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

PREt:ESSION OF THlE EQiINSOXEs,. 10'
small effect upon ftle equatorial rling, but thle result is sliglltly
to diumilnishl t1he amolunt of precession.'lThe whole effect of thle
sun and moon eillg o" 0'.it-, thlat of the plainets is 0".31, tleaving
thle actual amount of preeession 50". 1.
T.h.e law of compositions of motion inl rotattion is analogouts
to thlat for the composition of rectilinear motions (Nat. PIlil.,
Art. 12), anld may be stated thus:'ijf two forces a;'ce ap]p)lie" l to
a 0body, winch, seitrtely, sJwoultt cause itt to rotate on two /di/,frcnt avesN, thec'r jyoent action7 wl t)prtoducte rotationi n aOz: ( is
lying Ietwieent the others, a(t antiles whose Si't:es a:re intverstely as
thed fo)cs. [I the precession, of eq:tinoces,t the eartl rotates onl
thlle diurnall 1 axis by one force, and the sun tends to revolve it
on an a`Xis Ipassilng through tle equincoxes..As thle ltatter force
is minute, compared w\ith tle other, thte tnew axis is slhifted by
a very small angle each year froln the, axis of rotation toward
the line of equinoxes. And as this line slides along by tle
amoe quanttity, the two axes remain perpetually at right anmgles
Nwith eacht other7.
1.93. lthe tidme occ.)pied by the sunt in passinty tfi'onL an
eqtinox or solstice re'round to t he same poinJt again, is Calle(I the
TROPICAL, E-A..t. As the sun does not perform a complete revolution in thtis interval, blut falls short of it 50".1, thoe tropical
year is shorter ttan tlle sidereal by 20m. 20s. in melan solar
time, this being thle time of desciribing an are of 50".1. in the
Canlmltal revolution. T1h le cltanges produced by the prcccssion
of the equinoxes in the apparent places of tlhe circumpol ar
stars, have led to some interesting results in chronology. In
consequence of the retrograde nmotion of the equtinoct ial points,
tlhe sigys of tlhe ecliptic (Art. 35) do not correspto3l at )lresenlt
to the constellaltiones whlich bear the same names, but lie tabout
o0le whlole sign, or 300, westwa-rd of themn. tuls, tihat division
of the ecliptic which is called the sign TalrfwltS, lies in the coastellation Aries, and the sign Ge-mini iln thle constellation
" Taur us. Undoubtcdly, however, whenl the ecliptic Nwas thlus
lirst divided, and tfhe divisions namled, tle svetral constella"he precel'ssion of eqlluinoxes, and other cases of compound rotations, are
finely illustratedt by the gyrocope of FoulRtt:, andt still better by Jolhsson's
ro!ascylte.