what is comet ? eplain briefly ?
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Comet - Comets are the member of the solar system having orbits of highest eccentricity. These revolve round the sun in long and irregular orbits. These are the masses composed of atmospheric dust, ice and frozen gases and inhabit the cold dark regious far away from the sun. Tails of gases and of dust only appear when a comet passes near the sun, they point away from the sun and may be millions of kilometres long. Short period comets are the comets having orbital periods below 150 years. Orbital periods of some of the comets exceed 10,000 years. Halley's comet was discovered in 1757. It reappeared in 1986. Donatis (1858) and Kohoutek's comets are the other well known comets.
The shielding effect sometimes referred to as atomic shielding or electron shielding describes the attraction between an electron and the nucleus in any atom with more than one electron. The shielding effect can be defined as a reduction in the effective nuclear charge on the electron cloud, due to a difference in the attraction forces on the electrons in the atom. It is a special case of electric-field screening.