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the only Transit which woiild be visible in this country during a 

 period of 235 years, and it would probably be the last which 

 would be employed for the determination of the Sun's distance, as 

 there could be no doulit that long ere the next in 2004, a more 

 accurate result than this method could afford, would have been 

 already attained in a different way. 



The principle involved in tlie determination of the distance 

 of a celestial object, like the Moon, in no way differs from that 

 upon which a surveyor bases his determination of the distance of 

 some inaccessible object. Just as he takes the bearing of the 

 object from each end of a measured base line, so the astronomer 

 takes the bearing of the Moon from two observatories, say at 

 Greenwich and at the Cape, whose distance apart he already knows. 

 In this way the distances of the Moon, of Mars, and of some of the 

 minor planets, have been ascertained. But there are difficulties in 

 the way of observing the Sun, wliich prevent the method being 

 directly applied in its case. Since, however, the proportions of the 

 distances of the Sun, and of the various planets, are, well and 

 accurately known, to ascertain the distance of one is to ascertain 

 those of all ; and not a few astronomers consider tliat the best 

 method of obtaining the Sun's distance is first to find that of Mars, 

 or of a minor planet, and then to infer from it that of the Sun. 



Venus, however, comes much nearer to us than does Mars or 

 any other jjlanet, but as she turns her dark side towards \is at her 

 nearest approach, she is invisilile unless she is seen on the bright 

 background of the Sun. The lecturer then explained the laws 

 regulating the recurrence of Transits, and showed that they 

 must happen either in June or December, and at intervals of 8, 

 105^, 8, and 122^ years; the same intervals then occurring over 

 again. 



The most direct method for utilising a transit is as follows : — • 

 The following facts are already known — the proportio'U which 

 the distance of Venus from the Earth bears to the distance of the 

 Sun, the proportion the diameter of the Sun bears to its distance, 

 and the actual size of the Earth. If then, Venus is watched from 

 two stations on the Earth, the distance between the two positions 

 which she seems to occupy on the disk of the Sun, will bear a 

 known proportion to the distance between the stations. Thus, if 

 the stations on the Earth were 7,000 miles apart, the positions 



