PRESIDENTIAL ADDRESS SECTION A. 37 



Notes. — (1) The absolute magnitude of a star is defined as 

 the apparent magnitude it would have if placed at the standard 

 distance of 10 parsecs or 32-6 light years where the parallax would 

 be 01". The star in the above table is therefore of absolute 

 magnitude -5 at all distances and whatever its apparent magni- 

 tude. 



(2) Log distance factor =0-2 x (apparent mag. — absolute mag.). 



(3) Distance in light years = Distance ratio x 32-6. 



3-26 



(4) Parallax in seconds of arc= — ; ; 



distance in light-years. 



(5) Parallax 01" ==2,062,650 astronomical units. 



(6) One astronomical unit=approx. 93,000,000 miles. 



Even the nearest stars are so remote that it is a matter of 

 exceedingly delicate measurement to find their distances. 



The first principle employed is the same as that used by sur- 

 veyors in measuring: the distance of a tree on the opposite bank 

 of a broad river. Two stations are chosen at a measured distance 

 apart, and sights are taken at both ends of the line. The angles 

 between the lines and the length of the base-line give the distance. 

 In the case of a star, no terrestrial base-line is big enough, and 

 we have to use as base-line the diameter of the earth's orbit, 

 186,000,000 miles, and take sights from each end of a diameter 

 six months apart, as we swing from one side of our orbit to the 

 other. We are only now beginning to obtain these measures of 

 distances in any number. 



Forty years ago only about 20 such distances were measured, 

 20 vears ago only about 60. These figures had increased to 200 in 

 1915, and now we have measured distances of about 900 stars, 

 and the number may be expected to increase rapidly now that more 

 instruments are in use for taking photographs for the purpose. 

 This method, however, only enables us to measure the distances of 

 the nearest of our stellar neighbours. For the vast majoritv of 

 stars the angle subtended by 186,000,000 miles (twice the parallax) 

 at the star is too small to be directly measured by any means at 

 our command. 



There are, however, other methods by which the distances of 

 the stars, and therefore their absolute magnitudes, can be 

 measured. 



The sun in its journey through space is doing the same thing 

 as the surveyor when measuring his base-line along the bank of a 

 river; it is measuring out a base-line at the rate of nearly 400 

 million miles a year. In a century, therefore, the base-line will 

 be over 200 times as long as the diameter of the earth's orbit. It 

 is evident, then, that it is of the utmost importance to secure 

 photographs of the stars at the present time on a sufficiently large 

 scale to show changes of relative position when compared with 

 photographs to be taken a century hence. It is a debt we owe to 

 posterity. Comparisons can now be made with photographs taken 

 a quarter of a century ago for the detection of this "parallactic 



