In Kepler 1665 proposed his three laws of planetary motion in which explained HOW the
planets moved around the Sun but he was not able to explain WHY. This was left to Isaac
Newton who proposed his law of gravitation in 1666.
He realised that the same type
of force that made an apple fall to the earth kept the Moon in orbit round the Earth and the
Earth in orbit around the Sun.
Newton's law of gravitation says that there is a force
between all objects and that this force depends on the masses of both objects and the
distance between their centres. The force, known as the force of gravitation, is greater for
bigger masses and gets less as the distance between them increases.
The planets
are held in their orbits by the gravitational attraction between them and the Sun. This
gravitational attraction pulls the planets from moving in straight line and swings them into
almost circular orbits round the Sun.
The planets move slower around their orbits
the further out from the Sun they are. In fact a planet five times further away from the Sun
than another will only travel one fifth as fast. For example Mercury, closest to the Sun moves
at 49 km/s, the Earth travels at about 31 km/s while Neptune crawls along at only 5.4 km/s
which means that in 2001 it had only travelled once round the Sun since it was discovered in
1846.
The pull of gravity gets weaker and weaker with increasing distance but
because of the enormous masses of the planets the force is strong enough to keep them
travelling round the Sun.
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