Before Galileo or Copernicus or someoe like that aimed a telescope at the moon and saw craters, people believed that the moon was a perfect sphere. But there are obvious light and dark spots on it. How did people explain that?
The dark spots on the moon that we can see with our naked eyes aren’t shadows. They’re “seas”: dry lava beds. That the moon has some dark patches doesn’t preclude it from being spherical. It wasn’t until Galileo (Copernicus died before the invention of the telescope) looked at the moon with a telescope and saw shadows that we had any direct evidence that the moon wasn’t a perfect sphere.
The ancients believed very much in the perfection of the universe outside the Earth, and spheres and circles were seen as being representative of perfection. The planets (including the moon and the sun) were thought of as being perfectly spherical and moving around the Earth in perfect crystalline spheres. In Ptolemaic theory, the planets even moved in circles (epicycles) around a center of gravity in their orbit, thus spiraling around the Earth. (This was meant to explain retrograde motion.) The stars moved in a celestial sphere, beyond all the planets.
I believe it was thought that the dark patches and other blemishes on the moon were caused by its proximity to the imperfect Earth, but that it was a perfect sphere nonetheless.
Anyone in the ancient world could look at a total eclipse and see evidence of the irregular surface of the moon. Bailey’s Beads is the effect created when the light of the sun streams through the craters, mountains and valleys on the limb of the Moon.
For that matter, you can see the shadows with the naked eye, at half moon. But the Moon is, in fact, very close to a perfect sphere, close enough that it’s quite easy to overlook the subtle evidence otherwise.
Surface irregularities of the moon and/or the earth are less in proportion to the irregularities on the surface of an orange.
Quite a few different explanations were proposed in the medieval period for how the Moon could be a perfect sphere and yet show blotches. In no particular order:
[ul]Alexander of Neckham suggested that the Moon was stained by God as a symbol of original sin.[/ul]
[ul]It was dirt from the Earth drawn up by moisture.[/ul]
[ul]The Moon was acting as a giant mirror, reflecting the image of the Earth back at us. In particular, we were seeing the great ocean that was believed to surround Europe, Asia and Africa. Or perhaps it’s terrestrial mountain ranges that we’re seeing.[/ul]
[ul]That there was some sort of obstruction between us and the Moon, possibly “heavy vapours” rising to the latter.[/ul]
[ul]Or that the obstruction was between the Sun and the Moon, with the dark areas shadows.[/ul]
But the most influential suggestion was the Aristotelian one that the two types of areas represent differences in the substance that the Moon is made up of. In fact, Aristotle was quite vague on the whole issue and it’s not even clear that he believed that the Moon was a perfect sphere; in places he talks about how there may be animals living on it. But the issue was discussed by his medieval commentators, notably Averroes. He proposed that the Moon was denser in the places that were dark, less so where it was light. Albert the Great and Nicholas Oresme agreed, though Albert of Saxony thought that it was the denser bits that were light. Dante adopted the latter view.
It’s a version of this argument that Galileo has Simplicio argue in the Dialogue.
It should also be realised that, even before Harriot and Galileo tried looking at it with a telescope, there was a long tradition of speculation in which the Moon wasn’t regarded as a perfect sphere.
For a survey of different ideas about its nature from antiquity to the telescopic era, see The Moon and the Western Imagination (Arizona, 1999) by Scott L. Montgomery.
But while anybody could have noticed the phenomenon and used it as evidence, there’s no record that anybody did before Francis Baily - the more correct spelling; see either the old DNB entry for him or the 1844 obituary notice by John Herschel - in 1836.