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 km 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).
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 (top, center) 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). 
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. The figure at the left 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). | Year | Comet | Comments | Year | Comet | Comments |
| 1066 | Halley | Portent of Wm the Conqueror | 1106 | - | Widely visible in day - Europe & Orient |
| 1145 | Halley | Well documented by Chinese | 1265 | - | - |
| 1378 | - | - | 1402 | - | Comet visible in broad daylight |
| 1456 | Halley | Comet was excommunicated by the Pope! | 1531 | Halley | Observed by Kepler |
| 1556 | Heller | - | 1577 | - | Observed by Tycho Brahe; tail 80 deg long |
| 1607 | Halley | - | 1618 | - | Tail 104 degrees |
| 1661 | - | 6 degree tail & multiple nucleus structures | 1680 | Kirch | Max tail arc of 90 deg. |
| 1682 | Halley | Epoch of E. Halley's observations | 1689 | - | Discovered at sea, tail 68 deg |
| 1729 | Sarabat | Large perihelion distance | 1744 | De Cheseaux | Remarkable appearance with 6 tails |
| 1759 | Great Comet | Passed 0.07 AU from Earth | 1769 | Messier | Tail length exceeded 90 degrees |
| 1811 | Flaugergues | Unprecedented 17 mo. visibility | 1823 | Great Comet | Large sunward anti-tail |
| 1835 | Boguslawski | - | 1843 | Great Comet | Sungrazing comet |
| 1858 | Donati | Most beautiful comet on record | 1861 | Tebbutt | Daytime "auroral glow" reported |
| 1874 | Coggia | Unusual jet features | 1880 | Great S. Comet | Orbit resembles comet of 1843 |
| 1881 | Great Comet | Only comet spectrum observed before 1907 | 1882 | Great Comet | Orbit resembles comet of 1880 |
| 1887 | Great S. Comet | Orbit resembles comet of 1843 | 1901 | Great S. Comet | Brightness rivaled that of Sirius |
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).
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).