434 SCIENCE PROGRESS 



two directions is the angle at S, and this is the parallax of the 

 star for the observer. It is quite clear that, as the observer 

 moves about, the parallax will vary. If he goes down to the 

 line CS there will be no parallax, whilst if SO becomes a tangent 

 to the circle, the parallax is the maximum. This shows us 

 clearly that the star's parallax may be said to be the angle 

 between the observer and the centre of the earth when looked 

 at from the star. It would obviously be most inconvenient to 

 have to deal with a continually varying parallax, and one that 

 depended so much on every individual observer, and so it has 

 been agreed that when we speak of the parallax of a star we always 

 refer to this greatest parallax, which is made by the equatorial 

 radius of the earth. So the parallax of the sun is the angle the 

 radius of the earth makes at the sun, or, what is the same thing, 

 the very small portion of the circle of the heavens that the 

 radius of the earth would occupy if looked at from the sun. 

 This is a definite quantity, but is very small indeed, less than 

 one four-hundredth part of one degree, being, in fact, about 8*8 

 seconds of arc. All our calculations are directed to the determi- 

 nation of the exact value of this angle. The smallest alteration 



Fig. 2. 



in the number of seconds means a big variation in the number 

 of miles that gives the distance of the sun. Thus, for example : 



8'7S seconds correspond to a distance of 93,428,000 miles 

 8-8o „ „ „ 92,897,000 „ 



8-85 „ „ „ 92,372,000 



8-90 „ „ „ 91,852,000 „ 



There are a great many different ways of determining this 

 parallax, but the only ones that concern us at present are those 

 based on the transit of Venus. 



The first method was proposed in 1677 by Halley. Two 

 observers go to two stations separated as widely as possible in 

 latitude^ one as near the North Pole as convenient, the other as 

 near the South. These two men look at the transit and time its 

 duration. It is not necessary to know the exact times at which 

 the transit begins or ends, the clocks may be fast or slow, it does 

 not much matter, for the duration is the one thing to be ob- 

 served. 



