DISTANCE, SIZE, AND MASS.

17

in the same way that the sun or a star can be followed,by means of a suitable pointer carried round the axison which the celestial vault seems to rotate in what iscalled the diurnal motion; that is, around an axisdirected to the true pole of the heavens. Such apointer directed once upon a star would follow thestar from rising to setting (neglecting the effects ofatmospheric refraction); but directed on the moon,and corrected from time to time, so that the moon’sactual motions among the stars should be taken intoaccount, the pointer would not follow the moon by amere rotation around its polar axis. If pointed onthe moon when she first rose above the horizon, itwould be found to point below the moon when carried(round its axis) towards the place occupied by the moonwhen high above the horizon; for it would have to bedepressed by the full amount of the horizontal parallaxwhen the moon was on the horizon, and this depressionwould be too great when the moon was high above thehorizon. In like manner, if the pointer were directedupon the moon when she was high above the horizon,it would be carried to a place above that occupied bythe moon when setting beyond the western horizon.

It was in this way that the moon’s distance wasfirst ascertained. The reader will recognize in thedescription just given the principle of the equatorialtelescope, which, turning around a polar axis, followsa star by a single motion. But the astronomical prin-ciple of this instrument was understood and appliedlong before the telescope itself was invented. Ptolemy ,

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