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

:188 TE PLANE,,S:.
or0' rVens are found. Sinlce Venucs, foy example, completes one
revolution arou(nd the sun1 in 22-1.7 days, and the cartlI in
365.25i,) and silnce the number of times each will re-volve ill a
given period is inven-rsely as the timle of one revolutiolln, therefore, in 224t-,700 revolutions of the earth, and 3%5,256 rcvolutionls of Venus, t:le two bodies would meet exactly at the same
llode asc befol. IBut 224,700: 365,256:: 8::13 zetarly; so that
transits of Ve'nus are somlctimles repeated at intervals of 8 ycarls,
ald if the ratio of 8 to 13 were ecaetly that of the two first
termsll of the proportion, we should have a transit oft Venus
every 8 years. The ratio of 227 to 369 is still nearer tlat of
those terms; anLd lence a transllit after 227 years is stiql more
problalc; but since there are two nodes, tle clhance is doubled,
so that a transit is higllly probable after an interval of 1131
yenars.'.he two transits of'Venus in the 18th century occurred
int June, 176:1, and Junel:t179, 8 years apart; tile two transits
of the:19th century are ill )ecember, 1.874, and D)ccember,:.882, tle intervals bcilng 1,05- and 8 years.'hlie a-verlage long
interval is 113}- years, but it may be lengthicned to 121t- or
slhortened to 10l5-, according as tllhe pr'eceding transit took place
before or after passing the node.
3 1 7. I'The great interest attached by astronomersl to a tratinsit
of Vellus, arises frollm its furnishing thle most accurate imeans
in our power of detelrmining tile suln's.lo.izotr).d Ia'ta:.....ri -anl
clement of great importance, since it leads to a know\ledtge of
the distance of tile eartl- fiom thle suln, and consequently, by
tlhe application of Kepler's third law (Art. 183), of the distances of all the other planets. Itenlce, in 1769, great efforts
were made tbrougllout the civilized world, ulider the patronage of (liffrenlt governments, to observe thlis pheollmnelion
unller circumllstances the most ftvorable for determrtiling the
parallax of the sunll.'lie met.hod of finding tlte parallax of a
hleacvenly body, described in Art. 85, can not bte relied on to a
greatler (degree of accuraIcy tllan 4". In tihe ease of the mnoon,
-w\lhose greatest parallax amounts to about 1:., this deviation
from absolute accura cy is not; aterial, butt it amounts to
nearly lhalf thte entire parallax of thle sunll; and since the distalnce is inversely as the horizontal pa'rallax, sulcl an error
\would l:ltak tle: distance of thle slln citlcer twice as g:ctal, or