

Comets 



261 



FIG. 155. Photograph of a comet. The telescope was set to move with 

 the comet, and the stars show as lines on the plate. 



On the 3oth of June, 1861, the earth passed through 

 a comet's tail. The light of the sun was somewhat 

 obscured, so that artificial lights had to be used at seven 

 o'clock in the evening, although the sky was cloudless. 

 Yet the effect on the earth and its inhabitants was so 

 slight that the vast majority of persons never knew 

 that such an event had taken place. 



The head of a comet. The head of a comet is not a 

 single bright point like a star, but hazy, and keeps chang- 

 ing its size, usually becoming smaller as it approaches 

 the sun and expanding again as it recedes. The head 

 is often 40,000 or 50,000 miles in diameter, or half the 

 diameter of the planet Jupiter, though not nearly so 

 bright. Very few have been seen which were as broad 

 as the sun and few that were bright enough to be visible 

 when the sun was high above the horizon. 



