142 
THE GUIDE TO NATURE 
center of Betelgeuse the earth 93,000,- 
000 miles away could revolve about the 
sun as now and be but two-thirds of the 
way to the surface. Even the planet 
Mars could perform its revolutions in- 
side the surface of Betelgeuse. The vol- 
ume of the sun is 1.300,000 times that 
of the earth. The volume of Betelgeuse 
is roughly 27,000.000 times that of the 
sun, which means that it is a body with 
a volume 35,000,000.000,000 times that 
of the earth. The results depend upon 
the distance of the star, which has not 
been determined with great accuracy. 
The result has been obtained by a 
scientific method by a scientist of the 
highest standing, and it is thus worthy 
of great consideration. Astronomers 
had not previously suspected that any 
star was so large. Betelgeuse was not 
suspected of being especially large. 
More may be said on this subject later. 
^ ^ ^ ^ ^ 
The Planets. 
In addition to the bright stars shown 
on the map we have the brightest two 
planets, Venus and Jupiter. Venus is 
the exceedingly brilliant evening star 
seen in the west in the early evening. 
As it sets before nine o’clock its posi- 
tion at the first of the month cannot be 
shown on Figure 1. By March it has 
moved so that its position lies within 
the region of the map. As shown in 
the January article, this planet is at its 
greatest eastern elongation on Febru- 
ary 10, at which time it is nearly forty- 
seven degrees from the sun. At this 
time it may be seen for the longest 
Figure '2. — The greatest elongation of 
Mercury and V enus. 
time after sunset; that is, it sets latest. 
In the January article we showed the 
motions of the earth and Venus. In 
Figure 2 we now show the path of 
Venus relative to the earth and sun. 
In some ways this is easier to under- 
stand. Although the earth moves, it 
seems to us as though the earth re- 
mained at rest and Venus moves in 
such a way as to combine the earth’s 
motion with its own. It may be seen 
from the figure that when Venus is at 
S, where the line from the earth to 
Venus is tangent to the orbit of Venus, 
the angle at the earth between Venus 
and the sun is greatest, and Venus is 
then said to be at its greatest eastern 
elongation. The angle in any case is 
called the elongation of the planet. 
Mercury is at its greatest eastern 
elongation when it is at T. This occurs 
February 15. It is then only eighteen 
degrees from the sun. The angle is 
eighteen degrees. Usually the elonga- 
tion of Mercury is so small that we 
cannot see it because of the sun. It is 
only when near its greatest elongation 
that it can be seen. The orbit of Mer- 
cury, as will be seen, is much flattened, 
and Mercury is the point of its orbit 
nearest to the sun also on February 15, 
so that the greatest elongation has 
nearly its smallest value. The angle 
is sometimes as great as twenty-eight 
degrees. Venus reaches the position 
U, called inferior conjunction, April 
22. Mercury similarly reaches V 
March 2. After these times they are 
morning stars. Mercury may be seen 
for a few nights about February 15. It 
will be low in the west, between V enus 
and the western horizon. It will be 
seen only in the twilight. While not 
nearly as bright as Venus, it will be 
easily seen under proper conditions. 
Mars also lies between Venus and the 
horizon. It is closer to Venus and not 
as bright as Mercury. 
On the eastern side of our map Jupi- 
ter and Saturn have made their en- 
trance into the early evening sky. They 
will be visible in the early evening now 
for about six months. 
5$C 5-C 
Saturn’s Rings. 
The appearance of the rings of Sa- 
turn at the present time is very inter- 
esting. As is well known, Saturn is 
surrounded by a series of three concen- 
tric flat rings lying in the same plane. 
The rings have an extreme diameter of 
173.000 miles, but wide as they are their 
thickness is probably under fifty miles. 
