When did humans first learn the size of the Earth? Or, for that matter, when did it become clear that the Earth was a sphere?

To those not attuned to science history, it may come as a surprise to learn that the Earth’s size and shape were settled 2,200 years ago, and by the same person. In what was one of the great intellectual feats of antiquity, the Greek Eratosthenes took the measure of the Earth using only simple tools and mathematics, together with his creative mind. This post recounts how Eratosthenes did it.

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On June 5, 2012, a rare astronomical event will occur: a transit of Venus. This just means that Venus will be between the Earth and Sun, so that Venus will appear as a small dot on the Sun’s surface. The relative motion of Venus and the Earth will cause the silhouette of Venus to drift across the Sun’s face over a period of several hours. Venus transits happen in pairs, eight years apart, so that the 2012 event is the second of a pair. Once we have a pair of these transits, there will be no more for the next 105 years. This odd timing is due to the vagaries of the orbits of the Earth and Venus. If both orbits were exactly in the same plane, there would be a transit whenever Venus and Earth were in line with the Sun; roughly once every two years. As it happens, in almost every Earth-Venus-Sun alignment, Venus is in the sky either above or below the Sun’s disk.

Astronomers will observe the June event mostly as a curiosity, but the Venus transits that occurred in the eighteenth century were of intense interest, and the efforts to observe them constituted the biggest scientific enterprise of the century. Britain, Austria, and France each launched expeditions to observe and time the transits from several locations throughout the world.

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