INTRODUCTORY. J 



that its determination is the first step to any knowledge of 

 the dimensions and constitution of the sun itself. 



This parallax of the sun is simply the angular semi- 

 diameter of the earth as seen from the sun ; or, it may be 

 defined in another way as the angle between the direc- 

 tion of the sun ideally observed from the center of the 

 earth, and its actual direction as seen fr )m a station where 

 it is just rising above the horizon. 



We know with great accuracy the dimensions of the 

 earth. Its mean equatorial radius according to the latest 

 and most reliable determination (agreeing, however, very 

 closely with previous ones,) is 3962.720 English miles, and 

 the error can hardly amount to more than 0.0000 1 of the 

 whole — perhaps 200 feet one way or the other. Accord- 

 ingly, if we know how large the earth looks from any point, 

 or, to speak technically, if we know the parallax of the point, 

 its distance can at once be found by a very easy calculation ; 

 it equals simply (206265 x the radius of the earth) -^(the 

 parallax in seconds of arc). 



Now, in the case of the sun it is very difficult to find 

 the parallax with sufficient precision, on account of its small- 

 ness — it is less than 9", almost certainly between 8". 8 and 

 8".9. But this tenth of a second of doubtfulness is more 

 than.o.oi of the whole, although it is no more than the angle 

 subtended by a single hair at a distance of nearly 800 feet. 

 If we call the parallax 8".86, which is probably very near 

 the truth, the distance of the sun will come out 92,254,000 

 miles, while a variation of 2V of a second either way will 

 change it nearly half a million miles. 



When a surveyor has to find the distance of an inaccess- 

 ible object he lays off a convenient base line, and from its 

 extremities observes the directions of the object, considering 

 himself very unfortunate if he cannot get a base whose 

 length is at least iV of the distance to be measured. But the 



