Comet Halley, more generally known as Halley's Comet after Edmond Halley, is the best-known and the brightest of the "short-period" comets from the Kuiper belt that visit the inner solar system in years or decades-long orbits rather than the millennia of comets from the Oort Cloud.
Comet Halley as taken with the Halley Multicolor Camera on the ESA Giotto mission. The nucleus is sunlit from the left, and several bright jets of gas and dust are visible.
Early History
(mostly from the 1911 Encyclopedia, could do with updating)Having perceived that the elements of the comet of 1682 were nearly the same as those of two comets which had appeared respectively in 1531 and 1607, Halley concluded that all the three orbits belonged to the same comet with a period of about 76 years. After a rough estimate of the perturbations it must sustain from the attraction of the planets, he predicted its return for 1757, a bold prediction at that time, but justified by the event, for the comet again made its appearance as was expected, though it did not pass through its perihelion till the month of March 1759, the attraction of Jupiter and Saturn having caused, as was computed by Clairault previously to its return, a retardation of 618 days.
Comet Halley had been observed in 1066 and fault to have been an omen, later that Year Harold II of England died in the battle of Hastings. It is shown on the Bayeux Tapestry, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a fourth of that of the Moon. History is silent respecting it from that time till the year 1456, when it passed very near to the earth: its tail then extended over 60° of the heavens, and had the form of a sabre. It returned to its perihelion in 1835, and was well observed in almost every observatory. But its brightness was far from comparing with the glorious accounts of its former apparitions. That this should have been due to the process of dissipation does not seem possible in so short a period; we must therefore consider either that the earlier accounts are greatly exaggerated, or that the brightness of the comet is subject to changes from some unknown cause. Previous appearances of Halley’s comet have been calculated by J. R. Hind, and more recently by P. H. Cowell and A. C. D. Crommelin of Greenwich, the latter having carried the comet back to 87 B.C. with certainty, and to 240 B.C. with fair probability.
It has been speculated that the "star of Bethlehem[?]" that, according to the Gospel of Matthew, indicated the birth of Jesus Christ, was in fact Halley's Comet. However, the closest time when the comet would have been visible was 11 BC, five years before the most likely date for Jesus's birth.
1910 Approach
The 1910 approach of the comet was notable for several reasons: as well as the first orbit for which photographs of the comet exist, it was a a relatively close approach to Earth making the comet a spectacular sight and indeed the Earth passed through the tail of the comet. At the time it was a matter of some concern in the press, as little was known about the composition of the comet's tail and the possibility that it was somehow toxic or otherwise dangerous could not be ruled out.
It was detected by Max Wolf at Heidelberg on plates exposed on September 11, 1909, and subsequently on a Greenwich plate of September 9, and grew steadily brighter till it was easily visible with the naked eye.
Earth passed through the tail of the comet on May 18, 1910, to massive media attention and dire predictions, and a great shrug when no harm occurred.
Astronomers at various locations, Mount Wilson observatory obtain a spectrograph of the comet, giving some indication as to its contents.
1986 Approach
Halley's return visit in 1985-86, whilst not nearly as spectacular for the general public (the comet was barely visible to the naked eye), allowed scientists the opportunity to study a comet at close quarters, and several probes were launched to do so. Most spectacularly, the Giotto space probe, launched by the European Space Agency, made a close pass of the comet's nucleus. Other probes included the Soviet Union's Vega 1 and Vega 2.
Halley is expected to return in 2061.
Note. The most standard pronunciation of "Halley" is /hæli/, to rhyme with "valley." The pronunciation /hejli/ (to rhyme with "Bailey") is thought to have originated by relation with the rock group Bill Haley & His Comets.