Which of Galileo's initial observations directly provided evidence that celestial bodies were imperfect?

Answer

Seeing mountains and craters on the Moon.

Galileo observed surface irregularities like mountains and craters on the Moon, which countered the long-held Aristotelian and Ptolemaic view that celestial bodies were perfect and unchanging.

Which of Galileo's initial observations directly provided evidence that celestial bodies were imperfect?
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