A total lunar eclipse graces the evening sky on May 15, 2003.

On average, total lunar eclipses visible from any given locale occur about once every two years. Luckily for us, the year 2003 features two of them. Both eclipses take place (coincidentally and auspiciously) during the early-evening hours. The first one occurs Thursday, May 15, while the second one follows a half year later on November 8. For those who follow astronomical events, these two eclipses are regarded as the two most significant predictable celestial spectacles of the year.

Lunar eclipses occur during certain full moons, whenever there is a sufficiently precise linear alignment of the sun, planet Earth, and the moon. The requisite alignment occurs during three-week periods approximately six months apart. Only during those periods is it possible for the moon to partially or wholly pass through the cone-shaped shadow of Earth cast by the sun. If the alignment is somewhat inexact, the moon will only graze the shadow and a partial lunar eclipse will result. With a better alignment, the eclipse will include a total phase -- "totality" -- during which the moon can lie entirely within the shadow.

As observed locally, tonight's eclipse begins with a progression of diminishing partial phases, starting before sunset and ending at 8:14 p.m. in evening twilight. We may see none of these preceding partial phases -- or perhaps (depending on sky conditions) only the latter few minutes of them. Look for a thinning sliver of moon hovering low over the eastern horizon -- opposite the point where the sun has set.

Totality for this eclipse lasts 52 minutes, 8:14 to 9:06. During that period, the sky darkens while the moon steadily ascends in the east. By 8:30 or 8:45, it should be dark enough to spot the totally eclipsed moon with the naked eye. It should appear faint and reddish, glowing with a light some 10,000 to 100,000 times dimmer than normal full-moon illumination. The explanation for the reddish glow is this: A small fraction of the sun's light skims through Earth's atmosphere and is refracted (bent) into Earth's shadow. Because of its long passage through air, the light is reddened, like the reddish light we see from a setting sun. Once inside the shadow cone, the faint, red-tinged light continues toward the shadowed moon, and the moon bounces a fraction of it back to Earth.

When totality ends at 9:06, you'll spot a sliver of pure white sunlight making landfall along the edge of the moon. The closing partial phases ensue, ending at 10:17.

Residents of urban San Diego County may need to travel 20 to 40 miles east to position themselves above the low, gray "marine layer" clouds that typically move onshore during May afternoons or evenings. The spacious roadside turnouts along Sunrise Highway, one to two miles north of Interstate 8 in the Laguna Mountains, are good destinations -- assuming the skies are cloud-free above the marine layer. Bring binoculars -- they will enhance your view of the eclipse tremendously.

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