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Brief Description of a Comet - 12 and over
by Jeanette Cain
A comet's tail will only develop in the vicinity of the Sun. It will have its greatest expansion at its perihelion. It will be directed away from the Sun and the tail will curve away from the direction of the comet's head.
The gaseous material of the head and the tail will be expelled from the nucleus with a speed of several miles per second. The tail's curvature is explained by Kepler's law of equal areas. It states that the squares of the sidereal periods of any two planets are to each other as the cubes of their mean distances from the sun. Materials will be swept further from the Sun, opposing the force of gravity. The forcing away of light pressure effects by large objects is small when compared to gravitational attraction. However, with decreased size, surface area will increase.
A reduction in size goes to molecular levels where an area may become so large that light pressure is hundreds of times larger than the gravitational attraction. Matter in gas, which is formed near the Sun, can be driven back with great force, which explains a comet's tail being opposite the Sun.
Some comet tails form a hollow cone composed of material from a comet's head. After this material is pushed away by the Sun, it is unable to return to the head. Should this continue for an indefinite period, the comet's head will be destroyed.
When a comet is a great distance from the shine, it will shine by reflected sunlight. As the comet approaches the Sun, its brightness will increase, which results from glowing gases of the head and nucleus. The elements which are showing as the comet nears the Sun are hydrogen, nitrogen, hydrocarbons, carbon monoxide, cyanogen, sodium, iron and nickel.
Definitions
1. sidereal: simply put, means of the stars. It is measured by the daily motion of the stars, but a sidereal day is about 4 minutes less than a mean solar day.
2. cube: a solid with 6 square and equal sides.
Sources:
Barnhart, Clarence L. Thorndike-Barnhart Comprehensive Desk Dictionary Doubleday & Company,
Inc: NY. 1965
Editors. The World Book Encyclopedia. World Book-Childcraft International, Inc: Chicago. 1990
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