Annular vs. Total Solar Eclipses
- The angular sizes of the Moon and the Sun as seen from the Earth are nearly identical.
- The Moon just barely covers the Sun during a total eclipse
- Only a few select places on the Earth can see the Sun completely covered.
- The Moon's orbit is not circular. Its distance from the Earth
varies depending on where it is in its orbit.
- When the Moon is close to the Earth (perigee) its angular size is large
and it easily covers the Sun completely. This produces a total
- At the eclipse center it becomes very dark and the stars and
Sun's corona become visible.
- When the Moon is far from the Earth (apogee) its angular size is small
-- too small to completely cover the Sun -- and a ring of light is
left even when the Moon is centered on the Sun's disk. This produces an
- In this configuration the apex of the Moon's umbral shadow does not reach down to the Earth.
- Since a portion of the Sun's disk remains visible it does
not become dark enough to see the Sun's corona. A few bright planets might
- The Earth's orbit is also not circular and thus the angular size
of the Sun changes depending on our distance.
- The best total eclipses occur when the Moon is closest to the
Earth (perigee) and the Earth is farthest from the Sun (aphelion).
- In this configuration the Moon's umbral shadow is large and
can take 7 minutes to sweep across a particular spot -- 7 minutes of total eclipse!
- When the Earth is close to the Sun (perihelion) and the
Moon is far from the Earth (apogee) annular eclipses occur.
- The tidal coupling between the Earth and Moon is moving the Moon further from the Earth (about an inch each year). In a few tens of millions of years total solar eclipses will no longer occur!
Updated September 25, 2002