Instruct: Given a question, retrieve passages that can help answer the question.
Query: What is the circumference of the planet Earth?
Earth's circumference is the distance around the Earth, either around the equator (40,075.017km [24,901.461mi][1]) or around the poles (40,007.86km [24,859.73mi][2]).
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 ​71⁄2 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.[11] It is generally thought that the stadion used by Posidonius was almost exactly 1/10 of a modern statute mile. Thus Posidonius's measure of 240,000 stadia translates to 24,000mi (39,000km), not much short of the actual circumference of 24,901mi (40,074km).[11] 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 18,000mi (29,000km).[12] 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.[13]
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.[6] 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[7]) 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.[8] 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.[9] 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.[10] Depending on whether he used the "Olympic stade" (176.4m) or the Italian stade (184.8m), this would imply a circumference of 44,100km (an error of 10%) or 46,100km, an error of 15%.[10] In 2012, Anthony Abreu Mora repeated Eratosthenes's calculation with more accurate data; the result was 40,074km, which is 66km different (0.16%) from the currently accepted polar circumference.[9]
Both the metre and the nautical mile were originally defined as a subdivision of the Earth's circumference; today the circumference around the poles is almost exactly 40,000 km and 360 × 60 nautical miles long.[18]
A potentially habitable planet implies a terrestrial planet within the circumstellar habitable zone and with conditions roughly comparable to those of Earth (i.e. an Earth analog) and thus potentially favourable to Earth-like life. However, the question of what makes a planet habitable is much more complex than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host star’s plasma environment can influence the evolution of planets and life, if it originated.
According to Stephen Jay Gould, "there never was a period of 'flat Earth darkness' among scholars (regardless of how the public at large may have conceptualized our planet both then and now). Greek knowledge of sphericity never faded, and all major medieval scholars accepted the Earth's roundness as an established fact of cosmology." Historians of science David Lindberg and Ronald Numbers point out that "there was scarcely a Christian scholar of the Middle Ages who did not acknowledge [Earth's] sphericity and even know its approximate circumference".
No name is ever established for the mysterious planet, but the inhabitants seem to know of Earth, Venus and Mars, referring to them by name in one episode. Exactly where the planet is located is also never made clear. In the episode "On a Clear Night You Can See Earth," Captain Steve Burton (Gary Conway) claims to have seen Earth through a set of infrared goggles invented by the giants, implying that the two planets are near enough to see one from the other. The only established method by which Earth people may reach the giants' planet is high-altitude flight, passing through what one giant calls a "dimension lock".
Astrobiology is an interdisciplinary field that uses a combination of geobiological and planetary science data to establish a context for the search for life on other planets. The origin of life from non-living chemistry and geology, or abiogenesis, is a major topic in astrobiology. Even though it is fundamentally an earth-bound concern, and therefore of great geobiological interest, getting at the origin of life necessitates considering what life requires, what, if anything, is special about Earth, what might have changed to allow life to blossom, what constitutes evidence for life, and even what constitutes life itself. These are the same questions that scientists might ask when searching for alien life. In addition, astrobiologists research the possibility of life based on other metabolisms and elements, the survivability of Earth’s organisms on other planets or spacecrafts, planetary and solar system evolution, and space geochemistry.
In 2191, Earth made first contact with aliens called "Gamilas". First attempt at peaceful contact with the Gamilas failed, resulting in interstellar war. The United Nations Cosmo Navy, even though outmatched by the Gamilas space naval forces, was able to stop their direct assaults on Earth in the Second Battle for Mars, but suffered heavy losses in the process. The Gamilas, from their military base on Pluto, then started planetary bombardment with modified asteroids called "planet bombs". The planet bombs hampered United Nations efforts to rebuild their space fleet and forced Earth to build underground cities to protect humanity. The planet bombs altered the atmosphere and irradiated the planetary surface, causing the complete destruction of the planet's biosphere. The Gamilas then started what is believed to be their first step in terraforming—preparing the Earth to be inhabited by themselves—by introducing plant life that was lethal to any life on Earth. With mankind facing extinction, the United Nations started planning for a small colony of humans to leave Earth in an attempt at the survival of humanity, called the "Isumo Plan". But in early 2198 "Starsha", from the planet Iscandar, learned of Earth's situation and dispatched her sister "Yurisha" to Earth. Yurisha brought with her the designs to what is called the "Dimensional Wave Motion Engine", providing for interstellar flight along with other technological assistance to Earth. The Iscandarans revealed that they could reverse the damage done to the Earth with the "Cosmo Reverse System". For technical reasons they could not send the system directly and would need Earth to send a ship for it. The United Nations then scrapped the Isumo Plan in order to build a new Cosmo Navy ship to retrieve the Cosmo Reverse System. The new ship was designed as a heavily armed space battleship. To conceal the ship's construction from the Gamilas, Earth built the new ship at the same site as the sunken World War II Yamato battleship. The new space battleship was also named the "Yamato" for which the series Space Battleship Yamato 2199 is named.
Let "C"  be the Earth's circumference, "R"  be its radius, "c"  be the added string length and "r"  be the added radius. has a circumference of 2"R" ,
1,700 years after Eratosthenes's death, while Christopher Columbus studied what Eratosthenes had written about the size of the Earth, he chose to believe, based on a map by Toscanelli, that the Earth's circumference was one-third smaller. Had Columbus set sail knowing that Eratosthenes's larger circumference value was more accurate, he would have known that the place that he made landfall was not Asia, but rather the New World.[17]
A planet orbiting the Sun so that it was always on the other side of the Sun from Earth could have such an orbit because it was the same distance from the Sun and had the same mass as Earth (according to the theory). Thus, what would make it undetectable to astronomers (or any other human beings) on Earth would also make it habitable to beings at least similar to humans. With the same size and distance from the Sun as Earth, it could have the same (or very similar) surface environment—gravity, atmospheric pressure, and surface temperature range. At the same time such a planet could have the same orbiting velocity and path as Earth, so that if it was positioned 180 degrees from Earth, it would remain behind the Sun being blocked from view from Earth indefinitely. 
Although a Counter-Earth would not be directly visible from the Earth since the Sun is in the way, if such a planet actually existed, it could still be detected from the Earth for a number of reasons.
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 .
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).
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.
Due to the larger mass of super-Earths, their physical characteristics may differ from Earth's; theoretical models for super-Earths provide four possible main compositions according to their density: low-density super-Earths are inferred to be composed mainly of hydrogen and helium (mini-Neptunes); super-Earths of intermediate density are inferred to either have water as a major constituent (ocean planets), or have a denser core enshrouded with an extended gaseous envelope (gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth's interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different composition. Researchers at Harvard Astronomy Department have developed user-friendly online tools to characterize the bulk composition of the super-Earths. A study on Gliese 876 d by a team around Diana Valencia revealed that it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition is. For Gliese 876 d, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s). However, this planet is not known to transit its host star.
The project Overland was founded in 1995 with the ambitious aim of tracing a portrait of our planet, exploring the far corners and creating an exceptional diary broadcast on Rai. The numbers of Overland, already today, are impressive: 13 expeditions in 16 years, more than 421,000 km of total mileage (approximately 8 times the circumference of the Earth), covered in over 1460 days travel through 372 borders of countries belonging to the disparate geographical areas and climatic zones of the planet.
Eratosthenes, a Greek astronomer from Hellenistic Cyrenaica (276–194 BC), estimated Earth's circumference around 240 BC. He had heard that in Syene the Sun was directly overhead at the summer solstice whereas in Alexandria it still cast a shadow. Using the differing angles the shadows made as the basis of his trigonometric calculations he estimated a circumference of around 250,000 "stades". The length of a 'stade' is not precisely known, but Eratosthenes's figure only has an error of around five to fifteen percent. Eratosthenes used rough estimates and round numbers, but depending on the length of the stadion, his result is within a margin of between 2% and 20% of the actual meridional circumference, . Note that Eratosthenes could only measure the circumference of the Earth by assuming that the distance to the Sun is so great that the rays of sunlight are practically parallel.
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%.
A more accurate estimate was provided in Al-Biruni's Codex Masudicus (1037).[14][15] In contrast to his predecessors, who measured the Earth's circumference by sighting the Sun simultaneously from two different locations, al-Biruni developed a new method of using trigonometric calculations, based on the angle between a plain and mountain top, which yielded more accurate measurements of the Earth's circumference, and made it possible for it to be measured by a single person from a single location.[16]
In 1617 the Dutch scientist Willebrord Snellius 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 (1/60) of one degree of latitude. As one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle – such that the polar circumference of the Earth would be exactly 21,600 miles. 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.[19]
The Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC by Eudoxus of Cnidus and Callippus of Cyzicus. Their younger contemporary Heraclides Ponticus proposed that the Earth rotates around its axis. In the 3rd century BC Aristarchus of Samos was the first to suggest a heliocentric system. Archimedes in his treatise The Sand Reckoner revives Aristarchus' hypothesis that ""the fixed stars and the Sun remain unmoved, while the Earth revolves about the Sun on the circumference of a circle"". Otherwise, only fragmentary descriptions of Aristarchus' idea survive. Eratosthenes, using the angles of shadows created at widely separated regions, estimated the circumference of the Earth with great accuracy. In the 2nd century BC Hipparchus of Nicea made a number of contributions, including the first measurement of precession and the compilation of the first star catalog in which he proposed the modern system of apparent magnitudes.
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.[4] Other circles of latitude have smaller circumference; for example, the 60th parallel north or south is approximately half as long as the Equator.[4]
Aryabhatiya also estimates the circumference of Earth, with an error of less than 1%. He gave the circumference of the earth as 4 967 yojanas and its diameter as 1 5811/24 yojanas. Since 1 yojana = 5 miles this gives the circumference as 24 835 miles, which is an excellent approximation to the currently accepted value of 24 902 miles. Aryabhata gives the radii of the orbits of the planets in terms of the Earth-Sun distance as essentially their periods of rotation around the Sun. He also gave the correct explanation of lunar and solar eclipses and that the Moon shines by reflecting sunlight.
Earth's circumference lines are the foundation of the coordinate system of latitude and longitude, introduced in the mid-second century BC by Hipparchus.[5]