An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

LUNAlt Ift1IZEOtJULAT1T IECS.:I 37
236. The chlange of place in tlhe apsides of the moon's
orbit, like tle sllifting of tile nodes, is caused by the disturbilng
influCence of tile sunll. If lwhen thlle mloonl sets out fr'omI its pl'igee, it Awere urlged b))y no other force than tllt of pro jcction,
combiiincd itlI its gravitationl toward the eiartlh, it would describe a symmetrical curve (Art. 187), cominig to its at)ogee at
the distance of 1L800. 31ut as the mean d(istlirbing forcee ill the
lirection of the radius vector tends, oil the \whole, to dillinislh
tlhe gravittionl of tlth moolI to the earth, tlec portion of the
path d(escribed in an instant will be less deflected from lher
tangent, or less curved, tllan if this force did not exist. Hencoe
thle,ath of the moon will not intersect tihe radius vector at
righ]t angles, thlat is, shle will not larrive at her apogee until after
passilng more taitan 180 friom ler )peri ge, by whlich mlleans
tcse points will constantly shift their positions f'om \west to
east.*" The motion of the apsides is found to be 30:L 20," for
Ovelry sidereal revolution of t.llhe mool.
237. On account of t:llhgreatcr eccentricity of tlhe mnool's:)rbit above thatt of the sutll, t.te i u.l ation, qf the Ceei', or that
correction whlicll is applicOd to tce l ooll's imean anomaly to find
ielr trute alomaly (Art. 200), is nluttc greater than that of the
situl, beilng whe1Cn greatest more tha.n six degrees (6 C 1':12".),
while t:itt; of tlhe suitn is less tltan two tltegree (1~ 55' 26".8)..ltche irreg llarities ill tle mnotionls of thle moonl maty be compared to tlhose of tle lmagnetic needle. As a,fi,.t (pr'omt.'iion) \we\ say that tle neelle lplaces itself in a north. and south
line. Ot closer examinationo, however, wev fidll that, at difet'rent places, it deviates more or less firom thlis line, andl we illntrodice thel first grcat correction lunder, tlhe name of thle (ilctination, of tle needlCe. HBut observ ation shows us titat t]to
declination alternately increases and liminishes every day, anld
theretre Nwe appl)y to thle declination itself a, second correction
tfo; the itaitzual variation. Fi'ially, ve ascertain, frol longcontinulted observations, that tlte diurnal variation is aflected
by tlte cltangl of seasons, being greater it summlller than in
wintetr, and lhence we apply to the diurnal variation a thlird
correction for thle aznual variatiotn.
o Playfitir.