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

21.t0'TrIT rNAIN;T'fS.
quaintlltce with the simplest lllotions of the planet-ary system,
ttlld afterward to conduct him gradually through suchl as are
more complicated and difficult.
351.15. When viewced firomi the center of their motions, tilo
revolutilons of the planets would appear simple and hIaronious,
all coursilng round the spectator friom west to cast in rgulnlar
ordelr, ill nearly the same great highway, though with very
differeit degrees of vclocity. Iet u1S, thell, supl)ose Ourselves
standing on t~he sun, and contemplate the revolutions of the
})lanets, rilst, severally, and then as f'orming one glrand whole,
co(lsistilg of numl erous Iparts, but bound togtlhe'r 111der the
sante laws il. o1ne vast empire. rWe should see Mercury making?vel.ry perce'ltiblc progress aroundl thel hcavGns, like the moon in
its motions albout the carth, this rate of motion castward being
albotL one-tlhird as rapid as thlat of the moon, since hIe con)letes his entire revoliltion in about tllhre' moths.It wil, t at
Iirst, aidl otir conccptions of the respective positions of tle
p)lanmeta'ry orbits, to imagine the ccliptic to be marked out on
the face of the visiible hecavens in a l)alpable line distinctly visible to thei eye. if we, stationed at the sunl, wNatch t)lc 1motiolns
of IMrclleury, we shlall see it cross thl ecliptic in two opposite
poilnts of thle heavens, constituting its todechts; and w\e shall see
it, wheln alf way letwceen tihe nodes, at an angulair distance
front the ecliptic of about 7, this bCing,' the ineliwtlion of its
orbit. lKnowing the position of the orbit of Iecltlury with
lres)cct to tile eclil)tic, we may' now, in imalnination, represent
that orbit in a grcat cir'le assing tllrough the center of theo
p)lanet and the center of the sun, and cutting tim plane of
the eclittic in two opposite points in an an tgle of 7 degreeCs,. lhe patlls of )oth}t Iplanets appear as circles amlong
the stars; anlid if we suppose tlhem to be visibly traced oil
their respective pllanes, the observer, while at the suit, can
not distingullish tell fr'om circles, of which he is the ce1tre'. uht, if we imagine him transfported to a great distance
firom t1e suntl, in a lilne at rightt angles to the celiptic, line wilt
d(iscern the true forms of the orbits. The earth's orbit can not
evlml 1l()\ be d(istillgishedt fr'om al circle, but the sint is plainly
t little i ot oft its clter. Tlle orbit of M5tllerr'y, however, is
di::i:ntlc y vclli:tieal, witi thlt sun ill onet of its foci. On Lhis