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 have eclipse seasons that occur roughly every 6 months (during eclipse seasons).

Why don't we have eclipses every month?

The answer is straightforward:

• 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 Eclipses

A nice reference: sunearth.gsfc.nasa.gov/eclipse/eclipse.html