Lunar eclipse

Eclipses come in many forms

solar eclipse
solar eclipse

A solar eclipse is one of Nature’s most awesome spectacles. It occurs when the moon passes in front of the sun (as seen from Earth).

Amazing things happen in the heavens. In the hearts of distant galaxies, black holes swallow stars. Once every 20 years or so, on average, a star somewhere in our Milky Way galaxy explodes. For a few days, that supernova will outshine entire galaxies in our night sky. Near our solar system, things are thankfully quiet.

Nevertheless, awesome events happen in our neighborhood too.

Eclipse means to overshadow. And that’s exactly what happens during a solar or lunar eclipse. These celestial events take place when the sun, moon and Earth briefly make a straight (or very nearly straight) line in space. Then one of them will be fully or partially shrouded by another’s shadow. Similar events, called occultations and transits, occur when stars, planets, and moons line up in much the same way.

Scientists have a good handle on how planets and moons move through the sky. So these events are very predictable. If the weather cooperates, these events easily can be seen with the unaided eye or simple instruments. Eclipses and related phenomena are fun to watch. They also provide scientists with rare opportunities to make important observations. For instance, they can help to measure objects in our solar system and observe the sun’s atmosphere.

Solar eclipses

Our moon is, on average, about 3,476 kilometers (2,160 miles) in diameter. The sun is a whopping 400 times that diameter. But because the sun is also about 400 times further from Earth than the moon is, both the sun and moon appear to be about the same size. That means that at some points in its orbit, the moon can entirely block the sun’s light from reaching Earth. That’s known as a total solar eclipse.

This can happen only when there is a new moon, the phase that appears fully dark to us on Earth as it moves across the sky. This happens about once per month. Actually, the average time between new moons is 29 days, 12 hours, 44 minutes and 3 seconds. Maybe you’re thinking: That’s an awfully precise number. But it’s that precision that let’s astronomers predict when an eclipse will occur, even many years ahead of time.

So why doesn’t a total solar eclipse occur each and every full moon? It has to do with the moon’s orbit. It is slightly tilted, compared to Earth’s. Most new moons trace a path through the sky that passes near to — but not over — the sun.

Sometimes the new moon eclipses only part of the sun.

The moon creates a cone-shaped shadow. The totally dark part of that cone is known as the umbra. And sometimes that umbra doesn’t quite reach Earth’s surface. In that case, people along the center of the path of that shadow don’t see a totally darkened sun. Instead, a ring of light surrounds the moon. This ring of light is called an annulus (AN-yu-luss). Scientists call these events annular eclipses.

annular eclipse
Ring-like annular eclipses (lower right) occur when the moon is too far from the Earth to completely block the sun. In the early phases of this eclipse (proceeding from upper left), it is possible to see sunspots on the face of the sun.

Not all people, of course, will be directly in the center path of an annular eclipse. Those in line with only a portion of the shadow, its antumbra, will see a partially lit moon. The antumbra is also shaped like a cone in space. The umbra and antumbra are lined up in space but point in opposite directions, and their tips meet at a single point.

Why won’t the umbra reach Earth every time there’s a solar eclipse? Again, it’s due to the moon’s orbit. Its path around Earth isn’t a perfect circle. It’s a somewhat squished circle, known as an ellipse. At the closest point in its orbit, the moon is about 362,600 kilometers (225,300 miles) from Earth. At its furthest, the moon is some 400,000 kilometers away. That difference is enough to make how big the moon looks from Earth vary. So, when the new moon passes in front of the sun and is also located in a distant part of its orbit, it’s won’t be quite big enough to completely block the sun.

These orbital variations also explain why some total solar eclipses last longer than others. When the moon is farther from Earth, the point of its shadow can create an eclipse lasting less than 1 second. But when the moon passes in front of the sun and is also at its closest to Earth, the moon’s shadow is up to 267 kilometers (166 miles) wide. In that case, the total eclipse, as seen from any one spot along the shadow’s path, lasts a little more than 7 minutes.

The moon is round, so its shadow creates a dark circle or oval on Earth’s surface. Where someone is within that shadow also affects how long their solar blackout lasts. People in the center of the shadow’s path get a longer eclipse than do people near the edge of the path.

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shadow diagram
Partially lit portions of Earth’s shadow are known as the penumbra and antumbra. The cone-shaped umbra is completely dark. The shadows of all celestial objects, including the moon, are divided into similar regions.

Partial eclipses

People completely outside the path of the moon’s shadow, but within a few thousand kilometers on either side of it, can see what’s known as a partial solar eclipse. That’s because they’re within the partially lit portion of the moon’s shadow, the penumbra. For them, only a fraction of the sun’s light will be blocked.

Sometimes the umbra completely misses the Earth but the penumbra, which is wider, doesn’t. In these cases, no one on Earth sees a total eclipse. But people in a few regions can witness a partial one.

eclipse shadow
The moon’s shadow on Earth’s surface during a total solar eclipse, as seen from the International Space Station on March 29, 2006.

On rare occasions, a solar eclipse will start and end as an annular eclipse. But in the middle of the event, a total blackout occurs. These are known as hybrid eclipses. (The change from annular to total and then back to annular…