The Earth rotates in a period of slightly less than 24 hours on an axis that runs between the North and South Poles. The east-west circumference of the Earth decreases as the latitude increases. The rotational speed is 1,038 miles per hour (1,670 kph) at the equator, 519 mph (835 kph) at 60° and 0 at the poles (90°).

Standing on the Earth's surface, it appears that the planet is fixed and the rest of the universe is revolving around it. For a long time, this apparent motion was taken to be real, coupled with the doctrinal belief that man and the Earth were at the center of the universe.

One exception to early thinking was the Greek astronomer Heracleides of the fourth century B.C. who suggested that it might be possible that the Earth was traveling around the Sun. His main clue was the close relationship the inner planets, Mercury...