Your guide to an unusual solar eclipse

By Joe Rao

Night sky columnist

updated 4:55 p.m. ET April 8, 2005

Residents in parts of the United States will have a chance to watch the moon partially eclipse the sun on Friday. Within a very narrow corridor that extends for about 8,800 miles (14,000 kilometers), 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 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 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.

What you won't see
The path of the central eclipse (where the phenomena of annularity-totality may be observed) first touches 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 0.8 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 Earth about 1,400 miles (2,240 kilometers) south of Tahiti, and the eclipse becomes total. No Pacific islands of any appreciable size fall 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 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 (800 kilometers) 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.

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