NEW WA YS OF MEASURING THE SUN'S DISTANCE. 6? 



difference. Astronomers can measure this difference ; and 

 measuring it, they can ascertain what the actual numbers are 

 for which I have roughly given the numbers 389, 390, and 

 391 ; in other words, they can ascertain in what degree the 

 sun's distance exceeds the moon's. This is equivalent to 

 determining the sun's distance, since the moon's is already 

 known. 



Another way of measuring the sun's distance has been 

 "favoured" by Jupiter and his family of satellites. Few 

 would have thought, when Romer first explained the delay 

 which occurs in the eclipse of these moons while Jupiter is 

 further from us than his mean distance, that that explanation 

 would lead up to a determination of the sun's distance. But 

 so it happened. Romer showed that the delay is not in the 

 recurrence of the eclipses, but in the arrival of the news of 

 these events. From the observed time required by light to 

 traverse the extra distance when Jupiter is nearly at his 

 furthest from us, the time in which light crosses the distance 

 separating us from the sun is deduced; whence, if that distance 

 has been rightly determined, the velocity of light can be in- 

 ferred. If this velocity is directly measured in any way, and 

 found not to be what had been deduced from the adopted 

 measure of the sun's distance, the inference is that the sun's 

 distance has been incorrectly determined. Or, to put the 

 matter in another way, we know exactly how many minutes 

 and seconds light takes in travelling to us from the sun ; if, 

 therefore, we can find out how fast light travels we know 

 how far away the sun is. 



But who could hope to measure a velocity approaching 

 200,000 miles in a second ? At a first view the task seems 

 hopeless. Wheatstone, however, showed how it might be 

 accomplished, measuring by his method the yet greater 

 velocity of freely conducted electricity. Foucault and Fizeau 

 measured the velocity of light ; and recently Cornu has made 

 more exact measurements. Knowing, then, how many miles 

 light travels in a second, and in how many seconds it comes 

 to us from the sun, we know the sun's distance. 



