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THE MOON’S CHANGES

mean interval between successive conjunctions of theperigee and rising node is very nearly six years,falling short of six years in fact by but about threethousandths of a year, or almost exactly 1 X V days.*The mean interval between successive conjunctionsof the apses and nodes (without regard to the dis-tinction between apogee and perigee, rising node anddescending node) is three years, wanting only abouthalf a day, or more exactly wanting 13 h. 18’5m.

We are now in a position to discuss the effects ofthe moon’s rotation.

If the moon as she went round the earth turnedseveral times round upon an axis nearly square to thelevel of her path, she would present every part of hersurface several times successively towards the earth,precisely as the earth turns every part of her surfacetowai’ds the sun in the course of a year. On theother hand, if the moon did not turn round at allas she went round the earth, we should see in turnevery part of her surface, since at opposite sidesof the earth she would necessarily present twoopposite faces towards the earth. Since as a matterof fact it may be said (as a first rough account of themoon’s appearance) that she turns always the sameface towards the earth, it follows that she must turnonce on an axis nearly square to the level of her pathas she performs one complete circuit.

* The mean interval between successive conjunctions of theperigee and the rising node is 2190'343 days, and in six years thereare 2191'452 days ; so that the mean difference is 1T09 days.