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

205i'TIIPI r'iA N1's.
the disk of the primary, for, beipgp illuminated like tlte )rillary, it is not readilyv distillguishable frlom it; but sometimes,
wlhen it hlappensl to be projected on one of the belts, it is seen
as a b 6g/ tl spot, matkint its transit acrossthe disk. Occasionally), also, it is seen as a dAik skpot of smaller dimelnsions tlhat
thlo shadow. 11This curious fact 1 has led to the conclusion thlat
certail of the satellites have sometrimes on their o wn bodies, or
in their atmlospherels, obscure spots of great cxtent.'x'
33., A, very singullar relation subsists between the mean
motions of tlhe three first satellites of Jupiter. TI'hce mneant longitude of the first, plus twice thlat of the third, millus thltee times
that of tle second, always equalst 180 dlegrees. A curious
consequence, of tltis relation is, that the tllree satellites can
Ilever be all eclipsed at the samlle time; for tlhen, havino\ sevcrally the same lbngit.lud as the lprimary, tlheir longitudes
would be equal, and thlat of the first, plu) s twice thlat of tlhe
ttird(l miltus three times that of tlhe second, vwould be nothilng,
and of course could not be 180 dcgrees.1t The longitudes here
mentioncl (are such ans would be observed by a spectator on
Jiupiter~, anld not a spectator on thie ecatll.
3 33.'1.'lhe discoveryl of thle system of Jtpiter and lhis satellites, soon <after the invention of the telescope, lent a powerflld
support to the (oopelCrnican system of astronomy, tChen just, begitnling to be received by astron lomers, since it ptresented to
lth eye anl exact miniature of the solar system, a"'nd exhibited
an actual lmodel of that arrangement of the sun andl a lalects
which had lbefore only been contempllated )by tlhe eye of the
mind; and the laws of thoe planetary'y system, discoveLred by
K(eplcer, were here actually seen to be verified, in thle motiolns
of t his miniature system. i\MoreoveNr, the eclipses of Jupiter's
satellites, furnished. one of thlose instantaneous events, occur-:rinDg at the same moment of absolute time w\VlerCC r sCen,
wlichll; are available for: finding the longitudes of (lifitcrent
places; and at that period it was deemed a more eligiblo
Imethod of determining l this great practical problem of astronomly- t.shan anylr method thein in use.
S............................................................................
* Sir J. Ifcrschcl. t Mot.