Annular Eclipses of the Sun
A Ring Shaped Eclipse
The term "annular" derives from the Latin word "annulus" meaning a ring. Thus an annular eclipse of the sun is a "ring-shaped eclipse". Such an eclipse is a kind of partial eclipse in which the face of the sun is not completely covered by the dark body of the moon. A ring of sunlight is visible around the edge of the eclipsing moon as in the picture at the top of this page.
Total Solar Eclipses are Hard to See
Total eclipses of the sun only occur on average about once every year and a half. During an eclipse at any instant, the area on earth is quite restricted from which the face of the sun appears to be completely covered by the body of the moon. In order to see a total eclipse you must be observing from inside the shadow that moon casts on earth in the light of the sun as in the figure below.
More particularly you must be inside the deepest part of the moon's shadow - the umbra. See the figure below.
The umbra represents deep shadow precisely because the sun is eclipsed, that is, the light of the sun is entirely blocked by the body of the moon.
The Penumbra
The umbra is surrounded by an area of partial shadow called the penumbra. The penumbra represents partial shadow precisely because some sunlight reaches this area. The face of the sun is not completely blocked by the body of the moon. Thus if you are inside the penumbra, you will see the face of the sun only partially covered by the moon. In other words, inside the penumbra, you will see the sun in partial eclipse.
Only the Tip of the Moon's Umbra Brushes the Earth
The umbra of any round object like the moon is a cone that narrows down behind it. Dwellers on earth can see a total solar eclipse only when the moon's umbra is in contact with the surface of the earth. You have to be there inside the umbra to observe the eclipse. This area is quite restricted in size, because even under the most favorable conditions, it is only the very tip of the moon's umbra that contacts the earth.
Looking at this another way, we can observe that the sun and moon seem to have almost exactly the same angular size in the sky, namely about 1/2° or about 30 minutes of arc. Since the two objects are nearly identical in apparent size, you are only going to see the total eclipse if your eye is almost exactly on the line from the center of the sun through the center of the moon.
What a Coincidence!
The equality in apparent size between the sun and moon is really a rather miraculous coincidence. If the moon were only maybe 10% farther away, it would appear 10% smaller in the sky, and would then be too small in apparent size to cover the face of the sun. In that case there would be no such thing as a total eclipse of the sun. We would experience only partial eclipses.
The Earth Moon Distance Varies
It is not only the apparent equality in angular size between the sun and moon that makes it difficult to experience eclipses. Another problem is that the orbit of the moon is not a perfect circle around the earth. Consequently the distance between the earth and moon varies - from 363,000 km at the nearest, that is, at perigee to about 406,000 km at the furthest distance, that is, at apogee.
Eclipses Near Apogee
At or near apogee, the moon is in fact at too great a distance to create a total eclipse of the sun. At apogee, the distance between the earth and moon is greater than the length of the moon's umbra. Thus even though the sun, moon, and earth are perfectly aligned for a total eclipse of the sun, observers on earth will see a partial eclipse, because the umbra does not contact the earth. See below.
Observers directly beneath the tip of the moon's umbra will experience an annular eclipse of the sun. Because the moon is at a greater distance away, it appears to be of smaller size than normal. It appears too small to completely cover the face of the sun, and a ring of sunlight is observed around the edge of the moon at the time of maximum eclipse.
The image at the top of this page shows the Annular Solar Eclipse May 10, 1994. It was taken by the Solar Optical Observing Network (SOON) telescope operated by the U.S. Air Force at Holliman Air Force Base, New Mexico.
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