THE SUN AND OTHER STAKS 265 



seconds could run around the earth for one year without 

 stopping, he would in that time circle the earth over seven 

 times. If he could run at the same rate from the earth 

 straight to the sun, it would require over five hundred 

 years for him to make the trip one way; and we need not 

 discuss his return trip, for long before his return human 

 history would have undergone fundamental changes and 

 our generation would be all but forgotten. It is said that 

 the shortest time in which anyone has made the trip around 

 the earth is thirty-seven days. At that rate it would take 

 one year to reach the moon, four hundred years to reach the 

 sun, and one hundred million years to reach the nearest star. 



Possibly a better illustration is the time it would take for 

 sound to travel from the sun to the earth. As a matter of 

 fact, sound could not travel from sun to earth, since there 

 is no atmosphere through which it travels the earth's 

 atmosphere probably extending only a little more than a 

 hundred or at most a few hundred miles from the earth's 

 surface. In our atmosphere sound travels at the rate of 

 approximately one mile in five seconds. At this rate, with 

 an intervening atmosphere like ours, we should hear the 

 reports of the explosions which occur on the surface of the 

 sun after an interval of about fifteen years. That is, an 

 explosion which occurred on the sun's surface at the time 

 when you entered the first grade of school would be heard 

 by you at about the time you are graduated from college. 



268. The earth's orbit and the size of the sun. The distance 

 between the earth and the sun is not always the same, since 

 both bodies move constantly in certain courses known as 

 their orbits. 



The circumference of the earth's orbit is about 600,000,000 

 miles. When this number is divided by the number of sec- 

 onds in the year, it is found that the earth moves in its orbit 

 at the rate of about 18.5 miles per second. The orbit is so 

 immense that the curvature in a distance of 18.5 miles is 



