Page:Popular Astronomy - Airy - 1881.djvu/183

Rh meridian, may be compared with some fixed star, the same star being observed at the European Observatory and at the Observatory at the Cape of Good Hope. I am afraid that it will seem that I am dealing in generalities in this matter; but all that I can endeavour to do is to make the principles of the computation intelligible. It is nearly impossible to go into details. The method which I have just described is not, however, the best method, although it has been used with tolerable success.

The fourth method is, by observing the transit of Venus over the sun's disc. The transit of Venus occurs rarely. In explaining this, it was necessary to point out that the orbits of the different planets are inclined to each other, as in Figure 42, where EE' represents the orbit of the earth and VV' that of Venus. You will observe that at V, Venus is considerably elevated above the plane in which the earth moves. If the conjunction takes place at V, that is, if the sun, Venus, and the earth, are nearly in the line at S, V, E, still, however, they are not and cannot be exactly in the same direction; and if a spectator upon the earth looks at Venus, he will see her considerably above the sun. If the conjunction takes place at V', she will be seen below the sun. But if the conjunction takes place at V", Venus, the earth, and the sun, are exactly in a line when the conjunction takes place; at this time a spectator on the earth will see Venus on the sun's face as a black spot. I believe it is visible to the naked eye. With a telescope it is seen extremely well I have seen Mercury, which is a much smaller body, on the sun; and a very beautiful black spot it is.

In the observation of which I am going to speak, it is necessary to know beforehand the time when