TUTOR. A spheroid, that is, it is a little flattened at the poles, and is in shape not unlike an orange or a turnip. This you will not be surprized at when I tell you that the equatorial parts are about four thousand miles from the center of motion. PUPIL. I suppose then you infer that as the centrifugal force is greater the farther it is removed from the center, that the parts near the poles have a tendency to fly off towards the equator. TUTOR. I do. And as we have finished this part of our subject, I shall take leave of you. ------------------------------------------------------------------------ DIALOGUE XI. TUTOR. I now propose giving you a description of the moon, and I doubt not it will afford you some degree of pleasure. PUPIL. Indeed it will, as I know little more than that she is a secondary planet or satellite, revolving round the earth, and with it round the sun. TUTOR. You know her mean distance from the earth. PUPIL. I did not recollect that: 240 thousand miles. TUTOR. Right. Her diameter is about 2161 miles, and her bulk about a fiftieth part of the earth’s. Her axis is almost perpendicular to the plane of the ecliptic, consequently she can have no diversity of seasons. PUPIL. What is her period? TUTOR. The time she takes to revolve from one point of the heavens to the same again is called her _siderial_ or _periodical revolution_, and is performed in 27 days, 7 hours, 43 minutes; but _synodical revolution_, or the time taken up to revolve from the sun to the same apparent situation with respect to the sun again, or from change to change, is 29 days, 12 hours, and 44 minutes. PUPIL. I do not clearly comprehend it. TUTOR. If the earth had no annual motion, the period of the moon would be uniformly 27 days, 7 hours, 43 minutes; but you are to consider that whilst the moon is revolving round the earth, the earth is advancing in its orbit, and of course she must be so much longer in completing her synodical revolution as the difference of time between that and her siderial revolution. This I will make clear to you in a few minutes.—What is the situation of the hour-hand and minute-hand of a watch at twelve o’clock? PUPIL. They will be in conjunction. TUTOR. And will they be in conjunction at one? PUPIL. No, Sir. TUTOR. Yet the minute-hand has made a complete revolution: but before they can be in conjunction again the minute-hand must move forward till it overtakes the hour-hand. PUPIL. I now understand it, and must beg you to explain to me the different phases of the moon.