What Is a Solar Eclipse?
The Hinode spacecraft captured this stunning image of the maximum solar eclipse on May 20, 2012, which darkened the sky in parts of the Western United States and Southeast Asia, according to NASA.
Credit: Hinode/XRT

Solar eclipses are some of nature's most dramatic celestial performances. Solar eclipses occur when the Earth, moon and sun are aligned in the same plane and the moon passes between the Earth and the sun, partially or completely covering our closest star. The next total solar eclipse will occur on Aug. 21, 2017 — learn more on that below.

There are four types of solar eclipses: total, annular, partial and hybrid. Here's what causes each type:

These are a happy accident of nature. The sun's 864,000-mile (1.4 million kilometers) diameter is fully 400 times greater than that of Earth's moon, which measures just about 2,160 miles (nearly 3,500 km). But the moon also happens to be about 400 times closer to Earth than the sun (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. On the average a total eclipse occurs somewhere on Earth about every 18 months.

There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is blocked out. The umbra 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 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. A total solar eclipse typically only lasts for a few minutes, explained NASA solar astronomer Mitzi Adams of the Marshall Space Flight Center in Huntsville, Ala. The longest solar eclipse lasted about seven minutes, she said in a NASA webchat.

Total eclipses are seen rarely because totality — when the sun appears totally hidden by the moon — only exists along a narrow path on Earth's surface, as opposed to partial eclipses which can be viewed across a much wider region.

The next eclipse will be a total solar eclipse on Aug. 21, 2017.  According to NASA, during this eclipse event, the moon's umbral shadow will first inch across the northern Pacific before crossing the United States from west to east, taking it through parts of: Oregon, Idaho, Montana, Wyoming, Nebraska, Kansas, Iowa, Missouri, Illinois, Kentucky, Tennessee, North Carolina, Georgia and South Carolina. "Only a tiny chip of a tiny fraction of a tiny part of the tiniest southwestern tip of the beard of Montana will see totality," according to Eclipse2017.org.

However, if you're in another state, you may catch a glimpse of a partial eclipse as the moon's penumbral shadow will produce the effect across a larger region covering most of North America.

For the following areas in the U.S., totality starts at the following local times, according to Eclipse2017.org:

  • Beach just north of Newport, Oregon: 10:15 a.m.
  • Madras and Warm Springs, Oregon: 10:19 a.m.
  • Stanley, Idaho: 11:28:18 a.m. MDT
  • Mackay, Idaho: 11:30:19 a.m. MDT
  • Weiser, Idaho: 11:25:18 a.m. MDT
  • Grand Teton National Park, Wyoming: 11:35 a.m.
  • Pavillion, Wyoming: 11:38 a.m.
  • Alliance, Nebraska: 11:49 a.m.
  • Lincoln, Nebraska: 1:02 p.m.
  • Troy, Kansas: 1:05 p.m.
  • Atchison, Kansas: 1:06 p.m.
  • Kansas City, Missouri: 1:08 p.m.
  • Murphysboro, Illinois: 1:19:30 p.m.
  • Makanda, Illinois: 1:20:11 p.m.
  • Carbondale, Illinois: 1:20 p.m. 
  • Marion, Illinois: 1:20:40 p.m.
  • Paducah, Kentucky: 1:22 p.m.
  • Franklin, Kentucky: 1:26:48 p.m.
  • Clarksville, Tennessee: 1:25 p.m.
  • Nashville, Tennessee (at the State Capitol): 1:27 p.m.
  • Clayton, Georgia: 2:35:45 p.m.
  • Bryson City, North Carolina: 2:35:13 p.m.
  • Murphy, North Carolina: 2:34 p.m.
  • Greenville, South Carolina: 2:38 p.m.
  • Charleston, South Carolina: 2:46:22 p.m. 

(Find out where and when the solar eclipse will be visible to you at Eclipse2017.org)

A partial solar eclipse occurs when only the penumbra (the partial shadow) passes overhead. 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 the 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 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. The annular eclipse is a subspecies of a partial eclipse, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds.

An annular solar eclipse on Jan. 4, 2011, as seen by the Hinode spacecraft.
An annular solar eclipse on Jan. 4, 2011, as seen by the Hinode spacecraft.
Credit: Hinode/XRT

On Sept. 1, 2016, an annular eclipse was visible over most of Africa, the southern Arabian Peninsula and much of the Indian Ocean. [See Photos and Videos of the 'Ring of Fire' Solar Eclipse

The next annular solar eclipse will occur on Feb. 26, 2017, according to NASA.

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 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.

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.

Of all solar eclipses, about 28 percent are total; 35 percent are partial; 32 percent annular; and just 5 percent are hybrids.

When viewing a solar eclipse, looking at one through a telescope can be dangerous, Adams warned.

"You have to have a proper filter," she said. "The safest way is through a method called projection, where you actually take the eyepiece out and project the image onto a sheet of paper behind the telescope, without looking at the sun. You move the sheet of paper back and forth until you get a focused image."

Similarly, you can construct a "pinhole camera." A pinhole or small opening is used to form an image of the sun on a screen placed about 3 feet (or about 1 meter) behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. The farther away the card, the larger you can focus the image. Look for sunspots. Notice that the sun appears somewhat darker around its limb or edge. This method of solar viewing is safe so long as you remember not to look through the binoculars or telescope when they are pointed toward the sun; put another way, never look directly at the sun when any part of its blindingly bright surface is visible. [How to Make a Solar Eclipse Viewer

A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a quarter-inch hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun’s face. The farther away from the wall is the better; the image will be only 1 inch across for every 9 feet (or 3 centimeters for every 3 meters) from the mirror. Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror’s optical quality is good enough to project a clean, round image. Of course, don't let anyone look at the sun in the mirror.

If you're around leafy trees, look at the shadow cast by them during the partial phases. What do you see? Is it worth a photograph? You will see scores of partially eclipsed suns projected through pinhole gaps between the leaves. This is caused by diffraction, a property of light. According to Vince Huegele, an optical physicist at the NASA Marshall Space Flight Center, the light rays do not shoot straight by the rim of the gaps, or a pinhole, but bend around the edge. This wave effect creates a pattern of rings that resembles a bull's eye.

Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder’s glass, available for just a few dollars at welding supply shops. Of course, it is always a good idea to test your filters and/or observing techniques before eclipse day.

Unacceptable filters include sunglasses, old color film negatives, black-and-white film that contains no silver, photographic neutral-density filters and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through these types of filters, is no guarantee that your eyes are safe.

There is one time when you can safely look directly at the sun: during a total eclipse, when the sun's disk is entirely covered. During those few precious seconds or minutes, the magnificent corona shines forth in all its glory surrounding the darkened sun; a marvelous fringe of pearly white light. It differs in size, in tints and patterns from eclipse to eclipse. It is always faint and delicate, with a sheen like a pale aurora. It has a variable appearance. Sometimes it has a soft continuous look; at other times, long rays of it shoot out in three or four directions. It may stand out from the disk in filmy petals and streamers. But when the sun begins to again emerge into view, the corona quickly disappears and you’ll need to protect your eyes once again.

Joe Rao, Space.com Skywatching Columnist, contributed to this article.