THE YOUNG SCIENTIST. 
51 
to the sun ; this is called a synodic revo- 
lution. In 584 days after the planets 
start together from a and h they will again 
foe in the same relative i:)Osition with re- 
spect to the sun, and if, as represented, 
the orbits of these planets were in the 
same plane, a transit would occur every 
584 days. But such is not the case ; the 
orbit of Venus intersects the earth's path, 
as do the orbits of all the planets, and 
may be compared to barrel hoops shoved 
through each other, and the points of in- 
tersection, a and h in Fig. 2, are called the 
Nodes. The Node a, where the planet 
passes above the earth's path, is called 
the Ascending Node, and 180° from there, 
at b, where it passes beneath the ecliptic, 
the Descending Node. Now suppose Ve- 
nus in inferior conjunction <>x between the 
earth and sun, as at h in Fig. 1, and the 
earth to be at that x)oint, a, in its orbit 
wliere the orbit of Venus intersects it; 
then must Venus make a transit, and 
only at the points a and h can a transit 
occur. It is found that about thirteen of 
Venus' years equals eight of the earth's, 
and that about 235 of the earth's equals 
382 of Venus'. Hence, having known the 
date of a former transit, the years when 
otliers may occur can readily be obtained. 
They cannot recur in less than eight 
years, and if not then 235 years must 
elapse before another can occur at the 
m)}te Node, or 105 at the otlier Node. 
" How do you hope to arrive at the exact 
distance from the earth to the sun from 
observations of a transit of Venus?" We 
will endeavor to give a simple explanation 
of a very difficult problem. 
Two observers, situated at widely differ- 
ent points upon the eartli, as at n and s in 
Fig. 3, will see Venus projected upon the 
sun's disc at different places; the one at 
N will see Venus at h, and the other at w, 
and the two lines, a c and b d, show the 
apparent paths of Venus as seen at the 
two stations. The length of the line w h 
is the " parallax " of Venus, or the amount 
of apparent displacement suffered by that 
planet from being seen from widely sepa- 
rated points. The length of this line can 
be rea,dily computed. The rate of move- 
ment of Venus in transit is known with 
great precision, and when, from observa- 
tion, the instant of ingress or egress of 
the centre of the planet is known, the 
time of her describing the chords a c and 
b cl becomes known, and from that the 
length of those cords. Thus, if the rate of 
motion be 4' 3.5" per hour, and the time 
be 5h. 42m., or 5.7h., the chord a c must 
be 5.7 times 4' 3.5", or 23' 7.95" long; in a 
similar manner the length of the other 
chord, b d, becomes known. The angular 
radius (apparent semi-diameter) of the 
sun being known, say 14' 55.8", at that 
time, and taken = 1, then half of the 
chord a c or w c would be .7747. Now c c 
= 1, from which take the square of .7747 
and extract the square-root of the re- 
mainder, and the result will be .63233, the 
length of c w. By proceeding in a similat 
manner with the radius c d and the half* 
