Page:Popular Astronomy - Airy - 1881.djvu/155

Rh are combined, so as to affect the determination of the angle GMC by a larger error.

Now there is another way in which this observation can be shaped, in which the effect will not be quite so bad. This is by referring the place of the moon, as seen at each Observatory, not to the North and South Pole as I have spoken, but to a star. Let S, Figure 40, be a star at a very great distance, and suppose we observe the moon when she is nearly in the direction of that star. Now, the thing which it is our object to ascertain, is the angle made by the two lines GM, CM. At G the moon is seen somewhat below the star; we have then only to measure the angle SGM, about which there can be very little uncertainty, either from refraction or from any other cause. At C the moon is seen still more nearly in the same direction as the star; the angle SCM can therefore be measured with great accuracy. The angle GMC is the difference between the two angles SGM, SCM. Suppose, for instance, that at G the moon is seen two degrees below the star, and at C is seen only half a degree below the star, then the difference or the angle GMC must be 1$1⁄2$ degrees; and this angle is scarcely liable to any possible error. We have then got this angle GMC accurately, and we have got the directions which the two lines GM, CM, make in reference to the line GC; and the calculation is then much the same as with a triangle in a survey, where we have a base measured from which we can begin our computations. This is the way in which the distance of the moon is measured; and we may say, as a general result, that the distance of the moon from the earth is about thirty times the breadth of the earth.

It is necessary now to explain with a little more