CHAPTER I.
THE MOON : DISTANCE, SIZE, AND MASS.
Although the sun must undoubtedly have been thefirst celestial object whose movements or aspectattracted the attention of men, yet it can scarcelybe questioned that the science of astronomy had itsreal origin in the study of the moon. Her compa-ratively rapid motion in her circuit around the earthafforded in very early ages a convenient measure oftime. The month was, of course, in the first place,a lunar time-measure. The week, the earliest divi-sion of time (except the day alone) of which we haveany record, had also its origin, most probably, inthe lunar motions. Then the changes in the moon’sappearance as she circles round the earth must haveled men in very early times to recognize a distinctionbetween the moon and all other celestial objects.While inquiring into the nature of these changes,and perhaps speculating on their cause, the firststudents of the moon must have soon begun to
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