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What are Kepler's Laws?

Michael Anissimov
Updated Feb 25, 2024
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Kepler's laws are three equations which govern the motion of astronomical bodies. Kepler's laws were first discovered by the 17th century astronomer Johannes Kepler while analyzing data collected by Tycho Brahe. Kepler's laws are an extension of Copernicus's earlier heliocentric theory, and eventually paved the way for Isaac Newton's complete theory of how bodies interact. Newton's equations of gravity and motion can be used to derive Kepler's laws, if you assume that there are only two bodies, one of which is fixed, and one of which is orbiting at less than escape velocity. Although Kepler's laws were originally developed to explain planetary motions, they apply to any body which is in orbit around a much more massive body.

The first of Kepler's laws states that a planet, or any other object in orbit around the Sun, follows an elliptical path with the Sun at one focus. The shape of these ellipses depends on the Sun's mass, the planet's position, and the planet's velocity. A set of six numbers, called the Keplerian elements, can be used to specify the exact path that a planet traces out.

The second of Kepler's laws says that a planet in orbit traces out equal areas in equal times. If you draw a line from the planet to the Sun, and add up the area which the line sweeps over during a given time interval, it is always constant. This law is a consequence of the conservation of angular momentum; if the planet is moving faster, it also must be closer to the Sun. The increase in the area covered from the larger angular motion, and the decrease in the area covered from the shorter distance, must exactly cancel each other.

The third law states that the square of the period of the orbit must be directly proportional to the cube of the orbit's semi-major axis. The semi-major axis is half of the total distance between the perihelion, or closest approach to the Sun, and the aphelion, or farthest distance from the Sun. A planet very far from the Sun, such as Neptune, has a much larger orbit; it also moves more slowly, taking more time to cover the same distance than a planet such as Mercury. The exact relationship between orbital period, semi-major axis, mass, and the gravitational constant was later worked out by Isaac Newton.

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Michael Anissimov
By Michael Anissimov

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Discussion Comments

By anon245599 — On Feb 06, 2012

Kepler's area law for time taken can initially be expressed as the area of a triangle t=PXBX1/2. If r replaces both P and B we have t=rXrX1/2, but if the 1/2 becomes a power we have t=rXr^(1/2)which is Kepler's distance law v=r/t=1/r^(1/2) which applies throughout the whole universe.

By anon214263 — On Sep 14, 2011

I would appreciate it if someone could explain how to explain the third law. I'm doing a report and I'm stuck on the third law of planetary motion.

By anon118325 — On Oct 13, 2010

That was actually quite important. Mars is the most eccentric of the planets known at the time and so the elliptical nature of its orbit most visible.

Brahe was a very careful observer and Kepler knew this which meant that he could trust the data and had to move away from his original idea of nested geometrical solids.

By minombre — On Oct 13, 2010

It was while studying the orbit of planet Mars that gave Kepler the insight into two of his laws.

Michael Anissimov

Michael Anissimov

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology...

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