Since the beginning of history people have seen comets appear in the night sky, rising and setting with the stars. The ancient Greeks called them "hairy stars", and the name "comet" comes from the Greek word for hair. This is because the tail of a comet was thought to look like long hair streaming from a woman's head.
Comets vary greatly in brightness, some being so faint that they can be seen only through the largest telescopes, while other are so brilliant that they can be seen even in sunlight. The nature of comets and where they came from was a mystery for a long time, and it was only in the 17th century that astronomers finally came to realize that comets are members of the large family of heavenly objects forming the Solar System. Like planets, they travel round the Sun, and take anything from three and a half years too more than a million years to complete their journeys.
A brilliant comet with a bright head and a long tail reaching halfway across the sky is a rare and glorious sight. Many amateur astronomers are so fascinated by comets that they devote all their spare time to observing and studying them.
Nature and Origin of Comets
Space probes sent to examine comets during the 1980s have provided much information to help us form a picture of what a comet is. Most astronomers now believe it to be a fairly small object, up to 75 kilometers (46 miles) across, that travels around the Sun along a very eccentric (that is, elongated) path. Comets are composed of a mixture of solid, rocky material (apparently mainly carbon), dust, and ice, and frozen gases. A widely accepted theory says that comets are the remains of materials from which the planets Uranus and Neptune were formed. The gravitational force of these large planets pushed the comets into their present extended paths, forming a huge spherical cloud on the outer fringes of the Solar System. Scientists call this the Oort Cloud, after the Dutch astronomer who invented the theory. Occasionally the gravitational force of a nearby star disturbs a comet in the Oort Cloud and sends it into the inner Solar System. Scientists think that there are billions of comets in the Oort Cloud, but only a few approach the Sun and form tails that can be seen from Earth.
Comets of the Past
People now look forward with interest to the appearance of a comet. But for many centuries they were believed to have an evil influence on the affairs of men, and particularly they were thought to foretell plagues, wars, and death. This belief was part of the so-called science of astrology. It is true, however, that several times comets have chanced to appear before great events in history.
One was seen over the city of Rome after the murder of Julius Caesar in 44BC, and the people thought it was a golden chariot coming to carry the spirit of Caesar to the home of the gods. Another appeared just before Jerusalem was captured and destroyed by the Romans in AD70. A comet also appeared before the death of the Emperor Charlemagne in AD814. Comets are also said to have appeared before the conquest of Mexico by the Spaniards in the 16th century.
However, even as early as the 1st century AD some people refused to be frightened by comets. One of these was the Roman emperor Vespasian who said, when one appeared, "Fear nothing, this bearded star concerns me not; rather should it threaten my neighbor the king of the Parthians, since he is hairy and I am bald."
For a long time it was thought that comets were made of vapor and had risen from the Earth. In the 16th century the Danish astronomer Tycho Brahe proved by meticulous observations that comets lay not only beyond Earth's atmosphere but beyond the Moon as well. It was not until the 17th century, however, that comets began to be properly understood. In 1680 Sir Isaac Newton, who had discovered the law of gravity, watched a great comet that appeared that year and from his observations made an important discovery. He realized that comets are attracted to the Sun by the force of gravity and that they revolve round it as the planets do.
Two years later another comet appeared and was observed by the astronomer Edmond Halley, a friend of Newton. Halley studied the accounts of 24 comets that had been seen from time to time since 1337 and calculated their orbits. He found that the comets of 1531, 1607, and 1682 moved in almost the same paths. Thus he came to the conclusion that they were all appearances of the same comet and that it would return in about 1758 or 1759.
His forecast was correct, for the comet did appear again, on Christmas Day, 1758, although Halley himself did not see it as he had died in 1742. For the first time scientists realized that comets can be regular visitors, and the great comet of 1758-59 was named after Halley. It ahs since appeared regularly every 76 years or so, in 1834-35, 1910 and 1985-86, when scientists sent a series of spacecraft to intercept it and investigate its nature.
© 1988 Encyclopaedia Britannica, Inc.