Planetary Science Research Discoveries HOT IDEA HEADERposted February 14, 1997 1997 Apparition of Comet Hale-Bopp
Historical Comet Observations by Karen Meech

Comet 1857 To primitive man, a comet was something to be feared, a portent of an impending disaster. Because comets brighten relatively rapidly when they get close to the sun, and because bright comets (visible to the naked eye) are relatively rare, comets would appear in the sky suddenly and unexpectedly. In addition, near perihelion, comet tails can extend millions of kilometers in space (making them the largest objects in the solar system), thus depending on the geometry of the orbit, the tail can have a length projected against the sky which is a large fraction of the celestial sphere. In an era where the celestial realm was the realm of the gods, the sudden appearance of an unknown object which dominated the night sky was terrifying.

In the Greek Era, the nature of the comets was intensely debated, but the theme of fear was prevalent as seen in this cometary quote from the greatest Greek author of antiquity, Homer:

"[The helmut of Achilles shone] like the red star, that from his flaming hair shakes down disease, pestilence, and war" (Iliad, Bk. XIX, 11, 380-3).

Bayeux Tapestry One of the most familiar comets, Halley's comet, played a prominent role in history because of its large nucleus and therefore great brightness and longevity. In 1066 when King Harold was overthrown by William the Conquerer at the Battle of Hastings, the cause of the event seems to have been pegged on a celestial visitor as is shown by the appearance of Halley's comet in the Bayeux Tapestry (left) which chronicles the event. In 1456, on a return passage, Halley's comet was excommunicated as an agent of the devil by Pope Calixtus III, but it didn't do any good - the comet has continued to return! During this same apparition, while Turkish forces were laying seige to Belgrade, the comet was described as a fearsome celestial apparition "with a long tail like that of a dragon" which was perceived by some as being in the form of "a long sword advancing from the west ... " (Moore and Mason, 1984).

Great Comet 1843 According to Chambers (1909), there are only a handful of comets which may be considered to be "remarkable". The list, reproduced below, comprises only 32 comets in the past 1000 years, indicating that we might expect an exceptional comet on average only 3 times per century. These remarkable comets are noteworthy for their extended visibility (including daytime visibility), and their exceptional brightness and spectacular features, which included reddish colors, multiple tails, jets and haloes.

This figure shows the Great Comet of 1843 as seen from Kent, England (Chambers, 1909). Because these comets appear suddenly and are seen by a multitude of people, nobody can be claimed as the discoverer. One of the most spectacular historical comets was the Great Comet of 1811 (Flaugergues) which was observed for an unprecedented 17 months. When discovered, it was 5th magnitude and over 2 AU from the sun. The maximum tail length was estimated to be 100 million miles. This comet attracted the attention of Napoleon as presaging his invasion of Russia, yet others wondered "what misfortune does it bring?" (Chambers, 1909).

Comets in History
YearComet name (if known)Additional Comments
1066HalleyPortent of Wm the Conqueror
1106-Widely visible in day - Europe & Orient
1145HalleyWell documented by Chinese
1402-Comet visible in broad daylight
1456HalleyComet was excommunicated by the Pope!
1577-Observed by Tycho Brahe; tail 80 deg long
1618-Tail 104 degrees
1661-6 degree tail & multiple nucleus structures
1680KirchMax tail arc of 90 deg.
1682HalleyEpoch of E. Halley's observations
1689-Discovered at sea, tail 68 deg
1729SarabatLarge perihelion distance
1744De CheseauxRemarkable appearance with 6 tails
1759Great CometPassed 0.07 AU from Earth
1769MessierTail length exceeded 90 degrees
1811FlaugerguesUnprecedented 17 mo. visibility
1823Great CometLarge sunward anti-tail
1843Great CometSungrazing comet
1858DonatiMost beautiful comet on record
1861TebbuttDaytime "auroral glow" reported
1874CoggiaUnusual jet features
1880Great S. CometOrbit resembles comet of 1843
1881Great CometOnly comet spectrum observed before 1907
1882Great CometOrbit resembles comet of 1880
1887Great S. CometOrbit resembles comet of 1843
1901Great S. CometBrightness rivaled that of Sirius

Discovering The Nature of Comets

Edmond Halley Comets were objects of much speculation among the early Greek astronomers, some of whom considered them to be planetary in nature, and others, such as Aristotle, considered them to be more of an atmospheric phenomena, such as meteors. The first real scientific facts known about comets were due to the observations of the great observer Tycho Brahe (1546-1601). Brahe made measurements of the position of the Great Comet of 1577 and determined from its parallax that it was a distant object, much farther away than the Moon, and therefore not an atmospheric phenomenon as many had believed. It was Edmond Halley (1656-1742) along with his contemporary Sir Isaac Newton (1643-1727) who contributed to the first physical understanding of the nature of comets. Halley's first experience with comets was with the spectacular appearance of the comet of 1680, and of the comet of 1682 (which was later to bear his name) and he became very interested in understanding how they moved. After observing these bright comets, Halley began contemplating the theory of gravitation with others at the Royal Society, but they needed a mathematical basis for their discussions. Halley approached his friend, Isaac Newton, the only man capable of working out the proof - and was surprised to learn that Newton had solved the problem many years earlier, but had lost his notes. At Halley's urging, Newton was convinced to re-work his calcuations, and Halley paid for their publication in the Principia. Using this, Halley was able to calculate comet orbits, and he noticed that the orbits of the comets of 1531, 1607 and 1682 looked very similar. He proposed that they were the same comet returning every 76 years, and that the comet would return in 1759. Although he died before the prediction could be verified, the comet was recovered on January 21, 1758, and it was named in honor of him (usually comets are named after their discoverers).

New Discovery at Greenwich The next significant break-through in the fundamental understanding of the nature of comets came in 1950 with the appearance of 2 competing physical models for the comet nucleus. Lyttleton proposed the "Sandbank Model" which supposed that the comet was a loose swarm of ice and dust which developed the characteristic comet tail and coma as it was heated upon approaching the sun. In contrast, in attempt to explain the orbital behavior of periodic comet Encke, F. Whipple proposed that the comet was a solid nucleus composed of water ice and dust - a "dirty snowball". There were several pieces of evidence that suggested that the latter model was the correct one: (i) the survival of sungrazing comets is difficult to understand without a solid body; (ii) the radar echoes measured from comet Encke were interpreted as a return from a solid body; and (iii) the delay or advance of the perihelion passage date is best described as the result of a jet forces from localized outgassing from a solid body. Nevertheless, the Whipple comet model was not actually confirmed until the spacecraft encounters with comet Halley in 1986!

Both intense public interest in observing comets and fear over their apparitions has continued from the time of Halley and Newton. In the figure at the left the celebration of the discovery of a bright comet at Greenwich Observatory was published in Punch in 1906 (Chambers, 1909). At the same time people still feared comets as evidenced from the advertisements of comet pills to fend off the evils effects of the passage through comet Halley's tail in 1910, and the concern over the appearance of Biela's comet in 1872: "The fear which took possession of many citizens has not yet abated. The general expectation herabouts was that the comet would be heard from on Saturday night. As one result, the confessionals of the two Catholic churches here were crowded yesterday evening. As the night advanced there were many who insisted that they could detect a change in the atmosphere. The air, they said, was stifling..." (Chambers, 1909).

Chambers, G. F., 1909, The Story of the Comets. Simply Told for General Readers, The Clarendon Press, Oxford, 256 p. Kronk, G. W., 1984, Comets, A Descriptive Catalog, Enslow Pub., Inc., Hillside, N.J., 331 p. Moore, P. and J. Mason, 1984, The Return of Halley's Comet, W. W. Norton & Company, New York, 121 p.
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