| MadSci Network: Astronomy |
In the 4th Century BC, Eratosthenes had used the fact that, at Midsummers' Day, the Sun stood vertically above a well in the town of Syrene, in Egypt, to measure the displacement from the vertical at a location 500 stadi to the north, in Alexandria. At this time it was appreciated that the Earth was spherical to first approximation. WhilE Eratosthenes noted the sun's vertical, shadowless, postion on June 22 at Syrene, another observer in Alexandria noted a 7.5 degree displacement from the vertical, and thus showed that the Earth's circumference is 7.5/360 X 500 stadia in circumference, ie , approx, 24,000 miles!
Aristarchus, by 150 BC, had a pretty shrewd idea , from this figure and by extending parallax principles to the position of the Moon against the fixed stars at different latitudes, of the lunar distance.
Ptolemy, in his Almagest, showed a 2 degree displacement of the Moon against the background constellations from Alexandria to Greece, and so inferred that its distance is about 59 Earth radii from Earth( 224,000 miles) A good first approximation!
Nowadays, the Lunar distance is known, by reflected laser light using ASLEP instruments left by the Apollo missions to within 5 centimetres! Radar, likewise, gives modern accuracy.
[Moderator's note: In fact, laser ranging from the Earth to the Moon is so precise that it has been used to test Einstein's theory of General Relativity by looking for deviations of the Moon's orbit from that predicted by the theory. No such deviations have been found using this method.]
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