Annular Eclipse
Written by Jean Tate
An eclipse of the Sun is when at least part of the Moon is on a line of sight to the Sun (from the Earth). A total eclipse is when a point on the Earth's surface is in the Moon's shadow (and light from the Sun is completely blocked by the Moon). An annular eclipse is when the Moon and Sun are in the same direction (or very close to it) but the Moon appears smaller than the Sun. Such an eclipse is 'annular' because you see a ring of light, an annulus.
Annular eclipses, and solar eclipses in general, can only occur at New Moon. They do not occur at every New Moon because the orbital plane of the Moon (around the Earth) is tilted with respect to the orbital plane of the Earth (around the Sun).
Some other astronomical terms:
When one body appears to move across the face of another, it is called a transit … annular eclipses are transits of the Moon (across the Sun); transits of Venus across the Sun are famous for the black drop effect.
When one body moves into the shadow of another, it is called an occultation … annular eclipses are occultations of the Sun by the Moon; lunar eclipses are occultations of the Moon by the Earth.
If three (or more) bodies in a single system (bound by gravity) line up, it is called a syzygy (what a great Scrabble word! Pity you'd have to use a blank to make it). Annular eclipses are syzygies involving the Sun, Earth, and Moon.
The size of the Moon, as seen from the surface of the Earth, varies, because the Moon's orbit is elliptical; it seems smaller when it's at its apogee. Similarly, the size of the Sun also varies, as the Earth's orbit is also elliptical, being smallest when the Earth is at its apohelion. We see only annular eclipses when the Moon is at, or near, its apogee and the Sun is at, or near, its perihelion (and total eclipses for perigee/apohelion!).
Very occasionally a solar eclipse may be both total and annular, depending on where on Earth you see it; can you work out why?
Want more? Visit this NASA webpage on eclipses. And check out the 26 January Annual Solar Eclipse Photos here in Universe Today, and Annular Eclipse by Martin Whipp.
Filed under: Astronomy
Related stories on Universe Today
- Northern Europe's Annular Eclipse: May 31, 2003
- January 26 Annular Eclipse Photos
- Astrophoto: Annular Eclipse by Christophe Bogaert
- Watch a Solar Eclipse on Saturday
- Annular Eclipse by Martin Whipp
