Man's physical universe




Comets Move in Well-defined Orbits around the Sun.

The orbits of about four hundred comets are fairly well known, so

that the time of reappearance of such comets can be very accurately

predicted in some cases. Some of the comets have orbits that carry

them far outside of the orbit of Pluto.

The shortest period of revolution around the sun is about 3.3 years,

that of Encke's comet, while others travel

along orbits that require

thousands or even hundreds of thousands of years for one revolution.

The Nature of Comets.

The masses of the comets are much less than those of the planets;

comets with the largest mass probably weigh less than our atmosphere.

Within the heads of most comets nuclei are observed. These nuclei

are observed to consist of a swarm of meteors. Some of these swarms

display a definite nucleus of their own.

These nuclear swarms of meteors are thought to be surrounded with

an envelope of gas, so rarefied that stars can be readily seen through it.

Comets Are Visibly Affected as They Approach the Sun.

The speed of comets is greatly increased as they approach the sun;

at the same time the head contracts, and, perhaps because of this

increase in speed, a tail begins to form. The tail becomes longer as

Fig. 30. Variations in comets' tails. (From G. F. Chambers, Story of the

Comets, Clarendon Press, Oxford, England.)

the comet gets closer to the sun and always points away from the sun

as the comet leaves the sun its tail actually precedes its head. Sometimes

the comets' tails extend a length of as much as two hundred

million miles, while they spread out to a maximum of about ten million

miles. The tail probably consists of gas particles which are driven

outward by the pressure of the radiations from the sun. It is quite

probable that this gaseous material driven away from the head is

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