SOLAR AND LUNAR ECLIPSES
Eclipse Geometry and Eclipse Seasons
The geometries for solar and lunar eclipes are:
We see that we can have solar eclipses only during the new moon
phase and that lunar eclipses will only occur during the full
moon
phase. But, wait a minute. New moon and full moon occur every month. We
don't have eclipses every month; we have eclipse seasons which occur
roughly every 6 months. Why?
The answers are simple.
- The orbital plane of the Moon is not in the
ecliptic plane. The orbital plane of the Moon is inclined 5 degrees or
so
with respect to the ecliptic plane. This has the following consequence.
If
new (or full) Moon occurs when the Moon is not passing through the
ecliptic
plane then the shadow formed will not strike the Earth (Moon).
- The points where the Moon passes through the ecliptic plane are
referred
to as nodes. If we draw a line through the nodes, we define what
is
called the line of nodes. Eclipses will occur only when the line
of
nodes points roughly toward the Sun. The direction in which the line of
nodes
points is roughly fixed, i.e.,
which means that the line of nodes will point toward the Sun every 6
months
or so. (Actually the period is shorter than 6 months--the eclipse year
is 346.6 days long. What does this imply
about the line of nodes?)
Types of Eclipses
We see that there are two types of shadows cast. In the darkest region
(the
umbra), the Sun is completely blocked out. However, if you are in
the outer shadow (the penumbra), you can see part of the Sun.
- Solar Eclipses:
- umbra ===> total solar eclipse
- penumbra ===> partial solar eclipse
- umbra doesn't reach Earth ===> annular eclipse
- Lunar Eclipses:
- umbra ===> total lunar eclipse
- part of umbra ===> partial lunar eclipse
- penumbra ===> penumbral eclipse
In general, far more people have seen total lunar eclipses than total
solar eclipses. Why is this true?
Upcoming Solar Eclipses
- 1998, Aug 22 -- Indian Ocean, East Indies, Pacific; annular
- 1999, Feb 16 -- Indian Ocean, Australia; annular
- 1999, Aug 11 -- Atlantic, Europe, Middle East, India; total
