THE UNIVERSE 



11 



before they reach the ground. A few heavier ones have fallen 

 to the earth. Some of these weigh more than a ton each. 

 Specimens may be seen in almost any large museum. Comets 

 are probably aggregations of matter similar to that in meteorites 

 and often in a gaseous form. Some comets have a regular 

 orbit about the sun, and may come and go at different times. 

 Others may appear but once and, traveling past the solar sys- 



Autumn 



FIG. 1. Position of the earth's axis with reference to the sun during the year. 



tern, finally become too faint to be seen with the telescope, and 

 may be lost among the distant suns. 



17. Time on the Earth. The various motions of the earth 

 serve as convenient measures of time. Thus the time required 

 for the earth to make one revolution about the sun is a year, 

 while its rotation on its axis produces day and night. The 

 month is partly an artificial measure of time, but it nearly cor- 

 responds to a revolution of the moon about the earth, this 

 latter occurring in about 29 days. The movement of the earth 



