72 LOCAL PAETICULAKS OF THE TRANSIT OF TENUS, 1874. 



and have no convenient places near them for observation. At 

 the time of completed ingress, the transit will be visible from 

 the whole hemisphere of the earth of which Eockhampton is 

 the centre ; and, excepting Australia, and on the north-west, 

 where it includes India, China, and part of Russia, it is nearly all 

 water, hence the difl&culty of finding stations to fulfil the required 

 conditions. But at the end of the transit the earth will have 

 moved in its diurnal rotation, so that to all the stations on the 

 east and north-east which were well situated to .observe the 

 ingress the sun will have set, and to all those on the west he will 

 have risen higher. Many new places will have come within view 

 of the phenomena, and the centre of the hemisphere of the earth 

 from which it will be visible will be no longer Eockhampton, but 

 a point situated 88° 46' east of Q-reenwich, and 22° 58' s. latitude, 

 or not far from Amsterdam Island. This hemisphere includes 

 more than half of Africa, the south-east corner of the Mediter- 

 ranean, part of the Black Sea, nearly the whole of Russia and 

 •Japan, and all the countries to the south of them. Australia and 

 New Zealand are included in its south-east side, and the Southern. 

 Continent on the south. At a point not far from South Victoria 

 Land the egress will be first seen, and at a place near Orsk, in 

 Russia, it will be last seen. 



At the ingress Australia is not well situated for observation ; but 

 at egress, New Zealand and South-east Australia are very favour- 

 ably situated for observation. It thus appears that Europe and 

 the whole of America will see nothing of the transit, and it will 

 be for those who are favourably situated to make the most of 

 their opportunities for observing phenomena so simple but of 

 such vast importance to science. 



There are some facts in reference to the phenomena which will 

 repay a few moments' consideration, and it will greatly simplify 

 the matter if we for the time neglect the earth's motion, and 

 assume that Ave are on a fixed earth watching the passage of 

 Venus across the sun's disc. In diagram 6 we take E to 

 represent the earth, V Venus, and the large circle S the Sun, and 

 the arrow to indicate the direction in which Venus will appear to 

 move, Aaz., from east to west. 



If- two observers are placed at A and B, they will each see 

 Venus like a black spot crossing the sun's disc ; but A will of 

 course see it in the line A Va moA"ing in the direction o n, and 

 B will see it in the line BVb moving in the line and direction 

 m g, and if Venus Avould only trace the lines on the sun as definite 

 as these we have on paper all difficulty would be over, we could at 

 our leisure make all. the measures which would be required, and 

 thus determine the angular distance between the lines o n and 

 m y, which is all that we requii'e to know in order to find the 

 sun's distance. 



