THE GUIDE TO NATURE 
172 
this time Mercury can be seen very 
easily in the position indicated. But 
there need be no uncertainty, for just 
above Mercury lies Venus, the bright- 
est of all heavenly bodies except the 
sun and moon. This planet cannot be 
mistaken. As indicated, Mercury is 
just below it. Mercury is brighter than 
any other object in that part of the sky 
except Capella, at A, which is but little 
brighter. I here is no difficulty in see- 
ing the planet if one looks in the right 
place at the right time. 
Both Mercury and Venus change 
their positions rapidly. One need not 
wait until May 23 to see these planets. 
Venus can be seen at any time during 
the month and Mercury for a while be- 
fore May 23 and a few days after that 
time. Many will be interested in see- 
ing Mercury early and following it in 
its motions. In Figure 2 I have repre- 
sented the motions of the sun and these 
planets and the positions of a few of 
the bright stars. Capella, as stated, is 
the star marked A in Figure 1 ; the 
twins Castor and Pollux are marked B 
and C ; Procyon is at D and Betelgeuse 
at E. Mercury passed the sun and be- 
came an evening star April 24. The 
positions marked 1, 2, 3, etc., apply for 
the dates May 1, 5, 10, 15, 20, 23, 25, 31, 
June 5, 10, 13, 18. The sun and Venus 
move eastward at a nearly uniform rate, 
but Venus moves faster and each day 
is a little farther from the sun. At first 
Mercury moves rapidly and is farther 
from the sun each day until position 6, 
May 23, when it is farthest east of the 
sun or at its greatest eastern elonga- 
tion, as astronomers call it. Mercury 
then continues to move eastward, but 
not as fast as the sun, until June 5, 
position 9, when it ceases its “direct” 
or eastward motion and begins its 
westward or “retrograde” motion. At 
this time, June 5, Mercury is said to be 
“stationary.” With the sun moving 
eastward and Mercury moving west- 
ward they soon pass in position 12 on 
June 18. This is called inferior con- 
junction. After that time Mercury is 
west of the sun and is a morning star. 
The diagram shows why we see a 
planet much longer before its greatest 
elongation than after. At first Mercury 
gains rapidly on Venus, but it does not 
quite overtake it. The closest approach 
is on May 20, when it is a little over 
three degrees from Venus, position 5. 
They remain near each other for a 
while and then separate rapidly. Venus 
continues its eastward motion until 
November 4, at which time it will be 
south of Antares in Scorpio, a little 
beyond the southeastern boundary of 
higure 1. In the interval it moves half- 
way around the sky. 
What is the law or principle under- 
lying these curious motions? This 
question was asked as soon as such mo- 
tions were observed. A careful record 
of the facts of observation was neces- 
sary before the explanation was pos- 
sible. The cause is thoroughly under- 
stood. This cause is explained in Fig- 
ure 3. Mercury moves around the sun 
in an orbit which is nearly circular, 
but with the sun out of the center. 
Venus and the earth move in nearly 
circular paths with the sun at the cen- 
ter. It is the fact that we make our 
observations from the moving earth 
that complicates the apparent motions. 
The real motions are rather simple. 
Each planet is in the position shown by 
Figure 2. Apparent motions of the Sun, 
Mercury and Venus during May ard June. 
corresponding figures, the dates and 
figures corresponding with those used 
in Figure 2. On April 24, at O, Mer- 
cury was at superior conjunction on 
the opposite side of the sun from the 
earth. By May 1 it has moved to posi- 
tion 1 to the east of the sun, but the 
earth has also moved. The diagram 
shows the relative positions. Mercury 
appears farther from the sun in the 
successive positions until position 6, in 
which the line from the earth to Mer- 
cury is tangent to the orbit of Mercury. 
