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

TIRANSI'S OF Tl') INFt'ltIOR XLAFfNETS. 1 89
ont;l two-thlirds as great as thc tlrue distance, according as thlo
)arallax was 4" bclow or 4" above the truth.
3 1 8. If thle suill and Vclnus wcre equally distant from uls,
thelly would be equally aflected by parallax as viewed by spectators in diflerent parts of the ca'ttl, and conscquently thflir
elatkive situation would not be altecrcd by such a difierence in
the points of view; but since ~Venlus, at the inferior conjitiunction, is olnly about one-third as fatl' off Ias thle sn81l, }ter parallax
is proportionally greater, and therefore spectators, at distant
points, will see Venus projected on differentl parts of thle solar
disk; and as the planet travelrses the disk, she w\il appear to
describe cllolrds of diftlerent lengths, by means of whicll tle
duration of the transit may be estimated att diflterent placels.
The diference in the duration of the transit, as viewed from
opposite pyarts of tthe earth, does not amlount to many m-ilullt-s;
)but to malke it as large as possible, places very distant fr'om
each otlher are selected for observationl.'Thus, in the trallsit
of 1769, among the places selected, two of thie most favorable
wcrc Wlardllhusl in L, apland, and Otalheite (now written T.'ldtitdi),
one of tlhe Society Islands, in the South Paciice Ocean, to whlichl
place tle celebrated Captain Cook was dispattched by tle
IBritish governenlt for the express purpose of ob)servinlcg tlle
tranllsit.
Although the exact determination of the sun's lorizonltal
parallax by titis method is a very complicated and dilfficult
ploble lm, yet the pr2ignlpe; on which the process depends admnits of an easy illustration. |;Let E (Fig. 63) be the eart h, V
Vtnus, and S the center of tle sun. Suppose A and l1 two
d Fig. 63...... fe...............:.:...............::::::::::::::..:3
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observers at the extremities of that diameter of thle carth which11
is pel'plndicutlar to the orbit of ^Venus. At at certainll 11 ltc t
the spectaltor A vwill see Ve\Clus onl the st111's diS-k ll;t a(,) td(1 tite
spectator:.! will see it at b; and sinece A\ and BV' nmay be
considered as equal to eacll other, as also V\b and Vtu; tllerc