Gravity

The movements of all astronomical bodies are determined by gravitational forces. 

There were many different models of the Universe and Solar System, starting with Ptolemy's model (the Earth at the centre of the Universe) through Copernicus' model (with the planets having circular orbits around the Sun) to the accepted model proposed by Kepler who suggested that the planets described elliptical orbits around the Sun.

In 1687, Newton put forward a theory of gravity which successfully explained Kepler's model and the movement of the planets.

Newton's Laws of Gravity

Newton proposed that there was an attractive force between any two objects caused by the masses of the objects. He called this force gravitational force.

 The size of gravitational force depends upon:

1. the masses of the two objects
   
2. the distance between the masses.

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The distance follows an inverse square relationship; as the distance doubles, the force is quartered

Planets travel in orbits around the Sun; they are constantly changing direction because they are not travelling in a straight line. We know, from Newton's First Law of Motion that they must have a force being applied to them for this to happen. 

This force is the gravitational force between the mass of the Sun and the mass of the planet, (our Sun contains 99% of the mass of the Solar System). It is the gravitational attraction between the mass of the Sun and each of the planets that holds the Solar System together and causes the orbits of the planets.