Kepler’s laws of planetary motion revolutionized astronomy and physics. His first law stated that planetary motion is elliptical, not cyclical, and his second law explained how planets change speed as they follow their orbits. The third law compared the motions of planets to each other mathematically. Kepler’s laws have helped scientists understand not only the movements of planets but also the orbital patterns of satellites and man-made objects in space.
How the planets move was one of the first questions ancient scientists asked as they attempted to determine the rules of the universe. Early theories postulated that the Earth was the center of the universe and that all celestial objects orbited it. With Galileo’s discoveries, it was revealed that the sun, not the Earth, was the center of our solar system and that the planets moved around it at varying speeds and angles. Today’s theories of planetary motion are based on the work of 16th-century German astronomer Johannes Kepler.
Using the work of his mentor, Tycho Brahe, as a basis for his theories, Kepler changed the world of astronomy and physics through his three laws of planetary motion. Although only six planets were known at the time, theories of him were confirmed more than a century later by Newton and have held up well for over 400 years. While his theories are somewhat baffling to the non-astronomer, they have greatly changed the playing field for the world of planetary science.
The first law Kepler determined was that planetary motion is elliptical rather than cyclical. Instead of moving in a circular pattern around the sun, each planet moves in an oval-shaped orbit. This law was in complete disagreement with the prevailing theories of planetary motion that had existed since the time of Aristotle, but ultimately overwhelming scientific evidence proved Kepler’s new theory to be true.
Kepler’s second law concerns the speed at which planets move as they follow their orbits. The planets change speed with respect to their position relative to the sun; when they are closer they speed up and when they are farther away they slow down. Kepler’s second law states that for equal periods of time, a planet will move an equal distance. Basically, the distance it would travel in a month is longer but at a faster speed when it is close to the sun, while away from the sun it would move slower but have less distance to cover. According to this law of planetary motion, speed balances distance, so a planet will almost always travel the same distance in a given amount of time.
The third law of planetary motion that Kepler guessed is more mathematical and complicated in nature. While the first two laws deal with how a planet moves relative to the sun, the third law compares the motions of a planet to other planets. Essentially, if you square the time it takes for a planet to complete one orbit and divide it by the average cubic distance of the planet from the sun, you get an almost identical ratio for each planet. This means that a planet’s orbit time is directly proportional to how large the orbit is, so the ratio is almost exactly the same no matter which planet is being described.
Planetary motion helps describe the rules of the solar system, but its usefulness doesn’t end there. In addition to explaining how planets move, it also helps modern scientists determine the orbital patterns of satellites and other man-made objects put into space. Kepler’s laws have also helped explain the orbital pattern of newly discovered planets by advanced technology, even though we can’t observe them visually.
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