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Halley’s Comet

Comet 1P

Proper NameHalley
Designation1P
Orbital inclination to the Ecliptic162.3°
Orbital Period75.3 years
Distance from the SunSemi-Major Axis: 2,667,928,000 km (17.83 AU)
Perihelion: 87,668,000 km (0.59 AU)
Aphelion: 5,248,189,000 km (35.08 AU)
Eccentricity0.9671
NotesAn important comet following a narrow elliptical orbit that can carry it more than 35 AU from the Sun, but which has been regularly returning to the inner Solar System and shining prominently in Earth's skies since ancient times.

The progress of Halley's Comet between the years 2000 to 2050. During the first part of this period, until 9 December 2023, the comet recedes from the Sun until it reaches its aphelion point - tbe farthest extent of its orbit, marked by an orange circle on this map.

At this point it is more than 35 AU from the Sun, but then it begins its journey back into the inner Solar System. By 2050, when it falls close to the bright star Procyon in the sky, its distance has fallen to less than 24 AU, and it is rapidly returning to the region of the Sun and inner planets.

The most famous of all comets visits the inner Solar System in periods of approximately seventy-six years. At present, it is on the outward leg of its journey, beyond the orbit of Neptune, but it can be expected to return to the skies of Earth in July 2061.

History

In 1705, Edmund Halley* studied historical reports of a bright comet that had been reported in the years 1531, 1601 and 1682, and observed a regular pattern of some 75 to 76 years between sightings, and he calculated that the same comet should be seen again in the year 1758. Though Halley himself died in 1742, and so never discovered whether he was right, the comet did indeed appear as he had predicted, showing that (at least some) comets pursued a regular orbit around the Sun. The comet itself was designated '1P' (the first periodic comet to be identified) but is commonly known simply as 'Halley's Comet'.

Based on Halley's calculations, it became possible to search back in history for sightings preceding 1531, and many were found. The earliest official record of Halley's Comet came from China in 239 BCE, some two thousand years before Halley's own time. Perhaps the most famous of these historical sightings was in March 1066, several months before the Battle of Hastings, and the comet's appearance was recorded in the Bayeux Tapestry telling the story of the battle.

The Comet and Its Orbit

For such an important and well-known object, the physical nucleus of Halley's Comet is remarkably small. At the heart of the comet is an irregular rocky body just fifteen kilometres along its longest axis. As it nears the Sun, however, compounds on this small object begin to evaporate as they heat, forming a tenuous coma that can extend out to 50,000 km from the core. The solar wind acts on the comet, creating a tail of material pointing away from the Sun, reaching a length of some hundred million kilometres.

Halley's Comet does not in fact follow a perfectly periodic cycle, and the periods between its returns to the inner Solar System have varied between 74 and 79 years since it was first observed. Its orbit is highly eccentric, so that between its close approaches to the Sun it journeys into the far depths of the Solar System. At its closest approach, its perihelion, the comet reaches a distance from the Sun of 0.6 AU, well within Earth's orbit. At its most distant aphelion point, however, it reaches a distance of some 35 AU from the Sun, beyond the orbit of Neptune.

Meteor showers are typically associated with comets, and are caused by Earth's passing through particles left in the wake of a comet as it passed through the inner Solar System. In the case of Halley's Comet, the Orionid meteor shower seen in late October each year is directly associated with the comet's orbit. Halley's Comet is thought to affected the meteors of the Eta Aquariid shower or early May, though in this case it is less clear whether the comet is directly responsible for the shower, or influenced existing cometary debris.

Halley's Comet in the Future

The last return of the Halley's Comet to the inner Solar System was in 1986, but conditions were far from ideal for observation. The comet approached the Sun from the opposite direction to Earth, and so was not easily visible.

Observing conditions are expected to be much better for subsequent returns. In 2061 the comet will approach the Sun from the same approximate direction as Earth, and should be clearly visible to the naked eye. The same situation will apply on its next return in 2134, when the comet will pass within a tenth of an AU of Earth, and will shine brighter than any star.


* There is some dispute about the spelling and pronunciation of Edmund Halley's name, especially as during his lifetime the name was spelt in various different ways. Some sources prefer to give his forename as 'Edmond' rather than 'Edmund', and he himself seems to have used both spellings. His surname, and hence the name of the comet, is conventionally pronounced with a short 'a' sound (to rhyme with 'dally'). However, the name is sometimes pronounced to rhyme with 'daily' instead. There is reason to believe that in his own time Halley would have used neither of these modern pronunciations, instead preferring something like 'hawley'.

There is sufficient variation in the original sources that none of these spellings or pronunciations can be considered unequivocally 'correct', though in practice the short-'a' 'dally' pronunciation is the version most commonly used.

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