Half a century later, an English scientist, Isaac Newton (1642—1727), was inspired to think about gravity by another falling object, in this case an apple from a tree in his garden in Lincolnshire. He realized that the same force that made the apple fall to the ground must also be responsible for keeping the Moon in orbit around Earth.

Newton went on from this realization to deduce his law of gravity, publishing it in 1687 in Principia Mathematica. According to Newton, an object's gravitational attraction depends on its mass (that is, the amount of matter it contains), and the strength of the attraction falls off with the square of the distance from the object. This law

NEWTON'S PRINCIPIA

The laws of motion demonstrated by Newton in his Principia Mathematica of 1687 provided a sound mathematical basis for all subsequent students of physics and astronomy.

explained for the first time why the planets orbited the Sun as they did and why the Moon raised tides in Earth's oceans. In due course, it would apply also to the motions of artificial satellites and space probes.

Using Newton's theory of gravity, the English astronomer Edmond Halley (1656-1742) calculated that comets move around the sun on highly elliptical orbits. Convinced that the comets seen in 1531, 1607, and 1682 were one and the same, Halley predicted that the comet would return around 1758. When it duly reappeared 16 years after his death, it was named Halley's Comet.

HALLEY'S COMET

Halley correctly calculated that one elliptical orbit of the comet that bears his name (below) took around 76 years. Confirmation of this came from the 11th-century Bayeux tapestry (left), which records the appearance of the comet in 1066 shortly before King Harold of England was defeated at the Battle of Hastings by William, Duke of Normandy.

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