Page:The Scientific Monthly vol. 3.djvu/537

 WHAT WE KXOW ABOUT COMETS S3'

enjoyed seeing, was for a long time about 150,000 miles in diameter. It is a curious fact that the heads of eoraet« in general contrat-t in size as they approach the sun and expand as they recede from the sun. Encke's periodic comet, which has been observed on many returns, fre- quently had a diameter of 250,000 mites or more when the comet was at a great distance from the sun, whereas the diameter of the head reduced to 10,000 or 15,000 miles when the comet was nearest the sun.

��Pin. 4. DoNATi's r»UET. 18S8 Octobf.b S: Lead nnd beglnalDg of raU i brilliant ■Cellar nucl«UB near c«Dler al bead: envelopes Burroiiadlng aucleuE on Bide toward the SuD. Wblte circle to the left represeDta comparative size of the Eartb.

Before the disappearance into distant space the head resumed its origi- nal dimensions. A satisfactory explanation of the contraction and expansion of the heads of comets has not been found.

2. Near the center of the head of the comet there is usually a bril- liant, star-like point which we call the nucleus (Fig. 4). This is the point upon which accurate measures are made when it is a question of determining the position and the orbit of the comet. In general the nuclei are most sharply defined for those comets which have come in from great distances upon orbits nearly parabolic, and the nuclei are frequently hazy, poorly defined, and sometimes entirely lacking, in the comets composing Jupiter's comet family. Occasionally there is a double, a triple, or a quadruple nucleus, a division undoubtedly con- nected with the disintegration or breaking up of the comet into smaller masses. The size of the nucleus varies greatly, apparently from a few miles up to several thousand miles in diamet«r.

3. Most comets have tails. They frequently develop to enormous

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