Halley's comet during its 1910 visit to the inner Solar System.
COMET HALLEY
 
Halley's comet is probably the most famous comet of all. It has been known for much of recorded history. Chinese astronomers recorded an appearance by Halley's comet in 239 BC. It had very likely been observed even before then.
 
One of its most famous appearances was in 1066 AD when it was seen prior to the Battle of Hastings, a famous English battle which ended in the defeat of Harold II by William, Duke of Normandy, who then went to London and was crowned King. The comet is shown in the Bayeux Tapestry which was made to commemorate the events. It was observed on each return, but it was not known until Edmund Halley in 1705 calculated, using Isaac Newton's laws of motion, that the comets of 1531, 1607, 1682 were the same, returning every 76 years. Sadly, Halley died in 1742, so never saw the comet when it returned in 1758; but as a tribute to his prediction it was named after him.
 
The 1835 and 1910 returns were very bright. However in the January of 1910, a few weeks before the appearance of Halley there was another bright comet. Many people confused this comet with Halley's. The other comet was called the "Daylight Comet" because it was visible in the daytime.
 
The last return of Halley's comet was in 1986.
 

Halley's comet during its 1910 visit to the inner Solar System.
 
As one of the best known short-period comets, its orbital properties are fairly well understood. However it is not possible to determine the date when it will reappear because its orbit is affected by the gravitational pull of the larger planets, Jupiter and Saturn, which changes according to their location in the Solar System between each return. The comet can take between 76 and 79 years to make its orbit.
 

Hubble Space Telescope image of Halley's comet in the outer Solar System.
 
Halley is one of three comets which have been visited by spacecraft. Five spacecraft from Russia, Japan and Europe (Giotto) and two NASA probes, including the International Cometary Explorer (ICE) and another deep space satellite. The probes determined that the nucleus is an ellipsoid, egg-shaped, and that it measures 16 x 8 x 8 kilometres.
 

False-colour image of the nucleus of Halley's comet, observed by the Giotto spacecraft.
Giotto dived into the coma of Halley's comet.
The nucleus of Halley's Comet, and the surface sources of the coma, as seen by the Giotto spacecraft.
 
The dust stream left by Halley's comet produces two important annual meteor streams, the Eta Aquarids visible in the southern hemisphere and the Orionids which are visible in the northern hemisphere. The orbit of Earth about the Sun intersects with the incoming ascending path of the comet in May to give a shower with a radiant (point from which it radiates), in the south. In October as the Earth passes through the descending path or "node" of the comet, it produces a shower with a radiant in the north located in the constellation of Orion. We see a small leftover part of Halley's comet twice a year.