History of geodesy

The history of geodesy (/dʒiːˈɒdɪsi/) began during antiquity and ultimately blossomed during the Age of Enlightenment. Many early conceptions of the Earth held it to be flat, with the heavens being a physical dome spanning over it. Early arguments for a spherical Earth pointed to various more subtle empirical observations, including how lunar eclipses were seen as circular shadows, as well as the fact that Polaris is seen lower in the sky as one travels southward.

Hellenic world

Initial developments Though the earliest written mention of a spherical Earth comes from ancient Greek sources, there is no account of how the sphericity of Earth was discovered, or if it was initially simply a guess. A plausible explanation given by the historian Otto E. Neugebauer is that it was "the experience of travellers that suggested such an explanation for the variation in the observable altitude of the pole and the change in the area of circumpolar stars, a change that was quite drastic between Greek settlements" around the eastern Mediterranean Sea, particularly those between the Nile Delta and Crimea. Another possible explanation can be traced back to earlier Phoenician sailors. The first circumnavigation of Africa is described as being undertaken by Phoenician explorers employed by Egyptian pharaoh Necho II c. 610–595 BC. In The Histories, written 431–425 BC, Herodotus cast doubt on a report of the Sun observed shining from the north. He stated that the phenomenon was observed by Phoenician explorers during their circumnavigation of Africa (The Histories, 4.42) who claimed to have had the Sun on their right when circumnavigating in a clockwise direction. To modern historians, these details confirm the truth of the Phoenicians' report. The historian Dmitri Panchenko hypothesizes that it was the Phoenician circumnavigation of Africa that inspired the theory of a spherical Earth, the earliest mention of which was made by the philosopher Parmenides in the 5th century BC. However, nothing certain about their knowledge of geography and navigation has survived; therefore, later researchers have no evidence that they conceived of Earth as spherical. Speculation and theorizing ranged from the flat disc advocated by Homer to the spherical body reportedly postulated by Pythagoras. Anaximenes, an early Greek philosopher, believed strongly that the Earth was rectangular in shape. Some early Greek philosophers alluded to a spherical Earth, though with some ambiguity. Pythagoras (6th century BC) was among those said to have originated the idea, but this might reflect the ancient Greek practice of ascribing every discovery to one or another of their ancient wise men. Pythagoras was a mathematician, and he supposedly reasoned that the gods would create a perfect figure which to him was a sphere, but there is no evidence for this claim. Some idea of the sphericity of Earth seems to have been known to both Parmenides and Empedocles in the 5th century BC, and although the idea cannot reliably be ascribed to Pythagoras, it might nevertheless have been formulated in the Pythagorean school in the 5th century BC although some disagree. After the 5th century BC, just a few Greek writers of repute thought the world was anything but round. The Pythagorean idea was supported later by Aristotle. Efforts commenced to determine the size of the sphere.

Plato Plato (427–347 BC) travelled to southern Italy to study Pythagorean mathematics. When he returned to Athens and established his school, Plato also taught his students that Earth was a sphere, though he offered no justifications. "My conviction is that the Earth is a round body in the centre of the heavens, and therefore has no need of air or of any similar force to be a support." If man could soar high above the clouds, Earth would resemble "one of those balls which have leather coverings in twelve pieces, and is decked with various colours, of which the colours used by painters on Earth are in a manner samples." In Timaeus, his one work that was available throughout the Middle Ages in Latin, he wrote that the Creator "made the world in the form of a globe, round as from a lathe, having its extremes in every direction equidistant from the centre, the most perfect and the most like itself of all figures", though the word "world" here refers to the heavens.

Aristotle

Aristotle (384–322 BC) was Plato's prize student and "the mind of the school". Aristotle observed "there are stars seen in Egypt and [...] Cyprus which are not seen in the northerly regions". Since this could only happen on a curved surface, he too believed Earth was a sphere "of no great size, for otherwise the effect of so slight a change of place would not be quickly apparent". Aristotle reported the circumference of the Earth (which is actually slightly over 40,000 km or 24,000 miles) to be 400,000 stadia (45,000 miles or 74,000 km). Aristotle provided physical and observational arguments supporting the idea of a spherical Earth:

Every portion of Earth tends toward the centre until by compression and convergence they form a sphere. Travelers going south see southern constellations rise higher above the horizon. The shadow of Earth on the Moon during a lunar eclipse is round. The concepts of symmetry, equilibrium and cyclic repetition permeated Aristotle's work. In his Meteorology he divided the world into five climatic zones: two temperate areas separated by a torrid zone near the equator, and two cold inhospitable regions, "one near our upper or northern pole and the other near the [...] southern pole", both impenetrable and girdled with ice. Although no humans could survive in the frigid zones, inhabitants in the southern temperate regions could exist. Aristotle's theory of natural place relied on a spherical Earth to explain why heavy things go down (toward what Aristotle believed was the center of the Universe), and things like air and fire go up. In this geocentric model, the structure of the universe was believed to be a series of perfect spheres. The Sun, Moon, planets and fixed stars were believed to move on celestial spheres around a stationary Earth. Though Aristotle's theory of physics survived in the Christian world for many centuries, his geocentric model was eventually superseded by the heliocentric model as an explanation of the Solar System, while atomic theory took the place of the classical elements such as earth, water, air, fire, and aether.

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