A partial eclipse of the sun will darken Anchorage skies on Sunday afternoon. According to several astronomical websites, it will start at 3:17 p.m. (ADT) and end at 5:54 p.m.
At its height, at 4:38 p.m., the sun will appear to local viewers as a crescent with the moon obscuring two-thirds of the solar disk.
This will be the first central eclipse of the millennium in the continental United States and the first annular eclipse since May 10, 1994.
NASA defines a central eclipse as "a solar eclipse in which the central axis of the Moon's shadow cone traverses Earth thereby producing a central line in the eclipse track," with semi-shadowed areas falling on either side more or less equally. "Annular" means that, in certain places, the dark disk of the moon will appear to nearly fill the larger disk of the sun, producing a ring -- "annulus" in Latin.
There have been other partial solar eclipses in the U.S. since 2000, including some viewable in Alaska. But they were not "central" in that, as viewed from the ground, the moon was not centered in the sun like a small dish set on a larger one.
While Anchorage and the rest of Alaska will see the eclipse with the moon off-center from the sun, the full ring effect of the annular phase will be visible from Taiwan to Lubbock, Texas in an arc across the North Pacific south of Alaska. The maximum coverage of the sun -- hence the thinnest ring -- will be seen just south of uninhabited Amatignak Island in the Aleutians, about 125 miles southwest of Adak and the southernmost point in the Alaska.
Most of the western U.S. and Canada will experience a partial shadow similar to what Anchorage viewers will see, weather permitting. The effect will extend as far east as Atlanta, Ga., which may catch some of the eclipse as night falls. Atlantic coast locations, including Washington, D.C. and Boston, will miss it.
NASA warns, "It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques." Even a slim crescent or ring of sun can cause permanent retinal damage.
NASA offers projection as a safe and inexpensive method for viewing. A pinhole or small opening in a card is used to cast the image of the sun on a surface 18 inches or more beyond the opening.
"Projected images of the sun may even be seen on the ground in the small openings created by interlacing fingers, or in the dappled sunlight beneath a leafy tree," NASA said. "Binoculars can also be used to project a magnified image of the sun on a white card, but you must avoid the temptation of using these instruments for direct viewing."
Special filters, usually with a thin surface layer of aluminum, chromium or silver may be used. One of the most widely available filters for safe solar viewing is a No. 14 welder's glass, available through welding supply outlets," NASA said.
"More recently, aluminized mylar has become a popular, inexpensive alternative. Mylar can easily be cut with scissors and adapted to any kind of box or viewing device."
For those worried about missing precious sunbathing hours, be patient. Once the eclipse ends around 6 p.m., you'll still have five hours before sunset Sunday (10:52 p.m.). And twilight will linger past midnight.
Sunday's solar event is part of a long-observed series of 73 related eclipses that began on Aug. 29, 984 A.D. and is calculated to end on Nov. 1, 2282. Called Saros cycle 128, the series repeats every 18 years and just under 11 days.
"Saros" comes from a Sumerian counting word for the number of bricks needed to make a proper Babylonian garden.
Babylonian mathematicians were among the earliest people to record patterns in eclipses. The word "saros" to describe those patterns was first used circa 1691 by English astronomer Edmond Halley, who also calculated the return of the comet now named after him.
As applied by Halley, one saros is the amount of time after an eclipse when the sun, Earth and moon return to approximately the same relative positions and produce a nearly identical eclipse.
There are at least 90 distinct saros cycles in play for the next 2,000 years or so.
One half of a saros is a "sar."
Reach Mike Dunham at email@example.com or 257-4332.