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[edit]
Initial developments[edit]
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.[2] 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"[3] around the eastern Mediterranean Sea, particularly those between the Nile Delta and Crimea.[3]
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.[4][5] 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.[5] 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.[4]
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.[6] 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.[2] 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.[7] Some idea of the sphericity of Earth seems to have been known to both Parmenides and Empedocles in the 5th century BC,[8] and although the idea cannot reliably be ascribed to Pythagoras,[9] it might nevertheless have been formulated in the Pythagorean school in the 5th century BC[2][8] although some disagree.[10] After the 5th century BC, no Greek writer of repute thought the world was anything but round.[6] The Pythagorean idea was supported later by Aristotle.[11] Efforts commenced to determine the size of the sphere.
Plato[edit]
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."[12] 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."[13] 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",[14] though the word "world" here refers to the heavens.
Medieval Europe[edit]
Greek influence[edit]
In medieval Europe, knowledge of the sphericity of Earth survived into the medieval corpus of knowledge by direct transmission of the texts of Greek antiquity (Aristotle), and via authors such as Isidore of Seville and the Venerable Bede. It became increasingly traceable with the rise of scholasticism and medieval learning.[46]
Revising the figures attributed to Posidonius, another Greek philosopher determined 18,000 miles (29,000 km) 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 3,000 or 4,000 miles (4,800 or 6,400 km) west of Europe.[77]
Ptolemy's view was not universal, however, and chapter 20 of Sir John Mandeville's Travels (c. 1357) supports Eratosthenes' calculation.
Spread of this knowledge beyond the immediate sphere of Greco-Roman scholarship was necessarily gradual, associated with the pace of Christianisation of Europe. For example, the first evidence of knowledge of the spherical shape of Earth in Scandinavia is a 12th-century Old Icelandic translation of Elucidarius.[78] A list of more than a hundred Latin and vernacular writers from Late Antiquity and the Middle Ages who were aware that Earth was spherical has been compiled by Reinhard Krüger, professor for Romance literature at the University of Stuttgart.[46]
It was not until the 16th century that his concept of the Earth's size was revised. During that period the Flemish cartographer, Mercator, made successive reductions in the size of the Mediterranean Sea and all of Europe which had the effect of increasing the size of the earth.