Introduction to Eclipses
A Fortunate Coincidence
The fact that the Sun and the Moon are almost exactly the same angular size as we see them from Earth is a convenient coincidence, especially when it comes to eclipses of the Sun. The Moon is just about exactly the right size to blot out the Sun’s disk but little or none of the surrounding space, allowing us to see, during total solar eclipses, features of the Sun that would otherwise be invisible except from outer space. These include the corona , a pearly white mane of glowing, rarefied gases, and solar prominences , which are bright red or orange and look like flames leaping thousands of miles up from the solar surface.
Partial Solar Eclipses
The Moon’s shadow consists of two parts, called the penumbra and the umbra (the same names as are used with sunspots but with different meanings). Any object exposed to sunlight casts a shadow consisting of an umbra and a penumbra. From the point of view of an observer in the Moon’s penumbral shadow, part of the Sun is obscured, but not all. Observers within the Moon’s umbra see the entire solar disk covered up by the Moon.
The umbra of the Moon’s shadow is a long cone extending approximately 395,000 kilometers (245,000 miles) into space opposite the Sun. The penumbra extends much further from the Moon and gets wider and wider with increasing distance, as shown in Fig. 4-11. The penumbra, while much longer than the umbra, does not have infinite length; it fades away as the Moon’s apparent diameter shrinks with respect to that of the Sun. At a distance of several million miles, the penumbra dissipates as the Moon’s disk as viewed against the Sun becomes comparable with the size of a sunspot.
When an observer is located within the penumbra of the Moon’s shadow, a partial solar eclipse is observed. At the edge of the penumbra, the Moon seems to “take a bite out of the Sun.” Further within the penumbra, the Sun’s disk takes the shape of a crescent. The closer to the umbra the observer gets, the slimmer the crescent Sun becomes. However, even a narrow crescent Sun is bright and has the same observed brilliance per unit area as the full Sun. Even with three-quarters of the Sun covered by the Moon, daylight still looks quite ordinary. This is where careless observers get into trouble. The exposed portion of the Sun is likely to cause permanent eye damage if a partial eclipse is viewed directly. This is true even if 99.9 percent of the Sun is obscured. The only safe way to look at an eclipse is the hole-projection method. Punch a 1- or 2-millimeter hole in a piece of cardboard, let the Sun shine through the hole, and look at the Sun’s image on a piece of white paper held in the shadow of the cardboard. Never look at the Sun directly during an eclipse .
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