MOOD, or MODE. [VERB.] MOON (Latin Luna, Greek crovhvn, Wane), the satellite of the earth, a heavenly body which moves round the earth, sharing the motion of the earth round the sun.
In a day or two (depending on the state of the weather) from the time called in the almanacs " the new moon," a thin silver crescent is seen with its horns turned from the sun and placed to the eastward of the sun, after which it soon sets. Its distance from the sun in creases, the horns at the same time growing fuller, until, in 7i days, it is at ninety degrees (or as far as from the horizon to the zenith) distant from the sun, and the crescent has become a semicircle of white light. The distance still increases, until the moon is ISO° distant from the sun, or in the opposite part of the heavens, by which time the light part has become a full circle : this happens in somewhat more than 144 days from the new moon The satellite still continues its revolution among the stars, becoming westward of the sun after the full moon, and, decreasing by the same steps as it increased, is lost a day or two before the time which the almanacs point out as the next new moon. The whole of this process what is called a luna tion, or a lunar month the lunar months are slightly unequal, but their average period is 29 days, 12 hours, 44 minutes. 2?„- seconds, or 29'5305867215 mean solar days. To show the irregularity of the lunation, we give the times of all the new moons which take place in the years 1860 and 1861, with the intervals.
phenomenon occurred only 705 instead of 730 (twice 365) times. Now the motion of the moon round the heavens is found to take place (one time with another) in 27'32166142 solar days (we shall presently see why this is not a lunation), which gives 13" 10' 35" increase of right ascension in each solar day, or 13° 8' 23" iu a sidereal day, or actual revolution of the earth. Hence the meridian of the spectator, between two times at which the moon is on the visible side of that meridian, must make so much more revolution as is necessary to overtake a body which revolves through 13° 8' 23" while it revolves through 360°"; which gives 24" 55"' of a revolution of the earth for each lunar day, or 12" 274' for its half. Now the year contains 3o64 sidereal days, or
simple revolutions of the earth ; and it will be found that 12" 274"' is contained 705 times and a fraction in 366id. As every reader may not be acquainted with the distinction of sidereal and solar time. we may here simply state (referring to Su: and Time for detail) that the com mon day is not the simple revolution of the earth, but includes the additional time in which the meridian overtakes the sun, which has moved forward about a degree. Thus it appears, that even on a single year the coincidence of half a lunar day and the interval between two times of high water is sufficiently apparent. It may be said that we have assumed the question by counting the times of high water from an almanac constructed on the supposition which we wished to esta blish. This Would be true if we had talked of the year 1860; but we may consider an almanac for 1858 as now a verified prediction : it would have made no small noise in the public papers if there had been a tide more or less in the Thames thau was predicted in the almanacs. The theory of the tides is the most difficult in astronomy, owing to the disturbing action of winds, channels, &c., as well as its intrinsic mathematical difficulties ; but this one phenomenon has never had its exception in open sea—that every transit of the moon over (either side of) the meridian is followed by the rise of the water, though so high a wind has been known as to prevent the tide coming up a river.
We return to the phenomena of the phases (Greek for appearances), as they are called, of the moon, namely, the changes in the quantity of its illuminated part. These may be immediately explained on the supposition that the moon is not luminous itself, but receives its light from the sun.