The appearance of a bright comet is a rare event which cannot be predicted, as they travel round the Sun in huge orbits over periods that can last many thousands of years. There are therefore no records of previous appearances as their wanderings through space cannot be followed.

Comets of short periods that return at regular intervals are fainter objects but can be followed and recovered. Most of the short period comets travel in orbits of small inclination which reach out to Jupiter´s orbit. Jupiter is responsible for the severe perturbations that many of thes comets undergo. Unlike the planets, comets may be seen in any part of the sky.

Halley’s Comet

Christmas night 2008 saw the 250th anniversary of the recovery and predicted return of Halley´s Comet. Until that time comets had not been recognised as members of the Solar System. Even Newton thought they travelled in a straight line passing the Sun just once. Halley’s Comet was the first to make a predicted return to perihelion. Records of Halley´s Comet go back a very long way. The orbital period is about 76 years and all returns since 240BC have been observed. It may also have been recorded as early as 1059BC by the Chinese.

Generally a very bright object, this was unfortunately not the case during its last return in 1986. It was at its very best in 837AD when the tail stretched over 90º and the coma was as bright as Venus. It was also around in 1066AD and is shown in the Bayeux Tapestry, apparently regarded as an unfavourable omen by the Saxons.

In 1301AD the return was also favourable and the Florentine painter Giotto di Bondone used it as a model for the star of Bethlehem in his painting the Adoration of the Magi.

The return of 1456AD was also favourable, at a time when Turkish forces were on a rampage in Europe. It was said that on June 8th that year a fearsome apparition appeared in the sky with a long dragon´s tail. At the time Pope Calixtus III preached against it as an agent of the devil and, some say, excommunicated it.

On 15th August 1682 the comet was recovered by Dorffel. A few nights later Edmond Halley made a long series of observations and realised the orbit was strikingly similar to the comets of 1531 and 1607. Halley predicted a 1758 return which was picked up on Christmas night by the German observor Palitzsch (who has a formation on the Moon and an asteroid named after him) and came to perihelion in 1759.

Since then Halley´s Comet has returned in 1835, 1910 and 1986. It is due again in 2061, predicted as unfavourable and after that in 2137 when predictions are it will be magnificent.