Page:Popular Astronomy - Airy - 1881.djvu/164

150 distances of Mars and of the earth from the sun, and the form of their orbits) that at certain times, when Mars is in opposition to the sun, the distance of Mars from the earth is only one-third of the distance of the sun from the earth; therefore, if we can ascertain the distance of Mars from the earth at that time, and multiply it by three, we get the distance of the sun from the earth. The distance of Mars can be got at with considerable accuracy, by observations made at Greenwich and the Cape of Good Hope, in exactly the same method as that which I have explained for getting the distance of the moon. Our observations as to the angular distance of Mars from the celestial North Pole must be referred to the same fixed star at the two places. In the Nautical Almanack, for several years past (a work which is published three or four years in advance,) there is prepared a list of stars, which it is recommended should be observed and compared with Mars, not only at the European Observatories, but also at the Cape of Good Hope and elsewhere in the Southern Hemisphere. Accordingly, at the opposition in 1862, observations were made in pursuance of these recommendations: and from them it has been deduced, that the distance of the sun from the earth is rather more than 91$1⁄2$ millions of miles, a smaller value it will be observed than that found from the observations of the last transit. This method is a pretty good one; but it is not the most accurate method, which is that founded upon the transit of Venus, and to the explanation of it I now proceed.

There is one point with regard to the motion of the planets, to which I should wish to allude in the first place, as I have hitherto made no mention of it. It is rather important in reference to the transit of