102 



NEW MEASUREMENTS OF DISTANCE OF SUN. 



Di-sfaiicr of f<iiii (■orrcsiiouiVnin to iliffcrciit raltirs of the xoJtir ixiraUa.v und 



Ctarh-c's fi<jtirc of ISSO. 



A difference of 0-01" in the parallax is eciuivalent to 106,000 miles, or 170,000 kilometers. 



Let lis look at the matter for a moment as a prol)lem in pure sur- 

 veying. To measure the distance of the sun we have as a base a 

 chord somewhat less than the diameter of the earth, since observa- 

 tions can not be made on a heavenly body when it is actually on the 

 horizon. Suppose we put our base line at nine-tenths of the diam- 

 eter. Our problem is to determine the distance of a body so far away 

 that the whole diameter of the earth subtends at it an ang-le of only 

 about 17.6 seconds of arc; and with our somewhat diminished base 

 this angle is reduced to a little less than 15 seconds. I believe that 

 the length of your base upon the great lines of Chatham is about 

 1,730 feet. Imagine that from that base 3^ou had to determine with 

 an accuracy greater than one in a thousand the distance of an inter- 

 sected point about 4,500 miles away, as far away as Chicago, and you 

 have a problem which is l)v comparison simplicity itself. For the 

 ends of our 7,000-inile base are not visible from each other, being on 

 opposite sides of the world, and our angles at the base must be 

 determined by a complicated reference to the zenith, with all the 

 Avell-known impossibilities of determining absolute places in the sky 

 increased by the special difficulties that arise when the object to be 

 observed is the sun. You will readily grant that to determine the 

 distance of the sun by direct observation of that body is impossible, 

 unless you are content with an accuracy of about 1 in 10. 



Now, it is a curious fact that there is a way of obtaining the dis- 

 tance of the sun with an accuracy of 10 per cent with no other instru- 

 ment than a clock keeping accurate time. You do it by observing 

 the times of minima of the variable star Algol. Every two days 

 twenty-one hours Algol drops more than a magnitude, and does 

 this with a regularity which would be unfailing were it not for the 

 fact that at one season of the year we are nearer the star by nearly 

 the whole diameter of the earth's orbit than we are at the opposite 



