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Halley’s Comet, also known as Comet Halley, is the most well known periodic comet. A periodic or short-period comet is a comet that has an orbit time lasting less than 200 years or one whose orbit has been observed more than once. Halley’s Comet is formally known as 1P/Halley.
Centuries ago, astronomers did not know that comets orbited the Sun. Instead, they believed the comets simply traveled in a straight line, so we would only see the same comet pass Earth once. Isaac Newton thought that comets could travel in an orbit of some sort, but he never developed a solid model. Edmond Halley was the one who used Newton’s laws to calculate what effect the gravitational fields of Jupiter and Saturn would have on comets. Using this and records, he was able to figure out that a comet observed in 1682 followed the same path as a comet observed in 1607 and another one discovered in 1531 and that these were in fact all the same comet, predicting it would return in 76 years. The comet did return in 76 years and was spotted by Johann Georg Palitzsch, a farmer and astronomer. Later, scientists realized that the comet passes by Earth every 75 to 76 years. The comet was named after Halley because of his discovery.
Comet Halley has been observed since 240 B.C., although it may have been seen as early as 467 B.C. by astronomers in China. The comet has been recorded by ancient Chinese astronomers as well as in Babylonian tablets around 87 B.C. The orbit of the comet is a rough ellipse with the perihelion, its closest point to the Sun, at 0.6 astronomical units (AU). An astronomical unit is the average distance between the Earth and the Sun. The aphelion, furthest point from the Sun, is 35 AU, which is about how far Pluto is from the Sun.
Halley’s Comet is something of an irregularity for a short-period comet because they have an average orbit of 6.5 years and most of them have orbital periods shorter than 20 years. Comets with orbits similar to Halley’s Comet – with orbital periods longer than 20 years and inclinations from 0 to 90 degrees – are called Halley-type comets. Scientists believe that Halley-type comets used to be long-period comets until the gravity of the gas giants threw them into the inner Solar System. The last time Halley’s Comet was seen was in 1986, which means it will not reappear until 2061.
Universe Today has articles on famous comets and distant Halley’s Comet.
Astronomy Cast has an episode on comets.