Page:Popular Astronomy - Airy - 1881.djvu/176

162 angular distance of the moon from the South Pole of the heavens. The reason why we must make the observations in this way is, because at the European Observatory we can see the North Pole, while at the Cape of Good Hope, they can only see the South Pole, The angle between the two celestial poles is necessarily 180 degrees exactly, because the directions of the lines drawn from the two places to the celestial poles are parallel to the same line, namely, the axis of the earth. I then remarked that the moon, as viewed from the Cape of Good Hope, does not pass the meridian precisely at the same time as when viewed at the Observatory at Greenwich or Cambridge. But the difference of time is well known; and we have only to take into account the change in the moon's place during the interval which, elapses between the observations at the two places. That is done with very great accuracy, and we can reduce the angular distance of the moon from the South Pole, to what it would have been if it had been observed at the same time as at Greenwich. Thus we have, for the same instant of time, got the apparent angular distance of the moon from the North Pole, as seen at Greenwich or Cambridge, and the apparent angular distance of the moon from the South Pole, as seen from the Cape of Good Hope. The sum of these two angular distances, if a star were observed, would be 180 degrees; but when the moon is observed, the sum of these two angular distances is found to be more than 180 degrees, and the excess above 180 degrees is the effect of parallax. It is the same as the angle which is made at the moon by two lines, one drawn from the European Observatory and the other from the Cape of Good Hope. Here we have got everything in much the same state as when