Page:Popular Astronomy - Airy - 1881.djvu/173

Rh side next the sun. A station near the North Pole would describe a circle, of which the smaller part would be towards his eye, and the greater part towards the sun. The whole passage of Venus occupied about six hours. Now, it was possible to choose a station A (Wardhoe, in Lapland) such that, at the beginning of the transit Wardhoe was at A, that it then passed the dark part of its revolution, and arrived on the sunny side at a at the end of the transit; in other words, that the transit began in the evening and ended the next morning, And another station B (Otaheite) might be chosen, such that the transit began soon after Otaheite entered into the light at B, (or in the morning) and ended shortly before Otaheite came to the darkness at b. Now, we have already seen that Venus as seen from A described the longer path CD, merely because A is higher in the figure than B; but now we may see that, in consequence of A being to the left, Venus will be seen to enter at C sooner than if A had been in the central line; and, in consequence of a being to the right, Venus will be seen to leave the sun at D later than if a had been in the central line; and for both reasons, the time in which Venus appears to describe the path CD will be lengthened. In like manner, it will be found that as seen from B and b, the time in which Venus appears to describe the path EF will be shortened. And, therefore, the difference of the whole durations of the transit as seen at the two stations will be considerably increased.

I have mentioned specially Wardhoe and Otaheite. Many other stations were used, but these were the best of all. The whole duration of the transit at Wardhoe was about 5 hours 54 minutes. The whole duration at Otaheite was about 5 hours 32 minutes.