Page:Popular Astronomy - Airy - 1881.djvu/179

Rh altered.) This is called the horizontal parallax. And as the earth is not spherical, and therefore different places are at different distances from the earth's centre, the horizontal parallaxes will not be the same at different places; it is therefore convenient to fix on some one place as a standard. The place fixed on by the consent of astronomers is the equator; and the horizontal parallax of the moon at the equator is called the horizontal equatoreal parallax. This is the quantity used by astronomers in relation to the moon's distance; it is convenient for their calculations, and it amounts to the same thing as using the distance; for if the distance is known, the horizontal equatoreal parallax is known; or if the horizontal equatoreal parallax is known, the distance is known. The moon's horizontal equatoreal parallax varies (according to the moon's distance), from 54 minutes of a degree to 61$1⁄4$ minutes; these correspond respectively to the distances 252,390 miles and 222,430 miles.

I then stated that when every observation of the moon made at any one place is corrected for parallax, so as to inform us what would be the position of the moon as viewed from the centre of the earth, it is found that her orbit is sensibly a plane; and this conclusion may then be properly used as a basis for determining with greater accuracy the moon's horizontal parallax at that place. For the moon's angular distance from the Pole when she is nearest to it will fall short of 90 degrees, just as much as it will exceed 90 degrees when she is farthest from the Pole, provided that the proper correction for parallax is made so as to reduce the observed place to what it would have been as seen from the earth's centre. If the condition which I have mentioned is not satisfied, it