Galileo observed that the planet goes through phases like the moon.

Venus

Explanation:

Galileo observed in 1610 for the very first time with his first telescope that like moon Venus also exhibit different phases. Before that planets resembled bright and shining stars.  

This made him excited as then he could easily register the different phase of Venus as a result of reflection of sunlight from its surface and this pave a path for him to initiate the establishment of the fact that its not earth but the sun as the center of the cosmos around which planet revolves.

When Venus was at a distance closest to our Earth earth, it seems to be bigger, but we see more of the night side of Venus which seems to be crescent-shaped.

When Venus moves away from the Earth around the Sun, it seems to be smaller, but we register more of the day side which seems to be full and round.

fgravity=10x10=100n case 1

fgravity=30x20=600n case 2

6 times more