142 
THE GUIDE TO NATURE 
coveries of double stars. The first was 
Mizar, at D in Ursa Major, discovered 
in 1650; the second was Gamma. Arie- 
tis, at E, in 1665, and the third was 
Alpha Centauri, in 1689. Alpha Cen- 
tauri is a star which is not visible here. 
It is the star nearest to the sun, so far 
as we know. It is the finest of double 
stars with respect to the brightness of 
the stars, as one is of the first and the 
other of the second magnitude. One 
star moves about the other in a revolu- 
tion of eighty-one years. It has been 
observed to make nearly three com- 
plete revolutions since its discovery. 
About 1750 Bradley noted that Castor 
was double, also Gamma Virginis, at 
F, 61 Cygni and Beta Cygni. The 
stars in Cygnus are not now visible. 
Any of these stars can be seen double 
with small telescopes. 
Castor is the finest double star to be 
seen here, if only the brightness of the 
components is considered. The stars 
are of second and third magnitudes. It 
was the first double star in which one 
star was found to be moving about the 
other. This was noticed in 1803. How- 
ever, in spite of the long period dur- 
ing which it has been observed, we 
know little about the period required 
for a complete revolution except that 
it is between two hundred and one 
thousand years. 
Although both Sirius and Procyon 
are very bright stars and carefully ob- 
served by many observers, they were 
not seen to be double stars until recent 
years. In 1834 variations in the mo- 
tion of Sirius were detected, and in 1840 
a similar variation was noticed in 
Procyon. These variations were as- 
serted to be due to the presence of un- 
seen companions. A discussion of the 
variation enabled astronomers to pre- 
dict the position in which the compan- 
ions must be seen, the path, mass and 
period of the stars. The companion 
to Sirius was found in 1862 with a new 
telescope larger than those existing be- 
fore. It has now made a little more 
than one revolution. The companion of 
Procyon was not found until 1896, with 
the great telescope of the Lick Observa- 
tory. It is extremely difficult to see. 
This companion revolves in about forty 
years. It is a remarkable fact that the 
bright component of Sirius is twice as 
massive as the fainter but gives out 
more than five thousand times as much 
light. The mass of the companion of 
Procyon is about one-fourth of that of 
the sun, which means that it is among 
the stars of the smallest mass known. 
Both Sirius and Procyon are among 
the stars nearest to you. Sirius is 
forty-eight times as bright as the sun 
and Procyon ten times as bright. 
5}C Jjc ifC 
The Planets. 
The positions of the bright planets 
Jupiter and Saturn are now such that 
they can be shown on our map. The 
planets are not far apart in the constel- 
lation Virgo. The satellites of Jupiter 
and the rings of Saturn make them the 
most interesting planets. Saturn be- 
comes an evening star March 25. After 
that time it will be less than 180 de- 
grees east of the sun. Observers 
should recall that this is the time when 
the zodiacal light is seen best. This is 
a column of faint light extending up- 
ward along the ecliptic toward the 
Pleiades from the western horizon. It 
must be observed on a clear, dark night 
just after twilight. 
^ ^ ^ ^ 
Eclipse of the Sun. 
An annular eclipse of the sun occurs 
on March 28. The path in which the 
eclipse is seen as an annular eclipse 
crosses South America and Africa. The 
eclipse can be seen as a partial eclipse 
over these continents and also over 
Europe. A very minute portion of the 
eclipse can be seen under poor condi- 
tions from Florida. Elsewhere in the 
United States no part of the eclipse 
will be seen. The eclipse has little 
scientific value. 
This is the time of the year when we 
might expect an eclipse of the moon, 
but none occurs. The sun appears to 
move over the sky in a path which we 
call the ecliptic. The ecliptic is marked 
