Page:Popular Astronomy - Airy - 1881.djvu/160

146 which related to the ascertaining the distance of the sun from the earth, were defrayed from the private purse of George the Third.

The next transit of Venus will occur in the year 1874. It will be followed by one in 1882: after which there will be none for more than a century.

From the transits of Venus in 1761 and 1769, but especially from the latter, the sun's distance from the earth was ascertained to be about 95$1⁄2$ millions of miles. It was long believed that this determination was all but perfectly accurate; but recent investigations seem to point to the conclusion that the value of the sun's distance so found is too great by more than three millions of miles. The source of error appears to be traced to the untrustworthy nature of the observations made in Lapland; and astronomers now look to the coming transits in 1874 and 1882 for the precise settlement of this important question.

When the distance of the sun is obtained, the distances of the other planets are easily found by calculation from the proportion of distances, which, as I said, was known long before the real distance of any one was known.

Before entering on the explanation of the principles of this method, I will point out to you the nature of other attempts which had been made to ascertain the distance of the sun from the earth, some of which were totally unsuccessful. In the first place, we might use the same method to ascertain the distance of the sun from the earth as that used for ascertaining the distance of the moon from the earth. We might take the angular distance of the sun from the North Pole, as viewed from an Observatory on one part of the earth (as Greenwich); and the angular distance of the sun from the South Pole, as viewed from