Comet McNaught seen in the night skies of Chile, January 2007A comet is a potato-shaped lump of dust and ice measuring only a few kilometres across, accompanied by—when near to the Sun—two tails stretching out hundreds of millions of kilometres into space. Occasionally, we can spot a comet from Earth: a dramatic smear of light across the starry heavens. Like the planets and asteroids, comets orbit the Sun, although their orbits are very elliptical (squashed oval). Different comets take anything from a few years to hundreds of thousands of years to complete their circuit. Short-period comets originate in the Kuiper Belt or the Scattered Disc, which lie just beyond the orbit of Neptune. Long-period comets probably originate in the Oort Cloud, a cloud of icy bodies around the edge of the Solar System.
The English astronomer Edmond Halley (1656–1742) was the first to realise that comets were orbiting objects. He once made a famous prediction: a comet that he had observed in 1682 would return to the skies in 1758. Halley believed that comets recorded in 1531 and 1607 were simply earlier sightings of the one he saw in 1682. Halley did not live to see his prediction come true. Halley’s Comet, as it became known, was duly sighted on Christmas Day 1758 and has reappeared at intervals of 75 to 76 years ever since.
Halley's Comet was visible from Earth in 1066 and is shown in the famous Bayeux Tapestry that commemorates the Norman conquest of England in that year.
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