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

Velnus.s is visilblc from three to four hlours after the setting, o
before the rising of the sull.
3 1 4. ]Avewy eighit years, T': eusforfmns her' conjnuetions wliiA
the Stt i i1 t/e s pame'art fr the fieaverts.
1.tlc sidcrtcal period of Venus bcing 221.7Y days, and thlat of
the carltl 365.225 days, thirltteen revolutionls of VeCInuts are accomplilshed in nearly thte same tilt as ci/gh revolutions of the
earth: fot 224.$x13::::::2921, and 3G5.2S56 x8.:.:92.:At the
fend, therefore, of 29292 days, or eighlt years, tlhe tNwo bodies will
come round to the same pointt of th}e lecavens, and be inl the
sname situation in their rlesp)ectiv orbits, as at the lbeinnlinmt.
Consequently, whatevr appearances of this planct arise fiom
its positions withl respect to the carth and the sun (as, for example, b)eilng visible in the daytimc), they are repeated every
eight years in nearly the salmel form.
rltANs'rT OF T'in f EX IXOlt PL'.AN.F,,'S.
3 1 5. tf ie 1wtratssitf JliereW ro'Vewu:ts, is its passage across
the stn's (disk, as the?tnoon ptfasses ov'er it in2, a solta' eci pse.
As a transit takes place only when tie p)lanet is in inferior
conjunction, at whlich time her motion is retrograde (Art. 305),
it is always fiom left to righlt, atd the planet is seen pr'ojcted
on the solar d(isk in a black rounttd spot. Werce tleo orbits of
theese planets coincident with the carth's orbit, a transit would
occur at somie part of the earth at cvcry inferior conjunction,
as there would be all cli)pse of the suni at eveiry new moon,
\wer the mlnooln's revolltion ill the plane of the ecliptic. ]But
the orbit of'Venus makes an angle of 3-o with that of thic
eltlh, an(l the orbit of AMercury an angle of 77~; and, moreover, the appl)arent diameter of caech of these bodies is very
small, both of whic circtumstances conspire to lrender a transit
a contll)ratively rare occurrencc, silce it can happen only
NwhVen teil silun, at the titme of an inferior cojiunctioni, al)l)enls
to be at, or extremely near the planct'ts node.'The nlodes of
M:[ercury lie in thlat part of the eartl's orbit which it passes ill
thle mlonths of May and -November. It is only in these mo0nths,
therefore, that transits of Mlcrcury can occur. For a similar