Earth Astronomical symbol of Earth
The Earth seen from Apollo 17.jpg
The Blue Marble photograph of Earth, taken during the Apollo 17 lunar mission in 1972
Orbital characteristics
Epoch J2000[n 1]
Aphelion 152100000/
Perihelion 147095000/
149598023/
Eccentricity 0.0167086[1]
365.256363004/
29.78/
358.617 °
Inclination
−11.26064 °[3] to J2000 ecliptic
114.20783 °[3]
Satellites
Physical characteristics
Mean radius
6371.0/[5]
Equatorial radius
6378.1/[6][7]
Polar radius
6356.8/[8]
Flattening 0.0033528[9]
1/298.257222101 (ETRS89)
Circumference
Volume 1.08321×1012 km3 (2.59876×1011 cu mi)[3]
Mass 5.97237×1024 kg (1.31668×1025 lb)[14]
(3.0×10−6 solar mass)

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago.[19][20][21] Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.[n 4]

Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth.[22] The gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation.[23] Earth is the densest planet in the Solar System and the largest of the four terrestrial planets.

Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water, mostly by oceans.[24] The remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, and a convecting mantle that drives plate tectonics.

Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms. Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive.[25][26] In the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species[27] that ever lived on Earth are extinct.[28][29] Estimates of the number of species on Earth today vary widely;[30][31][32] most species have not been described.[33] Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.[34] Humans have developed diverse societies and cultures; politically, the world has about 200 sovereign states.


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  1. ^ Cite error: Invalid <ref> tag; no text was provided for refs named VSOP87
  2. ^ Cite error: Invalid <ref> tag; no text was provided for refs named Allen294
  3. ^ a b c d e f Cite error: Invalid <ref> tag; no text was provided for refs named earth_fact_sheet
  4. ^ Cite error: Invalid <ref> tag; no text was provided for refs named ucs
  5. ^ Cite error: Invalid <ref> tag; no text was provided for refs named hbcp2000
  6. ^ Cite error: Invalid <ref> tag; no text was provided for refs named usno
  7. ^ a b World Geodetic System (WGS-84). Available online from National Geospatial-Intelligence Agency.
  8. ^ Cite error: Invalid <ref> tag; no text was provided for refs named cazenave_ahrens1995
  9. ^ Cite error: Invalid <ref> tag; no text was provided for refs named IERS2004
  10. ^ Cite error: Invalid <ref> tag; no text was provided for refs named WGS-84-2
  11. ^ Earth's circumference is almost exactly 40,000 km because the metre was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
  12. ^ Cite error: Invalid <ref> tag; no text was provided for refs named Pidwirny 2006_8
  13. ^ Cite error: Invalid <ref> tag; no text was provided for refs named cia
  14. ^ Cite error: Invalid <ref> tag; no text was provided for refs named Luzum2011
  15. ^ Cite error: Invalid <ref> tag; no text was provided for refs named NIST2008
  16. ^ Cite error: Invalid <ref> tag; no text was provided for refs named Williams1994
  17. ^ Cite error: Invalid <ref> tag; no text was provided for refs named Allen296
  18. ^ Cite error: Invalid <ref> tag; no text was provided for refs named IERS
  19. ^ "Age of the Earth". U.S. Geological Survey. 1997. Archived from the original on 23 December 2005. http://pubs.usgs.gov/gip/geotime/age.html. Retrieved 10 January 2006. 
  20. ^ Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Special Publications, Geological Society of London 190 (1): 205–21. DOI:10.1144/GSL.SP.2001.190.01.14. 
  21. ^ Manhesa, Gérard (1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters 47 (3): 370–82. DOI:10.1016/0012-821X(80)90024-2. 
  22. ^ Cite error: Invalid <ref> tag; no text was provided for refs named yoder1995
  23. ^ Cite error: Invalid <ref> tag; no text was provided for refs named aaa428_261
  24. ^ National Oceanic and Atmospheric Administration. "Ocean". http://www.noaa.gov/ocean.html. Retrieved 3 May 2013. 
  25. ^ Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Associated Press (Yonkers, NY: Mindspark Interactive Network). http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html. Retrieved 20 October 2015. 
  26. ^ (19 October 2015) "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF). Proc. Natl. Acad. Sci. U.S.A. 112 (47): 14518–21. DOI:10.1073/pnas.1517557112. ISSN 1091-6490. PMID 26483481. Retrieved on 20 October 2015.  Early edition, published online before print.
  27. ^ The Biology of Rarity: Causes and consequences of rare—common differences. 31 December 1996. p. 110. ISBN 978-0412633805. https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110. Retrieved 26 May 2015. 
  28. ^ Stearns, Beverly Peterson; Stearns, S. C.; Stearns, Stephen C. (2000). Watching, from the Edge of Extinction. Yale University Press. p. preface x. ISBN 978-0-300-08469-6. https://books.google.com/books?id=0BHeC-tXIB4C. Retrieved 30 May 2017. 
  29. ^ Novacek, Michael J. (8 November 2014). "Prehistory's Brilliant Future". The New York Times. https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html. Retrieved 25 December 2014. 
  30. ^ Cite error: Invalid <ref> tag; no text was provided for refs named science_241_4872_1441
  31. ^ Miller, G.; Spoolman, Scott (1 January 2012). "Biodiversity and Evolution". Environmental Science. Cengage Learning. p. 62. ISBN 978-1-133-70787-5. https://books.google.com/books?id=NYEJAAAAQBAJ&pg=PA62. Retrieved 27 December 2014. 
  32. ^ Staff (2 May 2016). "Researchers find that Earth may be home to 1 trillion species". National Science Foundation. https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446. Retrieved 6 May 2016. 
  33. ^ (23 August 2011) "How many species are there on Earth and in the ocean?". PLOS Biology 9 (8): e1001127. DOI:10.1371/journal.pbio.1001127. PMID 21886479. 
  34. ^ Hwang, Andrew D. (9 July 2018). "7.5 billion and counting: How many humans can the Earth support?". The Conversation. https://theconversation.com/7-5-billion-and-counting-how-many-humans-can-the-earth-support-98797. Retrieved 28 July 2018. 
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