say, two telescopes at the end of the base) on the Moon is quite considerable, and by measuring it we find that the distance of the Moon is 240,000 miles, as I told you. And it was to find out this fact that our Government established the Royal observatories at Greenwich and at the Cape of Good Hope, one in the northern hemisphere and one in the southern. They are not quite as far apart as possible, because one might have been at the North Pole and one at the South Pole: but you will probably agree with
Fig. 13.
me that the astronomers would not have been quite so comfortable in that case, and we have to sacrifice something for comfort.
This old difficulty, however, is not yet done with: it crops up again in a most tiresome way when we want to find the distance of the Sun instead of that of the Moon, because the Sun is still further away—nearly a hundred million miles: and even when our base is the biggest we can get on our Earth, the "squint" required for the Sun is very small, and the difficulty of measuring it very great. The only thing then left for the astronomer is to take advantage of special occasions when the difficulty happens
