Comet

French cartoon from 1900

Etymology

The word comet derives from the Latin comēta. That, in turn, is a romanization of the Greek κομήτης 'wearing long hair'. The term means long-haired star.

Typical direction of tails during a comet's orbit near the Sun

Orbits of Comet Kohoutek (red) and Earth (blue)

Tycho Brahe's demonstration of the 1577 comet

Astronomy

Comets are celestial bodies with a frozen core that can measure up to 100 km in diameter. They have highly elliptical orbits around the Sun. When they approach their perihelion, the Sun's intensive energy releases glowing gases and other particles which form the nebulous coma around the solid core. When a comet's perihelion is particularly close to the Sun, the Solar wind forces it to eject streams of dust and gas which form the so-called comet's tail.

Comets probably formed at around the same time as the other planets in the solar system. Only a very small number of the numerous comets in our Solar System are visible to the naked eye. Visible comets appear on average around once or twice per decade.

Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one Astronomical Unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper Belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space.

The appearance of a comet is called an apparition. Comets have been observed and recorded since ancient times by many cultures and religions.

Comet Leonard on Dec. 24, 2021

Halley's Comet appeared in 1066

Astrology

Comets used to be considered significant for astrological interpretation, particularly in Mundane Astrology. The comet's orbit was plotted in the horoscope to see if it made any aspects to other planets. One of the first practitioners of the theory of comets was Aristotle. He thought their appearance represented fire that originated from the Milky Way and that their vapours sank down to Earth and polluted the atmosphere, causing natural catastrophes. In general, comets were held to be harbingers of all kinds of disaster including war, uprisings, floods, plagues and famine. However, there are also examples of comets being thought to herald the birth of an important individual or time of change ("Star of Bethlehem"). But their sudden appearance generally caused panic. Since around the time of the Enlightenment they have ceased to have any significance in astrological interpretation, although some populist astrologers continue to stoke up fear of comets in order to gain more publicity in the popular media. One example of this was the attempt by some astrologers to link the appearance of Halley's comet in April 1986 with the Chernobyl nuclear disaster which occurred around two weeks later.

The German astrologer Schubert-Weller on the decline of the importance of comets in astrology: "Present-day astrology, with its focus on the Sun, Moon and main planets, would have appeared very strange to the Ancients. It was not only folk religion that saw the sky as being populated by animals, spirits, angels and all kinds of strange creatures that appeared as meteorites, comets and other celestial phenomena. A whole heavenly kingdom, a "heavenly earth" was thought to exist - and its significance for humans was mantically and astrologically interpreted."

See also

• Star of Bethlehem

Deep Purple's Rock album Fireball (1971)

Weblinks

• Wikipedia on Comets
• Wikipedia: Elliptic orbit
• Periodic Comet Numbers (Current list of comets; Minor Planet Center (IAU))
• NASA: Near-Earth Object Program (Asteroid and comet orbits and their odds of Earth impact)
• Wikipedia: Divination by Astrological and Meteorological Phenomena (Chinese sightings of comets more than two thousand years ago)
• Asteroids and Comets (David McCann, 1997)
• Hale Bopp: Harbinger of Government Flop. (Comets in history; Deborah Houlding, 1997)
• Unexpected Visitors: The Theory of the Influence of Comets (Jonathan Flanery, 2005)
• Comets in Astrology - Possible meanings of these celestial events (Rod Chang, 2014; Astrodienst)