The _motions_ of the moon are most of them very irregular. The only equable motion she has, is her revolution on her own axis. The time in which she moves round her axis is about 27 days, 7 hours, 43 minutes, 5 seconds; and her revolution through an elliptical orbit is performed in the same time as her rotation on her axis, moving about 2,290 miles every hour. Her revolution round her axis exactly in the same time that she goes round the earth, is the reason she always turns the same face towards us: she has only one day and one night in the course of a month. From a long series of observations, it has been ascertained that the moon makes a complete revolution in 27 days, 7 hours, 43 minutes, 5 seconds; this is called the periodical month; but, if we refer to the time passed from new moon to new moon again, the month consists of 29 days, 12 hours, and 44 minutes, which is called the synodical month. This difference is occasioned by the earth’s annual motion in its orbit. Thus, if the earth had no motion, the moon would make a complete round in 27 days, 7 hours, 43 minutes, and 5 seconds; but while the moon is describing her journey the earth has passed through nearly a twelfth part of its orbit, which the moon must also describe before the two bodies come again into the same position that they before held with respect to the sun: this takes up so much more time as to make her synodical month equal to 29 days, 12 hours, and 44 minutes. The motions of the hour and minute hands of a watch may serve to give some idea of the periodical and synodical revolutions of the moon; for when the minute hand has performed a complete revolution, it has yet some distance to go to obtain a coincidence with the hour hand, similar to that which it had the preceding hour. We have observed that the same face is turned towards us during the whole of the moon’s revolution, and that the other half of her surface is never visible to us. This arises from the two motions we have noticed, which, with regard to our view of the moon, appear to counteract each other. Her revolution round the earth is performed towards the _east_; while the revolution upon her own axis is performed towards the _west_: so that, one of these motions turns as much of her face from us, as the other turns towards us. And from the moon’s axis being inclined to the plane of her orbit, sometimes one of her poles is inclined towards the earth, and sometimes the other: in consequence of which, we see more or less of her polar regions in different periods of her revolution. When the moon is in _perigee_, or nearest distance from the earth, her motion is quickest; and when in _apogee_, or most remote distance, her motion is slowest.