Residents in parts of the United States will have a chance to watch the Moon partially eclipse the Sun on Friday, April 8. Within a very narrow corridor that extends for about 8,800 miles, the disks of the Sun and the Moon will appear to exactly coincide, setting up the most unusual type of eclipse known as a hybrid.

Solar eclipses are caused when Earth, the Moon and the Sun line up just right and the Moon casts a shadow on our planet.

On rare occasions, the Moon is at such a distance from the Earth that its pointed shadow is just long enough to touch Earth for only a short distance along its projected path. The eclipse is only total where the shadow actually intersects the Earth's surface; at other points along the eclipse track, the Moon appears ever-so-slightly too small to obscure the Sun's face entirely.

From these places an annulus, or ring of the Sun's surface, remains to be seen, thus there is an annular eclipse. In essence, this is really nothing more than a fancy partial eclipse.

The effect is like a dark penny atop a shiny nickel. The Sun becomes a blazing ring of light at maximum effect.

The path of the central eclipse (where the phenomena of annularity-totality may be observed) first touches the Earth in the south Pacific Ocean at 18:54 GMT, just to the south and east of the South Island of New Zealand. The eclipse starts off as annular, with the Moon covering all but 8/10 of a percent of the Sun's disk, leaving only an exceedingly thin, and rapidly narrowing ring of sunlight shining at maximum effect.

Ten minutes later, at 19:04 GMT, the tip of the Moon's dark umbra strikes the Earth about 1,400 miles south of Tahiti, and the eclipse becomes total. No Pacific islands of any appreciable size falls within this narrow path of totality.

Traveling northeast across the vast expanse of the south Pacific, the duration of the total eclipse gradually increases, with the greatest eclipse, featuring all of 42 seconds of a glorious totality, occurring at 20:34 GMT, far out over inaccessible open ocean waters. The totality path will be less than 17 miles wide at this point. The magnitude of the eclipse is 1.007, which means the Moon completely covers the Sun and 0.007 of a Sun-width more.

The total phase then diminishes and the totality track narrows as it nears its end, the path then turning toward the east. At 22:00 GMT, about 500 miles due north of the Galapagos Islands, the tip of the Moon's shadow can no longer reach to the Earth's surface, so the Moon can no longer completely cover the Sun. Thus, it morphs back into an annular eclipse.

The thread of this annular eclipse path makes its first landfall in Central America, at the border of Costa Rica and Panama, over Panama's Azuero Peninsula, barely skimming the southern outskirts of the Panamanian City of David, as well as sweeping over the coastal town of Pedregal.

Interestingly, the silhouette of the Moon is not a perfect circle, but rather it is slightly prickly with mountains, which are relatively much higher than those on Earth.

So just before the transition from annular to total and later, just after the transition from total back to annular, the eclipse will become something neither annular nor total: it will be a broken annular. As lunar mountains protrude onto the hairline-thin ring of the Sun, it will be seen not as an unbroken ring but an irregular, changing, sparkling sequence of arcs, beads and diamonds very briefly encircling the Moon: a "diamond necklace" effect! This is a spectacle that viewers in the Panama and possibly Costa Rica might see.

The path then quickly slides across the base of the isthmus where Central joins to South America, then over the Gulf of Uraba; and on into northern Colombia and central Venezuela before finally coming to an end at local sunset, at 22:18 GMT.

Parts of North America will see this as a partial solar eclipse. However, there will also be locations that will see nothing of this eclipse.

If you have an atlas of the United States, draw a line starting from a point roughly from Imperial Beach, California and extend it northeast to Quincy, Illinois, and then east to Perth Amboy, New Jersey. All places above (or north) of this line will not have any view of the eclipse.

Meanwhile, those localities below (or south) of the line will be able to see at least a part of this eclipse near sunset, although for those places in the immediate vicinity of this line, the Moon's "bite" out of the lower edge of the Sun will be tantalizingly small.

For example, while the eclipse will not be visible from New York City, just 85 miles to the southwest, at Philadelphia, the edge of the Moon's dark silhouette will appear to encroach upon the Sun at 6:07 p.m. ET. Twelve minutes later, maximum eclipse will be attained, with the Moon only obscuring about 2 percent of the Sun's diameter (or just three-tenths of one percent of the total area of the Sun's disk). The "eclipse" -- if we can charitably call it that - will come to an end at 6:32 p.m. ET.

As one heads farther south, the eclipse will last longer and this slight dent will evolve into a more noticeable scallop out of the Sun's left rim.

From Washington, D.C., the eclipse will last 41 minutes, with just over 5 percent of the Sun's diameter covered at 6:19 p.m. ET. Continuing southward, from Raleigh, North Carolina, the eclipse will last 69 minutes from start to finish, the Moon covering a maximum of 15 percent of the Sun's diameter at 6:20 p.m. ET.

Along the Gulf Coast, the eclipse will last about 2½ hours; prospective observers will see anywhere from about 30 to 40 percent coverage, while for those in the Florida Keys, it will be a nearly three hour affair, with the Moon appearing to obscure about half of the Sun's disk.

From San Juan, Puerto Rico, nearly 68 percent of the Sun's diameter will be eclipsed, maximum eclipse coming at 6:22 p.m. AST. Eighteen minutes later, the Sun will drop down below the west-northwest horizon, making for a most unusual sunset!

For full prediction details for many cities are available from NASA.

In addition, NASA astronomer Fred Espenak has a website dedicated to the upcoming April 8 solar eclipse which contains maps, tables and additional prediction details.

To look at the Sun without proper eye protection is dangerous.

Unlike a total eclipse of the Sun, concentrating its excitement into a few fleeting minutes, a partial eclipse can be watched in a relaxed manner from wherever one happens to be. Providing proper protection is employed, bservations can be made with or without telescopes or binoculars. However, looking at the Sun is harmful to your eyes at anytime, partial eclipse or no. Most people are under the mistaken impression that when a solar eclipse is in progress that there is something especially insidious about the Sun's light.

But the true danger that an eclipse poses is simply that it may induce people to stare at the Sun, something they wouldn't normally do. The result can be "eclipse blindness," a serious eye injury that has been recognized at least since the early 1900's. About half of the reported victims of eclipse blindness recover their precious quality of eyesight after a few days or weeks. The other half carries a permanent blurry or blind spot at the center of their vision for the rest of their lives.

Public warnings by news media have vastly reduced solar eye injuries at eclipses in the last few decades. After the solar eclipse that crossed the United States on March 7, 1970, no fewer than 245 cases of retinal injury were reported. Of these people, 55 percent suffered permanent impairment of vision. In contrast, after the solar eclipse of May 30, 1984, Sky & Telescope magazine was able to locate only three cases of eclipse blindness in the entire United States. During any direct observation of the eclipse, your eye or must be protected by dense filters from the intense light and heat of the focused solar rays.

By far, the safest way to view a solar eclipse is to construct a "pinhole camera." A pinhole or small opening is used to form an image of the Sun on a screen placed about three feet 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. Just be sure not to look through the binoculars or telescope when they are pointed toward the Sun!

A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a ¼-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 one inch across for every 9 feet 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.

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 of dollars at welding supply shops. It also used to be widely advertised that two layers of fully exposed and developed black-and-white negative film was safe. This is still true but only if the film contains an emulsion of silver particles. But beware: some black-and-white films now use black dye, which is no longer safe. It is always a good idea to test your filters and/or observing techniques before eclipse day.

Unacceptable filters include sunglasses, 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 the filter, is no guarantee that your eyes are safe.

The next solar eclipse will occur Oct. 3 this year. It will be an annular solar eclipse with a maximum duration of just over 4½ minutes that will sweep across the Iberian Peninsula and stretches across the African Continent. Madrid, Spain finds itself directly in the center of the annular eclipse track and will see the mid-morning Sun turn into a blazing ring of fire for over four minutes.

But the next solar eclipse visible over a large swath of North America won't come until May 20, 2012, when the path of an annular solar eclipse passes across portions of eight southwestern states.

• How to Safely View the Sun
• How Solar Eclipses Occur
• Solar Eclipse Facts
• Night Sky Main Page: More Skywatching News & Features

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