Eclipses Help (page 2)
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 .
Total And Annular Solar Eclipses
The length of the Moon’s umbral shadow is almost exactly the same as the mean distance between Earth and the Moon. Thus, when the Moon passes between Earth and the Sun, the tip of the umbra sometimes reaches Earth’s surface, but not always. If the Moon is at perigee and Earth is at aphelion when a solar eclipse takes place, the Moon is at its largest possible angular diameter, whereas the Sun is at its smallest. This results in a spectacular total solar eclipse , and, under ideal conditions, it can last about 7 minutes.
The worst conditions for eclipses of the Sun occur when the Moon is at apogee and Earth is at perihelion. Then the Moon is at its smallest possible apparent size, and the Sun is at its largest. In this case, a total eclipse does not occur anywhere on Earth. For observers fortunate enough to have the tip of the Moon’s umbral shadow pass overhead, an annular eclipse takes place. The term annular means “ring-shaped,” a good description of the appearance of the Sun during such an event. During an annular eclipse, the landscape and sky appear as they would in ordinary daylight through dark sunglasses.
Earth, like the Moon, casts a shadow into space, but Earth’s shadow is longer and wider. The full Moon usually misses Earth’s shadow, but it sometimes enters the penumbra, and once in while it makes it into the umbra. If the Moon passes into Earth’s penumbral shadow, we usually don’t notice anything; for a few hours the full Moon might shine a little less brightly than it ought, but that is all. However, if part or all of the Moon moves into Earth’s umbral shadow, a lunar eclipse occurs, and anyone on Earth who can view the Moon will see the eclipse.
When the Moon first begins to enter Earth’s umbral shadow, the darkness “takes a bite out of the Moon” in much the same way as the Moon obscures the Sun during the beginning of a solar eclipse. The Moon might pass beneath or above the shadow core, so darkness never covers the Moon completely; in these cases we see a partial lunar eclipse . At mideclipse, the Moon seems to either “smile” or “frown” depending on which side of the umbra it passes.
The diameter of Earth’s umbral shadow at the Moon’s distance (381,000 kilometers, or 237,000 miles, on average) is several times the diameter of the Moon itself. For this reason, there is a fair chance that the Moon will plunge entirely into the umbra for a time, causing a total lunar eclipse (Fig. 4-12). These happen more often than total solar eclipses occur on Earth. The Moon rarely goes black during totality but exhibits a dim coppery or rusty glow caused by sunlight passing through Earth’s atmosphere; the redness occurs for the same reason that some sunrises or sunsets appear red. An observer on the Moon would see a total solar eclipse of a truly alien sort. Imagine it: a thin red, orange, and yellow ring hanging in a black sky filled with unblinking stars, the Moonscape aglow as if with energy of its own.
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