What is the circumference of the planet Earth?
Earth's circumference
In modern times it has been used to define fundamental units of measurement of length – the nautical mile in the seventeenth century and the metre in the eighteenth. Earth's polar circumference is almost exactly 40,000 km because the metre was originally calibrated on this measurement (1/10-millionth of the distance between the poles and the equator), which is almost exactly 21,600 nautical miles (being defined as one minute of a degree – i.e. 360 multiplied by 60).
3854 George
According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), "George" measures between 3.02 and 3.26 kilometers in diameter and its surface has an albedo between 0.22 and 0.46. In 2017, a WISE-study dedicated to Mars-crossing asteroids gave a somewhat larger diameter of 3.62 kilometers with an albedo of 0.308. The "Collaborative Asteroid Lightcurve Link" assumes an albedo of 0.30 and calculates a diameter of 3.67 kilometers based on an absolute magnitude of 14.1.
History of geodesy
From these observations, measurements, and/or "known" facts, Eratosthenes concluded that since the angular deviation of the sun from the vertical direction at Alexandria was also the angle of the subtended arc (see illustration), the linear distance between Alexandria and Syene was 1/50 of the circumference of the Earth which thus must be 50×5000 = 250,000 stadia or probably 25,000 geographical miles. The circumference of the Earth is 24,902 mi (40,075.16 km). Over the poles it is more precisely 40,008 km or 24,860 mi. The actual unit of measure used by Eratosthenes was the stadion. No one knows for sure what his stadion equals in modern units, possibly it was the Hellenic 185 m stadion.
Eratosthenes
Eratosthenes later rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia, likely for reasons of calculation simplicity as the larger number is evenly divisible by 60. In 2012, Anthony Abreu Mora repeated Eratosthenes's calculation with more accurate data; the result was 40,074 km, which is 66 km different (0.16%) from the currently accepted polar circumference of the Earth.
ED50
It used the International Ellipsoid of 1924 ("Hayford-Ellipsoid" of 1909) (radius of the Earth's equator 6378.388 km, flattening 1/297, both exact). That spheroid was an early attempt to model the whole Earth and was widely used around the world until the 1980s when GRS 80 and WGS 84 were established.
Angular diameter
Thus, the angular diameter of Earth's orbit around the Sun as viewed from a distance of 1 pc is 2″, as 1 AU is the mean radius of Earth's orbit.
Kepler-20f
Kepler-20f is very likely (>95% chance) a rocky planet because of its radius, which is notable as being the closest to Earth yet: 1.004 . However, although its radius is almost the same as Earth's, its surface conditions are not Earth-like in any way. The equilibrium temperature of Kepler-20f is approximately , too hot to support liquid water on its surface and hot enough to melt some types of metal. But because of its size, it can be expected to have an atmosphere of water vapor. The mass of the planet can be approximated to around 0.66–3.04 , depending on its composition. An Earth-like composition would have its mass to be around 1.2 .
Asteroid
Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as "Earth-crossers". , 14,464 near-Earth asteroids are known and the number over one kilometer in diameter is estimated to be 900–1,000.
Earth's circumference
Earth's circumference is the distance around the Earth, either around the equator () or around the poles ().
Metre
The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth's circumference is approximately 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted.
History of geodesy
Around AD 830 Caliph al-Ma'mun commissioned a group of astronomers led by Al-Khwarizmi to measure the distance from Tadmur (Palmyra) to Raqqa, in modern Syria. They found the cities to be separated by one degree of latitude and the distance between them to be 66 miles and thus calculated the Earth's circumference to be 24,000 miles. Another estimate given was 56 Arabic miles per degree, which corresponds to 111.8 km per degree and a circumference of 40,248 km, very close to the currently modern values of 111.3 km per degree and 40,068 km circumference, respectively.
Earth
The Hill sphere, or the sphere of gravitational influence, of the Earth is about in radius. 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.
Milankovitch cycles
The Earth's orbit approximates an ellipse. Eccentricity measures the departure of this ellipse from circularity. The shape of the Earth's orbit varies between nearly circular (with the lowest eccentricity of 0.000055) and mildly elliptical (highest eccentricity of 0.0679) Its geometric or logarithmic mean is 0.0019. The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ±0.012). Other components have 95,000-year and 125,000-year cycles (with a beat period of 400,000 years). They loosely combine into a 100,000-year cycle (variation of −0.03 to +0.02). The present eccentricity is 0.017 and decreasing.
Kepler orbit
where
Kepler-11g
Its radius is estimated to be 3.33 times that of Earth, somewhat smaller than Neptune's radius. 
According to formation models, the planet has a gaseous envelope comprising about 10% of its mass.
Kepler-11g has an estimated surface equilibrium temperature of 400 K, over 1.5 times (50% higher) that of Earth's equilibrium temperature. Kepler-11g orbits Kepler-11 every 118.37774 days (over 2.5 times that of the fifth planet from Kepler-11, Kepler-11f) at a distance of 0.462 AU, almost half of the distance from which Earth orbits the Sun. Its eccentricity is unknown. In comparison, planet Mercury orbits the Sun every 87.97 days at a distance of 0.387 AU. With an orbital inclination of 89.8°, Kepler-11g is seen almost edge-on with respect to Earth.
Figure of the Earth
The simplest model for the shape of the entire Earth is a sphere. The Earth's radius is the distance from Earth's center to its surface, about . While "radius" normally is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth".
Earth radius
The Earth's mean radius of curvature at latitude is:
Kepler-68c
Kepler-68c is an Earth-sized planet orbiting the star Kepler-68 in the constellation of Cygnus. It was discovered by planetary-transit methods by the Kepler space telescope in February 2013. It has a mass of 4.8 times that of Earth (0.015 M) and a radius of 0.953 Earth radii. It has an orbital period of 9.605085 days at a distance of about 0.09059 AU from its star. Relatively wide constraints on Kepler-68c's mass are the result lack of detection of the planet through radial-velocity and transit-timing-variation methods.
Spherical Earth
The works of the classical Indian astronomer and mathematician, Aryabhatta (476–550 AD), deal with the sphericity of the Earth and the motion of the planets. The final two parts of his Sanskrit magnum opus, the "Aryabhatiya", which were named the "Kalakriya" ("reckoning of time") and the "Gol" ("sphere"), state that the Earth is spherical and that its circumference is 4,967 yojanas. In modern units this is , close to the current equatorial value of .
List of Earth-crossing minor planets
An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. The known numbered Earth-crossers are listed here. Those Earth-crossers whose semi-major axes are smaller than Earth's are Aten asteroids; the remaining ones are Apollo asteroids. "(See also the Amor asteroids.)"
Earth radius
The International Union of Geodesy and Geophysics (IUGG) gives three global average radii, the arithmetic mean of the radii of the ellipsoid (R), the radius of a sphere with the same surface area as the ellipsoid or authalic radius (R), and the radius of a sphere with the same volume as the ellipsoid (R). All three IUGG average radii are about . A fourth global average radius not mentioned by the IUGG is the rectifying radius, the radius of a sphere with a circumference equal to the perimeter of the polar cross section of the ellipsoid, about . The radius of curvature at any point on the surface of the ellipsoid depends on its coordinates and its azimuth, north-south (meridional), east-west (prime vertical), or somewhere in between.
Figure of the Earth
The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947 – 3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of . Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 – 3,963 mi). The difference correspond to the polar radius being approximately 0.3% shorter than the equator radius.
Earth radius
This "radius of curvature in the prime vertical" which is perpendicular (normal or orthogonal) to at geodetic latitude is:
This radius is also called the transverse radius of curvature. At the equator, .
Kepler-12b
Kepler-12b has a mass of 0.431 Jupiters. Its radius of 1.695 Jupiter radii, however, indicates that the planet is almost 70% more than the size of Jupiter. With an orbit of 0.0556 AU, Kepler-12b's average orbit is approximately 5% the average distance between the Earth and Sun. The orbit lasts 4.4379637 days. Kepler-12b has an orbital inclination of 88.86º, indicating that the planet is seen as nearly edge-on with respect to the Earth and to its host star. According to Kepler's official website, the mass and radius of the planet can be compared to 137 Earths (in mass) and 19 Earths (for its radius). The planet's density is 0.111 grams/cm, about a tenth of the density of water, and its equilibrium temperature is 1481 K (some 5.8 times greater than Jupiter's equilibrium temperature). Additionally, Kepler-12b has an almost totally circular orbit, with an orbital eccentricity of less than 0.01.
Structure of the Earth
The average density of Earth is
. Because the average density of surface material is only around , we must conclude that denser materials exist within Earth's core.
This result has been known since the Schiehallion experiment, performed in the 1770s.
Charles Hutton in his 1778 report concluded that the mean density of the Earth must be about formula_1 that of surface rock, concluding that the interior of the Earth must be metallic. Hutton estimated this metallic portion to occupy some 65% of the diameter of the Earth.
Hutton's estimate on the mean density of the Earth was still about 20% too low, at 
Henry Cavendish in his torsion balance experiment of 1798 found a value of , within 1% of the modern value.
Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ≈1,220 km and a liquid outer core extending beyond it to a radius of ≈3,400 km. The densities are between 9,900 and 12,200 kg/m in the outer core and 12,600–13,000 kg/m in the inner core.
Kepler-10
Per the usual exoplanet nomenclature, the first planet discovered to be orbiting Kepler-10 is called Kepler-10b. Announced in 2011, it was the first rocky planet identified outside the Solar system. The planet has a mass that is 3.33±0.49 times that of Earth's and a radius that is 1.47 times that of Earth. The planet orbits Kepler-10 at a distance of 0.01684 AU every 0.8375 days; this can be compared to the orbit and orbital period of planet Mercury, which circles the Sun at a distance of 0.3871 AU every 87.97 days. Because the planet orbits so closely to its star, its eccentricity is virtually zero. It, thus, has an extremely circular orbit.
Voyages of Christopher Columbus
A spherical Earth had been the general opinion of Ancient Greek science, and this view continued through the Middle Ages (for example, Bede mentions it in "The Reckoning of Time"). In fact Eratosthenes had measured the diameter of the Earth with good precision in the 2nd century BC. Where Columbus did differ from the generally accepted view of his time was in his incorrect arguments that assumed a significantly smaller diameter for the Earth, claiming that Asia could be easily reached by sailing west across the Atlantic. Most scholars accepted Ptolemy's correct assessment that the terrestrial landmass (for Europeans of the time, comprising Eurasia and Africa) occupied 180 degrees of the terrestrial sphere, and dismissed Columbus's claim that the Earth was much smaller, and that Asia was only a few thousand nautical miles to the west of Europe. Columbus's error was attributed to his insufficient experience in navigation at sea.
Kepler-9d
Kepler-9d, formerly known as KOI-377.03, is a planet in orbit around the sunlike star Kepler-9. Initially discovered by Kepler spacecraft, a terrestrial planet-searching satellite built and operated by NASA, Kepler-9d is most likely a Super-Earth, with an estimated radius approximately 60% larger than that of Earth's, although its exact mass cannot be determined. Kepler-9d orbits Kepler-9 every 1.56 days at a distance of .0273 AU from its star, an extremely close distance. Although Kepler-9d is the closest planet to its star in its system, it is named Kepler-9d instead of Kepler-9b because two gas giants, Kepler-9b and Kepler-9c, were confirmed first. The original studies into the system first suggested that Kepler-9d might be a planet, but a follow-up investigation made by the Kepler team later confirmed that it was; the confirmation of Kepler-9d as a planet was made public with the team's paper, which was published in the Astrophysical Journal on January 1, 2011 . The team used telescopes at the W.M. Keck Observatory in Hawaii to follow up on the Kepler space telescope's initial discovery.
History of geodesy
Revising the figures attributed to Posidonius, another Greek philosopher determined 18,000 miles as the Earth's circumference. This last figure was promulgated by Ptolemy through his world maps. The maps of Ptolemy strongly influenced the cartographers of the Middle Ages. It is probable that Christopher Columbus, using such maps, was led to believe that Asia was only 3 or 4 thousand miles west of Europe.
Minute and second of arc
A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn – it is for this reason that the Earth's circumference is almost exactly 21,600 nautical miles. A minute of arc is of a radian.
Close to Critical
Revolving on an orbit about the star Altair, Tenebra is an example of one of Hal Clement's "hellworlds". 27 times the mass of Earth, Tenebra is three times Earth's diameter (38,226 kilometers; 23,746 miles) and has three times Earth's gravity at its surface (29.43 meters per second per second; 96.6 feet per second per second), which gives Tenebra an escape velocity of 33.558 kilometers per second (nearly 21 miles per second). Clement also describes the planet as rotating on its axis in a little less than four Earth days.
Absolute rotation
In his "Principia", Newton proposed the shape of the rotating Earth was that of a homogeneous ellipsoid formed by an equilibrium between the gravitational force holding it together and the centrifugal force pulling it apart. This effect is more easily seen with the planet Saturn which has a radius 8.5 to 9.5 times that of Earth but has a rotational period of only 10.57 hours. The ratios of Saturn's diameters is approximately 11 to 10.
Khālid ibn ʿAbd al‐Malik al‐Marwarrūdhī
Together with ʿAlī ibn ʿĪsā al-Asṭurlābī in 827, he measured at 35 degrees north latitude, in the valley of the Tigris, the length of a meridian arc and thus the Earth's circumference, getting a result of 40,248 km (or, according to other sources, 41,436 km). The two researchers measured in Arabian ell, and determined the geographical latitudes of the end points they used from the star altitudes in a celestial horizontal coordinate system. We believe that 1 Arabian ell was 49 1/3 cm. Thus, they found the length of 1° of meridian to be 111.8 km (115.1 km), which differs from the actual value by 850 metres.
Minute and second of arc
Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. Because of the effects of atmospheric seeing, ground-based telescopes will smear the image of a star to an angular diameter of about 0.5 arcsecond; in poor seeing conditions this increases to 1.5 arcseconds or even more. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond.
Earth mass
Before the direct measurement of the gravitational constant, estimates of the Earth mass were limited to estimating Earth's mean density from observation of the crust and estimates on Earth's volume. Estimates on the volume of the earth in the 17th century were based on a circumference estimate of to the degree of latitude, corresponding to a radius of 5,500 km (86% of the Earth's actual radius of about 6,371 km), resulting in an estimated volume of about one third smaller than the correct value.
Early world maps
Posidonius (or Poseidonius) of Apameia (c. 135–51 BCE), was a Greek Stoic philosopher who traveled throughout the Roman world and beyond and was a celebrated polymath throughout the Greco-Roman world, like Aristotle and Eratosthenes. His work "about the ocean and the adjacent areas" was a general geographical discussion, showing how all the forces had an effect on each other and applied also to human life. He measured the Earth's circumference by reference to the position of the star Canopus. His measure of 240,000 stadia translates to , close to the actual circumference of .
He was informed in his approach by Eratosthenes, who a century earlier used the elevation of the Sun at different latitudes. Both men's figures for the Earth's circumference were uncannily accurate, aided in each case by mutually compensating errors in measurement. However, the version of Posidonius' calculation popularised by Strabo was revised by correcting the distance between Rhodes and Alexandria to 3,750 stadia, resulting in a circumference of 180,000 stadia, or . Ptolemy discussed and favored this revised figure of Posidonius over Eratosthenes in his "Geographia", and during the Middle Ages scholars divided into two camps regarding the circumference of the Earth, one side identifying with Eratosthenes' calculation and the other with Posidonius' 180,000 stadion measure.
Kepler-37b
Kepler-37b is a sub-Earth, an exoplanet with a radius and mass smaller than Earth. Its surface temperature is . Because of this, it is not expected to have an atmosphere. Its radius is approximately 0.34 (about a diameter of ), about a little larger than the Moon. Due to its small size, it is very likely Kepler-37b is a rocky planet with a solid surface. Furthermore, it is too hot to support liquid water on its surface.
Earth radius
where and are, respectively, the equatorial radius and the polar radius.There are two principal radii of curvature: along the meridional and prime-vertical normal sections.
IAU definition of planet
According to an IAU draft resolution, the roundness condition generally results in the need for a mass of at least 5 kg, or diameter of at least 800 km. However, Mike Brown claims that these numbers are only right for rocky bodies like asteroids, and that icy bodies like Kuiper Belt objects reach hydrostatic equilibrium at much smaller sizes, probably somewhere between 200 and 400 km in diameter. It all depends on the rigidity of the material that makes up the body, which is in turn strongly influenced by its internal temperature. Assuming that Methone's shape reflects the balance between the tidal force exerted by Saturn and the moon's gravity, its tiny 3 km diameter suggests Methone is composed of icy fluff.
372 Palma
Since 2000, it has been observed 14 times in an Asteroid Occultation event, a number of which produced multiple chords revealing the asteroid's size and shape. On September 13, 2018 it was revealed to be 120 miles long (193 kilometers long). However it will never hit the Earth since it's in a fixed orbit around the Sun between the orbits of Mars and Jupiter.
Kepler orbit
where
Rings of Chariklo
The minor planet and centaur 10199 Chariklo, with a diameter of about , is the smallest celestial object with confirmed rings and the fifth ringed celestial object ever discovered in the Solar System, after the gas giants and ice giants. Orbiting Chariklo is a bright ring system consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of . The rings orbit at distances of about from the centre of Chariklo, a thousandth the distance between Earth and the Moon. The discovery was made by a team of astronomers using ten telescopes at various locations in Argentina, Brazil, and Chile in South America during observation of a stellar occultation on 3 June 2013, and was announced on 26 March 2014.
Megagon
The perimeter of a regular megagon inscribed in the unit circle is:
which is very close to 2π. In fact, for a circle the size of the Earth's equator, with a circumference of 40,075 kilometres, one edge of a megagon inscribed in such a circle would be slightly over 40 meters long. The difference between the perimeter of the inscribed megagon and the circumference of this circle comes to less than 1/16 millimeters.
Earth's circumference
Earth's circumference lines are the foundation of the coordinate system of latitude and longitude, introduced in the mid-second century BC by Hipparchus.
History of geodesy
Since the spherical shape was the most widely supported during the Greek Era, efforts to determine its size followed. Aristotle reported that mathematicians had calculated the circumference of the Earth (which is slightly over 40,000 km) to be 400,000 stadia (between 62,800 and 74,000 km or 46,250 and 39,250 mi) while Archimedes stated an upper bound of 3,000,000 stadia (483,000 km or 300,000 mi) using the Hellenic stadion which scholars generally take to be 185 meters or of a geographical mile.
History of Mars observation
In the 5th century CE, the Indian astronomical text "Surya Siddhanta" estimated the angular size of Mars as 2 arc-minutes (1/30 of a degree) and its distance to Earth as 10,433,000 km (1,296,600 yojana, where one yojana is equivalent to eight km in the "Surya Siddhanta"). From this the diameter of Mars is deduced to be 6,070 km (754.4 yojana), which has an error within 11% of the currently accepted value of 6,788 km. However, this estimate was based upon an inaccurate guess of the planet's angular size. The result may have been influenced by the work of Ptolemy, who listed a value of 1.57 arc-minutes. Both estimates are significantly larger than the value later obtained by telescope.
In 1543, Nicolaus Copernicus published a heliocentric model in his work "De revolutionibus orbium coelestium". This approach placed the Earth in an orbit around the Sun between the circular orbits of Venus and Mars. His model successfully explained why the planets Mars, Jupiter and Saturn were on the opposite side of the sky from the Sun whenever they were in the middle of their retrograde motions. Copernicus was able to sort the planets into their correct heliocentric order based solely on the period of their orbits about the Sun. His theory gradually gained acceptance among European astronomers, particularly after the publication of the "Prutenic Tables" by the German astronomer Erasmus Reinhold in 1551, which were computed using the Copernican model.
Spherical Earth
Knowing the difference in angle between two points on the Earth's surface and the surface distance between them allows a calculation of the Earth's size. Using observations at Rhodes (in Greece) and Alexandria (in Egypt) and the distance between them, the Ancient Greek philosopher Posidonius actually did use this technique to calculate the circumference of the planet to within perhaps 4% of the correct value (though modern equivalents of his units of measure are not precisely known).
Earth radius
Another mean radius is the "rectifying radius", giving a sphere with circumference equal to the perimeter of the ellipse described by any polar cross section of the ellipsoid. This requires an elliptic integral to find, given the polar and equatorial radii:
Great-circle distance
The Earth is nearly spherical (see Earth radius), so great-circle distance formulas give the distance between points on the surface of the Earth correct to within about 0.5%. (See .)
Earth bulge
The mean radius of the earth is about . (See Earth radius) Using the same units for both the altitude of the station and the radius of the earth,
Kepler-186e
The exoplanet is only slightly larger than Earth, with a radius 1.27–1.33 times that of Earth. Its mass is not known but it is likely to have a similar composition to Earth, giving it a mass of about 2.29–2.72 times the mass of the Earth. Kepler-186e orbits an M-dwarf star with about 4% of the Sun's luminosity with an orbital period of 22.4077 days and an orbital radius of about 0.11 times that of Earth's (compared to 0.39 AU for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.22 to 0.40 AU).
Observable universe
The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or ) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs (93 billion light-years or ). Assuming that space is roughly flat (in the sense of being a Euclidean space), this size corresponds to a comoving volume of about ( or ).
Mercator projection
Although the surface of Earth is best modelled by an oblate ellipsoid of revolution, for small scale maps the ellipsoid is approximated by a sphere of radius "a". Many different methods exist for calculating "a". The simplest include (a) the equatorial radius of the ellipsoid, (b) the arithmetic or geometric mean of the semi-axes of the ellipsoid, (c) the radius of the sphere having the same volume as the ellipsoid. The range of all possible choices is about 35 km, but for small scale (large region) applications the variation may be ignored, and mean values of 6,371 km and 40,030 km may be taken for the radius and circumference respectively. These are the values used for numerical examples in later sections. Only high-accuracy cartography on large scale maps requires an ellipsoidal model.
Air mass (astronomy)
Using a scale height of 8435 m, Earth’s mean radius of 6371 km,
and including the correction for refraction,
2010 AU118
WISE estimates the asteroid to be in diameter. is the largest object listed on the Sentry Risk Table. It has a poorly constrained orbit with an uncertainty parameter of 9. Virtual clones of the asteroid that fit the uncertainty region in the known trajectory, show a 1 in 770 million chance that the asteroid could impact the Earth on 2020 October 20. With a Palermo Technical Scale of −3.14, the odds of an impact by in 2020 are about 1400 times less than the background hazard level of Earth impacts, which is defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact. NEODyS lists the nominal 20 October 2020 Earth distance as .
It has now been removed from the sentry list on 03/10/2018 as it poses no danger to earth
Minimum orbit intersection distance
The only object that has ever been rated at 4 on the Torino Scale (since downgraded), the Aten asteroid (99942) Apophis, has an Earth MOID of 0.000316 AU. This is not the smallest Earth MOID in the catalogues; many bodies with a small Earth MOID are not classed as PHO's because the objects are less than roughly 140 meters in diameter (or absolute magnitude, "H" < 22). Earth MOID values are generally more practical for asteroids less than 140 meters in diameter as those asteroids are very dim and often have a short observation arc with a poorly determined orbit. The only objects that have been detected and had their Earth-MOID calculated before Earth impact were the small asteroids and 2014 AA. was listed with a MOID of 0.00001 AU in the Minor Planet Center database, and is the smallest MOID calculated for an Apollo asteroid. It is even smaller at the more precise JPL Small Body Database (0.0000078 AU).
Ahmad ibn Muhammad ibn Kathir al-Farghani
In the 15th century, Christopher Columbus used al-Farghani's estimate for the Earth's circumference as the basis for his voyages to America. However, Columbus mistook al-Farghani's 7091-foot Arabic mile to be a 4856-foot Roman mile, causing him to underestimate the Earth's circumference, believing he could take a shortcut to Asia.
Mathematical coincidence
When it was discovered that the circumference of the earth was very close to 40,000,000 times this value, the meter was redefined to reflect this, as it was a more objective standard (because the gravitational acceleration varies over the surface of the Earth). This had the effect of increasing the length of the meter by less than 1%, which was within the experimental error of the time.
Earth's circumference
Latitude lines, known as circles of latitude are parallel to each other; the equatorial circumference is the largest circle of latitude, and the Earth’s longest circumference distance. Other circles of latitude have smaller circumference; for example, the 60th parallel north or south is approximately half as long as the Equator.
Oerth
According to the paper by Gary Holian in "Oerth Journal", Oerth's circumference is about 42,024 km, and its radius measures 6,714 km, about 1% more than that of Earth. There is no flattening of the planet sphere due to rotation, and divine power is again hinted at.
North African climate cycles
Obliquity, otherwise known as tilt, refers to the angle that Earth's axis of rotation makes with a line that is perpendicular to Earth's orbital plane. The current tilt of Earth's axis is roughly 23.5°. However, over long periods of time the tilt of Earth's axis of rotation changes because of the uneven distribution of mass across the planet and gravitational interactions with the Sun, Moon, and planets. Due to these interactions the tilt of Earth's axis of rotation varies between 22.2° and 24.5° on a 41,000 year cycle.
Earth radius
In geophysics, the International Union of Geodesy and Geophysics (IUGG) defines the mean radius (denoted ) to be
For Earth, the mean radius is .
(388188) 2006 DP14
The asteroid has an Earth minimum orbit intersection distance (MOID) of , which corresponds to 6.4 lunar distances (LD). On 10 February 2014, it passed Earth close to this theoretical minimum distance at 6.25 LD, or . This makes is a potentially hazardous asteroid (PHA), a body with a threatening close approach to the Earth, due to its low MOID and large size (absolute magnitude of 18.9). PHAs are defined as objects with an absolute magnitude of 22 or brighter – which generically corresponds to a diameter of approximately 140 meters – and a MOID that is smaller than 0.05 AU or 19.5 LD.
Kepler-10c
Kepler-10c orbits its host star every forty-five days at a quarter of the average distance between the Sun and Earth. Initial observations showed that it has a radius more than double that of Earth, and suggested a higher density, suggesting a mainly rocky composition with around 5–20% ices by mass. For comparison, the Earth's oceans represent only 0.02% of our planet's mass, with an additional amount potentially a few times this stored in the mantle. However, in 2017, more careful analysis using both HARPS and HIRES data revealed that Kepler-10c is not a large terrestrial planet, but instead a typical volatile-rich planet of about seven Earth masses.
Seconds pendulum
In 1790, one year before the metre was ultimately based on a quadrant of the Earth, Talleyrand proposed that the metre be the length of the seconds pendulum at a latitude of 45°. This option, with one-third of this length defining the "foot", was also considered by Thomas Jefferson and others for redefining the yard in the United States shortly after gaining independence from the British Crown.
Instead of the seconds pendulum method, the commission of the French Academy of Sciences – whose members included Lagrange, Laplace, Monge and Condorcet – decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris. Apart from the obvious consideration of safe access for French surveyors, the Paris meridian was also a sound choice for scientific reasons: a portion of the quadrant from Dunkirk to Barcelona (about 1000 km, or one-tenth of the total) could be surveyed with start- and end-points at sea level, and that portion was roughly in the middle of the quadrant, where the effects of the Earth's oblateness were expected to be the largest. The Spanish-French geodetic mission combined with an earlier mesurement of the Paris meridian arc and the Lapland geodetic mission had confirmed that the Earth was an oblate spheroid. Moreover observations were made with a pendulum to determine the local acceleration due to local gravity and centrifugal acceleration; and these observations coincided with the geodetic results in proving that the Earth is flattened at the poles. The acceleration of a body near the surface of the Earth, which is measured with the seconds pendulum, is due to the combined effects of local gravity and centrifugal acceleration. The gravity diminishes with the distance from the center of the Earth while the centrifugal force augments with the distance from the axis of the Earth's rotation, it follows that the resulting acceleration towards the ground is 0.5% greater at the poles than at the Equator and that the polar diameter of the Earth is smaller than its equatorial diameter.
Earth
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. 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.
Earth radius
The Earth's polar radius of curvature is:
Great-circle distance
The shape of the Earth closely resembles a flattened sphere (a spheroid) with equatorial radius formula_17 of 6378.137 km; distance formula_18 from the center of the spheroid to each pole is 6356.752 km. When calculating the length of a short north-south line at the equator, the circle that best approximates that line has a radius of formula_19 (which equals the meridian's semi-latus rectum), or 6335.439 km, while the spheroid at the poles is best approximated by a sphere of radius formula_20, or 6399.594 km, a 1% difference. So long as a spherical Earth is assumed, any single formula for distance on the Earth is only guaranteed correct within 0.5% (though better accuracy is possible if the formula is only intended to apply to a limited area). Using the mean earth radius, formula_21 (for the WGS84 ellipsoid) means that in the limit of small flattening, the mean square relative error in the estimates for distance is minimized.
Willebrord Snellius
In 1615 Snellius, after the work of Eratosthenes in Ptolemaic Egypt in the 3rd century BC, probably was the first to try to do a large-scale experiment to measure the circumference of the earth using triangulation. He was helped in his measurements by two of his students, the Austrian barons Erasmus and Casparus Sterrenberg. In several cities he also received support of friends among the city leaders ("regenten"). In his work "The terrae Ambitus vera quantitate" (1617) under the author's name ("The Dutch Eratosthenes") Snellius describes the methods he used. He came up with an estimate of 28,500 Rhineland rods – in modern units 107.37 km for one degree of latitude. 360 times 107.37 then gives a circumference of the Earth of 38,653 km. The actual circumference is 40,075 kilometers, so Snellius underestimated the circumference of the earth by 3.5%.
LHS 1140 b
The orbit of LHS 1140 b takes 24.737 days to complete, much quicker than Earth's year of 365 days. Its orbital radius is at 0.0875 AU, or 8.75% the distance between Earth and the Sun. While this is quite close, the star LHS 1140 is so dim that the planet only get 0.41 times the incident flux of Earth at this distance. With an albedo of 0, LHS 1140 b has an equilibrium temperature of , compared to Earth's at . If LHS 1140 b has an albedo similar to that of Earth, the equilibrium temperature would be even lower, at . However, with a greenhouse effect at least as strong as Earth's LHS 1140 b would have a surface temperature greater than for an albedo of 0. Due to the high mass of the planet, it likely has a thicker atmosphere with a more powerful greenhouse effect. In addition, the orbit of LHS 1140 b is rather eccentric. The discovery team found that a circular orbit is impossible due to tidal forces from the star, and that the planet's eccentricity is less than 0.29 with a 90% confidence. This would cause significant temperature variations as the planet swings closer and farther from its star over its 24-day orbit.
54 Piscium
The orbit of an Earth-like planet would need to be centered within 0.68 AU (around the orbital distance of Venus), which in a Keplerian system means a 240-day orbital period. In a later simulation with the brown dwarf, 54 Piscium b's orbit "sweeps clean" most test particles within 0.5 AU, leaving only asteroids "in low-eccentricity orbits near the known planet’s apastron distance, near the 1:2 mean-motion resonance". Also, observation has ruled out Neptune-class or heavier planets with a period of one year or less; which still allows for Earth-sized planets at 0.6 AU or more.
Surya Siddhanta
The "Surya Siddhanta" describes rules to calculate the motions of various planets and the moon relative to various constellations, diameters of various planets, and calculates the orbits of various astronomical bodies. The text asserts, according to Markanday and Srivatsava, that the earth is of a spherical shape. It treats earth as stationary globe around which sun orbits, and makes no mention of Uranus, Neptune or Pluto. It calculates the earth's diameter to be 8,000 miles (modern: 7,928 miles), diameter of moon as 2,400 miles (actual ~2,160) and the distance between moon and earth to be 258,000 miles (actual ~238,000). The text is known for some of earliest known discussion of sexagesimal fractions and trigonometric functions.
Earth bulge
But the radius of Earth is given in kilometers. Converting the equation for distance in kilometers and height in meter,
Spherical Earth
Al-Farghānī (Latinized as Alfraganus) was a Persian astronomer of the 9th century involved in measuring the diameter of the Earth, and commissioned by Al-Ma'mun. His estimate given above for a degree (56 Arabic miles) was much more accurate than the 60 Roman miles (89.7 km) given by Ptolemy. Christopher Columbus uncritically used Alfraganus's figure as if it were in Roman miles instead of in Arabic miles, in order to prove a smaller size of the Earth than that propounded by Ptolemy.Abu Rayhan Biruni (973–1048) used a new method to accurately compute the Earth's circumference, by which he arrived at a value that was close to modern values for the Earth's circumference. His estimate of 6,339.6 km for the Earth radius was only 31.4 km less than the modern mean value of 6,371.0 km. In contrast to his predecessors, who measured the Earth's circumference by sighting the Sun simultaneously from two different locations, Biruni developed a new method of using trigonometric calculations based on the angle between a plain and mountain top. This yielded more accurate measurements of the Earth's circumference and made it possible for a single person to measure it from a single location.
Biruni's method was intended to avoid "walking across hot, dusty deserts," and the idea came to him when he was on top of a tall mountain in India. From the top of the mountain, he sighted the angle to the horizon which, along with the mountain's height (which he calculated beforehand), allowed him to calculate the curvature of the Earth.
He also made use of algebra to formulate trigonometric equations and used the astrolabe to measure angles.
Nautical mile
In 1617 the Dutch scientist Willebrord Snell assessed the circumference of the Earth at 24,630 Roman miles (24,024 statute miles). Around that time British mathematician Edmund Gunter improved navigational tools including a new quadrant to determine latitude at sea. He reasoned that the lines of latitude could be used as the basis for a unit of measurement for distance and proposed the nautical mile as one minute or one-sixtieth () of one degree of latitude. As one degree is of a circle, one minute of arc is of a circle (or, in radians, ). These sexagesimal (base 60) units originated in Babylonian astronomy. Gunter used Snell's circumference to define a nautical mile as 6,080 feet, the length of one minute of arc at 48 degrees latitude. Since the earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about at the poles and at the Equator, with a mean value of . France and other countries measured the minute of arc at 45 degrees latitude, making the nautical mile .
Gliese 436
In 2008, a second planet, designated "Gliese 436 c" was claimed to have been discovered, with an orbital period of 5.2 days and an orbital semimajor axis of 0.045 AU. The planet was thought to have a mass of roughly 5 Earth masses and have a radius about 1.5 times larger than the Earth's. Due to its size, the planet was thought to be a rocky, terrestrial planet. It was announced by Spanish scientists in April 2008 by analyzing its influence on the orbit of Gliese 436 b. Further analysis showed that the transit length of the inner planet is not changing, a situation which rules out most possible configurations for this system. Also, if it did orbit at these parameters, the system would be the only "unstable" orbit on UA's Extrasolar Planet Interactions chart. The existence of this "Gliese 436 c" was thus regarded as unlikely, and the discovery was eventually retracted at the Transiting Planets conference in Boston, 2008.
History of the metre
Jean Richer and Giovanni Domenico Cassini measured the parallax of Mars between Paris and Cayenne in French Guiana when Mars was at its closest to Earth in 1672. They arrived at a figure for the solar parallax of 91/2 inches, equivalent to an Earth–Sun distance of about 22000 Earth radii. They were also the first astronomers to have access to an accurate and reliable value for the radius of Earth, which had been measured by their colleague Jean Picard in 1669 as 3269 thousand "toises". Isaac Newton used this measurement for establishing his law of universal gravitation. Picard's geodetic observations had been confined to the determination of the magnitude of the earth considered as a sphere, but the discovery made by Jean Richer turned the attention of mathematicians to its deviation from a spherical form. The determination of the figure of the earth became a problem of the highest importance in astronomy, inasmuch as the diameter of the earth was the unit to which all celestial distances had to be referred.
Posidonius
Strabo noted that the distance between Rhodes and Alexandria is 3,750 stadia, and reported Posidonius's estimate of the Earth's circumference to be 180,000 stadia or . Pliny the Elder mentions Posidonius among his sources and without naming him reported his method for estimating the Earth's circumference. He noted, however, that Hipparchus had added some 26,000 stadia to Eratosthenes's estimate. The smaller value offered by Strabo and the different lengths of Greek and Roman stadia have created a persistent confusion around Posidonius's result. Ptolemy used Posidonius's lower value of 180,000 stades (about 33% too low) for the earth's circumference in his "Geography". This was the number used by Christopher Columbus in order to underestimate the distance to India as 70,000 stades.
Earth radius
The Earth's radius of curvature along a course at an azimuth (measured clockwise from north) at is derived from Euler's curvature formula as follows:
Kepler-11g
Kepler-11g is an exoplanet discovered in the orbit of the sunlike star Kepler-11 by the Kepler spacecraft, a NASA satellite tasked with searching for terrestrial planets. Kepler-11g is the outermost of the star's six planets. The planet orbits at a distance of nearly half the mean distance between Earth and the Sun. It completes an orbit every 118 days, placing it much further from its star than the system's inner five planets. Its estimated radius is a little over three times that of Earth, i.e. comparable to Neptune's size. Kepler-11g's distance from the inner planets made its confirmation more difficult than that of the inner planets, as scientists had to work to exhaustively disprove all reasonable alternatives before Kepler-11g could be confirmed. The planet's discovery, along with that of the other Kepler-11 planets, was announced on February 2, 2011. According to NASA, the Kepler-11 planets form the flattest and most compact system yet discovered.
Circumstellar habitable zone
On 7 January 2013, astronomers from the "Kepler" team announced the discovery of Kepler-69c (formerly "KOI-172.02"), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to our Sun, in the CHZ and expected to offer habitable conditions. The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.
(6037) 1988 EG
The asteroid has an Earth minimum orbit intersection distance (MOID) of . In combination with its size, this makes it a potentially hazardous asteroid, which require an intersection distance with Earth of less than 0.05 AU, which is about 19.5 times the distance to the moon, and a diameter of at least 150 meters. On 27 February 2041, it will pass from Earth. It also makes close approaches to Mars and Venus.
Kepler-42
On 10 January 2012, using the Kepler Space Telescope three transiting planets were discovered in orbit around Kepler-42. These planets' radii range from approximately those of Mars to Venus. The Kepler-42 system is only the second known system containing planets of Earth's radius or smaller (the first was the Kepler-20 system pictured at left). These planets' orbits are also compact, making the system (whose host star itself has a radius comparable to those of some hot Jupiters) resemble the moon systems of giant planets such as Jupiter or Saturn more than it does the Solar System. Despite these planets' small size and the star's being one of the faintest stars in Kepler field with confirmed planets, the detection of these planets was possible due to small size of the star, causing these planets to block a larger proportion of starlight during their transits.
Spherical cap
For example, assuming the Earth is a sphere of radius 6371 km, the surface area of the arctic (north of the Arctic Circle, at latitude 66.56° as of August 2016) is 2·6371²|sin 90° − sin 66.56°| = 21.04 million km², or 0.5·|sin 90° − sin 66.56°| = 4.125% of the total surface area of the Earth.
Flat Earth
Pythagoras in the 6th century BC and Parmenides in the 5th century stated that the Earth is spherical, and this view spread rapidly in the Greek world. Around 330 BC, Aristotle maintained on the basis of physical theory and observational evidence that the Earth was spherical, and reported an estimate of its circumference. The Earth's circumference was first determined around 240 BC by Eratosthenes. By the second century AD, Ptolemy had derived his maps from a globe and developed the system of latitude, longitude, and climes. His "Almagest" was written in Greek and only translated into Latin in the 11th century from Arabic translations.
LHS 1140 b
LHS 1140 b is a massive, dense rocky planet orbiting within the conservative habitable zone of the red dwarf LHS 1140. Discovered in 2017 by the MEarth Project, LHS 1140 b is nearly 7 times the mass of Earth and over 40% larger in radius, putting it within the Super-Earth category of planets. It is one of the densest planets found, with a density over twice that of Earth, along with a high surface gravity of about 3.25g. LHS 1140 b orbits entirely within the star's habitable zone and gets 41% the incident flux of Earth. The planet is only 40 light-years away and transits its star, making it an excellent candidate for atmospheric studies with ground-based and/or space telescopes.
Axial precession
The rotation axis of the Earth describes, over a period of 25,700 years, a small circle (blue) among the stars, centered on the ecliptic north pole (the blue "E") and with an angular radius of about 23.4°, an angle known as the obliquity of the ecliptic. The direction of precession is opposite to the daily rotation of the Earth on its axis. The orange axis was the Earth's rotation axis 5,000 years ago, when it pointed to the star Thuban. The yellow axis, pointing to Polaris, marks the axis now.
TRAPPIST-1e
TRAPPIST-1e was detected with the transit method, where the planet blocked a small percentage of its host star's light when passing between it and Earth. This allowed scientists to accurately determine the planet's radius at 0.910 , with a very small uncertainty of about 166–172 km. Transit-timing variations and advanced computer simulations helped constrain the planet's mass, which turned out to be 0.772 . With both the radius and mass of TRAPPIST-1e determined with low error margins, scientists could accurately calculate the planet's density, surface gravity, and composition. TRAPPIST-1e is unusual in its system as it is the only planet with a pure rock-iron composition, and the only one with a higher density than Earth (TRAPPIST-1c also appears to be just about entirely rock, but it has an extremely thick atmosphere that makes it less dense than TRAPPIST-1e). It has a density of 5.65 g/cm, about 1.024 times Earth's density of 5.51 g/cm. The higher density of TRAPPIST-1e implies a very Earth-like composition and a solid rocky surface. This is also unusual among the TRAPPIST-1 planets, as most are completely covered in either a thick steam atmosphere, a global liquid ocean, or an ice shell. TRAPPIST-1e has 93% the surface gravity of Earth, the second highest in the system. Its radius and mass are also the third least among the TRAPPIST-1 planets.
Kepler-442
Kepler-442b is a super-Earth with a radius 1.34 times that of Earth, and orbits well within the habitable zone. It is likely a rocky planet due to its radius. According to NASA, it was described as being one of the most Earth-like planets, in terms of size and temperature, yet found. It is outside of the zone (around 0.02 AU) where tidal forces from its host star would be enough to tidally lock it.
Voyages of Christopher Columbus
The true circumference of the Earth is about , a figure first established approximately by Eratosthenes in the 2nd century BC, and the distance from the Canary Islands to Japan . No ship that was readily available in the 15th century could carry enough food and fresh water for such a journey. Most European sailors and navigators concluded, probably correctly, that sailors undertaking a westward voyage from Europe to Asia non-stop would die of thirst, scurvy or starvation long before reaching their destination. Spain, however, having just completed the expensive Reconquista, was desperate for a competitive edge over other European countries in trade with the East Indies. Columbus promised such an advantage.
Earth's circumference
According to Cleomedes' "On the Circular Motions of the Celestial Bodies", around 240 BC, Eratosthenes, the librarian of the Library of Alexandria, calculated the circumference of the Earth in Ptolemaic Egypt. Using a scaphe, he knew that at local noon on the summer solstice in Syene (modern Aswan, Egypt), the Sun was directly overhead. (Syene is at latitude 24°05′ North, near to the Tropic of Cancer, which was 23°42′ North in 100 BC) He knew this because the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water. He then measured the Sun's angle of elevation at noon in Alexandria by using a vertical rod, known as a gnomon, and measuring the length of its shadow on the ground. Using the length of the rod, and the length of the shadow, as the legs of a triangle, he calculated the angle of the sun's rays. This angle was about 7°, or 1/50th the circumference of a circle; taking the Earth as perfectly spherical, he concluded that the Earth's circumference was 50 times the known distance from Alexandra to Syene (5,000 stadia, a figure that was checked yearly), i.e. 250,000 stadia. Depending on whether he used the "Olympic stade" (176.4m) or the Italian stade (184.8m), this would imply a circumference of 44,100 km (an error of 10%) or 46,100 km, an error of 15%. In 2012, Anthony Abreu Mora repeated Eratosthenes's calculation with more accurate data; the result was 40,074 km, which is 66 km different (0.16%) from the currently accepted polar circumference.
Globe of Peace
The Globe of Peace (Mappamondo della Pace in Italian) is a large globe located in Apecchio, Pesaro, Italy. It was the Guinness Book of World Records record holder for the world's largest rotating globe until 1999, when it was succeeded by Eartha. It was built over a period of six years by Orfeo Bartolucci with the stated goal of diffusing a message of peace and liberty to all people. It measures 10 metres in diameter and is reported to weigh 170 quintals, or 17,000 kilograms. Reportedly 250 quintals of putty, 30 cubic metres of wood, and a ton (unclear whether English or Metric) of nails were used in its construction. The globe is located outdoors and has a fibreglass skin. It can hold approximately 600 people and internally contains descriptive tables listing every country of the world and their flag.
Figure of the Earth
The concept of a spherical Earth dates back to around the 6th century BC, but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes’s measurement ranging from 2% to 15%.
Proxima Centauri b
The apparent inclination of Proxima Centauri b's orbit has not yet been measured. The minimum mass of Proxima b is , which would be the actual mass if its orbit were seen edge on from the Earth. Once its orbital inclination is known, the mass will be calculable. More tilted orientations imply a higher mass, with 90% of possible orientations implying a mass below . The planet's exact radius is unknown. If it has a rocky composition and a density equal to that of the Earth, then its radius is at least . It could be larger if it has a lower density than the Earth, or a mass higher than the minimum mass. Like many super-Earth sized planets, Proxima Centauri b could have an icy composition like Neptune, with a thick layer of hydrogen on its surface; the likelihood that this is the case has been calculated to be greater than 10%. The planet has an equilibrium temperature of .
COROT-7b
COROT-7b (previously named COROT-Exo-7b) is an exoplanet orbiting around the star COROT-7, in the constellation Monoceros, at 489 light years from Earth. It was first detected photometrically by the French-led COROT mission and reported in February 2009. Until the announcement of Kepler-10b in January 2011, it was the smallest exoplanet to have its diameter measured, at 1.58 times that of the Earth (which would give it a volume 3.95 times Earth's), and the first potential extrasolar terrestrial planet to be found. The planet has a very short orbital period, revolving around its host star in about 20 hours.
Super-Earth
In May 2014, previously discovered Kepler-10c was determined to have the mass comparable to Neptune (17 Earth masses). With the radius of 2.35, it is currently the largest known planet likely to have a predominantly rocky composition. At 17 Earth masses it is well above the 10 Earth mass upper limit that is commonly used for the term 'super-Earth' so the term mega-Earth has been proposed. However, in July 2017, more careful analysis of HARPS-N and HIRES data showed that Kepler-10c was much less massive than originally thought, instead around 7.37 (6.18 to 8.69) with a mean density of 3.14 g/cm. Instead of a primarily rocky composition, the more accurately determined mass of Kepler-10c suggests a world made almost entirely of volatiles, mainly water.
History of geodesy
A parallel later ancient measurement of the size of the Earth was made by another Greek scholar, Posidonius. He noted that the star Canopus was hidden from view in most parts of Greece but that it just grazed the horizon at Rhodes. Posidonius is supposed to have measured the angular elevation of Canopus at Alexandria and determined that the angle was 1/48 of circle. He used a distance from Alexandria to Rhodes, 5000 stadia, and so he computed the Earth's circumference in stadia as 48 times 5000 = 240,000. Some scholars see these results as luckily semi-accurate due to cancellation of errors. But since the Canopus observations are both mistaken by over a degree, the "experiment" may be not much more than a recycling of Eratosthenes's numbers, while altering 1/50 to the correct 1/48 of a circle. Later, either he or a follower appears to have altered the base distance to agree with Eratosthenes's Alexandria-to-Rhodes figure of 3750 stadia since Posidonius's final circumference was 180,000 stadia, which equals 48×3750 stadia. The 180,000 stadia circumference of Posidonius is suspiciously close to that which results from another method of measuring the earth, by timing ocean sunsets from different heights, a method which is inaccurate due to horizontal atmospheric refraction.
Posidonius
Posidonius calculated the Earth's circumference by reference to the position of the star Canopus. As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7½ degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes). Since he thought Rhodes was 5,000 stadia due north of Alexandria, and the difference in the star's elevation indicated the distance between the two locales was 1/48 of the circle, he multiplied 5,000 by 48 to arrive at a figure of 240,000 stadia for the circumference of the earth.
54 Piscium b
The orbit of an Earth-like planet would need to be centered within 0.68 AU (around the orbital distance of Venus), which in a Keplerian system means a 240-day orbital period. In a 2006 simulation with the brown dwarf, 54 Piscium b's orbit "sweeps clean" most test particles within 0.5 AU, leaving only asteroids "in low-eccentricity orbits near the known planet’s apastron distance, near the 1:2 mean-motion resonance". Also, observation has ruled out Neptune-class or heavier planets with a period of one year or less; which still allows for Earth-sized planets at 0.6 AU or more.
Planet Nine
The planet is estimated to have 10 times the mass of Earth, and a diameter of 26,000 to 52,000 km, or two to four times that of Earth. Brown thinks that if Planet Nine exists, its mass is sufficient to clear its orbit of large bodies in 4.6 billion years and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Astronomer Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.
11409 Horkheimer
According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, "Horkheimer" measures 15.355 kilometers in diameter and its surface has an albedo of 0.053, typical for carbonaceous asteroids.