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who is usually credited with the invention of the equa-torially mounted pointer, was the first to apply theinstrument to the determination of the moon’s dis-placement or parallax.* The result contrasts strikinglywith the ill success which he and other ancient astrono-mers experienced when they attempted to apply thisand other methods to the determination of the sun’sdistance. He assigned 57' as the moon’s parallaxwhen she is on the horizon,—in other words, hisobservations led him to the conclusion that theangle EM H P (fig. 6, Plate I.) is one of 57', a valuewhich would set the moon’s distance at almostexactly sixty times the earth’s radius. We shallsee presently that this is very close to the true value, fOther observations were made by this method; andit is probable that the value given for the lunar
* A trace of this early application of the principle remains inthe name parallactic instrument still sometimes given to theequatorial. The principle of the instrument is given in theAlmagest , and the instrument, as made before the telescope wasinvented, was sometimes called Ptolemy ’s Rule.
f Before this Aristarchus of Samos had set the moon’s distanceat two million stadia , which, according to Buchotte’s estimate of thelength of the Greek stadium, would be equal to about 230,000 miles.The method by which he deduced this result is not well known ; butit is believed to have been based on the consideration of the lengthof time occupied by the moon in passing from horizon to horizon ;in fact, it would seem to have been a modification of the methodhypothetically considered in pp. 10—12. If so, it corresponded toa certain degree with the method he applied to determine the sun’sdistance. (See “ The Sun,” p. 25.) Hipparchus considered that themoon’s distance lay between 62 and 72£ times the radius of theearth. The above evaluation of Ptolemy is inferred from thenumbers given at p. 211 of Prof. Grant’s “History of PhysicalAstronomy.”