Gilmore Girls]."[87]

text: In May 2009, Blair received attention from Forbes, which interviewed her via the series' writers.[88] Television Without Pity listed Meester in their "Golden Globes 2009: Overlooked TV Shows and Performances" article, labeling Blair "so multi-faceted, well-dressed and beautifully played that she elevates this teen soap to something we don't even feel guilty about admitting we love."[89] Meester won the Teen Choice Award for "Choice TV Actress Drama" in 2009,[90] and again in 2010.[91] In 2009, she was voted the "Best Mean Girl" in Zap2it's first poll of the best television characters in the 2000s.[92] In February 2012, Zap2it held another poll to determine TV's Most Crushworthy. Blair was elected TV's Most Crushworthy 1% Female over Temperance "Bones" Brennan.[93] The 1 percenters are people that "have everything going for them with their fantastic good looks and their opulent lifestyle".[94] Her relationship with Chuck Bass was included in TV Guide's list of "The Best TV Couples of All Time"[95] as well as Entertainment Weekly's "30 Best 'Will They/Won't They?' TV Couples".[96]

text: The character's wardrobe—credited to designers Abigail Lorick and Eric Daman[97][98]—is popular, earning mentions from periodicals such as InStyle[99] and New York,[100] along with recognition from websites.[101] TV Guide listed Blair among its "Best Dressed TV Characters of 2007".[102] Entertainment Weekly named Blair Waldorf and Chuck Bass the "Most Stylish" characters of 2008.[103] Lifetime television ranks Blair first in its listing of "The Top 10 Best-Dressed TV Characters",[104] while Glamour has named her among its best-dressed TV characters of all time.[105] In a Vanity Fair interview, costume designers Eric Daman and Meredith Markworth-Pollack named Audrey Hepburn and Vogue editor-in-chief Anna Wintour as their inspirations when dressing Meester as Blair. The designers also cited New York socialites Tinsley Mortimer and Arden Wohl as influences.[106] TV Guide named her the sixth most fashionable TV character.[107]


document_title: Centroid

question_text: where is the center of gravity in a triangle

root

text: Centroid

text: Properties[edit]

text: The geometric centroid of a convex object always lies in the object. A non-convex object might have a centroid that is outside the figure itself. The centroid of a ring or a bowl, for example, lies in the object's central void.

tag: p

text: If the centroid is defined, it is a fixed point of all isometries in its symmetry group. In particular, the geometric centroid of an object lies in the intersection of all its hyperplanes of symmetry. The centroid of many figures (regular polygon, regular polyhedron, cylinder, rectangle, rhombus, circle, sphere, ellipse, ellipsoid, superellipse, superellipsoid, etc.) can be determined by this principle alone.

text: In particular, the centroid of a parallelogram is the meeting point of its two diagonals. This is not true for other quadrilaterals.

text: For the same reason, the centroid of an object with translational symmetry is undefined (or lies outside the enclosing space), because a translation has no fixed point.

text: Triangle centroid[edit]

text: The centroid of a triangle is the intersection of the three medians of the triangle (each median connecting a vertex with the midpoint of the opposite side). It lies on the triangle's Euler line, which also goes through various other key points including the orthocenter and the circumcenter.[1][2]

text: Any of the three medians through the centroid divides the triangle's area in half. This is not true for other lines through the centroid; the greatest departure from the equal-area division occurs when a line through the centroid is parallel to a side of the triangle, creating a smaller triangle and a trapezoid; in this case the trapezoid's area is 5/9 that of the original triangle.[3]

text: Let P be any point in the plane of a triangle with vertices A, B, and C and centroid G. Then the sum of the squared distances of P from the three vertices exceeds the sum of the squared distances of the centroid G from the vertices by three times the squared distance between P and G:

text: The sum of the squares of the triangle's sides equals three times the sum of the squared distances of the centroid from the vertices:

text: A triangle's centroid is the point that maximizes the product of the directed distances of a point from the triangle's sidelines.[6]

text: For other properties of a triangle's centroid, see below.

text: Locating the centroid[edit]

text: Plumb line method[edit]

text: The centroid of a uniform planar lamina, such as (a) below, may be determined, experimentally, by using a plumbline and a pin to find the center of mass of a thin body of uniform density having the same shape. The body is held by the pin inserted at a point near the body's perimeter, in such a way that it can freely rotate around the pin; and the plumb line is dropped from the pin (b). The position of the plumbline is traced on the body. The experiment is repeated with the pin inserted at a different point of the object. The intersection of the two lines is the centroid of the figure (c).

text:
(a)
(b)
(c)

tag: table

text: This method can be extended (in theory) to concave shapes where the centroid lies outside the shape, and to solids (of uniform density), but the positions of the plumb lines need to be recorded by means other than drawing.

text: Balancing method[edit]

text: For convex two-dimensional shapes, the centroid can be found by balancing the shape on a smaller shape, such as the top of a narrow cylinder. The centroid occurs somewhere within the range of contact between the two shapes. In principle, progressively narrower cylinders can be used to find the centroid to arbitrary precision. In practice air currents make this unfeasible. However, by marking the overlap range from multiple balances, one can achieve a considerable level of accuracy.

text: Of a finite set of points[edit]

text:
The centroid of a finite set of

k

{\displaystyle {k}}

 points

x

1

,

x

2

,
…
,

x

k

{\displaystyle \mathbf {x} _{1},\mathbf {x} _{2},\ldots ,\mathbf {x} _{k}}

 in

R

n

{\displaystyle \mathbb {R} ^{n}}

 is

text: This point minimizes the sum of squared Euclidean distances between itself and each point in the set.

text: By geometric decomposition[edit]

text:
The centroid of a plane figure

X

{\displaystyle X}

 can be computed by dividing it into a finite number of simpler figures

X

1

,

X

2

,
…
,

X

n

{\displaystyle X_{1},X_{2},\dots ,X_{n}}

, computing the centroid

C

i

{\displaystyle C_{i}}

 and area

A

i

{\displaystyle A_{i}}

 of each part, and then computing

text:
Holes in the figure

X

{\displaystyle X}

, overlaps between the parts, or parts that extend outside the figure can all be handled using negative areas

A

i

{\displaystyle A_{i}}

. Namely, the measures

A

i

{\displaystyle A_{i}}

 should be taken with positive and negative signs in such a way that the sum of the signs of

A

i

{\displaystyle A_{i}}

 for all parts that enclose a given point

p

{\displaystyle p}

 is 1 if

p

{\displaystyle p}

 belongs to

X

{\displaystyle X}

, and 0 otherwise.

text: For example, the figure below (a) is easily divided into a square and a triangle, both with positive area; and a circular hole, with negative area (b).

text: The centroid of each part can be found in any list of centroids of simple shapes (c). Then the centroid of the figure is the weighted average of the three points. The horizontal position of the centroid, from the left edge of the figure is

text: The vertical position of the centroid is found in the same way.

text:
The same formula holds for any three-dimensional objects, except that each

A

i

{\displaystyle A_{i}}

 should be the volume of

X

i

{\displaystyle X_{i}}

, rather than its area. It also holds for any subset of

R

d

{\displaystyle \mathbb {R} ^{d}}

, for any dimension

d

{\displaystyle d}

, with the areas replaced by the

d

{\displaystyle d}

-dimensional measures of the parts.

text: By integral formula[edit]

text:
The centroid of a subset X of

R

n

{\displaystyle \mathbb {R} ^{n}}

 can also be computed by the integral

text:
where the integrals are taken over the whole space

R

n

{\displaystyle \mathbb {R} ^{n}}

, and g is the characteristic function of the subset, which is 1 inside X and 0 outside it.[8] Note that the denominator is simply the measure of the set X. This formula cannot be applied if the set X has zero measure, or if either integral diverges.

text: Another formula for the centroid is

text: where Ck is the kth coordinate of C, and Sk(z) is the measure of the intersection of X with the hyperplane defined by the equation xk = z. Again, the denominator is simply the measure of X.

text: For a plane figure, in particular, the barycenter coordinates are

text: where A is the area of the figure X; Sy(x) is the length of the intersection of X with the vertical line at abscissa x; and Sx(y) is the analogous quantity for the swapped axes.

text: Bounded region[edit]

text:
The centroid

(

x
¯

,

y
¯

)

{\displaystyle ({\bar {x}},\;{\bar {y}})}

 of a region bounded by the graphs of the continuous functions

f

{\displaystyle f}

 and

g

{\displaystyle g}

 such that

f
(
x
)
≥
g
(
x
)

{\displaystyle f(x)\geq g(x)}

 on the interval

[
a
,
b
]

{\displaystyle [a,b]}

,

a
≤
x
≤
b

{\displaystyle a\leq x\leq b}

, is given by

text:
where

A

{\displaystyle A}

 is the area of the region (given by

∫

a

b

[
f
(
x
)
−
g
(
x
)
]

d
x

{\displaystyle \int _{a}^{b}[f(x)-g(x)]\;dx}

).[11][12]

text: Of an L-shaped object[edit]

text: This is a method of determining the centroid of an L-shaped object.

text:
Divide the shape into two rectangles, as shown in fig 2. Find the centroids of these two rectangles by drawing the diagonals. Draw a line joining the centroids. The centroid of the shape must lie on this line AB.
Divide the shape into two other rectangles, as shown in fig 3. Find the centroids of these two rectangles by drawing the diagonals. Draw a line joining the centroids. The centroid of the L-shape must lie on this line CD.
As the centroid of the shape must lie along AB and also along CD, it is obvious that it is at the intersection of these two lines, at O. The point O might not lie inside the L-shaped object.

tag: ol

text: Of a triangle[edit]

text:
The centroid of a triangle is the point of intersection of its medians (the lines joining each vertex with the midpoint of the opposite side). The centroid divides each of the medians in the ratio 2:1, which is to say it is located ⅓ of the distance from each side to the opposite vertex (see figures at right). Its Cartesian coordinates are the means of the coordinates of the three vertices. That is, if the three vertices are

L
=
(

x

L

,

y

L

)
,

{\displaystyle L=(x_{L},y_{L}),}

M
=
(

x

M

,

y

M

)
,

{\displaystyle M=(x_{M},y_{M}),}

 and

N
=
(

x

N

,

y

N

)
,

{\displaystyle N=(x_{N},y_{N}),}

 then the centroid (denoted C here but most commonly denoted G in triangle geometry) is

text:
The centroid is therefore at

1
3

:

1
3

:

1
3

{\displaystyle {\tfrac {1}{3}}:{\tfrac {1}{3}}:{\tfrac {1}{3}}}

 in barycentric coordinates.

text: In trilinear coordinates the centroid can be expressed in any of these equivalent ways in terms of the side lengths a, b, c and vertex angles L, M, N:[13]

text: The centroid is also the physical center of mass if the triangle is made from a uniform sheet of material; or if all the mass is concentrated at the three vertices, and evenly divided among them. On the other hand, if the mass is distributed along the triangle's perimeter, with uniform linear density, then the center of mass lies at the Spieker center (the incenter of the medial triangle), which does not (in general) coincide with the geometric centroid of the full triangle.

text: The area of the triangle is 1.5 times the length of any side times the perpendicular distance from the side to the centroid.[14]

text: A triangle's centroid lies on its Euler line between its orthocenter H and its circumcenter O, exactly twice as close to the latter as to the former:

text: In addition, for the incenter I and nine-point center N, we have

text: If G is the centroid of the triangle ABC, then:

text:
(

Area of triangle ABG

)
=
(

Area of triangle ACG

)
=
(

Area of triangle BCG

)
=

1
3

(

Area of triangle ABC

)

{\displaystyle \displaystyle ({\text{Area of triangle ABG}})=({\text{Area of triangle ACG}})=({\text{Area of triangle BCG}})={\frac {1}{3}}({\text{Area of triangle ABC}})}

tag: ul

text: The isogonal conjugate of a triangle's centroid is its symmedian point.

text: Centroid of a polygon[edit]

text: The centroid of a non-self-intersecting closed polygon defined by n vertices (x0,y0), (x1,y1), ..., (xn−1,yn−1) is the point (Cx, Cy), where

text: and where A is the polygon's signed area,

text: In these formulas, the vertices are assumed to be numbered in order of their occurrence along the polygon's perimeter. Furthermore, the vertex ( xn, yn ) is assumed to be the same as ( x0, y0 ), meaning i + 1 on the last case must loop around to i = 0. Note that if the points are numbered in clockwise order the area A, computed as above, will have a negative sign; but the centroid coordinates will be correct even in this case.

text: Centroid of a cone or pyramid[edit]

text: The centroid of a cone or pyramid is located on the line segment that connects the apex to the centroid of the base. For a solid cone or pyramid, the centroid is 1/4 the distance from the base to the apex. For a cone or pyramid that is just a shell (hollow) with no base, the centroid is 1/3 the distance from the base plane to the apex.

text: Centroid of a tetrahedron and n-dimensional simplex[edit]

text: A tetrahedron is an object in three-dimensional space having four triangles as its faces. A line segment joining a vertex of a tetrahedron with the centroid of the opposite face is called a median, and a line segment joining the midpoints of two opposite edges is called a bimedian. Hence there are four medians and three bimedians. These seven line segments all meet at the centroid of the tetrahedron.[16] The medians are divided by the centroid in the ratio 3:1. The centroid of a tetrahedron is the midpoint between its Monge point and circumcenter (center of the circumscribed sphere). These three points define the Euler line of the tetrahedron that is analogous to the Euler line of a triangle.

text:
These results generalize to any n-dimensional simplex in the following way. If the set of vertices of a simplex is

v

0

,
…
,

v

n

{\displaystyle {v_{0},\ldots ,v_{n}}}

, then considering the vertices as vectors, the centroid is

text: The geometric centroid coincides with the center of mass if the mass is uniformly distributed over the whole simplex, or concentrated at the vertices as n equal masses.

text: Centroid of a hemisphere[edit]

text: The centroid of a solid hemisphere (i.e. half of a solid ball) divides the line segment connecting the sphere's center to the hemisphere's pole in the ratio 3:8. The centroid of a hollow hemisphere (i.e. half of a hollow sphere) divides the line segment connecting the sphere's center to the hemisphere's pole in half.


document_title: Earth

question_text: what is the surface area of the earth

question_tokens: what is the surface area of the earth

text: Earth

text: Name and etymology

text: The modern English word  Earth developed from a wide variety of Middle English forms,[n 6] which derived from an Old English noun most often spelled eorðe.[39] It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (gē): the ground,[n 7] its soil,[n 8] dry land,[n 9] the human world,[n 10] the surface of the world (including the sea),[n 11] and the globe itself.[n 12] As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus,[48] and later Norse mythology included Jörð, a giantess often given as the mother of Thor.[49]

text: Originally, earth was written in lowercase, and from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets.[39] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (e.g. "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g. "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[50]

text: Chronology

text: Formation

text: The oldest material found in the Solar System is dated to 7000456720000000000♠4.5672±0.0006 billion years ago (Bya).[51] By 7000454000000000000♠4.54±0.04 Bya[52] the primordial Earth had formed. The formation and evolution of Solar System bodies occurred along with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that disk along with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides). According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–7001200000000000000♠20 million years (Mys) to form.[53]

text: A subject of research is the formation of the Moon, some 4.53 Bya.[54] A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.[55] In this view, the mass of Theia was approximately 10 percent of Earth,[56] it hit Earth with a glancing blow and some of its mass merged with Earth.[57] Between approximately 4.1 and 7000380000000000000♠3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.

text: Geological history

text: Earth's atmosphere and oceans were formed by volcanic activity and outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets, and comets.[58] In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.[59] By 7000350000000000000♠3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[60]

text: A crust formed when the molten outer layer of Earth cooled to form a solid. The two models[61] that explain land mass propose either a steady growth to the present-day forms[62] or, more likely, a rapid growth[63] early in Earth history[64] followed by a long-term steady continental area.[65][66][67] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, the supercontinents have assembled and broken apart. Roughly 7002750000000000000♠750 million years ago (Mya), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia 7002600000000000000♠600–540 Mya, then finally Pangaea, which also broke apart 7002180000000000000♠180 Mya.[68]

text: The present pattern of ice ages began about 7001400000000000000♠40 Mya and then intensified during the Pleistocene about 7000300000000000000♠3 Mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 7012126230400000000♠40,000–100,000 years. The last continental glaciation ended 7011315576000000000♠10,000 years ago.[69]

text: Origin of life and evolution

text:
Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose.[70] The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen (O
2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O
3) in the upper atmosphere.[71] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[72] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized Earth's surface.[73] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,[74] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,[75] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.[30][31] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.[76][77]

text: During the Neoproterozoic, 7002750000000000000♠750 to 580 Mya, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity.[78] Following the Cambrian explosion, 7002535000000000000♠535 Mya, there have been five mass extinctions.[79] The most recent such event was 7001660000000000000♠66 Mya, when an asteroid impact triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared some small animals such as mammals, which at the time resembled shrews. Mammalian life has diversified over the past 7001660000000000000♠66 Mys, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[80] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day.[81]

text: Future

text:
Earth's expected long-term future is tied to that of the Sun. Over the next 7000110000000000000♠1.1 Bys, solar luminosity will increase by 10%, and over the next 7000350000000000000♠3.5 Bys by 40%.[82] The Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing CO
2 concentration to levels lethally low for plants (6995099999999999999♠10 ppm for C4 photosynthesis) in approximately 7002500000000000000♠500–900 Mys.[83] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.[84] After another billion years all surface water will have disappeared[85] and the mean global temperature will reach 70 °C (158 °F).[84] From that point, the Earth is expected to be habitable for another 7016157788000000000♠500 Ma,[83] possibly up to 7016725824800000000♠2.3 Ga if nitrogen is removed from the atmosphere.[86] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges.[87]

text: The Sun will evolve to become a red giant in about 7000500000000000000♠5 Bys. Models predict that the Sun will expand to roughly 1 AU (150 million km; 93 million mi), about 250 times its present radius.[82][88] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from the Sun when the star reaches its maximum radius. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level).[82] A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized.[88]

text: Physical characteristics

text: Shape

text: The shape of Earth is approximately oblate spheroidal. Due to rotation, the Earth is flattened at the poles and bulging around the equator.[90] The diameter of the Earth at the equator is 43 kilometres (27 mi) larger than the pole-to-pole diameter.[91] Thus the point on the surface farthest from Earth's center of mass is the summit of the equatorial Chimborazo volcano in Ecuador.[92][93][94][95] The average diameter of the reference spheroid is 12,742 kilometres (7,918 mi). Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: The maximum deviation of only 0.17% is at the Mariana Trench (10,911 metres (35,797 ft) below local sea level), whereas Mount Everest (8,848 metres (29,029 ft) above local sea level) represents a deviation of 0.14%.[n 13]

text: In geodesy, the exact shape that Earth's oceans would adopt in the absence of land and perturbations such as tides and winds is called the geoid. More precisely, the geoid is the surface of gravitational equipotential at mean sea level.

text: Chemical composition

text:
Chemical composition of the crust[97]

Compound
Formula
Composition

Continental
Oceanic

silica
SiO
2
60.2%
48.6%

alumina
Al
2O
3
15.2%
16.5%

lime
CaO
5.5%
12.3%

magnesia
MgO
3.1%
6.8%

iron(II) oxide
FeO
3.8%
6.2%

sodium oxide
Na
2O
3.0%
2.6%

potassium oxide
K
2O
2.8%
0.4%

iron(III) oxide
Fe
2O
3
2.5%
2.3%

water
H
2O
1.4%
1.1%

carbon dioxide
CO
2
1.2%
1.4%

titanium dioxide
TiO
2
0.7%
1.4%

phosphorus pentoxide
P
2O
5
0.2%
0.3%

Total
99.6%
99.9%

text: Earth's mass is approximately 7024597000000000000♠5.97×1024 kg (5,970 Yg). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[98]

text: The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash, and soda.[97][98][99]

text: Internal structure

text: Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity. The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are composed. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[100] The Earth's inner core might rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[101] The radius of the inner core is about one fifth of that of Earth.

text:
Geologic layers of Earth[102]

Earth cutaway from core to exosphere. Not to scale.
Depth[103]
km
Component layer
Density
g/cm3

0–60
Lithosphere[n 14]
—

0–35
Crust[n 15]
2.2–2.9

35–60
Upper mantle
3.4–4.4

  35–2890
Mantle
3.4–5.6

100–700
Asthenosphere
—

2890–5100
Outer core
9.9–12.2

5100–6378
Inner core
12.8–13.1

text: Heat

text: Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[104] The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.[105] At the center, the temperature may be up to 6,000 °C (10,830 °F),[106] and the pressure could reach 360 GPa (52 million psi).[107] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately 7016946728000000000♠3 Ga, twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[104][108]

text:
Present-day major heat-producing isotopes[109]

Isotope
Heat release
W/kg isotope
Half-life
years
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle

238U
6995946000000000000♠94.6×10−6
7009447000000000000♠4.47×109
6992308000000000000♠30.8×10−9
6988291000000000000♠2.91×10−12

235U
6996569000000000000♠569×10−6
7008704000000000000♠0.704×109
6990220000000000000♠0.22×10−9
6987125000000000000♠0.125×10−12

232Th
6995263999999999999♠26.4×10−6
7010140000000000000♠14.0×109
6993124000000000000♠124×10−9
6988327000000000000♠3.27×10−12

40K
6995292000000000000♠29.2×10−6
7009125000000000000♠1.25×109
6992369000000000000♠36.9×10−9
6988108000000000000♠1.08×10−12

text: The mean heat loss from Earth is 7001870000000000000♠87 mW m−2, for a global heat loss of 7013442000000000000♠4.42×1013 W.[110] A portion of the core's thermal energy is transported toward the crust by mantle plumes, a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[111] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans because the crust there is much thinner than that of the continents.[112]

text: Tectonic plates

text:
Earth's major plates[113]

Plate name
Area
106 km2

  Pacific Plate

103.3

  African Plate[n 16]

78.0

  North American Plate

75.9

  Eurasian Plate

67.8

  Antarctic Plate

60.9

  Indo-Australian Plate

47.2

  South American Plate

43.6

text: Earth's mechanically rigid outer layer, the lithosphere, is divided into tectonic plates. These plates are rigid segments that move relative to each other at one of three boundaries types: At convergent boundaries, two plates come together; at divergent boundaries, two plates are pulled apart; and at transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.[114] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates.[115]

text: As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 7015315576000000000♠100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 7015631152000000000♠200 Ma old.[116][117] By comparison, the oldest dated continental crust is 7017127177128000000♠4,030 Ma.[118]

text: The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 7001500000000000000♠50 and 55 Mya. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[119] and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of 21 mm/a (0.83 in/year).[120]

text: Surface

text: The total surface area of Earth is about 510 million km2 (197 million sq mi).[13] Of this, 70.8%,[13] or 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water.[121] Below the ocean's surface are much of the continental shelf, mountains, volcanoes,[91] oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system. The remaining 29.2%, or 148.94 million km2 (57.51 million sq mi), not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. Tectonics and erosion, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape the Earth's surface over geological time.[122][123]

text: The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[124] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[125] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[126] Common carbonate minerals include calcite (found in limestone) and dolomite.[127]

text: The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[128]

text: The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. The total arable land is 10.9% of the land surface, with 1.3% being permanent cropland.[129][130] Close to 40% of Earth's land surface is used for agriculture, or an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[131]

text: Hydrosphere

text: The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from other planets in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m (6,600 ft). The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of 10,911.4 m (35,799 ft).[n 17][132]

text: The mass of the oceans is approximately 1.35×1018 metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8 million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080 ft), resulting in an estimated volume of 1.332 billion km3 (320 million cu mi).[133] If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km (1.68 to 1.74 mi).[134][135]

text: About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[136]

text: The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[137] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[138] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[139] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[140] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[141]

text: Atmosphere

text: The atmospheric pressure on Earth's surface averages 101.325 kPa (14.6959 psi), with a scale height of about 8.5 km (5.3 mi).[3] It is composed of 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide, and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at the equator, with some variation resulting from weather and seasonal factors.[142]

text:
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 7000270000000000000♠2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[71] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O
2 into O
3. The ozone layer blocks ultraviolet solar radiation, permitting life on land.[143] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[144] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C (0 °F), in contrast to the current +15 °C (59 °F),[145] and life on Earth probably would not exist in its current form.[146] In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[147][148]

text: Weather and climate

text: Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.[149]

text: The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[150] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[151]

text: Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as precipitation.[149] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features, and temperature differences determine the average precipitation that falls in each region.[152]

text: The amount of solar energy reaching Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator.[153] Earth's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[154]

text: This latitudinal rule has several anomalies:

text:
Proximity to oceans moderates the climate. For example, the Scandinavian Peninsula has more moderate climate than similarly northern latitudes of northern Canada.
The wind enables this moderating effect. The windward side of a land mass experiences more moderation than the leeward side. In the Northern Hemisphere, the prevailing wind is west-to-east, and western coasts tend to be milder than eastern coasts. This is seen in Eastern North America and Western Europe, where rough continental climates appear on the east coast on parallels with mild climates on the other side of the ocean.[155] In the Southern Hemisphere, the prevailing wind is east-to-west, and the eastern coasts are milder.
The distance from the Earth to the Sun varies. The Earth is closest to the Sun (at perihelion) in January, which is summer in the Southern Hemisphere. It is furthest away (at aphelion) in July, which is summer in the Northern Hemisphere, and only 93.55% of the solar radiation from the Sun falls on a given square area of land than at perihelion. Despite this, there are larger land masses in the Northern Hemisphere, which are easier to heat than the seas. Consequently, summers are 2.3 °C (4 °F) warmer in the Northern Hemisphere than in the Southern Hemisphere under similar conditions.[156]
The climate is colder at high altitudes than at sea level because of the decreased air density.

text: The commonly used Köppen climate classification system has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[150] The Köppen system rates regions of terrain based on observed temperature and precipitation.

text: The highest air temperature ever measured on Earth was 56.7 °C (134.1 °F) in Furnace Creek, California, in Death Valley, in 1913.[157] The lowest air temperature ever directly measured on Earth was −89.2 °C (−128.6 °F) at Vostok Station in 1983,[158] but satellites have used remote sensing to measure temperatures as low as −94.7 °C (−138.5 °F) in East Antarctica.[159] These temperature records are only measurements made with modern instruments from the 20th century onwards and likely do not reflect the full range of temperature on Earth.

text: Upper atmosphere

text: Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[144] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.[160] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[161]

text: Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady loss of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily, and it leaks into outer space at a greater rate than other gases.[162] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[163] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[164] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[165]

text: Gravitational field

text: The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within the Earth. Near the Earth's surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad, regional differences in the Earth's gravitational field, known as gravitational anomalies.[166]

text: Magnetic field

text: The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[167] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[168][169]

text: Magnetosphere

text: The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail.[170] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bowshock precedes the dayside magnetosphere within the solar wind.[171] Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates;[172][173] the ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[174] and the Van Allen radiation belt are formed by high-energy particles whose motion is essentially random, but otherwise contained by the magnetosphere.[170][175]

text: During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the aurora.[176]

text: Orbit and rotation

text: Rotation

text: Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[177] Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 SI ms longer.[178][179]

text: Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[2][n 18] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[180] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[181] and 1962–2005.[182]

text: Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[183][184]

text: Orbit

text: Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.[3]

text: The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the north poles of both the Sun and Earth, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[3][185]

text: The Hill sphere, or the sphere of gravitational influence, of the Earth is about 1.5 million km (930,000 mi) in radius.[186][n 19] This is the maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

text: Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm.[187]

text: Axial tilt and seasons

text: The axial tilt of the Earth is approximately 23.439281°[2] with the axis of its orbit plane, always pointing towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and winter taking place when the Tropic of Capricorn in the Southern Hemisphere faces the Sun. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone and sets south of true west.

text: Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year, up to six months at the North Pole itself, a polar night. In the Southern Hemisphere, the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole. Six months later, this pole will experience a midnight sun, a day of 24 hours, again reversing with the South Pole.

text: By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[188]

text: The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[189] The orientation (rather than the angle) of Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[190]

text: In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9%[n 20] in solar energy reaching Earth at perihelion relative to aphelion. Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.[191]

text: A study from 2016 suggested that Planet Nine tilted all Solar System planets, including Earth's, by about six degrees.[192]

text: Habitability

text: A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[193] The distance of Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere, and magnetic field all contribute to the current climatic conditions at the surface.[194]

text: Biosphere

text: A planet's life forms inhabit ecosystems, whose total is sometimes said to form a "biosphere". Earth's biosphere is thought to have begun evolving about 7000350000000000000♠3.5 Gya.[71] The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.[195]

text: In July 2016, scientists reported identifying a set of 355 genes from the last universal common ancestor (LUCA) of all organisms living on Earth.[196]

text: Natural resources and land use

text:
Estimated human land use, 2000[197]

Land use
Mha

Cropland
1,510–1,611

Pastures
2,500–3,410

Natural forests
3,143–3,871

Planted forests
126–215

Urban areas
66–351

Unused, productive land
356–445

text: Earth has resources that have been exploited by humans. Those termed non-renewable resources, such as fossil fuels, only renew over geological timescales.

text: Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion, and plate tectonics.[198] These bodies form concentrated sources for many metals and other useful elements.

text: Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[199] In 1980, 50.53 million km2 (19.51 million sq mi) of Earth's land surface consisted of forest and woodlands, 67.88 million km2 (26.21 million sq mi) was grasslands and pasture, and 15.01 million km2 (5.80 million sq mi) was cultivated as croplands.[200] The estimated amount of irrigated land in 1993 was 2,481,250 km2 (958,020 sq mi).[14] Humans also live on the land by using building materials to construct shelters.

text: Natural and environmental hazards

text: Large areas of Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year.[201] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

text: Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.

text: There is a scientific consensus linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.[202]

text: Human geography

text:
North America
  South America
  Antarctica

  Europe
  Africa

  Asia
  Australia

v
t
e

text: Cartography, the study and practice of map-making, and geography, the study of the lands, features, inhabitants and phenomena on Earth, have historically been the disciplines devoted to depicting Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

text: Earth's human population reached approximately seven billion on 31 October 2011.[204] Projections indicate that the world's human population will reach 9.2 billion in 2050.[205] Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[206]

text: It is estimated that one-eighth of Earth's surface is suitable for humans to live on – three-quarters of Earth's surface is covered by oceans, leaving one-quarter as land. Half of that land area is desert (14%),[207] high mountains (27%),[208] or other unsuitable terrains. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[209] (82°28′N) The southernmost is the Amundsen–Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

text: Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica, a few land parcels along the Danube river's western bank, and the unclaimed area of Bir Tawil between Egypt and Sudan. As of 2015[update], there are 193 sovereign states that are member states of the United Nations, plus two observer states and 72 dependent territories and states with limited recognition.[14] Earth has never had a sovereign government with authority over the entire globe, although some nation-states have striven for world domination and failed.[210]

text: The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[211] The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.[212]

text: The first human to orbit Earth was Yuri Gagarin on 12 April 1961.[213] In total, about 487 people have visited outer space and reached orbit as of 30 July 2010[update], and, of these, twelve have walked on the Moon.[214][215][216] Normally, the only humans in space are those on the International Space Station. The station's crew, made up of six people, is usually replaced every six months.[217] The farthest that humans have traveled from Earth is 400,171 km (248,655 mi), achieved during the Apollo 13 mission in 1970.[218]

text: Moon

text:
Characteristics

Diameter
7006347480000000000♠3,474.8 km

Mass
7022734900000000000♠7.349×1022 kg

Semi-major axis
7008384400000000000♠384,400 km

Orbital period
27d 7h 43.7m

text: The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto. The natural satellites of other planets are also referred to as "moons", after Earth's.

text: The gravitational attraction between Earth and the Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

text: Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs/yr—add up to significant changes.[219] During the Devonian period, for example, (approximately 7002410000000000000♠410 Mya) there were 400 days in a year, with each day lasting 21.8 hours.[220]

text: The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[28] Some theorists think that without this stabilization against the torques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[221]

text: Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[184] This allows total and annular solar eclipses to occur on Earth.

text: The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.[222]

text: Asteroids and artificial satellites

text: Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29.[223][224] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in the Earth's orbit around the Sun.[225][226]

text: The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.[227]

text: As of August 2017[update], there were 1,738 operational, human-made satellites orbiting Earth.[5] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 3] Earth's largest artificial satellite is the International Space Station.

text: Cultural and historical viewpoint

text: The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, ,[228] representing the four corners of the world.

text: Human cultures have developed many views of the planet. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity,[229] and by the mid-20th century, the Gaia Principle compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[230][231][232] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[229]

text: Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. In the West, belief in a flat Earth[233] was displaced by the idea of spherical Earth, credited to Pythagoras in the 6th century BC.[234] Earth was further believed to be the center of the universe until the 16th century when scientists first theorized that it was a moving object, comparable to the other planets in the Solar System.[235] Due to the efforts of influential Christian scholars and clerics such as James Ussher, who sought to determine the age of Earth through analysis of genealogies in Scripture, Westerners before the 19th century generally believed Earth to be a few thousand years old at most. It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[236]

text: Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[237][238] The perception of Earth shifted again in the 20th century when humans first viewed it from orbit, and especially with photographs of Earth returned by the Apollo program.[239]