In the early 17th century, the concept of planetary motion was based on the ideas of Copernicus. People believed that planets orbited the Sun at a constant speed in perfect circles. This belief was challenged by the astronomer Johannes Kepler. He used the data meticulously gathered by his former employer Tyco Brahe, who had worked, without even the advantage of a telescope. Kepler calculated that rather than circles, the planets’ paths were ovals or ellipses and they did not have a constant speed. His discoveries certainly made people aware that the universe was not as simple as initially thought.
Kepler’s first two laws on planetary motion were published in 1609 and the third one in 1619. The application of Kepler’s laws extends to the motions of natural and artificial satellites and unpowered space crafts in orbits in stellar systems or near planets. It is to be noted that these laws, as formulated by Kepler, do not take gravitational interactions between planets into consideration.
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