Vanderbilt University Register : ASK THE EXPERTS

ASK THE EXPERTS

Q: What's the difference between a total solar eclipse and an annular solar eclipse, like the one scheduled to begin around sunset Monday, June 10?

A total solar eclipse (left) occurs when the moon completely covers the sun's disc. An annular eclipse (right) occurs when the apparent size of the moon is smaller than that of the sun.

A: A solar eclipse occurs when a new moon is in direct alignment between the sun and the earth. Although the sun may be 400 times larger than the moon, it is also 400 times farther away from the Earth, often causing both bodies to appear to be the same size, about half a degree wide in the sky. Yet, this distortion is not constant. Due to the elliptical orbits of both the sun and moon, there is a small percentage of discernable alteration within their angular diameters, which allows for the much smaller moon to be able sometimes to totally block the sun's light.

Full blockage, or "total" eclipse, occurs when the moon's angular diameter is greater than or equal to that of the sun and its orbit is closer to the earth, while the sun's is farther away.

However, if at the time of alignment the moon's angular diameter is less than that of the sun's, and its orbit is farther away from the earth while the sun's is closer, a partial, or "annular" eclipse occurs.

The solar eclipse scheduled to occur on June 10 will be an annular one, meaning that around 5:30 p.m., when the eclipse is scheduled to begin, viewers will be able to witness the moon slowly crossing the sun, taking "bites" out of it, until the sun is just a thin crescent. Unfortunately for Nashville, the fully annular sun will not occur until after the time of sunset, making it impossible to view.

Douglas Hall
Professor of Physics and Astronomy

-- Kofi Outlaw

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