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

1 91.'rOi3 IT, NEAVT.
to the simt.i' I t has been commonIllO: to aSC'ribe the ruddy ligh't
of this planict to anl extensive and dense atmloslphere, w\lichl was
sutpposetd to be distinctly indicated by the g'radualdimintution
of lighl observetl in a star as it. approachlld verynear to the
planet ill ulIrgoing ant occulta tion; but more recent observations afll rd no such evidence of an atmoslpherc.'l |:rBy observations onl the sl)ots, we learn tlhat Mars revolves on his axis in
very nearly the same time \ithl the earth (21-h. 39m. 21'.3);
and that the angle betwccnt hlis equator aind the plane of his
orbit is also nearly the same as betw\\e thle arttl's cequator atnd
thet celiptic, thle former lbeing 28~ 42', the lattter 9 23 28', so that
the changlt es of SCasons onl Maris must resembllle our ownl.
No satellite has ever bleel discorvered l)elonlging to A:ars,
altolfl1ghl being situated at a greater distance fiolml tihe sun than
our globe, it mnigtt seem mlore especially to need such a
luminalry to chtccr its dallrk nighlts. As the diurnal rotation of
Mars is pe fo lmcd illn nearly the same tiime as the earth, we
slhould expcct a similar flattcening of the poles. Such is the
fact, and thie cllipticity of.:ars exceeds thlat of thle cartlh, being
about onc-fitticth, 1 whlile tle cearth's ellil)ticity is one threetundtl'reth. Thiis dilrlerence in tlhe conjulgate (ia lcters mayr be
readily observed whenlc thle plalnt; is inl opposition, th1e whole
cnlightecnet d (isk being then, presented to Ius.324. AtXars being compalratively near to us when on the
same lside of the sunt witll the eart;h, and the ratio of lhis distancc froilm the sun1 to t!hat of tle eartht leing easily obtained,
alstronolers hlav soughlt by metans of his )arallax, as by thatl
of iVc1lus, to find the stun's ]horizontal pl)arallax. But the method l)y observations on Vclnus, as described in Art. 318, is moro
to )e relied otn.
3:25. Th1e Asteroids or Prl,:rToI)S compose a group1) of very
smaltl p1lants, indefinite in number, whose, orbits lio beyond
tllat of tM'N[ars, at the distance of aboutlt 250,000,00 milcs firom
the sun.'liThe discovery of them commenced withl the begi;nning of thI precscnt coltury. I(epler hla(d long before noticed
a large interval betwceen Matrs and JupitCe,,twhich slced to
* Phil. Trs., 1181.'{ Sir James South, Phil. Trans., 1833. $ Illiad.