They have been called long haired stars, from which the word comet derives. They have been called bad omens, as for King Harold. They have been called good omens, as for William, Duke of Normandy. They have been called the fossils of the solar system, holding the secrets of the origin of the planets. They are called comets.
Comets have undergone an interesting past. Early philosophers, believing that objects in the sky were fixed, perfect, and unchanging, could not reconcile this belief with comets' notorious unpredictability. So they conjectured that comets were cloud-like phenomena in our own atmosphere. This unpredictability also led to the notion that they were omens subject to interpretation. One story, told on the famous Bayeux Tapstry, recounts the events leading to the Battle of Hastings in 1066 AD. Upon the death of England's King Edward, Harold Godwinson, a Saxon, ascended to the throne, in spite of swearing an oath of fealty to William, Duke of Normandy, whom Edward had named his successor. The appearance of Halley's comet in 1066 was considered a serious problem for Harold. It was. Shortly thereafter, William was on his way to Hastings with a group of very determined soldiers in order to stop Harold.
One comet has a very special place in history. The distinguished British astronomer Edmund Halley noticed that the comets of 1531, 1607, and 1682 shared similar characteristics. He theorized that these three were in fact the same object, following an eccentric obit that returns the comet to the inner part of the solar system every 76 years. Halley was surprised to learn that his friend Isaac Newton had already discovered the mathematical principles of this type of orbit, as well as the calculus. At Halley's insistence, and with Halley's funding, Newton published these findings in perhaps the greatest work in physics, the Principia. Comet Halley's last apparition, in 1985, was a disappointment to the public, due to the fact that the Earth and the comet were never very close to each other, and the comet remained dim and appeared insignificant. Such was not the case for Hale-Bopp.
Modern observations of comets reveal that they are like large, dirty snowballs in space. Much of their time is spent in a region of space located far from the Sun. Occasionally, the orbit of a comet can be changed somewhat, and the comet begins a long, slow plunge towards the Sun. As it nears the orbit of Mars, the Sun's energy becomes strong enough to vaporize some of the ices at the surface, resulting in a cloudlike structure called the coma which then obscures the actual cometary nucleus. Nearing the Sun, some of the coma is pushed backwards by the Sun's wind, resulting in a tail that characteristically points away from the Sun.
Astronomers are keenly interested in studying comets because we believe that they represent the raw materials, fossils, from which the planets formed early in the development of the solar system. It has been speculated that a least some of the water in the Earth's oceans came from space - cometary material. If there are Rosetta stones in space, they are the comets.
On July 23, 1995, an object was discovered independently by Alan Hale, New Mexico and Thomas Bopp, Arizona. Preliminary analyses revealed that it was probably a comet, and it was given the designation C/1995 O1. The analysis of its orbit revealed that Hale-Bopp is the farthest comet ever discovered by amateurs and appeared thousands of times brighter than Comet Halley did at the same distance from the Sun. Normally, comets are extremely dim when they are beyond the orbit of Jupiter, so it has been speculated that Comet Hale-Bopp could be a rather large cometary object. As the comet approaches the Sun, its brightness increased. This comet was one of the brightest naked eye comets since Comet Hyakutake in 1996.
