Primitive man knew and comets. The brightest are looking good and not resemble any other object in the sky. They look like spots of light, often blurred, they leave a trail or hair. This makes them attractive and surrounded by magic and mystery. Comets are fragile bodies and small, irregularly shaped, formed by a mixture of substances hard and frozen gases.
In general, the orbit of the comet is much more elongated than that of the planets. At one point the can closer to the Sun and in the other, remove them beyond the orbit of Pluto.
When comets approach the sun and heat, gases evaporate, emit solid particles and form the hair. When it moves away, cools, gases freeze and the tail disappears.
In each pass lose matter. Finally, there remains only the rocky core. It is believed that there are asteroids that are bare nuclei of comets.
There are comets with short orbital periods, others long. There are those who never exceed the orbit of Jupiter and others who are far, until they leave the solar system and never return.
The photo on the left is the comet Kohouotek, which passed close to Earth in January 1974. It had been detected far away, when it crossed the orbit of Jupiter.
Comet Encke, short orbit, is about three years and three months. Only go with a good telescope. By contrast, Comet Halley, who visit us every 76 years, and the Rigollet, which makes every 156, are even brighter.