Lunar 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.

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.

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.

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.

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.

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 eclipse of 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.

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Eclipse Models 

Historical Studies of the Moon

Every civilization has studied the Moon. Not only is the Moon the brightest object in the night sky, its location and appearance changes rapidly from night to night in a predictable way, making it easy and appealing to track. And although it appears to be quite far away, it seems to have a strong, mysterious effect on the Earth, in the form of daily tides. Ancient people watched the Moon each night and learned to predict its phases and movements, even if they didn’t understand the reasons for those motions.

The first person to correctly explain the phases of the Moon is lost in history. By the time Pythagoras wrote in 600 B.C., the ancient Greeks knew that the Moon is spherical and that it revolves around the Earth. The Greeks understood how that motion causes the monthly changes in the Moon’s appearance. In fact, they even had pretty good measurements of the Moon’s relative size and distance from the Earth.

Eclipses, though more intermittent than the monthly cycle, were also predictable by the ancients. As early as 1000 B.C., several cultures were predicting the occurrence of lunar and solar eclipses. The ancient Greeks, again, were among the first to understand what was happening during an eclipse, and they used that understanding to learn more about the natural world. In 270 B.C., Aristarchus used a clever method to measure the size of the Moon. If you assume the Earth’s shadow is about the same size as the Earth (not a bad assumption, since the source of light – the Sun – is so far away), and measure the time the Moon spends traveling through the Earth’s shadow during a lunar eclipse, you can calculate the relative sizes of the Earth and Moon. According to this method, Aristarchus found that the Moon’s diameter is about a third of the Earth’s diameter. This is remarkably close to the modern measurement of 0.272 Earth’s diameter. Once the size of the Moon and its angular size in the sky are known, the small angle formula can be used to measure the distance from the Earth to the Moon.

The Moon is the only celestial body with features that are visible to the human eye. What we call “the man in the moon” was a source of discord among ancient scientists. Some observers suggested that the markings were shadows of mountains and valleys. But this would have meant a heavenly object was “imperfect” by being non-spherical. In Aristotle’s view, the heavens were the source of everything perfect. So Aristotle and others of his school tried to find an explanation of the markings which still permitted the moon to be a “perfect” sphere. What if the Moon was so smooth that it was actually reflecting terrestrial features, like a giant mirror? But in that case, the features on the Moon would change as it revolves around the Earth, and they do not. The favored explanation was that variations in density caused the Moon to appear non-uniform, even though it was perfectly spherical. Eventually, telescopes provided a better picture of the Moon. The mountains, valleys, and craters revealed by telescopes finally made this argument obsolete.

Despite this early controversy about the features on the Moon, no detailed drawings of those features were made until the advent of the telescope. The first to observe the Moon with a telescope and record what he saw was Thomas Harriot, in 1609. He beat Galileo by a few months, but he did not publish his work, so Galileo’s drawings were more widely recognized. Many other mapmakers of the Moon followed, including Johannes Hevelius in 1647. He was the first to show the surface of the Moon as if all the features were lit from one side (which is impossible to see at one time, since the Moon is spherical). He also started the practice of showing every part of the Moon that can be seen from the Earth – which is slightly more than a single hemisphere. This is because small irregularities in the Moon’s orbit make different areas visible to the Earth at different times.

In 1651, a Jesuit astronomer named Giovanni Battista Riccioli published a system of names for features on the Moon, which we still use today. The smooth, dark areas that make up the man in the Moon, he incorrectly called “maria,” or seas. Thus we get the “Sea of Tranquility,” and others. Not until centuries later, when humans actually traveled to the Moon, did we learn the true origins of those “seas.”

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