Page:Popular Astronomy - Airy - 1881.djvu/158

144 its path (which is in a plane passing through the centre of the earth) would appear to he a great circle inclined to the equator; and if we compared its places when nearest to the celestial North Pole, and when furthest from it, one of these angular distances from the celestial North Pole would be as much less than 90 degrees as the other is greater than 90 degrees; and their sum would he 180 degrees. But, in consequence of parallax, each of these angular distances as viewed at Greenwich is increased and therefore their sum is greater than 180 degrees. By ascertaining, therefore, what each of these distances is, and what their sum is, and how much that sum exceeds 180 degrees, we have the sum of two parallaxes; and from this we can find the moon's distance by calculation, (assuming a distance for trial, and altering it as often as may he  necessary, and for every alteration computing the two parallaxes, adding them together, and seeing whether their sum agrees with the observed sum), nearly in the same way as when the moon was observed at Greenwich and the Cape of Good Hope. It is remarkable that this principle was used as long ago as by Ptolemy, (about A.D. 130) and a respectable estimation of the proportion of the moon's distance to the earth's diameter was obtained by him; but, as he did not know the dimensions of the earth, he was unable to express the moon's distance by an absolute measure.

I shall now proceed to a subject of much greater difficulty, viz., the computation of the distance of the sun from the earth. This most difficult problem might not have been accurately solved but for a suggestion of Dr. Halley, who, in the year 1716, published a paper in the