132 



LECTURES 



earth will necessarily be periodic and the star must return to the 

 same place at the expiration of a year. A proper motion has been 

 detected in many of the stars, amounting in some to 4" or 5" in a 

 year. 



Any change due to the motion of the earth is called the annual 

 parallax. The only means within the reach of astronomy of finding 

 the distance is by means of this parallax. 



An obvious proof that the stars are immensely distant is that, except 

 under the severest possible scrutiny, they appear to hold the same 

 relative places at all seasons of the year. Our change of position^ 

 involving a distance of nearly 200,000,000 of miles, dwindles down to 

 nothing in comparison with the line which extends from the earth to 

 the star. 



In figure 19, let S be the sun, A E B the earth's orbit, and s a 



star ; then the angle A s S, or the angle subtended by the radms ofl 

 the earth's orbit, as seen from the star, would be the annual parallax; 

 of the star. If this angle were so much as 1", it would follow thati 

 the distance of the star must be 206,265 times the distance of the; 

 sun. In that case light would require more than 31 years m coming; 

 from the star to us. But in no case is the parallax of a star so large 

 as 1". As Biot has remarked, "an angle so large as \" could not 

 have escaped universal recognition." What we are now entitled to 

 say then is, that light from the nearest fixed star would require at least 

 more than 3| years to reach our earth. ^ 



There is no presumption that the stars are all equally distant. On 

 the contrary there is every probability that their distances are very: 

 unequal, and heu<3e that some stars of the first order of intrinsic splen- 

 dor appear faint from their vast distance. Huyghens, assuming that 

 Sirius was equal in intrinsic brightness to our sun, concluded from a 

 computation based upon the well known law that the intensity of light 

 diminishes inversely as the square of the distance, that Sinus must be 

 28,000 times more distant than the sun ; in other words that our sun 

 at that distance would appear no brighter than Sirius does now. But 

 this estimate of the distance, as we now know, is far too small. At 

 only that distance the parallax would be four times as great as that 

 of the nearest known star. Dr. WoUaston, by very ingenious photo- 



