BROWN : 



lax. It is nothing in the zenith and greatest in the horizon. 

 When calculated in the latter position it is called the sun's 

 horizontal parallax. 



The sun's horizontal parallax, therefore, is the angle 

 subtended at the sun's centre when he is in the horizon by the 

 Earth's semi-diameter. The equatoreal semi-diameter is chosen 

 because it is the longest. Observations taken at any altitude 

 can be reduced to the horizon by a simple formula, and so we 

 are free to speak of the horizontal parallax at all times, mean- 

 ing the horizontal equatoreal parallax. It is plain that if this 

 angle be known we shall have in order to calculate the sun's 

 distance from us to calculate the parts of a right angled tri- 

 angle, right angled at the Earth's centre, all the three angles 

 and the shorter leg of which are known — a problem entirely 

 within the scope of the most elementary trigonometry. 



It may be remarked, by the way, that the Earth's semi- 

 diameter, 4000 miles, bears the same proportion to the distance 

 of the sun from the earth, 93,000,000 miles, that one-sixteenth 

 of an inch does to 121 feet (nearly). We might conceive our- 

 selves, therefore, as measuring at 121 feet distance the date 

 figures on a twenty-five cent piece. 



i 



SUN 



In this diagram, the proportions of which are of course 

 enormously exaggerated, let C be an observer at the centre 

 and S an observer at the surface of the Earth. V is Venus. 

 S will see Venus on the sun's disc at Y' and C will see her at 



