Introduction

All material objects show a natural tendency to move towards the Earth -- a coconut falling from a tree, raindrops falling from clouds, a dropped pencil -- and, remarkably, every falling body accelerates towards the Earth at the SAME constant rate, irrespective of its own mass. This fact was first demonstrated experimentally by Galileo, who is said to have released two balls of very different masses from the top of the Leaning Tower of Pisa and found they reached the ground together.

Centuries earlier, the Indian astronomer-mathematician Aryabhata (476-550 CE) had already concluded, from careful study of planetary and lunar motion, that the Earth spins on its own axis and moves in a roughly circular path around the Sun, and that the Moon similarly circles the Earth -- ideas recorded in his treatise 'Aryabhatiya'. About a thousand years later, Tycho Brahe (1546-1601) compiled decades of precise naked-eye observations of planetary positions, and Johannes Kepler (1571-1630) analysed this data meticulously to arrive at three empirical laws that every planet's orbit obeys. Kepler could describe HOW the planets moved, but not WHY.

It was Isaac Newton who supplied the 'why': he proposed that a single force -- gravitation -- both keeps the planets in their orbits around the Sun and pulls objects down to the Earth's surface, and that this SAME force also binds the Moon to the Earth and, on the largest scale, causes dispersed matter to coalesce into stars, planets and galaxies in the first place.

Gravitation is the force of mutual attraction that every massive object in the universe exerts on every other massive object, by virtue of their masses alone. It is always attractive (never repulsive), acts over an infinite range, is unaffected by any intervening medium, and is by far the weakest of the four fundamental forces of nature -- roughly $10^{-39}$ times weaker than the strong nuclear force. Despite being so weak at the scale of everyday objects, its infinite range and purely attractive nature let it dominate on astronomical scales, where the other three fundamental forces are either short-ranged or self-cancelling for large neutral bodies.