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

INSFERI}OR(),:'i..r...s M'-RCUILY AND V*NUS. 79
Th'le distance of one of the near planets, as'Venus or Maris,
Imay be detcrmined fr'om its parallax; and thle distance bing
lknown, its real dianmctelr can hbe estimalttcd firoml its apparenvt
diameter, in the samne mamlc as we cstim, ate the diamcete of
tlhe sun. (Art. 146.)
30 1. RIIO)DmIO'fllS.
Sldereal rovoltlon. Moean dally mttotion.
Melrcury, 3 monthllts, or1 88 dayst 4~ 5' 39".6
enusll8 7' " 22(I 924" 10 36' 7".8
I, c ear, " 365 " 0~ 59' 8".3
NMarTs, 2 " 1 68 " 00 39t, 2t3.17
Cel'es, 4a2 " " it 1)681 00 12' 501".9
Ju1)iter, 4:2 " " -4,)332 9 00 4' 59".3
Satu1rn, 9 (IC " 10,759 0~ 2' 0~".6
UrTanuts, 814 " " 30,686 00 0' 42".4,
Nceptunc, 1t6L1- " " 0, 12'( " 00 01 21".5
11romi thlis view it appears ttat the planets nearcst thlc sun
move most rapidly.'lhulls At ccllry pe1)rf'rms lnetarly 350 revolutions while Uranus peril'tns olne. This is evidently not
owing mrtcely to tlhe greater ditltnsions of tllC orit Of Ulranus,
for the length of its orbit is not 50 times tlhat of the o`rbit of
Aercury, rllwhile tihe time cmplloyed in dlescribing it is 350
times that of MAerculry. Indleed, this ouglht to follow fiom
KClelelr's law, that tlmhe squa'ees of the periodic times are as the
cutb&es of t}e (listances; fiom w\tichl it is nmanifest that the. times
of revolution increase faster thlan the (iltmlnsions of tlhe orbit.
Accordildltvy, the applaarent progress of thle most distant planets
is exsued(linglt slow, thte 1rate of Ui'raltns being only 421".4 per
day; so that ftbr weeks an(d ontltmls, and t even years, this planet
but slightly; changesl its p)lace among the stars.
T'h1 11)linetts are divid(ed into two classes' first, the iifol*ior,
whilch lhave their 6rbits nlealrer to tile sunm than that of the
cartlt; and sccndly, the se'qriow, whlichl have their orbits Cxterior to the cartht's olrbit.'1tl Til INFEIOR PI'ANXETS> _ME11RCURtY AND) VYEtN1.
302. mThe inferior nalt:ct'rl d' anVenus, lhaving their
orbits flr' witlin tlat of the cartlh, appea to us as attcndants