1 



MOOX. 



MOON. 



and the mean longitude at any other time U found by adding in 

 UM proportion of 4808 -3S4S8 for every 365 day*, and making the 

 naoMsary additire allowance for the precession of the equinoxes. 

 [PRECESSION ] 



In the aame way the node and perigee of the moon have their mean 

 place*, and, a* we hare seen, their mean motions. The mean longitude 

 of the perigee, at the commencement of the century, was 266* 10' 7"-5 ; 

 that of the uoendtng node IS* 63' . 



To the above murt be added that them average motion*, a* they are 

 called, are subject to a (light acceleration, which hardly show* itself 

 in a century ; that of the longitude was detected by Halley from the 

 comparison of some Chaldean eclipse* with those of modern time*. 

 This acceleration would, in a century, increase the mean longitude of 

 the moon by 11% that of the perigee by 50", and that of the amending 

 node by 7". 



The mean longitude being ascertained for the given time, the true 

 longitude is found by applying a large number of corrections, as they 

 are called, some determined from the theory of gravitation, but the 

 larger one*, as might be supposed, detected by observation before that 

 theory was discovered, and since continued by it Into this subject it 

 will be impossible to enter at length ; we shall therefore merely 

 instance a few of the principal correction* for the longitude, observing 

 that the l.ii'ide. the distance, Ac., are all determined by adding or 

 subtracting a number of corrections from the results of the supposition 

 that the moon moves uniformly in the ecliptic at her average distance 

 from the earth. 



The first correction is one which brings the motion nearer to an 

 elliptical one, and is called the equation of ike centre. It depends upon 

 the moon's distance fioin her perigee, called the anomaly. The 

 mean anomaly is the distance of the moon's mean place from 

 that of the perigee. The mathematical expression is (we give only 

 rough constants) 



6 1"' x din (mean anomaly). 



The second correction, known as the tttrtvm, and discovered by 

 Ptolemy, is 



1*16' x sin j 2 (<) mean anomaly | 



when C and Q stand for the mean longitudes of the moon and sun. 



The variation and the annual equation (discovered by Tycho Brand) 

 are represented by 



3'xsin2(<C-O) 

 and ll'x sin (Q's mean anomaly.) 



Many such corrections (but those which remain, of less amount) 

 must be added to or subtracted from the mean longitude before the 

 true longitude can be determined. 



Having thus noticed the actual motions of the moon, we proceed to 

 the phenomena of eclipses and of the harvest-moon, as it is called. 

 An eclipse of the moon has now lost most of its astronomical im- 

 portance, and can only be useful as an occasional method of finding 

 longitude, when no better is at hand. Eclipses of the sun, observed in 

 a particular way, may be made useful in the correction of the theory 

 both of the sun and moon ; in this case matter i absolutely hid from 

 view by matter, and the moment of disappearance can be distinctly 

 perceived. But in the case of the moon, which is eclipsed by entering 

 the shadow of the earth, the deprivation of light is gradual ; so th.it it 

 i* hardly possible to note, with astronomical exactness, the instant at 

 which the disappearance of the planet'* edge takes place. 



In a lunar eclipse the first thing to be ascertained is the diameter 

 nf the earth's shadow at the distance of the moon. Suppose this 

 shadow, that U, its section at the distance of the moon, to be repre- 

 sented by the circle whose centre is o : it is directly opposite to the 

 sun, it* centre is on the ecliptic, and moves in the direction of the 

 un' general motion, or from west to east 



Let c A be the ecliptic, and let B c be a part of the moon 1 , orbit, with 

 the node at D. It must be remembered that the whole takes place on 

 a rrrj small part of the sphere, so that all the portions of the orbits 



which actually come into use may be represented by straight lines. 

 Let the centre of the moon be at r. when that of the sha low is at c ; 

 and let the hourly motions of the sun (that is, of the shadow) and of 

 the moon be or and EO. If then we communicate to the whole 

 system a motion equal and contrary to CF [MOTION], the shadow will 

 be reduced to rest, and the relative motion of the moon with respect 

 to it will remain unaltered. Take E B equal to c p, and contrary in 

 direction ; then E L will represent the quantity and direction of the 

 hourly motion of the moon relatively to the shadow at rest. By 

 geometrical construction therefore, x, N, and r may be ascertained, the 

 position* of the moon's centre at the beginning, middle, and mi! of the 

 eclipse ; and r. x, r N, and t: r, at the rate of F i. to an hour, represent 

 the times elapsed between that of the moon being at r. and the 

 mena in question. Such is the geometrical process : the i-ne en. 

 in practice is algebraical, and takes in several minor circumstances 

 which it is not worth while here to notice. 



An eclipse of the moon is a universal phenomenon, since the moon 

 actually loses her light, in whole or in part ; while in an eclipse of the 

 sun, the moon hides the sun from one part of the earth, but nut 

 another. The former can only take place when the con i 

 sameness of longitude) of the moon and c:irth's shadow, that 

 opposition of the sun and moon, or the full moon, happens when the 

 moon is near her node. [Ect.iPSK; Sus ; SARDS. | Ki.r the ph.-i 

 of the occupation of a star by the moon, see Orel I.TATIUX. 



By the harr'tt-moon i meant a phenomenon observed in our Lit! 

 t the time of the full moon nearest to the autumnal eijuiimx. 

 when it happens for a few days tbst the moon, instead <{ rising fifty 

 two minutes Liter every day. rises for several days nearly at th. 

 time. Something of the same sort takes place always wl'ii-n tin 

 is near her node; but the circumstance is most remarkable when it 

 happens at the time of greatest moonlight. The reason is that the 

 increase of declination (which is most rapid when the moon i 

 the equator, which she must be when full moon comes nearly at the 

 time of the equinox) compensates the retardation which would other- 

 wise arise from her orbital motion. [Srni-KK.] See the treat!*-- 

 cited, pp. 80, 81. 



The discovery of the telescope, and the examination of the moon 

 which followed, soon showed that the planet always turns th. 

 face towards the earth, or very nearly. From hence it immediately 

 follows that the moon muxt revolve round an axis in the same time n* 

 that axis revolves round the earth. If any one should walk round a 

 circle without tuniing himself round, that is, keeping his face always 

 in the same direction he would present alternately his front and back 

 to the interior of the circle. But if he desires to turn his face always 

 inwards, he must turn round in the same direction as he walks round. 

 [MOTION, DmonOB OF.] If the moon moved uniformly round in 

 her orbit, and had a uniform rotation of the siime duration, thi-n if 

 her axis were perpendicular to the plane of the orbit, and tin- 

 spectator were always at the earth's centre, the fa< f tin- 



would be always actually the same. None of these suppositions are true. 

 1. The motion in the orbit is irregular while the rotation is uniform 

 and exactly the sidereal month : the consequence will U- that when the 

 moon is moving quicker than the average, a little of the western i-id.- 

 will be coming into view, and a small portion of the eastern side will 

 be disappearing, and riet rertd. 2. The axis of the moon is n< 

 pendicular to her orbit, but is out of the perpendicular bv an an 

 5 8' 49*; the consequence is, that as she rero}Tei in li.-r orl.ir. the 

 north and south poles of the moon will alternately become im 

 each during half a revolution. 8. The spectator is in motion 

 the earth's axis, which will slightly vary the part seen of the moon in 

 the course of the day. These effects are called librations : (1) the 

 libration in longitude, (2) the li oration in latitude, (3) the diurnal 

 libration. The second will !>e elucidated in SEASONS. 

 the third in PRECESSION and NOTATION. 



The way in which we know that the face presented is always nearly 

 the same, is by observation of that face, which is varied by numberless 

 spots and utreaks. The following cut represents ihe general appearance 

 of the moon at full, being a view of its average face in the mean state 

 of libration, that is to say, no part of the present edge is ever hidden 

 by lil-r.it ion without as much of the opposite edge being hid. 

 some other time. 



It may here be mentioned that photography has been applied to tin- 

 production of views of the lunar surface with great success. A 

 these, the beautiful photographs by Professor Bond in America, ami 

 Mr. Warren De La Hue in this country, hold a distinguish' '1 

 Indeed the stereoscopic views of the moon in different states ) 

 latter gentleman, leave but little to be clone by succeeding photogra- 

 phers, their effect being such as to excite the surprise and admiration 

 of all by whom they are viewed. 



For particulars respecting the spots on the moon, the reader is 

 referred to a work lately puMi.hcd by the Rev. T. \V. Webb, entitled 

 'Celestial Objects for Common Telescopes' In this little work, win. 1, 

 contains a vaut amount of information on practical subjects, a map of 

 the moon is given, exhibiting the places and delineations of 404 of the 

 principal craters and other appearances on the lunar surface carefully 

 reduced from the large map published by Beer and Miidler, and 

 verified for the most part by actual observation. 

 | Casual observers of 'the lunar surface must however bear in mind, 



