The name comet is derived through the Latin from the Greek word for “hair”. It suggests an imagined resemblance between the tail of a comet and long hair streaming in the wind. The term used in China and Japan, translated “besom (broom) star,” may originate either from a comparison between a comet’s tail and the bundle of twigs in a broom over a floor suggested by the motion of a comet’s tail across the constellations.

      As we know, comets are the heavenly bodies which revolve around the Sun and consist of a nucleus. When they are close to the Sun a tail of great length develops. Most comets move in very elongated orbits that differ little from parabolas and require thousands of years for completion. Some in fact , appear to leave the solar system forever because of changes produced in their orbits by gravitational attractions of the major planets.

Distances and periods of comets

      Distances of comets from the Sun vary within extremely wide limits; the comets 1880 I and 1873 I approached within 1200000 km. of the Sun's surface its the Lexel's comet, and several other comets approached almost as closely. There appears to be a great reservoir of comets, belonging to the solar system, at distances that are large fraction of the distance from the Sun to nearest star (40000000000000 km.). Periods required by comets to complete a single revolution in their orbits also vary enormously. the shortest verified period is that of Encke's comet, 32 years. the longest computed periods have little real meaning, for comets moving in very large orbits can be observed over so small a fraction of the total path that the value found for the period has little significance. But periods certainly amount to millions of year for some of observed comets.

Law of gravitation applied to comets

      It was impossible to ascertain the true nature of the orbits of comets until the law of gravitation had been established Isaac Newton proved that, under a force that varied in proportion to the inverse square of the distance, a body would move in one of the family of curves known as conic section; that is, the circle, ellipse, parabola or hyperbola. It was soon recognized that the observed movements of comets could be explained on the hypothesis that they were traveling around the Sun in elongated ellipses or parabolas, being visible only when they described the small portion of their orbits in the neighborhood of the Sun. Newton himself applied the new principle of the brilliant comet of the 1680; subsequently he obtained the assistance of Edmund Halley, who in 1704 collected the observations of 24 comets, commencing with that of the year 1337, and calculated their orbits. On studying the orbits,  Halley noticed that three , those of the comets of 1531, 1607 and 1682, where practically identical. The interval between appearances of the comets were not the same, the first being longer by 15 months, but Halley saw that the differences could be readily explained by the disturbing action of the large planets Jupiter and Saturn. Examination of the records then revealed another appearance of the same comet in 1456. It was confidently assumed that all four apparitions belonged to a single object, whose return might be accepted about 1758. The comet did in fact return 1759, and again in 1835 and in 1910, establishing for the first time the fact of comets return. This comet is now known as Halley's comet which last time was seen in 1986.we expect that we will see the comet next time in 2078...

      The longest comet tail recorded was that of the Great comet of 1843, which was 330 million km (205 million miles) long.

      The brightest comet seen in the 20th century was the Daylight comet of 1910.

Discovery and observations of comets

      Most new comets are discovered by professional astronomers on photographs taken with wide-field telescopes for programs not connected with cometary research. Some comets are found as the result of systematic searches by amateur and professional astronomers, but many hours of searching usually required for one comet discovery. Returns of periodic comets can be predicted, however, and generally only a very limited area in the sky need be examined to ensure recovery of the comet when it becomes sufficiently bright. Such comets are normally recovered when they are still very faint and at large distances from the sun. The brightest of new comets often are first seen very close to the sun in the sky, a region neglected by professional astronomers. Hence they are often found by a casual observer as he looks at the sky at sunset or sun rise. Suspected comet discoveries should always be reported promptly to the nearest astronomically observatory for confirmation and communication of the time of observation, position, rate and direction of motion, brightness and a description of the appearance of the object to astronomers throughout the world.