Page:Popular Astronomy - Airy - 1881.djvu/151

Rh of an object D, Figure 38, as observed with the two eyes, I said that it depends on this: that the object is seen by the two eyes in two different directions. In like manner the measure of the distance of the moon, by means of observations made at the two observatories G and C, will be based upon this circumstance: that the moon is seen in two different directions.

How can that difference of direction be ascertained? It can be ascertained by observing, at each of these observatories, the Polar distance of the moon. It will be remembered (see page 33) that, by the use of the mural circle at Cambridge, or at Greenwich, we observe the elevation of the celestial North Pole; and by the use of the same mural circle, we observe the elevation of the moon on the other side of the Zenith; or to use a more convenient measure, which we more frequently employ, we measure the angular distances both of the Pole and of the moon from the Zenith. Thus, suppose that by observations made at G on one side the Pole is 38 degrees from the