6a PLEASANT WA YS IN SCIENCE. 



direct surveying method, is the planet Mars when he comes 

 into opposition (or on the same line as the earth and sun) in 

 the order 



Sun Earth _ _Mars, 



at a favourable part of his considerably eccentric orbit His 

 distance then may be as small as 34$ millions of miles ; and 

 we have in his case to make no reduction for the displace- 

 ment of the background on which his place is to be deter- 

 mined. That background is the star sphere, his place being 

 measured from that of stars near which his apparent path on 

 the heavens carries him ; and the stars are so remote that the 

 displacement due to a distance of six or seven thousand 

 miles between two observers on the earth is to all intents 

 and purposes nothing. The entire span of the earth's orbit 

 round the sun, though amounting to 184 millions of miles, 

 is a mere point as seen from all save ten or twelve stars ; 

 how utterly evanescent, then, the span of the earth's globe 

 less than the 23,oooth part of her orbital range ! Thus the 

 entire displacement of Mars due to the distance separating 

 the terrestrial observers comes into effect. So that, in com- 

 paring the observation of Mars in a favourable opposition 

 with that of Venus in transit, we may fairly say that, so far 

 as surveying considerations are concerned, the two planets 

 are equally well suited for the astronomer's purpose. 

 Venus's less distance of 25 millions of miles is effectively 

 increased to 34^ millions by the displacement of the solar 

 background on which we see her when in transit; while 

 Mars's distance of about 34 J millions of miles remains 

 effectively the same when we measure his displacement from 

 neighbouring fixed stars. 



But in many respects Mars is superior to Venus for the 

 purpose of determining the sun's distance. Venus can only 

 be observed at her nearest when in transit, and transit lasts 

 but a few hours. Mars can be observed night after night 

 for a fortnight or so, during which his distance still remains 

 near enough to the least or opposition distance. Again, 



