For most of human history, comets appeared without warning and were read as omens. One is stitched into the Bayeux Tapestry, blazing over the court of King Harold in 1066 while his courtiers point at it in alarm. Nobody imagined that a comet might be predictable, until Edmond Halley.
Three comets, one orbit
Halley was a friend of Isaac Newton. He had persuaded Newton to write up his theory of gravity, and when the Royal Society ran out of money, Halley paid for the printing of the Principia himself. Armed with Newton's laws, he set about working out the orbits of comets from old observations.
In 1705 he published his results and pointed out something striking. The comets of 1531, 1607 and 1682 had followed almost exactly the same path through the sky. They were, he argued, the same object, returning every 75 or 76 years on a long, stretched orbit around the Sun. If he was right, it would be back at the end of 1758.
Christmas night, 1758
Halley died in 1742, at the age of 85. Sixteen years later, on Christmas night 1758, a German farmer and amateur astronomer named Johann Palitzsch spotted the returning comet, just as predicted. It was the first time anyone had shown that at least some comets were members of the Solar System, bound to the Sun like the planets. The comet has been known as Halley's ever since.
It was the first proof that Newton's gravity reached beyond the planets, to the icy wanderers at the edge of the Solar System.
Later visits
In 1910 Earth passed through the comet's tail. Newspapers warned of poisonous gases and some enterprising sellers offered "comet pills" for protection. Nothing happened. Mark Twain, who had been born in 1835 in a year of the comet, had predicted he would go out with it too. He died the day after it made its closest approach to the Sun.
In 1986 the comet was greeted by a small fleet of spacecraft. Europe's Giotto, built by British Aerospace in Bristol, flew within about 600 km of the nucleus and photographed a dark, peanut-shaped lump of ice and dust around 15 km long. It was named after the Italian painter Giotto di Bondone, who is thought to have used the comet as his model for the Star of Bethlehem.
Meet it every year
You do not have to wait for the comet to come back. Twice a year Earth ploughs through the dust it has left along its orbit, giving two meteor showers: the Eta Aquariids in early May and the Orionids in late October. Every one of those meteors is a speck of Halley's Comet. Check dates on our events calendar.
The comet itself will next swing closest to the Sun on 28 July 2061. This time Earth will be much better placed than in 1986, and it should be a far finer sight. Find out more about Halley and other British astronomers on our British astronomy page.
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