Why eclipses occur

When the moon passes in front of the sun and blocks it completely, it casts a shadow onto our planet that plunges areas into a darkness that feels like nighttime in mid-day.

What causes this extraordinary phenomenon? Several things need to occur at once to create a total solar eclipse. First, the moon needs to be exactly the right size to block the sun. Next, the sun, moon, and Earth need to line up in a straight line, or nearly so, with the moon between the sun and Earth. Finally, to see the full eclipse, you need to be standing in the correct spot on Earth.

Lucky for us, our moon is the perfect size. The motions of sun, moon, and Earth bring the three bodies into the correct alignment about once every 18 months. These are the times when eclipses can happen. As the daytime moon passes in front of the sun, it begins to cast a partial shadow called the penumbra onto Earth. During a total solar eclipse, the moon casts its umbra upon Earth's surface; that shadow can sweep a third of the way around the planet in just a few hours.

Those who are fortunate enough to be positioned in the direct path of the umbra will see the sun's disk diminish into a crescent as the moon's dark shadow rushes toward them across the landscape. During the brief period of totality, when the sun is completely covered, the beautiful corona — the tenuous outer atmosphere of the sun — is revealed. Totality may last as long as 7 minutes 31 seconds, though most total eclipses are usually much shorter.

A partial solar eclipse occurs when only the penumbra the partial shadow passes over you. In these cases, a part of the sun always remains in view during the eclipse. How much of the sun remains in view depends on the specific circumstances.

Usually the penumbra gives just a glancing blow to our planet over the polar regions; in such cases, places far away from the poles but still within the zone of the penumbra might not see much more than a small scallop of the sun hidden by the moon.

In a different scenario, those who are positioned within a couple of thousand miles of the path of a total eclipse will see a partial eclipse. The closer you are to the path of totality, the greater the solar obscuration. If, for instance, you are positioned just outside of the path of the total eclipse, you will see the sun wane to a narrow crescent, then thicken up again as the shadow passes by.

An annular eclipse, though a rare and amazing sight, is far different from a total one. The sky will darken The annular eclipse is a subspecies of a partial eclipse, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds. However, an annular solar eclipse is similar to a total eclipse in that the moon appears to pass centrally across the sun. The difference is, the moon is too small to cover the disk of the sun completely. Because the moon circles Earth in an elliptical orbit, its distance from Earth can vary from , miles to , miles.

But the dark shadow cone of the moon's umbra can extend out for no longer than , miles; that's less than the moon's average distance from Earth. So if the moon is at some greater distance, the tip of the umbra does not reach Earth. During such an eclipse, the antumbra , a theoretical continuation of the umbra, reaches the ground, and anyone situated within it can look up past either side of the umbra and see an annulus, or "ring of fire" around the moon.

A good analogy is putting a penny atop a nickel, the penny being the moon, the nickel being the sun. These are also called annular-total "A-T" eclipses. This special type of eclipse occurs when the moon's distance is near its limit for the umbra to reach Earth. In most cases, an A-T eclipse starts as an annular eclipse because the tip of the umbra falls just short of making contact with Earth; then it becomes total, because the roundness of the planet reaches up and intercepts the shadow tip near the middle of the path, then finally it returns to annular toward the end of the path.

Because the moon appears to pass directly in front of the sun, total, annular and hybrid eclipses are also called "central" eclipses to distinguish them from eclipses that are merely partial. Eclipses do not happen at every new moon, of course. This is because the moon's orbit is tilted just over 5 degrees relative to Earth's orbit around the sun. For this reason, the moon's shadow usually passes either above or below Earth, so a solar eclipse doesn't occur.

But as a rule, at least twice each year and sometimes as many as five times in a year , a new moon will align itself in just such a way to eclipse the sun. That alignment point is called a node. Depending on how closely the new moon approaches a node will determine whether a particular eclipse is central or partial. And these alignments don't happen haphazardly, for after a specific interval of time, an eclipse will repeat itself or return. This interval is known as the Saros cycle and was known as far back as the days of the early Chaldean astronomers some 28 centuries ago.

After this interval, the relative positions of the sun and moon relative to a node are nearly the same as before. That third of a day in the interval causes the path of each eclipse of a series to be displaced in longitude a third of the way around Earth to the west with respect to its predecessor.

For example, on March 29, , a total eclipse swept across parts of western and northern Africa and then across southern Asia. One Saros later, on April 8, , this eclipse will recur, except instead of Africa and Asia, it will track across northern Mexico, the central and eastern United States and the Maritime provinces of Canada. As a solar eclipse approaches, the mainstream media often will provide a variety of warnings and advisories against looking at the sun with bare eyes, as blindness could ensue.

This has given most people the idea that eclipses are dangerous. The sun constantly emits invisible infrared rays that can damage your eyes. Ordinarily, we have no reason to gaze at the sun. An eclipse gives us a reason, but we shouldn't. By far, the safest way to view a solar eclipse is to construct a "pinhole camera. Binoculars or a good telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card.

The Moon orbits Earth; both swing around the Sun. Both the Sun and Moon vary in angular size throughout the year Sun or month Moon. The Moon's variation is more pronounced. Our planet is closest to the Sun perihelion in early January and farthest aphelion in early July. When near perigee, the Moon can easily cover the entire solar disk and create a total solar eclipse.

But at apogee the Moon is too small to cover all of the Sun's brilliant face. At mideclipse an annulus ring of sunlight surrounds the lunar silhouette, resulting in an annular eclipse. This path is called the ecliptic, for reasons that will become obvious in a moment.

More often than not, the new Moon passes above or below the Sun, and the lunar shadow misses Earth completely. But every