Atil RONOMY. 241 



whence it is clear that if the number of miles in one second is known, the diam- 

 eter of the Sun in miles and its distance can be told by simple arithmetic. 



But men have been striving for 2000 years to find the linear value of one 

 second at the Sun's distance. Really it is the greatest problem ever undertaken 

 by the human mind, it has become of world wide importance, while the nations of 

 earth have taken up the question, organizing expeditions to send to the bounds 

 of the world. As we write, every civilized people is making up parties to 

 journey throughout the earth to make accurate measurement of the transit of 

 Venus across the solar disc on December 6th, next. 



Forty expeditions are already announced, while those contemplated by the 

 United States, Italy and Austria have not yet been published. France will send 

 out eight fully equipped parties, four souih and four north of the Equator. The 

 United States will have six groups of astronomers in the central line of the trans- 

 it, while all the permanent observatories in the country will make close watch 

 of the phenomenon. The occasion is worthy the combined effort of mankind, 

 because soon as the precise distance of the Sun is known, the dimensions of the 

 universe become known, for the solar distance is the measuring line used by as- 

 tronomers 10 let fall in unheard of solitudes of space. 



The Transit of Venus.— The sole object in measuring the displacement on 

 the solar disc, of the planet Venus in transit, is to find the number of miles in 

 one second of angular measure at the distance of the Sun. 



In other words how large does the earth look if seen by an observer stand- 

 ing on the Sun, and what is its apparent angular diameter. The diameter of the earth 

 in miles is known with great precision, the probable error being within 200 feet. 

 It is self evident that if it can be ascertained how great an angle the radius of the 

 earth subtends if viewed from the Sun, we shall at once know the value of one 

 second of arc in miles. The method made use of is as follows : Let the line A 

 B be a wall, C and D observers having telescopes, and V a ball resting on a sup- 

 port. Now let C rest his glass on V and he will see it in apparent projection on 

 the wall at B, while D will see V at A. 



A 

 C 



V 

 D 



B 



But the distance from C to D is known, therefore the distance from C and D 

 to V and to the wall can be told in feet providing we know the relative distance. 



The relative distances between the Sun and planets have been known since 

 the time of Kepler, while the real distance is what men are now searching after 



