A comet is a small celestial body which has at least in the solar neighborhood parts of its orbit an image produced by outgassing coma.
At the time of its discovery, the comet is mostly a small fog cloud, which can be identified as a comet only by relatively rapid movement among the stars. Within days and weeks the comet approaches the sun. In perihelion the only a few kilometers wide comet nucleus (diameter between 1 and 100 km) is sourrounded by a diffuse, misty, sheat said coma, which can reach a distance of up to 2.7 million kilometers core and coma together are also called the comet’s head. However, the most distinctive feature of from the earth visible comets is the tail which can in large and sun nearby objects reach a length of more than 100 million kilometers and which can drag across the whole sky and even be visible in daylight. In most cases, it reaches only a maximum of several 10 million km.
An average of 20 to 30 comets per year are discovered in the sky. The most comets rarely achieve such brightness that they are visible with the naked eye.
The most comets move along parabolic orbit ellipses around the sun and have orbital periods of more than 200 years (long-period comets). There are also comets with short orbital periods (short period comets).