DRIFTING LIGHT- WA VES. 



101 



observation is as yet little more than a quarter of a century 

 old, we may fairly hope that in the years to come the new 

 method, already successfully applied to measure motions of 

 recession and approach at the rate of 20 or 30 miles per 

 second, will be employed successfully in measuring much 

 smaller velocities. Then will it give us a new method of 

 measuring the great base-line of astronomical surveying 

 the distance of our world from the centre of the solar 

 system. 



That this will one day happen is rendered highly pro- 

 bable, in my opinion, by the successes next to be related. 



Besides the motions of the planets around the sun, thert 

 are their motions of rotation, and the rotation of the sun 

 himself upon his axis. Some among these turning motions 

 are sufficiently rapid to be dealt with by the new method. 

 The most rapid rotational motion with which we are 

 acquainted from actual observation is that of the planet 

 Jupiter. The circuit of his equator amounts to about 

 f. 6 7, ooo miles, and he turns once on his axis in a few minutes 

 less than ten hours, so that his equatorial surface travels at 

 the rate of about 26,700 miles an hour, or nearly 7^ miles 

 per second. Thus between the advancing and retreating 

 sides of the equator there is a difference of motion in the 

 line of sight amounting to nearly 15 miles. But this is not 

 all Jupiter shines by reflecting sunlight Now it is easily 

 seen that where his turning equator meets the waves of light 

 from the sun, these are shortened, in the same sense that 

 waves are shortened for a swimmer travelling to meet them, 

 while these waves, already shortened in this way, are further 

 shortened when starting from the same advancing surface 

 of Jupiter, on their journey to us after reflection. In this 

 way the shortening of the waves is doubled, at least when 

 the earth is so placed that Jupiter lies in the same direction 

 from us as from the sun, the very time, in fact, when Jupiter 

 is most favourably placed for ordinary observation, or is at his 

 highest due south, when the sun is at his lowest below the 

 northern horizon that is, at midnight. The lengthening 



