In the Ptolemaic cosmology the earth is regarded as a fixed point with the other planets revolving around it .One reason for supposing that the earth is not moving is that if you drop an object it falls straight to the ground; if the earth was moving, Ptolemy reasoned that it would fall in a different place because the earth would move in between when you released the object and when it hit the ground. In fact, this is incorrect as can be demonstrated by dropping an object (keys for example) while walking. The keys do not fall to the ground at the place you stood when you released them but at the place you are, roughly speaking, when the keys hit the ground. In other words the keys move with you after you let them go, as long as you keep going with the same speed. This was first explained by Galileo in his famous work "Dialogue Concerning the Two Chief World Systems," When the keys are released they have same forward velocity with respect to the earth as you, the walker, do. In addition they experience a gravitational force pulling them down. So they drop to the ground while maintaining the same forward velocity, assuming we can neglect the drag of the air, and arrive at the same place as your feet. In a similar manner Galileo argued that you would not be able to tell if the earth is moving or not by experiments done on earth, thereby removing an important obstacle to the heliocentric (sun centered) cosmology of Copernicus. Today the principle that the laws of mechanics are the same in any frame moving with a constant velocity is called "Galilean invariance," and would play a crucial role in Einstein's development of special relativity.