He also found that when the planets were nearer the sun in their orbits, they move faster than when they were farther from the sun. He found that the Sun was positioned at one 'focus' of the ellipse (there are two 'foci', both located on the major axis). An ellipse is a sort of a squashed circle with a short diameter (the 'minor axis') and a longer diameter (the 'major axis').

He discovered that they move in ellipses. Using these observations, Kepler discovered that the planets do not move in circles, as 2000 years of 'Natural Philosophy' had taught. Tycho's observations of planetary motion were the most accurate of the time (before the invention of the telescope!). After Tycho's death in 1601, Kepler was able to obtain Tycho's observations. Tycho was a great and extremely accurate observer, but he didn't have the mathematical capacity to analyze all of the data he collected. Kepler briefly worked with the great Danish observational astronomer, Tycho Brahe. Why the huge differences in periods? We need to go back to the time of Galileo, except that we're not going to look at his work, but rather at the work of one of his contemporaries, Johannes Kepler (1571-1630). Refer to the table with the rotation rates and revolution rates of all the planets. Poor, ponderous, and distant Pluto takes a whopping 248 years for one revolution. While earth takes 365 days to make one circuit, the closest planet, Mercury, takes only 88 days. We all learn in grade school that the planets move at differing rates around the sun.

A year is the time it takes the earth to make one revolution - a little over 365 days. The revolution of the earth around the sun is how we define the year. Giant Jupiter has lots of spin, turning once on its axis every 10 hours, while Venus takes 243 days to spin once. There are no rules that govern the rotation rates of the planets, it all depends on how much 'spin' was in the original material that went into forming each one. We further divide this period of time into 24 hours, each of which is divided into 60 minutes, each of which is broken into 60 seconds. The time it takes the earth to rotate from noon until the next noon we define as one day.

The top-like rotation of the earth on its axis is how we define the day. Second, the earth revolves around the sun, like a tetherball at the end of a string going around the center pole. First, the earth rotates on its axis, like a spinning top. There are two that specifically interest us. Actually, several different motions all at once. This brings up the question of how we define the time intervals we measure. Looking at the numbers above, you'll immediately notice that you are different ages on the different planets.

Animated gif spinning earth