Why Do Planets Circle The Sun?

Perhaps there is a simple answer to this basic question. Maybe planets circle the sun because they have to. But some people want to know more than this simple law of physics. They want to understand fully what makes Earth, Jupiter, Saturn and the others maintain their defined orbits around the star we know as our Sun.

For these curious folks, the answer lies in the gravitational pull of the Sun, as far as scientists have been able to determine to date. The planetary orbits around the Sun are comparable to the moon’s consistent orbit of the Earth. Some believe that true orbits would be perfectly circular around an object that has the force of gravity working for it. But as many know, the planets move in orbits that are elliptical, that is they are slightly irregular rather than perfect circles. Planets orbit in several different elliptical paths. The speed at which a planet moves causes it to try to pull away from the sun and there are times during the orbit when the gravitational pull is not equal to the pull at other times. In addition, the other planets exert some gravitational pull on each other.

Astronomers and physicists have also found that the center of the Sun is not the true center of mass that exerts gravitational force. In fact, they theorize, the Sun also orbits slightly around this mass center. The Sun has more mass than the planets, thus it has the stronger pull of gravity. While the planets seem to try to move away from this pull at certain places in their orbits, the Sun always pulls them back. The planets have gravitational pull on the Sun as well, but that force is not strong enough to affect the Sun’s movement.

At one time, centuries ago, scientists thought the Earth was the center of the universe, with the other planets and the Sun revolving around it. This has been disproved, beginning with the work of Copernicus in the 1500s. There was great distress over this model of the universe and the controversy continued for some time.

Some years later, Sir Isaac Newton came up with some theories that indicated the planets had to orbit the Sun, according to certain laws of physics. He figured in such factors as acceleration, the speed with which an object will move and the direction it will move unless affected by the pull of another object. One thing that Newton and others discovered was that the speed the object was moving wouldn’t change, though the direction would. This is the function of the force that the Sun exerts on each planet.

One other key figure in our current understanding of planetary movement was Johannes Kepler. He discovered a number of very specific and important facts about planetary orbits, including some ideas of how speed affects orbit. His work also helped future astronomers understand how a planet that was closer to the Sun orbited in comparison to a planet that was further from the sun. For example, he helped astronomers understand how and why the planet Pluto completes an orbit measure in years, while others orbit in a much shorter time because they are closer to the sun.

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