# wy do all the planets revolve around the sun?

Why do the planets rotate around the sun?

The basic reason why the planets revolve around, or orbit the sun (rotate actually is used to describe their spin, for example, the Earth completes one rotation about its axis every 24 hours, but it completes one revolution around the Sun every 365 days), is that the gravity of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity.

Why, then, does it travel in an elliptical orbit around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun's pull. If the Sun weren't there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle. This is a little hard to visualize, so let me give you an example of how to visualize an object in orbit around the Earth, and it's analogous to what happens with the Earth and the Sun.

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Stars and planets form in the collapse of huge clouds of interstellar gas and dust. The material in these clouds is in constant motion, and the clouds themselves are in motion, orbiting in the aggregate gravity of the galaxy. As a result of this movement, the cloud will most likely have some slight rotation as seen from a point near its center. This rotation can be described as angular momentum, a conserved measure of its motion that cannot change. Conservation of angular momentum explains why an ice skater spins more rapidly as she pulls her arms in. As her arms come closer to her axis of rotation, her speed increases and her angular momentum remains the same. Similarly, her rotation slows when she extends her arms at the conclusion of the spin.

As an interstellar cloud collapses, it fragments into smaller pieces, each collapsing independently and each carrying part of the original angular momentum. The rotating clouds flatten into protostellar disks, out of which individual stars and their planets form. By a mechanism not fully understood, but believed to be associated with the strong magnetic fields associated with a young star, most of the angular momentum is transferred into the remnant accretion disk. Planets form from material in this disk, through accretion of smaller particles.

In our solar system, the giant gas planets (Jupiter, Saturn, Uranus, and Neptune) spin more rapidly on their axes than the inner planets do and possess most of the system's angular momentum. The sun itself rotates slowly, only once a month. The planets all revolve around the sun in the same direction and in virtually the same plane. In addition, they all rotate in the same general direction, with the exceptions of Venus and Uranus. These differences are believed to stem from collisions that occurred late in the planets' formation. (A similar collision is believed to have led to the formation of our moon.)

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Some visitors to the Gheens Science Hall & Rauch Planetarium at the University of Louisville in Kentucky recently asked why the planets go around the Sun. Most people take this fact for granted, but the answer involves many interesting ideas and important concepts. I'll touch on a few.

First of all, saying the planets go around the Sun is just another way of saying the planets are in orbit around the Sun. A planet orbiting the Sun is like the moon or a NASA satellite orbiting Earth. Now why does a planet orbit the Sun and not the Sun orbit the planet? The lighter object orbits the heavier one, and the Sun is, by far, the heaviest object in the solar system. The Sun is 1000 times heavier than the largest planet, Jupiter (which also happens to be my favorite planet), and it is more than 300,000 times heavier than Earth (another planet I am very fond of). In the same way, the moon and satellites we launch orbit Earth because they are so much lighter than our planet.

But now we still have the question of why anything orbits something else. The reasons are complicated but the first good explanation was provided by one of the greatest scientists ever, Isaac Newton, who lived in England about 300 years ago. He was very well known when he was alive, being admired by many people for answering some of the most difficult and fascinating scientific questions of his day, but I'm sorry to say his long life generally was not a happy one. I wonder how he would have felt to know that even hundreds of years after his death, he is widely considered to be one of the most brilliant, important, and productive scientists ever to have lived.

Newton realized that the reason the planets orbit the Sun is related to why objects fall to Earth when we drop them. The Sun's gravity pulls on the planets, just as Earth's gravity pulls down anything that is not held up by some other force and keeps you and me on the ground. Heavier objects (really, more massive ones) produce a bigger gravitational pull than lighter ones, so as the heavyweight in our solar system, the Sun exerts the strongest gravitational pull.

Now if the Sun is pulling the planets, why don't they just fall in and burn up? Well, in addition to falling toward the Sun, the planets are moving sideways. This is the same as if you have a weight on the end of a string. If you swing it around, you are constantly pulling it toward your hand, just as the gravity of the Sun pulls the planet in, but the motion sideways keeps the ball swinging around. Without that sideways motion, it would fall to the center; and without the pull toward the center, it would go flying off in a straight line, which is, of course, exactly what happens if you let go of the string.

Here are some other Space Place pages to explore:

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BECAUSE ALL PLANETS NEED SUN LIGHT

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because sun binds the whole solarsystem.if planets want that they don't resolve than also they had to resolve

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because sun binds the whole solarsystem.if planets want that they don 't resolve than also they had to resolve.........

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The basic reason why theplanets revolvearound, ororbit the sun.Why, then,doesit travel in an ellipticalorbit around the Sun,

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