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In 1684, the astronomer Edmond Halley visited Isaac Newton in Cambridge to discuss his ideas about the force of gravity. Halley was amazed to discover that Newton had already developed a mathematical theory to explain gravitation and the movement of the planets. Halley persuaded the reclusive Newton to publish his theory. Newton spent the next two years setting out his ideas in a book that is probably the most important book ever written. The English title of Newton’s book is ‘Mathematical Principles of Natural Philosophy’. This title is usually abbreviated to ‘The Principia’.

In 1705, Halley used Newton's theory to calculate the orbits of the comets seen in the years 1531, 1607, and 1682, and demonstrated that they were all, in fact, the same comet. Halley predicted that this comet would return again in 1758. He was correct, but unfortunately he did not live long enough to see the comet’s return. The comet is now named Halley’s comet in his honour. It returns every 76 years and last visited the inner solar system in 1986. It will return again in 2061.

Halley’s comet has been producing spectacular sights in the skies for thousands of years. Its return in the year 1066 is recorded on the Bayeaux tapestry, which was woven after the Norman Conquest. The artist Giotto depicted Halley’s comet as the Star of Bethlehem in his “Adoration of the Magi” painted in 1304, three years after the comet’s return in 1301. In 1986 Halley's comet was visited by the European Space Agency probe Giotto, named after the early Renaissance artist.

Comets are often described as 'dirty snowballs'. They are thought to have formed in the earliest days of the solar system. The planets grew by accumulating material from the impacts of millions of such bodies of rock and ice. Many more cometary bodies were despatched to the outer reaches of the solar system after close encounters with the proto-planets. Every so often gravitational perturbations return one of these bodies to the inner solar system.

As comets approach the Sun, some of their ice vapourises to form their characteristic tail that may extend for millions of kilometres through space. The photograph on the right, taken by the Giotto probe, shows vapours escaping from the nucleus of Halley's Comet on its approach to the Sun in 1986.

Text by Nick Mee from 'Forces and Electricity' (Virtual Image CD-ROM).