NEW MEASUREMEISTTS OF DISTANCE OF SUN. 103 



season; and light takes about sixteen minutes to traverse that dis- 

 tance. In the nuddle of Xoveniber the eclipses of Algol are taking 

 j)lace eight minutes earlier than the average. In May, could we 

 ol)serve the star so near the sun, they would be found eight minutes 

 behind their time ; and a practised observer could, on a long series of 

 observations, determine that ine(|uality, Avith a total range of sixteen 

 minutes, well within two minutes — that is to sav, with an accuracy 

 of about 10 per cent. We have then only to combine this quantity 

 with the known velocity of light and we have a measure of the sun's 

 distance. A mere curiosity in itself, it will serve to introduce us to 

 some indirect ways of determining the distance of the sun which 

 have, both practically and historically, a peculiar interest and im- 

 portance. ' 



At the present time we are in the thick of a new determination of 

 the distance of the sun on a scale of operations greater than has l)een 

 known before. More than fifty observatories of the Northern Hemi- 

 sphere are engaged more or less deeply on the work, which has occu- 

 ])ied a great many of us closely for the last four years and Avill give 

 plenty of trouble to some of us for several years to come. Before 

 we enter upon the consideration of the new method and the new 

 o})i)ortunities we might well pause to answer the question, which is 

 l)V no means superfluous, ITow does it come about that, at the end of 

 the nineteenth century, which had seen attempts almost innumerable 

 'to measure the distance of the sun, the result was still so nnich in 

 doubt that it was worth while to concentrate quite a large proportion 

 of the total astronomical energy of the world upon a new attempt 'i 

 I believe that' we shall find some explanation of this fact if we 

 examine the history of the various values of the solar i)arallax that 

 Avere used in the Nautical Almanac during the nineteenth century. 



A determination of the distance of the sun by direct observation of 

 the sun itself is impractical)le; the sun is too difficult an object to 

 observe with any great accuracy; its distance is too great, and our 

 base is too small for any method of direct trigonometrical survey to 

 be possible. But we can in effect diminish its distance by substitut- 

 ing for it one of the planets, which can be more accurately observed, 

 for when the distance of any one planet from the earth is known, the 

 dimensions of other orbits follow by the application of Kepler's third 

 law. And at the same time we can, as we shall see. secure the ines- 

 timal)le advantage that the measures to be made are relative and not 

 absolute. 



Let me digress for a moment to insist upon the importance of this 

 distinction. If you wished to find the difference in latitude and lon- 

 gitude between your Institute and the trigonometrical point at Dar- 

 land, you might determine the latitude and longitude of each and 

 take the differences, or vou mii>:ht triangulate from one to the other. 



