The Universe in the Classroom

Shadow Play: 1998's Total Solar Eclipse

James C. White II
Astronomical Society of the Pacific

On the morning of Thursday, 26 February, amid the Galápagos Islands, the Sun will fade from view. Not the culmination of ancient prophecy, but none less mystical, Earth's small Moon will be executing a slow slide past the enormous Sun. From the middle Pacific to nearly the western coast of Africa, the Moon's shadow will sweep quickly across a little earth and a lot of sea. And anyone living in southern North America, Central America, northern South America, or on islands and cruise ships in the Caribbean Sea will, weather permitting, have the opportunity to experience the wonder of a solar eclipse.
Hiding behind a mask (almost). The Moon's orbit around the Earth is elliptical, so its distance to our planet changes. When the Moon is more distant, it appears slightly smaller in the sky, and during an eclipse, it may not completely cover the Sun's disk. Conversely, when the Moon is very close to us, it appears appears larger in the sky and can cover more of the Sun. In that situation we would not be able to see the prominence structures. Photo courtesy of NASA.
The dark, inner part of the Moon's shadow, called the umbra, will strike the Earth first in the warm Pacific waters southeast of the Hawaiian Islands and trace a smooth curve across the Pacific until Galápagos landfall. From there the shadow races toward the western coasts of Panama and Colombia, crosses them, and heads through Venezuela before it returns to water as it plunges across the Caribbean Sea. After a fleeting, dark visit to several islands there, the shadow heads out over the Atlantic Ocean, finally leaving the Earth west of Morocco and returning to the cold, familiar darkness of space.

The eclipse's path is determined by the position of the Moon relative to the Earth and by subtleties in the perpetual waltz of these two bodies. Always occurring during a new Moon, a solar eclipse is visible from only limited portions of the Earth due to the inclination of the Moon's orbital plane with respect to the Earth's orbital plane about the Sun (called the ecliptic). At the time of a solar eclipse and when, say, the Moon is slightly above the ecliptic, the Moon's shadow will generally strike regions of the Earth's northern hemisphere. But the Earth is not flat, much as some might still like to believe, and the lunar shadow falls on a slightly squashed, spinning ball. This, combined with the Moon's own orbital motion, culminates in an eclipse path that gracefully curves across the world and a shadow that changes size and shape and speed during its visit. For those individuals who feel the chilling sensation of the dark umbral shadow's passage, the Sun will disappear, obliterated for up to several minutes by the Moon's disk as it covers Sol's face.

But what about those individuals who will not experience totality? They will witness a partial solar eclipse as the outer, or penumbral, shadow crosses them. A person in Coro, Venezuela, just south of the umbra's path, will see a Sun whose face is over 99% obscured, whereas persons at increasing distances from the eclipse path will see decreasing coverage of the Sun's face (e.g., someone in Nashville, Tennessee will view a Sun with 17% coverage). Near but just outside the path, the sky will reach an appearance like that at twilight. Inside the path, however, the Moon will appear to completely consume the Sun, providing anyone there with breathtaking views of the different layers of the solar atmosphere.

Eclipse Diagram

The inner layer of the Sun's tenuous atmosphere, the photosphere, is both the coolest and the brightest. As more and more of this "surface" is hidden by the Moon, the sky dims as Earth spins toward an unnatural twilight. The sky darkening at an ever-increasing rate, the Moon's edge will appear to leap into tendrils of fire and light as the last vestiges of photosphere are masked, its light slipping through mountains and crater rims on the ragged lunar limb. This is the one opportunity for humans to view the next, higher layer of the solar atmosphere, called the chromosphere. Immense plumes of hot gas propelled through the chromosphere appear as tiny luminous threads on the Sun's unraveling edge. But then, as quickly as it flashed into view, the thin, hot chromosphere disappears as it too is covered by the Moon. What is left is the majestic crown worn surreptitiously yet continuously by the Sun. The hottest, highest, and most tenuous portion of Sol's atmosphere is called the corona, and when the photosphere's garish light is shielded during a total solar eclipse, the corona glows in splendid radial streamers that give an eerie glow to the darkened day.

JAMES C. WHITE II is ASP staff astronomer and editor of Mercury magazine. His research concerns cataclysmic variables, and his passions are writing and trying to hear the music of the spheres.

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