Why Sunday's Solar Eclipse 'Ring of Fire' Will Be Rare Sight
As the solar eclipse on May 20, 2012, progresses, its partial and annular phases will look very similar to this eclipse on May 10, 1994
Credit: Fred Espenak/SkyandTelescope.com

When the moon blots out the sun in a solar eclipse on Sunday (May 20), producing a "ring of fire" in the sky, it will cover roughly 94 percent that of the sun and offer a rare view of our nearest star.

The remaining sun ring visible at the peak of the solar eclipse will  appear with a width of 3 percent of the sun’s diameter. While that might not sound like a lot, it will still be enough to drown out the beautiful solar corona, as well as the sun’s ruby red chromosphere and any prominences — such as plumes of hydrogen gas, leaping off of the limb of the sun.

Meanwhile, the sky (while getting somewhat dimmer) will never darken enough to see any bright stars or planets. All of these sights are reserved solely for a total eclipse.

Or are they?

As it turns out, we can have a wide variety of annular solar eclipses and that ring of fire can appear quite different depending on specific circumstances, namely the distance of the Earth to the sun and of the moon to the Earth. 

In fact, if the Earth revolved around the sun in a perfect circle, at its mean distance of 92.9 million miles (149.5 million kilometers), and the moon revolved around the Earth in a perfect circle equal to its mean distance of 238,857 miles (384,320 km), then a total eclipse of the sun would be impossible, because the moon as seen from the Earth would appear too small to completely cover the sun. So we would always see an annular or ring eclipse.

Fortunately, the moon can come closer to Earth than its mean distance. As a result, there are times when it will be large enough to totally eclipse the sun. In fact, there are occasions when the moon will be at a point in its orbit where its apparent diameter literally matches that of the sun (or very nearly so). The result is an annular eclipse that displays some of the characteristics of a total eclipse. [Annular Solar Eclipse of May 20 (A Photo Guide)]

Skinny ring: Some of our readers may remember an annular eclipse that swept across parts of the Southeast and Mid-Atlantic States on May 30, 1984. On that day, the tip of the moon’s dark umbral shadow missed the Earth by less than 500 miles (800 km) and the moon’s disk had an apparent diameter less than two-tenths of 1 percent that of the sun. 

And because of the moon’s rugged topography, instead of a ring, observers who were positioned along the eclipse track that day (which included Atlanta and Greensboro, N.C.) saw an exceedingly thin ring, punctuated here and there by bright beads of sunlight. Those who blocked out the beads with their thumb could actually glimpse the corona. And even though it was only a few degrees from the sun, Venus popped into view. 

Because the ring was so delicately thin, annularity did not last very long – less than 10 seconds.

This NASA graphic of the  United States depicts the path of the annular solar eclipse of May 20, 2012, when the moon will cover about 94 percent of the sun's surface as seen from Earth.
This NASA graphic of the United States depicts the path of the annular solar eclipse of May 20, 2012, when the moon will cover about 94 percent of the sun's surface as seen from Earth.
Credit: NASA/JPL, Jane Houston Jones

Fat ring: In contrast, on Dec. 24, 1973, an annular eclipse swept across parts of Mexico, Central America and Africa. It occurred around the time that the moon was at its most distant from Earth (called apogee), so it appeared to be very small. But it also took place near the time that Earth was near its closest point to the sun (called perihelion), so the sun appeared larger than normal. 

In the end, the moon appeared 9 percent smaller than the sun, creating a very "fat" ring nearly 5 percent of the sun’s diameter in width and causing the ring phase to last  almost 12 minutes for some. With so much of the sun left uncovered, the sky didn't get overly dim; some who witnessed it later said that if you didn't know in advance that an eclipse was taking place, you wouldn’t have noticed it at all. [Quiz: How Well Do You Know the Sun?]

Sunday’s eclipse: A typical annular

Sunday’s eclipse falls in the middle of these two extremes. 

The moon will be near its apogee point, making it appear rather small, but the sun is also approaching its farthest point from Earth (aphelion) in early July, making it appear somewhat smaller than normal and thus compensating for the moon’s smaller than normal size. So the ring will not be as wide as it was in 1973, but it will be nowhere near as thin as it was when it was almost total in 1984.

Next year: Interestingly, next year on Nov. 3, 2013, there will be a very special kind of solar eclipse — a "hybrid eclipse."

The hybrid solar eclipse will start as an annular over the Atlantic Ocean about midway between Bermuda and Puerto Rico then transition into a total eclipse as the moon’s shadow heads for Africa. That eclipse will result in a partial eclipse at sunrise for the U.S. East Coast.  And for some places, like Boston (which will not see any of this Sunday’s solar show), it will be their first solar eclipse since the year 2000. 

Mark your calendars!

Editor's note: If you snap any amazing solar eclipse photos that you'd like to be considered for use in a story or gallery, please send images and comments to SPACE.com managing editor Tariq Malik at tmalik@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.