NEWS PROM THE STARS — ABBOT. 



159 



the eye can scarcely see them. For this purpose the rays were col- 

 lected by a concave mirror of 3 feet diameter and focused on the 

 surface of a very delicate electrical thermopile (pi. 3 and pi. 4, fig. 2). 

 This instrument acts on the principle that a difference of temperature 

 between the junctions of two metals made up into a closed wire cir- 

 cuit, produces an electric current. The apparatus used was so deli- 

 cate that if the experiment could be made in a vacuum the heat from 

 rays of a candle at 53 miles could be observed. Further work along 

 similar lines is proposed. 



DISTANCE. 



When a surveyor measures the distance of an inaccessible object 

 he selects two convenient stations and measures their distance apart. 

 This is called the base line. 

 At each end of the base 

 line he observes the angle 

 the base line makes with a 

 line sighted toward the 

 inaccessible object. The 

 angles and the base of his 

 triangle being thus meas- 

 ured, the two remaining 

 sides can be calculated. 

 By such a process, using 

 the earth's diameter, or a 

 large part of it, as the 

 base line, the distance of 

 the moon is readily de- 

 termined, and comes out 

 243,000 miles. 



Even the length of a diameter of the earth is too small a base line 

 from which to triangulate for the distances of the stars. Astrono- 

 mers use the diameter of the earth's orbit round the sun, 186,000,000 

 miles, for this. Astronomers also take advantage of the fact that 

 very faint stars are usually much farther away (though not invari- 

 ably so) than bright ones. Thus it comes about that if photographs 

 of a bright star are made with the same telescope at two dates six 

 months apart, and exact measurements of the distance of the bright 

 star from its faint neighbors are made on both photographs, a slight 

 displacement of the bright star will often be found to have occurred. 

 The angular measure of displacement gives the vertical angle of 

 the isosceles triangle of which the base line is the diameter of the 

 earth's orbit, and from these data the star's distance is easilj'^ found. 

 Seen from the nearest star, a Centauri, the radius of the earth's 



Fig. 1. — Method of triangulating for distances of 

 heavenly bodies. Prom " The Sun," by C. G. 

 Abbot. Published by Appleton & Co., 1911. 



