Archaic conception of Earth's shape
Flat Earth is an archaic and scientifically disproven conception of the Earth's shape as a plane or disk. Many ancient cultures subscribed to a flat-Earth cosmography, notably including the cosmology in the ancient Near East. The model has undergone a recent resurgence as a conspiracy theory.
The idea of a spherical Earth appeared in ancient Greek philosophy with Pythagoras (6th century BC). However, the early Greek cosmological view of a flat Earth persisted among most pre-Socratics (6th–5th century BC). In the early 4th century BC, Plato wrote about a spherical Earth. By about 330 BC, his former student Aristotle had provided strong empirical evidence for a spherical Earth. Knowledge of the Earth's global shape gradually began to spread beyond the Hellenistic world. By the early period of the Christian Church, the spherical view was widely held, with some notable exceptions. In contrast, ancient Chinese scholars consistently describe the Earth as flat, and this perception remained unchanged until their encounters with Jesuit missionaries in the 17th century. It is a historical myth that medieval Europeans generally thought the Earth was flat. This myth was created in the 17th century by Protestants to argue against Catholic teachings. Traditionalist Muslim scholars have maintained that the earth is flat, though, since the 9th century, Muslim scholars tended to believe in a spherical Earth. 
Despite the scientific facts and obvious effects of Earth's sphericity, pseudoscientific flat-Earth conspiracy theories persist, and from the 2010s at latest, believers in a flat earth have increased, both as membership in modern flat Earth societies, and as unaffiliated individuals using social media. In a 2018 study reported on by "Scientific American", only 82% of 18 to 24 year old American respondents agreed with the statement "I have always believed the world is round". However, a firm belief in a flat Earth is rare, with less than 2% acceptance in all age groups. ;templatestyles src="Template:TOC limit/styles.css" / ;
History.
Belief in flat Earth.
Near East.
In early Egyptian and Mesopotamian thought, the world was portrayed as a disk floating in the ocean. A similar model is found in the Homeric account from the 8th century BC in which "Okeanos, the personified body of water surrounding the circular surface of the Earth, is the begetter of all life and possibly of all gods."
The Pyramid Texts and Coffin Texts of ancient Egypt show a similar cosmography; Nun (the Ocean) encircled "nbwt" ("dry lands" or "Islands").
The Israelites also imagined the Earth to be a disc floating on water with an arched firmament above it that separated the Earth from the heavens. The sky was a solid dome with the Sun, Moon, planets, and stars embedded in it.
Greece.
Poets.
Both Homer and Hesiod described a disc cosmography on the Shield of Achilles. This poetic tradition of an Earth-encircling ("gaiaokhos") sea (Oceanus) and a disc also appears in Stasinus of Cyprus, Mimnermus, Aeschylus, and Apollonius Rhodius.
Homer's description of the disc cosmography on the shield of Achilles with the encircling ocean is repeated far later in Quintus Smyrnaeus' "Posthomerica" (4th century AD), which continues the narration of the Trojan War.
Philosophers.
Several pre-Socratic philosophers believed that the world was flat: Thales (c. 550 BC) according to several sources, and Leucippus (c. 440 BC) and Democritus (c. 460–370 BC) according to Aristotle.
Thales thought that the Earth floated in water like a log. It has been argued, however, that Thales actually believed in a spherical Earth. Anaximander (c. 550 BC) believed that the Earth was a short cylinder with a flat, circular top that remained stable because it was the same distance from all things. Anaximenes of Miletus believed that "the Earth is flat and rides on air; in the same way the Sun and the Moon and the other heavenly bodies, which are all fiery, ride the air because of their flatness". Xenophanes (c. 500 BC) thought that the Earth was flat, with its upper side touching the air, and the lower side extending without limit.
Belief in a flat Earth continued into the 5th century BC. Anaxagoras (c. 450 BC) agreed that the Earth was flat, and his pupil Archelaus believed that the flat Earth was depressed in the middle like a saucer, to allow for the fact that the Sun does not rise and set at the same time for everyone.
Historians.
Hecataeus of Miletus believed that the Earth was flat and surrounded by water. Herodotus in his "Histories" ridiculed the belief that water encircled the world, yet most classicists agree that he still believed Earth was flat because of his descriptions of literal "ends" or "edges" of the Earth.
Northern Europe.
The ancient Norse and Germanic peoples believed in a flat-Earth cosmography with the Earth surrounded by an ocean, with the axis mundi, a world tree (Yggdrasil), or pillar (Irminsul) in the centre. In the world-encircling ocean sat a snake called Jormungandr. The Norse creation account preserved in Gylfaginning (VIII) states that during the creation of the Earth, an impassable sea was placed around it:
 ;templatestyles src="Template:Blockquote/styles.css" / ;And Jafnhárr said: "Of the blood, which ran and welled forth freely out of his wounds, they made the sea, when they had formed and made firm the Earth together, and laid the sea in a ring round. about her; and it may well seem a hard thing to most men to cross over it."
The late Norse Konungs skuggsjá, on the other hand, explains Earth's shape as a sphere:
 ;templatestyles src="Template:Blockquote/styles.css" / ;If you take a lighted candle and set it in a room, you may expect it to light up the entire interior, unless something should hinder, though the room be quite large. But if you take an apple and hang it close to the flame, so near that it is heated, the apple will darken nearly half the room or even more. However, if you hang the apple near the wall, it will not get hot; the candle will light up the whole house; and the shadow on the wall where the apple hangs will be scarcely half as large as the apple itself. From this you may infer that the Earth-circle is round like a ball and not equally near the sun at every point. But where the curved surface lies nearest the sun's path, there will the greatest heat be; and some of the lands that lie continuously under the unbroken rays cannot be inhabited.
East Asia.
In ancient China, the prevailing belief was that the Earth was flat and square, while the heavens were round, an assumption virtually unquestioned until the introduction of European astronomy in the 17th century. The English sinologist Cullen emphasizes the point that there was no concept of a round Earth in ancient Chinese astronomy:
 ;templatestyles src="Template:Blockquote/styles.css" / ;Chinese thought on the form of the Earth remained almost unchanged from early times until the first contacts with modern science through the medium of Jesuit missionaries in the seventeenth century. While the heavens were variously described as being like an umbrella covering the Earth (the Kai Tian theory), or like a sphere surrounding it (the Hun Tian theory), or as being without substance while the heavenly bodies float freely (the Hsüan yeh theory), the Earth was at all times flat, although perhaps bulging up slightly.
The model of an egg was often used by Chinese astronomers such as Zhang Heng (78–139 AD) to describe the heavens as spherical:
 ;templatestyles src="Template:Blockquote/styles.css" / ;The heavens are like a hen's egg and as round as a crossbow bullet; the Earth is like the yolk of the egg, and lies in the centre.
This analogy with a curved egg led some modern historians, notably Joseph Needham, to conjecture that Chinese astronomers were, after all, aware of the Earth's sphericity. The egg reference, however, was rather meant to clarify the relative position of the flat Earth to the heavens:
 ;templatestyles src="Template:Blockquote/styles.css" / ;In a passage of Zhang Heng's cosmogony not translated by Needham, Zhang himself says: "Heaven takes its body from the Yang, so it is round and in motion. Earth takes its body from the Yin, so it is flat and quiescent". The point of the egg analogy is simply to stress that the Earth is completely enclosed by Heaven, rather than merely covered from above as the Kai Tian describes. Chinese astronomers, many of them brilliant men by any standards, continued to think in flat-Earth terms until the seventeenth century; this surprising fact might be the starting-point for a re-examination of the apparent facility with which the idea of a spherical Earth found acceptance in fifth-century BC Greece.
Further examples cited by Needham supposed to demonstrate dissenting voices from the ancient Chinese consensus actually refer without exception to the Earth being square, not to it being flat. Accordingly, the 13th-century scholar Li Ye, who argued that the movements of the round heaven would be hindered by a square Earth, did not advocate a spherical Earth, but rather that its edge should be rounded off so as to be circular. However, Needham disagrees, affirming that Li Ye believed the Earth to be spherical, similar in shape to the heavens but much smaller. This was preconceived by the 4th-century scholar Yu Xi, who argued for the infinity of outer space surrounding the Earth and that the latter could be either square or round, in accordance to the shape of the heavens. When Chinese geographers of the 17th century, influenced by European cartography and astronomy, showed the Earth as a sphere that could be circumnavigated by sailing around the globe, they did so with formulaic terminology previously used by Zhang Heng to describe the spherical shape of the Sun and Moon (i.e. that they were as round as a crossbow bullet).
As noted in the book "Huainanzi", in the 2nd century BC, Chinese astronomers effectively inverted Eratosthenes' calculation of the curvature of the Earth to calculate the height of the Sun above the Earth. By assuming the Earth was flat, they arrived at a distance of (approximately ). The "Zhoubi Suanjing" also discusses how to determine the distance of the Sun by measuring the length of noontime shadows at different latitudes, a method similar to Eratosthenes' measurement of the circumference of the Earth, but the "Zhoubi Suanjing" assumes that the Earth is flat.
Alternate or mixed theories.
Greece: spherical Earth.
Pythagoras in the 6th century BC and Parmenides in the 5th century BC 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 2nd 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.
Lucretius (1st century BC) opposed the concept of a spherical Earth, because he considered that an infinite universe had no center towards which heavy bodies would tend. Thus, he thought the idea of animals walking around topsy-turvy under the Earth was absurd. By the 1st century AD, Pliny the Elder was in a position to say that everyone agreed on the spherical shape of Earth, though disputes continued regarding the nature of the antipodes, and how it is possible to keep the ocean in a curved shape.
South Asia.
The Vedic texts depict the cosmos in many ways. One of the earliest Indian cosmological texts pictures the Earth as one of a stack of flat disks.
In the Vedic texts, Dyaus (heaven) and Prithvi (Earth) are compared to wheels on an axle, yielding a flat model. They are also described as bowls or leather bags, yielding a concave model. According to Macdonell: "the conception of the Earth being a disc surrounded by an ocean does not appear in the Samhitas. But it was naturally regarded as circular, being compared with a wheel (10.89) and expressly called circular (parimandala) in the "Shatapatha Brahmana"."
By about the 5th century AD, the "siddhanta" astronomy texts of South Asia, particularly of Aryabhata, assume a spherical Earth as they develop mathematical methods for quantitative astronomy for calendar and time keeping.
The medieval Indian texts called the Puranas describe the Earth as a flat-bottomed, circular disk with concentric oceans and continents. This general scheme is present not only in the Hindu cosmologies, but also in Buddhist and Jain cosmologies