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THE EAKTH.

the dates of those days do not occur at equal intervals apart.Our present purpose, however, only requires that we shouldclearly understand that the time of the Earth ’s rotation uponits axis is termed a sidereal day; and that the differencebetween a sidereal day and a mean solar, or ordinary day,arises from the Earth ’s motion round the Sun, in exactly thesame way that a traveller, who should sail at an uniform speed,so as to journey round the world in 365 days, would find thathe would gain or lose a day in the whole journey, or ^ T thpart of a day (i.e. about four minutes) in each day of hisprogress, according as his voyage might be made from westto east, or from east to west. Moreover, the Earth ’s journeyround the Sun being made from west to east, it does not losethe one day, hut gains it in each year. Only 365j- ordinarydays are therefore reckoned wdiile it revolves 366] times uponits axis ; in other words, 365] ordinary, or mean solar days, areequal to 366] sidereal days.

'We have especially referred to this fact in order to he ablethe more clearly to state that it is a supposed gradual lengthen-ing of the sidereal day, or (which is really the same thing) anextremely slow diminution of the speed of the Earth ’s rotationupon its axis, that has of late years been suspected. Thesuspicion has arisen in consequence of a peculiarity which hasbeen detected .in the Moon ’s apparent motion. Some observa-tions of the Moon ’s place recorded more than 2,000 years agohave come down to us with great accuracy, owing to theirconnection with ancient eclipses. Now, if the Moon had con-tinued, since those times, to move with the same average velocitywhich it at present possesses, it may be calculated that it wouldhave attained to a position in advance of that in which we nowfind it. The most natural explanation of this fact might seemto he, that, in some way, the Moon must have slowly andgradually increased its speed since the date of those ancientrecords. But it is evident that the effect would he exactly thesame if the increase in speed were not real, but only apparent;which would be the case, if the Earth ’s rate of rotation, bywhich we measure our time, and consequently estimate theMoon ’s speed, has been getting slower.