When was it first realised that the lit portion of the moon always faces the sun, so moonlight must be reflected sunlight?
James Ladyman, University of Bristol, UK; Kelli Rudolf, University of Kent, UK
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Thinking about how objects can shine with reflected light, such as the way oil, water, metal or the eye reflect light when illuminated in the dark, is likely to have led many people over the ages to the conclusion that the moon doesn鈥檛 shine on its own.
We can鈥檛 even know for sure who first wrote this idea down because so many ancient philosophical texts are lost. However, we do know that Anaxagoras was one of the first to do so because a fragment of his writings translates as 鈥渢he sun indues the moon with brightness鈥� (it was hinted at a little earlier by Parmenides, who said the moon is a 鈥渘ight-shining, foreign light, wandering around the Earth鈥�).
Anaxagoras was born in approximately 500 BC in Clazomenae in Ionia (which is now in Turkey and was then part of the Persian Empire). He moved to Athens, from where he was eventually exiled for denying that the moon and the sun are gods. He thought that Earth was a flat disc, but his ideas about the moon and the sun led him to the correct explanation for eclipses, and he also claimed correctly that there are mountains on the moon.
Anaxagoras, born around 500 BC, was eventually exiled from Athens for denying that the moon and the sun are gods
That the same side of the moon always faces Earth is apparent by observation, especially with a telescope. The reason for this is gravitational tidal locking, which was first posited by Isaac Newton. A little more than half the moon鈥檚 surface is visible from Earth at different times.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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