252 



ELEMENTS OF GENERAL SCIENCE 



short time. If one were living on the planet Mercury and 

 counted his age in terms of its years, he would be four times 

 as old as he would be if he were living on the earth. If people 

 lived there and their activities were divided up as ours are, 

 instead of a year of school being about one hundred and eighty 

 days long, as it is on the earth, it would be only forty days, or 



FIG. 126. Diagram of relative sizes of the sun and planets 



about one and one third of our months. Since the length of the 

 day of Mercury is unknown, the number of days there might be 

 quite different from the numbers given. On the other hand, 

 the remote planets move in such large orbits and so slowly that 

 immense intervals of time are required for them to make theii 

 journey around the sun. In terms of the year of Neptune we 

 should all still be very young. These calculations and others 

 which you can make should help to give an understanding of 

 the periods of revolution and of rotation of the planets. 



