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

IPRECESSION OF TIlE E(iQUIN(XI'IS. 105
190, Suppose lnow we fix to tile center of eachl of tlhe two
rings (A. t. SS) a wire representilng its axis, Olte clrr101sp:ondilnlg
to thle axis of the celiptic, the other to thltt of lte equator, tlhe
extremity of each being t.he pole of its circle. As tlhe ritng (dcnotiln the equator turns round on the celintic, whiclt witht its
axiS remainSs fixcdl it is easy to conceivc that tIhe axis of thle
equator revolves around ttat of the ecliltic, adl tlhe pole of
thle equatorl around thle pole of tthe ecliptic, an(l Consttantly at a
distance equatl to thei illnclinatiol of tihe two circles. Tlo transfer ourl cotnceptions to thte celestial sphere, \Vwe l11may elasilty see
that ttte axis of tile diurnall l sphere (tlat of the earth prodluced,
Art. 28) would not ltve its pole constatnutly in thte samtlc place
amn1olng the stars, but tktat this pole 1would i)erlkrln a slow revolutioll 1arould tlc pole of tlhe ecliptic f1roml clast to wvest, conlpletingl tite circuit in about 26,)00 ycars. ienllee the star
whlic1h wetv now call thte pole-sttar, has not alw\ays etljoycd that
distinction, nor will it always enjoy it lhereafter.'\lWhlen the
earliest catalogtues of the starls wtre made, thlis star, was 172
firom the pol. It is now1"'24, auid will alpproach still nicarvr; or, to speak,mnore accurately, the pole \ ill come still nearer to this star, after whlichl it will letave it, antidt sutccessivcly
pass b)y others. In about 13,000 years, thie )right star'Lyra
wrlliclh lies oin the circle of revolution opposite to tt )l'resetlt
p}ole-star, will be within 050 of tIelt pole, and w-ill constitute t-he
Pole-star. As L yra now pasI)es near our zenith, tlte learlner
miglht, suppose that the clhange of tposition of the pole utmonlg
tile stars, would be attended withl a change of altitude of tl1
northl pole above the horizon. Thiis mistaken i(dea is one of
the mallny liS'appre)tlnsions whlic] result fi'o01 the lhabit of
considering the horizon as a fixed circle iln sipace. I I:owCeelr
the pole mtighlt sllift its position it space, \e should still beI at
the same distance frtom it, and oulr hlorizon w\ould al \Vways reach
the sainte distance beyond it.
191...'Te precess/ont qf tfle eqbilos.ces is ant gfl!I qf tie
spl.)heoidalt figt oe f the eart1h, and areises frac; tle aittrtzctionl 3f
fthe saZn3 an-ad moon uponZto thte excess csf iatteri aboutt the carth,'s
equator'.'Were the eartht a perfect sphere, thet attractions of thle slnI
ard mloon upon the earth w\ould be in equilibriumn among