What method did Galileo use to prove that lunar features were elevated structures rather than mere variations in brightness?

Answer

Observing long shadows cast near the terminator line

Galileo noticed that sunlight caught the edges of irregularities, casting long, distinct shadows across the darker parts of the surface. The angle and length of these shadows revealed the height of the objects casting them.

What method did Galileo use to prove that lunar features were elevated structures rather than mere variations in brightness?
discoveryGalileoastronomysurfaceMoon