TO KNOW THE STARRY HEAVENS 
03 
These rings lie in the plane of Saturn’s 
equator. Just as the earth in its motion 
about the sun keeps its axis always in 
parallel positions and hence its equator 
also in parallel positions, so also does 
Saturn in its motion about the sun keep 
its equator and the rings always in 
parallel positions. Just as the earth’s 
equator crosses the sun twice in each 
revolution (year), March 21 and Sep- 
tember 22, so, too, does Saturn’s equa- 
when visible. However, the circum- 
stances are very favorable indeed, since 
Saturn is visible on each of the three 
occasions, and three is the largest num- 
ber possible. Moreover, Saturn is al- 
most directly opposite from the sun on 
April 10, when the plane passes through 
the sun. 
The rings of Saturn can be seen well 
only with moderately large telescopes. 
In such an instrument they are prob- 
tor, the plane of its rings, cross the sun 
twice in each of its revolutions. Sa- 
turn’s revolution requires 29.5 years. 
The earth, too, must lie in the plane of 
the rings twice in each of Saturn’s rev- 
olutions, but since the earth also moves 
and it is possible for it to be in the 
plane of the rings whenever they cross 
the earth’s orbit, it is possible for the 
earth to be in the plane of the rings 
more than twice. The earth may be in 
the plane of the rings as many as three 
times, while the plane is crossing the 
earth’s orbit. The plane of the rings 
passes through the sun on April 10. 
Since 1907 the sun has been south of 
Saturn’s equator, as it is now south of 
our equator. After April 10, 1921, it 
will be north of Saturn’s equator for 
about fifteen years. 
In Figure 3 we show the motions of 
the earth and Saturn during this par- 
ticularly interesting period when the 
plane of the rings is crossing the earth’s 
orbit. The plane first cut the earth’s 
orbit October 12, 1920. The plane re- 
quires 359 days, or almost a year, to 
cross the orbit. On November 5, 1920, 
the earth first crossed the plane of the 
rings, as we pointed out at that time. 
On February 22 this month it crosses 
again in the opposite direction, as 
shown in Figure 3. It crosses again 
on August 3. At that time, however, 
Saturn will be rather low in the west 
ably the most impressive object to be 
seen. At the present time we are look- 
ing at the rings almost edgewise and 
hence get a poor view in some respects, 
but most interesting in others. Both 
Saturn and the rings get their light 
from the sun. Before November 5 the 
earth and the sun were on the same side 
of the ring, hence we could see it. When 
the earth is in the plane of the ring 
it often disappears, as it is so thin that 
it cannot be seen. Between November 
5 and February 22 the earth and sun 
are on opposite sides of the ring, hence 
the side of the ring turned toward us 
is not illuminated. Under these cir- 
cumstances the ball may be seen 
crossed by a narrow black line which 
is the shadow of the ring. From Feb- 
ruary 22 to April 10 the earth and the 
sun are again on the same side and the 
ring appears as a needle of light 
through the planet. 
On April 10 the sun shines only on 
the edge and the ring disappears for a 
time. From April 10 until August 3 
the earth and sun are again on opposite 
sides of the ring. After August 3 they 
are again on the same side. After this 
the rings open, as we say, wider and 
wider until they are open widest after 
seven years. These times when we see 
the rings edgewise are particularly fa- 
vorable for studying the nature of the 
rings, 'l'he appearance and disappear- 
