When you think of a comet, what picture comes to mind? You might envision something like a shooting star--a bright circle with a long tail trailing behind. Indeed, the word comet comes from a Greek word meaning "long-haired."
To understand comets we need to look at the makeup of our solar system. At the edges of the solar system lie long asteroid belts full of chunks of ice and dust. As an object, such as a star, passes through the asteroid belt, the object's gravity might cause one of these "chunks" to break away from the belt and gravitate towards the sun. As this body, the nucleus of the comet, approaches the sun, solid gases trapped inside it sublimate (turn directly from solid to gas). These escaped gases form a cloud around the nucleus called a coma (from the Greek word for "hair"), which can measure thousands of miles across--the nucleus alone can range from just a couple of miles to twenty miles in diameter. As the comet moves closer to the sun it encounters the solar wind, a stream of particles coming off the sun's surface. This solar wind "blows" gases and dust from the coma out behind the comet to form the tail. No matter what direction a comet is traveling, its tail nearly always faces away from the sun.
Comets travel in extremely elongated elliptical orbits with the sun at one focus. The point at which a comet is closest to the sun is called its perihelion. The period of a comet is the length of time it takes to travel once around its orbit. The orbits of some comets are so large that their periods are thousands of years. Sometimes the orbits are so elongated that the comet never returns. This happens when the comet has travelled so far from the sun that, by the time it reaches the other end of the ellipse it escapes the sun's gravity and is lost; these comets have parabolic orbits.
In 1705, the English astronomer Edmund Halley used Newton's laws of motion to prove that the comet seen in 1531, 1607, and 1682 was the same comet returning each time, and that it would return again in 1758. His prediction came to pass; and although Halley did not live to see it, the comet was named in his honor. Halley's Comet has a period of 76 years, and it most recently returned in 1986.
Until astronomers were able to explain comets, most people regarded comets as bad omens and signs of great change, and thought that even looking at them would bring misfortune. The most famous recording of Halley's Comet was in 1066 at the Battle of Hastings, in England. The Bayeux Tapestry, which documents the battle, depicts the comet flying ominously overhead in the scenes leading up to the battle.