A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth. A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and Earth and its shadows fall upon Earth’s surface.
But whether the alignment produces a total solar eclipse, a partial solar eclipse or an annular solar eclipse depends on several factors, all explained below.
The last solar eclipse was an annular eclipse on May 9, 2013. The next solar eclipse will be a hybrid eclipse on Nov. 3, 2013. It will be visible from eastern North America, southern Europe and Africa.
The fact that an eclipse can occur at all is a fluke of celestial mechanics and time. Since the moon formed about 4.5 billion years ago, it has been gradually moving away from the Earth (by about 1.6 inches, or 4 centimeters per year). Right now the moon is at the perfect distance to appear in our sky exactly the same size as the sun, and therefore block it out. But this is not always true.
Be careful: Watching a solar eclipse requires some safety measures. While it’s perfectly safe to view a total solar eclipse — when no direct sunlight is reaching your eye — you should never look directly at the sun itself. See more on this below, as well as a list of upcoming solar eclipses.
There are four types of solar eclipses: total, annular, partial and hybrid. Here’s what causes each type:
Total solar eclipses
These are a happy accident of nature. The sun's 864,000-mile diameter is fully 400 times greater than that of our puny moon, which measures just 2,160 miles. But the moon also happens to be about 400 times closer to the sun than the Earth (the ratio varies as both orbits are elliptical), and as a result , when the orbital planes intersect and the distances align favorably, the new moon can appear to completely blot out the disk of the sun.
There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is completely blocked out and takes the shape of a dark, slender cone. It is surrounded by the penumbra, a lighter, funnel-shaped shadow from which sunlight is partially obscured.
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 Earth 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. On the average a total eclipse occurs somewhere on Earth about every 18 months.
Partial solar eclipses
On Jan. 4, 2011, the joint Japanese-American Hinode satellite captured breathtaking images of an annular solar eclipse.
[Pin It] On Jan. 4, 2011, the moon passed in front of the sun in a partial solar eclipse - as seen from parts of Earth. Here, the joint Japanese-American Hinode satellite captured the same breathtaking event from space. The unique view created what's called an annular solar eclipse.
A partial solar eclipse occurs when only the penumbra (the partial shadow) passes you by. 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’re positioned just outside of the path of the total eclipse, you’ll see the sun wane to a narrow crescent, then thicken up again as the shadow passes you by.
Annular solar eclipses
An annular solar eclipse is similar to total eclipses in that the moon appears to pass centrally across the sun, but it’s too small to cover the disk of the sun completely. Because the moon circles the Earth in an elliptical orbit its distance from Earth can vary from 221,457 miles to 252,712 miles. But the dark shadow cone of the moon’s umbra can extend out for no longer than 235,700 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. An annular eclipse, though a rare and amazing sight, is far different from a total one. The sky will darken . . . somewhat; a sort of weird “counterfeit twilight” since so much of the Sun still shows. It is really more of a day, not a night sky; the eclipse is a subspecies of a partial, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds.
Hybrid solar eclipses
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 the Earth; then it becomes total, because the roundness of the Earth 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.
Of all solar eclipses, about 28 percent are total; 35 percent are partial; 32 percent annular; and just 5 percent are hybrids.
Predictions of solar eclipses
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 the Earth’s orbit around the sun. For this reason, the moon’s shadow usually passes either above or below the 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 of course, the moon’s distance from the Earth – and to a lesser degree the Earth’s distance from the sun – will ultimately determine whether a central eclipse is total, annular or a hybrid.
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. The word Saros means “repetition” and is equal to 18 years, 11 1/3 days (or a day less or more depending on the number of leap years that have intervened). 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 the Earth to the west with respect to its predecessor.
For example, on Mar. 29, 2006, a total eclipse swept across parts of western and northern Africa and then across southern Asia. One Saros later, on April 8, 2024, 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.