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Eclipses Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 16, 2011

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.

Lunar Eclipses

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.

The Moon and the Sun Eclipses Lunar Eclipses

Figure 4-12. Progress of a lunar eclipse. This drawing is not to scale.

Practice problems of this concept can be found at: The Moon and the Sun Practice Problems

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