At what point in our history did mankind first realize that planets do not produce light on their own but rather merely reflect the light of the sun.
I recently saw somewhere where someone made the claim that this was first discovered by Gallileo. I would think, however, that it was probably well known even before his time - but I don’t know for sure.
Does anyone have any info on this (preferably with sources?)
Thanks,
Zev Steinhardt
I’m really not sure that it was well-known prior to Galileo.
The big complication is that there was a plausible intermediate hypothesis: that stars and planets both depended on light from the Sun and were also self-luminous. For example, light from the Sun could heat a celestial body causing it to glow by re-radiating that heat. And, to be clear, there were those who realised that this was different from it simply reflecting the light in the manner of a mirror.
Such a mechanism could even explain the hard case: the phases of the Moon. It would only re-radiate from those areas that the Sun was shining on, so what would be observed was exactly the same as from reflection.
Given that ideas about the composition/nature of the celestial bodies varied enormously, there was a wide range of variations on this basic idea. The influential medieval authority who rather clearly held this hypothesis in the form explained above was Averroes, who completely denied that reflection had any role in the light from the Moon, the planets or stars. He had been influenced by Aristotle, though his explanations on the matter hadn’t been particularly clear. I find Ptolemy’s account of the nature of the planets - in Tetrabiblos I,4 - not entirely unambiguous either, but it’s consistent with him believing that they were re-radiating rather than reflecting.
On the other hand, the very diversity of opinions meant that there were those who did believe that the planets were just directly reflecting the light of the Sun. (As there were those who believed that planets were intrinsically luminous.) That idea can be traced back into antiquity, but without anyone being able to prove it.
Once Galileo shows both that the Moon has a surface like the Earth with mountains casting shadows and that Venus has phases, then the idea that planets are just reflecting light quickly became universal and uncontroversial. Despite all the fuss, people actually do seem to have rapidly adapted to simply accepting what they saw through telescopes as self-evident.