**Johannes Kepler** (1571-1630) was an astronomer of the early 1600s who developed and** perfected the theory of Copernicus.** He **worked with Tycho Brahe,** who carefully observed and recorded the positions of the planets in the sky. From this Kepler found that the theory of Copernicus was not that good at predicting where planets would be. It needed work.

Even Copernicus knew that planets did not move in simple circles.** Like Ptolemy, Copernicus had** circles within circles – **epicycles.** But even then it still did not quite fit the facts.

**Kepler** figured out what was really going on: planets did not move in circles or even circles within circles, but in** ellipses:** less than perfect circles that were longer one way than another. **In addition, planets moved both fast and slow** in a predictable way.

**Kepler’s system was not only better, it was simpler.**

Kepler came out for the theory of Copernicus 37 years before Galileo did! What is more, he knew the theory was not perfect and set about to make it better.

Even though **Galileo** knew of Kepler’s improvements, he** still preferred Copernicus.**

**Kepler was Protestant** not Catholic. The Catholic Church could not silence him as it did with Galileo. **The Protestant churches** were against the theory of Copernicus too, but were too weak. They** could not silence Kepler: all they could do was to make life difficult.** He often lost his home and his position at universities. He was always on the move, it seemed.

**Tycho’s 20 years of observations** of the planets were not perfect, as Kepler knew, but they were by far the best in the world. **Based on them** Kepler wrote his **“Nova Astronomia” in 1609.** It laid out the first two of his three laws –** the first laws of nature given by anyone:**

- A planet moves round the sun in an
**ellipse.** - The line between a planet and the sun sweeps across
**equal areas in equal time.**

The effect of the second law meant that **as a planet got closer to the sun, it began to move faster** and as it got farther away it moved more slowly. Because a planet moves in an ellipse it is not always the same distance from the sun.

He came out with **his third law** some years later:

- To figure out
**a planet’s year**(the time it takes to go round the sun) take**the cube of the square root of its average distance.**

**For example, Jupiter** is **5.2 times farther from the sun than the Earth.** The square root of 5.2 is 2.28 (because 2.28 times 2.28 equals 5.2). The cube of 2.28 – 2.28 times 2.28 times 2.28 – is 11.85. **So Jupiter takes 11.85 years** to go round the sun.

**Kepler had no idea** why any of this was true or **how it was possible physically** – no one did till Newton in the late 1600s. But to his great credit he followed the facts as he knew them.

**See also:**

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