HEW WA YS OF MEASURING THE SUN'S DISTANCE. 65 



observer can, in the course of a single favourable night, 

 determine the sun's distance. And in passing, it may be 

 remarked that this is the only general method of which so 

 much can be said. By some of the others an astronomer 

 can, indeed, estimate the sun's distance without leaving his 

 observatory at least, theoretically he can do so. But many 

 years of observation would be required before he would have 

 materials for achieving this result On the other hand, one 

 good pair of observations of Mars, in the evening and in the 

 morning, from a station near the equator, would give a very 

 fair measure of the sun's distance. The reason why the 

 station should be near the equator will be manifest, if we 

 consider that at the poles there would be no displacement 

 due to rotation ; at the equator the observer would be carried 

 round a circle some twenty-five thousand miles in circumfer- 

 ence ; and the nearer his place to the equator the larger the 

 circle in which he would be carried, and (catoris paribus) the 

 greater the evening and morning displacement of the planet. 



Both these methods have been successfully applied to 

 the problem of determining the sun's distance, and both have 

 recently been applied afresh under circumstances affording 

 exceptionally good prospects of success, though as yet the 

 results are not known. 



It is, however, when we leave the direct surveying method 

 to which both the observations of Venus in transit and Mars 

 in opposition belong (in all their varieties), that the most 

 remarkable, and, one may say, unexpected methods of deter- 

 mining the sun's distance present themselves. Were not my 

 subject a wide one, I would willingly descant at length on the 

 marvellous ingenuity with which astronomers have availed 

 themselves of every point of vantage whence they might 

 measure the solar system. But, as matters actually stand, I 

 must be content to sketch these other methods very roughly, 

 only indicating their characteristic features. 



One of them is in some sense related to the method by 

 actual survey, only it takes advantage, not of the earth's 

 dimensions, but of the dimensions of her orbit round the 



