284 sun's distance. 



ON THE MEANS WHICH WILL BE AVAILABLE FOR COR- 

 RECTING THE MEASURE OF THE SUN'S DISTANCE IN 

 THE NEXT TWENTY-FIVE YEARS. 



By the Astronomer Royal. From the Monthly Notices of the Royal Astronomical Society. 



At the meeting of the Society, on the 8th of April, the Astronomer 

 Royal gave an oral statement "on the means which will be available 

 for correcting the measure of the Sun's distance in the next twenty- 

 five years ;" the substance of which is contained in the following 

 abstract: 



The members of the Society will not be surprised at our looking so 

 far in advance as twenty-five years. The special opportunity, which 

 will then present itself, is the last which will occur for nearly a cen- 

 tury and a half from the present time. Some years of preparation 

 will be required to enable us to secure the full advantages which may 

 then be within our reach. But, with all possible care, it will be found 

 that the risk of total failure is not inconsiderable. The recognition 

 of this danger naturally leads us to examine Avhether there will not be 

 some earlier opportunity, of a different kind, for arriving at the same 

 determination. And it will appear (in the judgment of the Astrono- 

 mer Royal) that circumstances will be favorable, in the course of a 

 few years, for obtaining a very good measure by the use of a different 

 principle; less accurate, undoubtedly, in each of its individual appli- 

 cations than the method upon which reliance has usually been placed, 

 but admitting of almost indefinite repetition, demanding no co-opera- 

 tion of distant observers, and requiring only that, in each instance, 

 the observations which are to be compared be made with the same 

 instrument and by the same observer (or with observers only so far 

 changed that any personal equation would correct itself.) But even 

 this method requires appliances, which cannot be constructed at the 

 moment of observation; and it is necessary to study well, some time 

 before the operations shall actually commence, what equipment, in- 

 strumental and literary, is desirable for giving the best chance of suc- 

 cess. It will appear that we are not beginning too soon to direct our 

 attention to these matters in the present year. 



The measure of the Sun's distance has always been considered the 

 noblest problem in astronomy. One reason for this estimation is, that 

 it must be commenced as a new step in measures. It is easy to 

 measure a base-line a few miles long upon this Earth, and easy to 

 make a few geodetic surveys, and easy to infer from them the dimen- 

 sions of the Earth with great accuracy; and, taking these dimensions 

 as a base common to every subsequent measure, it is easy to measure 

 the distance of the Moon with trifling uncertainty. But the measure 

 of the Moon's distance in no degree aids in the measure of the Sun's 

 distance, which must be undertaken as a totally independent opera- 

 tion. A second reason is that, in whatever way we attack the prob- 

 lem, it will require all our care and all our ingenuity, as well as the 

 application of almost all our knowledge of the antecedent facts of as- 



