745 



MOOD. 



MOON. 



718 



MOOD, or MODE. [VERB.] 



MOON (Latiu Luna, Greek at\i]i>Tt, selene), 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 

 tune called in the almanacs " the new moon," a thin silver crescent 

 is seen with its horns turned from the sun and plaeed 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 7.1 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 180 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 bj 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 takes up 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 2J-, seconds, or 

 29'53u58a7215 mean solar days. To show the irregularity of the 

 lunations, we give the times of all the new moons which take place in 

 the years 1860 and 1861, with the intervals. 



NEW MOONS. I860, 1861. 



8 45 



10 13 



It appeirs theu not only that the lunar month varies, but that there 

 is no yearly cycle of variation. Before however we make any remark 

 on the preceding, we shall place by its side materials for confidence in 

 the almanac from whence the preceding times were quoted. Taking 

 :it hazard a volume of astronomical observations, and opening the part 

 where the results of the lunar observations are found, we took the first 

 right ascensions [ASCENSIONS] of the moon which we came to, opposite 

 to which, for comparison, were written the predicted right ascensions 

 of the moon for the same times. The dates matter nothing, since it is 

 only the accordance of prediction with observation which is to be 

 noticed. (' Camb. Obs.,' 1835.) 



Predicted R.A. 

 h m 

 14 23 57-87 

 23 1 59 84 

 4 66 13-31 

 18 7 40-74 



Observed R.A. 



I. m I 



14 23 57-01 



23 1 68 55 



4 56 14-01 



18 7 40-50 



Difference. 



2ii-100thB of a second. 

 1- ai.il 29-100tbs. 

 7-10ths of a second. 

 24-100ths of a second. 



The lunar theory then, resting upon the Newtonian doctrine, enables 

 astronomers to find the position of the moon within a part of the 

 heavens answering to a second of diurnal revolution, while the rough 

 observations with which astronomy must always commence would not 

 give the length of a lunation within an hour. It is also confidently 

 expected that the lunar tables recently computed by Professor Hansen 

 of Gotha, in accordance with his lunar theory, and printed at the ex- 

 pense of the British government, will, from their superiority, ensure a 

 degree of accuracy of prediction hitherto unprecedented. 



Taking Jhe lunar phenomena in the order of discovery, we next 

 notice that this planet writes its mark on the earth in terms which 

 have been understood from the earliest ages of astronomical inquiry. 

 The alternate rise and fall of the waters, called the tides, is found to 

 follow its motions, so that high water is al ways found to succeed the 

 time when the moon comes on the meridian, whether on the visible or 

 invisible aide of it. At first sight it would appear that there is high 

 water twice a-day (that is, in the common solar day), but it is found 

 on farther examination that the interval between high water and high 

 water \ a little more than twelve hours | o that in the year 1868 that 



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 27'321b'6142 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" in 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 h 55'" of a revolution of the earth for each lunar day, or 

 12 h 274'" f r its half. Now the year contains 3u6J sidereal days, or 

 simple revolutions of the earth ; and it will be found that 12' 1 274'" i* 

 contained 705 times and a fraction in 366 j d . 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 wo 

 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 I860; 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 than 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 p/uues (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. 



To show how this may be. suppose a ball, illuminated by another ball 

 at a great distance in the direction KS, to be carried round the specta 

 tor at E. This ball may be always subdivided into a visible and in- 



visible half, since one-half must hide the other in all positions. But it 

 may also be divided into an illuminated and unilluminated half. At A 

 the visible half is all unilluminated, and though we have called it the 

 visible half (meaning in a position to be seen, if thera were light), it 

 will not be seen. But when the ball arrives at B, a small portion of the 

 illuminated half is in the visible half, as much as is intercepted 

 between the arrows. At D a larger portion of the illuminated part is 

 visible, and at v a full half of the visible surface is Illuminated. A 

 little consideration of this scheme (which is moreover explained iu all 

 popular works) will show not only the occurrence of phases precisely 

 similar to those of the moon, but also that the circular boundary of the 

 enlightened part is towards the illuminating body. We copy from 

 Riccioli his collection of the Latin and Greek terms used with respect 

 to the different phases : 



A Noviluuium, luna silens, Conjunctio, Cougressus ! '"Ml 1 

 cum sole, Neomenia, Sy nodus, Luna; accensio. 



B Prinia phasis, Nova luua. 



C Ultima phasis. 



Luna novissima, Interlunium, sou iutermeustruum, 

 est tempus inter ultimam et primam 

 \ C d B. 



f Trp-Jirri 

 \ vta fft 

 f f 

 \ H 



astruum, 1 

 ph.isini, 1 1 



