AST 102 Lecture Notes - Lecture 6: Orbital Speed, Tidal Force, Binary Star
Document Summary
Now, the moon is in a roughly circular orbit around the earth. This is because any change in velocity requires an acceleration. To change the direction that an object is moving in requires a force, even if the object"s speed remains constant. Among the consequences of newton"s laws of motion are rules that govern objects in circular motion (and you"ll need to take a physics class if you want to see the derivation of this). So, he knew that any object in circular motion must have an acceleration that depends on its speed and the radius of the circle involved: Newton knew the distance to the moon (this was known from parallax measurements), and he knew the moon"s orbital speed (from its distance and its orbital period). Moon has to be experiencing to be in the orbit it is in:
