Comets

Evidence of a human fascination with the night sky goes back as far as recorded history. Records on Babylonian clay tablets unearthed in the Middle East, dating to at least 3000 B.C. and rock carvings found in prehistoric sites in Scotland, dating to 2000 B.C., record astronomical phenomena that may have been comets. Until the Arabic astronomers of the eleventh century, the Chinese were by far the world's most astute sky-watchers. By 400 B.C., their intricate cometary classification system included sketches of 29 comet forms, each associated with a past event and predicting a future one. Comet type 9, for example, was named Pu-Hui, meaning "calamity in the state, many deaths." Any comet appearing in that form supposedly foretold such a calamity. In fact, from Babylonian civilization right up until the seventeenth century and across cultures, comets have been viewed as omens portending catastrophe.

Of all the Greek and Roman theories on comets—"comet" is a Greek word meaning "long-haired one"—the most influential, though entirely incorrect, was that of Greek philosopher, Aristotle (384–322 B.C.). His view of the solar system put Earth at the center circled by the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn (in order of increasing distance from the earth). Stars were stationary, and temporary bodies like comets traveled in straight lines. Aristotle believed that comets were fires in the dry, sublunar "fiery sphere," which he supposed to be a combustible atmosphere "exhaled" from Earth which accumulated between Earth and the Moon. Comets were therefore considered technically terrestrial—originating from Earth—rather than celestial heavenly bodies.

Aristotle's model left many unexplained questions about the movement of bodies through the solar system. His theory came in the Middle Ages to be so strongly supported by the Christian Church, however, that those who challenged it were often called heretics. His theory remained standard for over 2,000 years, bringing European investigations into comets virtually to a halt. Fortunately, prolific and accurate cometary records were kept by the Chinese during this period.

Sporadic European scientific investigations into comets also had some influence, however. Although the Church held his theory in check, Polish astronomer Nicolaus Copernicus (1473–1543) suggested that a heliocentric (Sun-centered) solar system would help explain the motions of the planets and other celestial bodies. Through acute observation of the "Great Comet" of 1577, Danish astronomer Tycho Brahe (1546–1601) calculated that it must be four times further away from Earth than the Moon, refuting Aristotle's sublunar theory of comets. Also, Brahe found that the comet's tail pointed away from the Sun and that its orbit might be oval.

Study of the Great Comet by Brahe and his contemporaries was a turning point for astronomical science. Throughout the seventeenth and eighteenth centuries, mathematicians and astronomers proposed conflicting ideas on the origin, formation, orbits, and meaning of comets. In the early 1600s, English physicist and mathematician Isaac Newton (1642–1727) built on theories from the likes of German astronomer Johannes Kepler (1571–1630), who developed the three laws of planetary motion; Polish astronomer Johannes Hevelius (1611–1687), who suggested comets move on parabolas around the Sun; and English physicist Robert Hooke (1635–1703), who introduced the possibility of a universal gravitational influence. Newton developed an mathematical model for the parabolic motion of comets, published in 1687 in his book Principia, one of the most important scientific works ever written.

By this time, comets were viewed as celestial rather than terrestrial, and the focus turned from superstition to science. They were, however, still viewed as singular rather than periodic occurrences. In 1687, English astronomer Edmond Halley (1656–1742) suggested to Newton that comets may be periodic, following elliptical paths. Newton did not agree. Using Newton's own mathematical model, Halley argued that the comets of 1531, 1607, and 1682—the latter observed by both he and Newton—were actually one and the same, and predicted that this comet should return late in 1758. It did, and was subsequently named Halley's comet. Halley's comet has continued to return on a regular schedule.

By the end of the eighteenth century, comets were believed to be permanent celestial bodies composed of solid material, the movement of which could be calculated using Newton's laws of planetary motion. The return of two more comets in 1822 and 1832 was accurately predicted. The first, comet Enke, did not follow Newton's law of planetary motion, as its orbital period of recurrence Optical image of Halley's comet. Royal Observatory, Edinburgh/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.

was decreasing. In 1835, German astronomer Friedreich Bessel (1784–1846) accurately suggested that this was because gases given off by the comet as it passed near the Sun acted like a rocket, thrusting the comet either closer to or further away from the Sun and so affecting its orbital length.

The second predicted periodic comet, comet Biela, with a periodic orbit of 6.75 years, upset Newton's idea of comets'when it split in two in 1846. The now-twinned comet reappeared in 1852 for the last time.

Earlier in the nineteenth century, scientists had speculated that meteor showers may be flying debris from disintegrating comets. In November 1872, when Biela should have returned, the meteor shower predicted by some astronomers did indeed appear, strengthening the connection between meteors and dying comets.