Kepler is very important to the story of gravity because in 1609 he published a book that really got people thinking about the subject all over again.

Using his own observations of the heavens and those made by Tycho Brahe, he saw that Mars couldn't move around in circles as Copernicus had thought previously. It was moving in a flattened circle called an ellipse!

Kepler came up with three laws that would help both Galileo and Newton in the future.

These were:

1) When a planet goes round the Sun, it moves in closer then away again. In an ellipse.

2) When a planet gets nearer the Sun it speeds up.

3) Planets further away from the Sun take longer to go round than those that are further in.

Kepler knew that some sort of "force" was acting on the planets and suspected it must be the Sun. He thought that the force was magnetism but Newton worked out what the force actually was.

Astronomers were finally beginning to look at the universe in a more rational way and this in turn, would eventually lead to the space missions of the 1960s becoming a possibility.

In the meantime Galileo was working on experiments that would later help Newton with his "discovery".