THE EARTH AS A PLANET 353 



earth. Yet great as is this distance, the earth in six months 

 time will be at an equal distance on the other side of the sun. 

 In other words the earth will have traveled about half way 

 around in its orbit. When one calculates the number of miles 

 in half of the earth's path about the sun, and divides the 

 result by the number of minutes in six months of time, it is 

 not surprising to find that the earth's velocity in its orbit is 

 about one thousand miles per minute. This is about a 

 thousand times faster than the swiftest express train. 



In this revolution of the earth about the sun it must be 

 kept in mind that the moon accompanies the earth in its 

 advance. The earth is at all times in rotation as it moves on, 

 and the moon is all the time revolving about the earth as a 

 centre. Though the rate at which the earth moves varies 

 considerably in different parts of its orbit, the time period 

 for complete revolutions is of unvarying length, and it 

 constitutes the natural time unit known to us as the year. 

 In this year period there are about three hundred sixty-five 

 and one-fourth days. In the arrangement of days into 

 months to make up the calendar it has been necessary to let 

 every fourth year 1 contain 366 days, February then having in 

 these "leap years" twenty-nine days. The times of changes 

 of seasons are thus made to occur on the same dates every 

 year. The periods of revolution of the other planets or their 

 " years" vary widely, depending upon their distances from 

 the sun. Mercury, the innermost of them, gets around in 

 about eighty-eight of our days, while Jupiter requires twelve 

 and Neptune one hundred sixty-five of our years. 



SUMMARY 



The paths of the sun across the sky are not the same day after day. 

 It is a matter of common observation that from June to December the 



1 An exception to this is that the first year of each new century as 1800, 

 1900, 2000, etc., is a leap year only when divisible by four hundred. 

 23 



