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# 2.9: Eclipses

For thousands of years, people have observed a phenomenon that is rarer and apparently less predictable than the phases of the Moon or the annual cycles of constellations and seasons. Every generation or so, they saw the Sun blotted out by a solar eclipse, and every year or so they saw the Moon blotted out by a lunar eclipse. The sudden disappearance of the Sun and darkening of the sky was a terrifying spectacle. Ancient Greek warriors were frightened into interrupting their battle by a solar eclipse, and Christopher Columbus used prior knowledge of an eclipse in 1504 to gain control over the inhabitants of the island of Hispaniola. Mark Twain borrowed this idea in his book "A Connecticut Yankee in King Arthur's Court." A lunar eclipse is less spectacular, but can also be a fearful sight when the cause is not understood.

Lunar eclipse. Click here for original source URL.

Although several ancient civilizations had the ability to predict eclipses, they didn’t necessarily understand what they were predicting.  Superstitions surround eclipses, some of which are still observed.  These rare events were seen as bad omens — signs of evil, disease, and death. The ancient Chinese name for eclipse meant “to eat,” and indeed, an eclipsed Moon looks like it’s being devoured, especially when the shadowed Moon turns the color of blood.  Eclipses have been linked in history with earthquakes, although there is no scientific evidence they are related.  Connotations of danger and disease are still evident today — for example, some Japanese cover their wells to protect them from disease. Eskimos turn over their eating utensils, and many cultures bang pots or make loud noises in an attempt to frighten away eclipse-incited evil.

Total solar eclipse, revealing the Sun's corona. Click here for original source URL.

We know today that a solar eclipse occurs when the Moon passes between the Earth and the Sun, such that the Moon’s shadow falls on the Earth. A lunar eclipse occurs when the Earth passes between the Moon and the Sun, such that the Earth’s shadow falls on the Moon.

A full explanation of eclipses is more subtle. For example, why don’t we see lunar and solar eclipses every month? The reason is that the Sun and the Moon don’t travel on exactly the same path through the sky. The Moon’s orbit is tilted by about 5° from the ecliptic, which means that eclipses can only occur during the two times each year that the Moon’s path crosses the ecliptic.

Comparison of angular diameter of the Sun, Moon and planets. To get a true representation of the sizes, view the image at a distance of 102.6 [= 1 / tan(33.5/60*pi/180)] times the width of the largest (Moon: max.) circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.26 m away. Planetary angular diameters are from fact sheets at http://nssdc.gsfc.nasa.gov/planetary/factsheet/ and Sun/Moon ones are from http://education.gsfc.nasa.gov/eclipse/pages/faq.html . Click here for original source URL.

Total solar eclipses occur due to an amazing coincidence: the Sun and the Moon are the same angular size in the sky! Angular size alone is no indicator of true physical size — two celestial objects with very different sizes just happen to be at distances where they both subtend a ½° angle. So when the Moon passes between the Sun and the Earth, the lunar disk can completely cover the Sun.

Geometry of a Lunar eclipse. Click here for original source URL.

If you watch a lunar eclipse closely, first you’ll see the Moon go into partial shadow (the penumbra), and then into full shadow (the umbra).  The penumbra is caused by the finite size of the Sun. It’s an extended disk of light, rather than a point source, so there are areas where only part of the light is blocked. Partial eclipses occur when an object passes through the penumbra, and total eclipses occur when the object is completely within the umbra.  Partial solar eclipses sometimes also occur due to slight variations in the Earth-Sun distance. Sometimes the Moon appears to be slightly smaller than the disk of the Sun.  When this happens, the Moon does not completely cover the Sun, and a ring of light shows around the edge of the Moon.  This is called an annular eclipse.

The blood-red color of the Moon during an eclipse, which was so frightening to ancient observers, can also be explained.  Light from the Sun gets scattered by the Earth’s atmosphere, and only the longer (red) wavelengths fall on the Moon.

You may not have noticed it, but lunar eclipses occur only at full Moon. Why is that? The Moon must be opposite the Sun, as seen from Earth, in order for Earth’s shadow to fall on it. That’s also when the Moon appears fully illuminated, or full. For similar reasons of geometry, solar eclipses happen only at new Moon. The Moon must pass directly in front of the Sun, and its illuminated half then faces towards the Sun, and away from the Earth.

Geometry of a total Solar eclipse. Click here for original source URL.

Why are lunar eclipses so much more common than solar eclipses? The Greeks observed the Earth's curved shadow during a lunar eclipse. By measuring the amount of time the Moon spends in shadow and the shape of the shadow, they could tell the Earth's shadow is much bigger than the Moon. This suggests that the Earth itself is bigger than the Moon. Therefore, it’s more likely for the Moon to pass through the Earth's large shadow than it is for a given point on the Earth to pass through the Moon's small shadow.  Also, the Moon’s shadow is small and tapers nearly to a point, so that only a small area on Earth a few kilometers wide is fully darkened by a total eclipseof the Sun. A solar eclipse only lasts for a few minutes as the Earth's rotation sweeps the shadow across the surface at close to 1000 miles per hour.  But a lunar eclipse can last longer than an hour, and is visible to inhabitants on the entire night side of Earth.