Which observation provided empirical ammunition confirming that the celestial sphere was corruptible, similar to the Moon?

Answer

He observed sunspots on the surface of the Sun.

Observing blemishes or spots on the Sun demonstrated that even the most distant celestial bodies were not perfect or unchanging, supporting the erosion of the ancient dualism between terrestrial and celestial realms.

Which observation provided empirical ammunition confirming that the celestial sphere was corruptible, similar to the Moon?
physicsdiscoveryGalileoastronomy