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

110 TrtE, SUN.,MOTION O' tilE AlrS! i:s.1
1 496. The two poinits of the eclip)tic, where the,earthl is at
the greates tt d least (listanees fromt the sun resptectivel, (10o
not aItvays maintain the saRme ples among tlhe siHgs, but
grat lally shift their positions from Nwest to east. If we acenIttmy obrtsve the place among tlte stars, wlhere tile eart l is at
tlle time of its peri}hetlion the presentt year, we sltall fitnd tlat
it \will not be )reisely at thatt point the next Tyear whentll it
arrives at its perihelioM, blut about 12" (li1".'6) to the east of
it. A nd since tle equillox itself; firoml which longitude is reikoned, moves ill the opposite direction f')".t nt-tally, tte longitude of the perihelion increases every year (t".'t i, or a little
more tllhan one inulte. Titims ct i s c.sexr ssed!by saylf thlat
ttle line of the apsidles of tllhe eartht's orbit lhts a slow motion
fi'rm west to east. It collmpletes onle entire revolution ill its
own il prlte in about 100,000) years (111,149).
The mean longitude of the perilhelion at tile commueilement
of the present century \as 90 30' 5", 9and of course in the
ninthl' degree of Cancer, a little past thle winter solstice. 1n
te ycar 12148, the perimhelion was at tile )place of this solstice.
l.lthe advance of apsides is tcatused by thle attt:ractiont of the
other planets. A.s their weitglt is mostly outside- of tile eartl's
orbit, their effect is to diminish the earth's tendent y to wardt
the still. nBut tlis itllullence is oil thie lwhole, greater wilten
the carth is most distailtt; that is, at apllhelion. (,olnseqtuelttly
thlle el'rth passes a little furthl.er onwtr'd ttan at, the p)reee(ding
revolution, beft're Ltuiing to tapp)roact thle pterihellio ). Tus
the taplelion advatnces, and thel law1 of revolution requirels that
thte perihelion be opposite to it; hence thlat advances dtlso.
1. 9.'The anuh ular distance of a body fi'r1om its perillelionl is
called its.notnmay; and the interval betweell thle sun's passing the point of the ecliptic correspontding to thle earth]'s perihemlion, and returning to thle same poinit agail, is called thle
anomanlltL'ie ygear. This period nmlust be a little longer thatt
the sidereal year, since, in order to complete the anollalistic
revolutionl thie stln tmust traverse an are of 1".66 in additio(l
to 360(~, N'ow 3o60 30 65.256 11 ".66' 4m. 44s.,