HOW DO THE PLANETS STAY IN ORBIT?

 

The Solar System was formed from a rotating cloud of gas and dust which spun around a newly forming star, our Sun, at its center. The planets all formed from this spinning disk-shaped cloud, and continued this rotating course around the Sun after they were formed. The gravity of the Sun keeps the planets in their orbits. They stay in their orbits because there is no other force in the Solar System which can stop them.

The reason why the planets stay in orbit is contained within Isaac Newton’s theory of gravity; the central concept of which is ‘universal gravitation’, which states that everything with mass generates gravity: the Earth, the Moon, the Sun, all the planets and the moons, all the stars – everything. The amount of gravity generated is proportional to an object’s mass, and affects anything else nearby with mass; in other words, everything pulls everything else.

 

 

 

Newton’s Universal Gravitation was partly based upon the pioneering work of Johannes Kepler, who had mathematically described planetary motion several decades earlier.

Kepler’s first of three laws states that each planet moves in an elliptical orbit, with the Sun at one focus. The second law is that when the planet is far away from the Sun, it does not move as fast as when it is close to the Sun. This implied that whatever force was moving the planet weakened with distance. The third law follows on from the second, expressing as an equation the link between the size of the planet’s orbit and the time it takes to complete the orbit. But Kepler could not explain why the planets moved in this way.

Newton could. He visualized a cannon atop a tall tower, pointing horizontally and firing its projectile. Ignoring air resistance, the cannonball would zoom off parallel to the ground and the gravity of the Earth will immediately begin to pull it downwards, eventually dragging it to the floor. The greater the explosive charge, the faster the projectile will be ejected and the further it will travel before gravity pulls it down. Newton imagined sufficient explosive to eject the cannonball so fast that, by the time it started to fall, the curvature of the Earth resulted in the ground beneath dropping away and so the cannonball finds itself always at the same altitude above the ground. Without air resistance, the projectile would still be travelling at the same speed as when it left the cannon, and the whole situation starts again. Every time the cannonball drops a little, so the curvature of the Earth compensates, allowing the projectile to continue around the Earth forever; in effect, placing it in orbit.

This gives us the solution to the question of what stops the Moon crashing into the Earth. The Moon is falling towards us, but also travelling along so fast that it ‘overshoots’ the Earth and continues in a circular path.

Source and further reading:
http://cosmoquest.org/x/365daysofastronomy/2010/03/17/march-17th-why-do-the-planets-stay-in-orbit/
http://www.stuartclark.com/