Page:Popular Astronomy - Airy - 1881.djvu/175

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N last evening's lecture I treated more especially of the measures of the distance of the moon and the sun from the earth; and first of the distance of the moon from the earth. I endeavoured to point out to you that the method of parallax, by which the distance of the moon from the earth is measured, might be illustrated by the combined operation of the two eyes in the head, by which the distance of objects near us may be approximately estimated; but that in measuring the distance of the moon from the earth, instead of making use of the two eyes in the head, we make use of two observatories on the earth, placed at a considerable distance from each other, from which places we observe the same celestial object. I then pointed out to you two places that are peculiarly adapted for this measure; one of these is the Observatory at the Cape of Good Hope, and the other is Greenwich or Cambridge, or any other European Observatory. At the European Observatory, by means of the mural circle, we observe the apparent angular distance of the moon from the North Pole, at the time when it passes the meridian; and we also observe on the same day, at the Observatory at the Cape of Good Hope, what is the apparent