Why is retrograde motion considered an effect of revolution, but not a proof?
Retrogration is not a “proof” of revolution, since there are other ways of explaining retrograde motion. For example, the ancient believed that the Earth was stationary and explained retrograde motion by incorporating a series of epicycles in their model of the solar system (Mpg video – 267 KB). 22. Why doesn’t the Sun rise at the same time every day? What is the time differency between the rising times of the Sun on two consecutive days? There is a difference of 4 minutes. This is due to the revolution of the Earth around the Sun. The period of the Earth’s revolution is 365 days. Therefore the Earth travels approximately 1 degree per day in its orbit (360 degrees/365 days). This means that the Earth must rotate 1 degree more each day before the Sun rises. Since the Earth rotates 360 degrees every 24 hours, we can determine that it takes 4 minutes to travel that 1 degree (see below). Therefore, the 4 minute difference.
