The Solar System

44 PART 1^ |^ EXPLORING THE SKY

spinning on the ground. Th is precession is caused mostly by the

gravitational infl uence of the sun, and it makes the line of nodes

rotate once every 18.6 years. People back on Earth see the nodes

slipping westward along the ecliptic 19.4° per year. Consequently,

the sun does not need a full year to go from a node all the way

around the ecliptic and back to that same node. Because the

node is moving westward to meet the sun, it will cross the node

after only 346.6 days (an eclipse year). Th is means that, accord-

ing to our calendar, the eclipse seasons begin about 19 days ear-

lier every year (■ Figure 3-15). If you see an eclipse in late

December one year, you will see eclipses in early December the

next year, and so on.

Once you see a few eclipses, you know when the eclipse

seasons are occurring, and you can predict next year’s eclipse

seasons by subtracting 19 days. New moons and full moons near

those dates are candidates for causing eclipses.

Th e cyclic pattern of eclipses shown in Figure 3-15 gives you

another way to predict eclipses. Eclipses follow a pattern, and if

you were an ancient astronomer and understood the pattern, you

could predict eclipses without ever knowing what the moon was

or how an orbit works.

The Saros Cycle

Ancient astronomers could predict eclipses in an approximate

way using eclipse seasons, but they could have been much more

accurate if they had recognized that eclipses occur following cer-

tain patterns. Th e most important of these is the saros cycle

Th is makes eclipse prediction easy. All you have to do is keep
track of where the moon crosses the ecliptic (where the nodes of
its orbit are). Th is system works fairly well, and ancient astrono-
mers such as the Maya may have used such a system. You could
have been a very successful ancient Mayan astronomer with what
you know about eclipse seasons, but you can do even better if
you change your point of view.

The View from Space

Change your point of view and imagine that you are looking at the
orbits of Earth and the moon from a point far away in space. You
can imagine the moon’s orbit tipped at an angle to Earth’s orbit. As
Earth orbits the sun, the moon’s orbit remains fi xed in direction.
Th e nodes of the moon’s orbit are the points where it passes
through the plane of Earth’s orbit; an eclipse season occurs each
time the line connecting these nodes, the line of nodes, points
toward the sun. Look at ■ Figure 3-14a and notice that the line of
nodes does not point at the sun in the example at lower left, and no
eclipses are possible; the shadows miss. At lower right, the line of
nodes points toward the sun, and the shadows produce eclipses.
Th e shadows of Earth and moon are long and thin, as shown
in ■ Figure 3-14b. Th at is why it is so easy for them to miss their
mark at new moon or full moon and fail to produce an eclipse.
Only during an eclipse season do the long, skinny shadows pro-
duce eclipses.
If you watched for years from your point of view in space,
you would see the orbit of the moon precess like a hubcap

Node New moon

Moon’s orbit

Sun Ecliptic

5 °09'

Moon’s orbit

Ecliptic

Earth’s umbral shadow

Full moon

Node

ab

■ Figure 3-13

Eclipses can occur only when the sun is near one of the nodes of the moon’s orbit. (a) A solar eclipse occurs when the moon meets the sun near a node.
(b) A lunar eclipse occurs when the sun and moon are near opposite nodes. Partial eclipses are shown here for clarity.