USE OF PARALLAX. 485 



A, and take up our position at another point, B, at some distance in a 

 straight line, and measure that distance very carefully. By means of the 

 theodolite we can calculate the angles which our eye, or a supposed line 

 drawn from our eye to the top of the object (c) we wish to find the distance 

 of, makes with that object. We now have an imaginary triangle with the 

 length of one side, A B, known, and all the angles known ; for if all three 

 angles are equal to 180, and we have calculated the angles at the base, we 

 can easily find the other. We can then complete our triangle on paper to 

 scale, and find out the length of the side of the triangle by measurement ; 

 that is, the distance between our first position, A, and the object, C. It is of 

 course necessary that all measurements should be exact, and the line we 

 adopt for a base should bear some relative proportion to the distance at 

 which we may guess the object to be. 



In celestial measurements two observers go to different points of the 

 -earth, and their distance in a straight line is known, and the difference of the 

 latitudes. By calling the line between the observers a base line, a figure 

 may be constructed and angles measured ; then by some abstruse calculation 

 the distance between the centre of the earth and the centre of the moon may 

 be ascertained. The mean distance is sixty times the radius of the earth. 

 The measurement of the sun's distance is calculated by the observations of 

 the transit of Venus across his disc, a phenomenon which will again occur on 

 6th December, 1882, and on 8th June, 2004, the next transit will take 

 place ; there will be no others for a long time after 2004. 



All astronomical observations are referred to the centre of the earth, 

 but of course can only be viewed from the surface, and correction is made. 

 In the cut above, let E be the earth and B a point on the surface. From B 

 the stars, abed, will be seen in the direction of the dotted lines, and be 

 projected to e i k I respectively. But from the centre of the earth they 

 would appear at efgh correctly. The angles formed by the lines at bed 

 are the parallactic angles, /**// and h I show the parallax. An object on 

 the zenith thus has no parallax. (See fig. 524.) 



Fg- 525. Halo Nebulae. 



