Lunar Eclipse to Be Visible at Sunset : Astronomy: Earth’s shadow is expected to be especially dark because of ash in the atmosphere from volcanic eruptions.

Weather and volcanic dust permitting, Southern Californians will be able to see the moon rise partially in Earth’s shadow tonight as the East Coast glimpses the first total lunar eclipse visible in North America since 1989.

When the moon pops above the northeast horizon in Los Angeles at 4:41--three minutes before sunset--a shadow should still be covering its northeastern part.

This partial eclipse will end at 5:30, a few minutes after twilight begins, although the faint penumbral eclipse will be visible until after 6 p.m. The penumbra is the lighter, outer portion of Earth’s shadow.

The eclipse should be plainly visible to the naked eye, although binoculars or telescopes would enhance the effect. The city’s Griffith Observatory in the Hollywood Hills will be open to the public, but a crowd is expected and people are encouraged to arrive early.

Unlike an eclipse of the sun, a lunar eclipse presents no danger to the eyes.

Amateur astronomers will study Earth’s outline on the moon to see if recent volcanic eruptions--the Philippines’ Mt. Pinatubo and Chile’s Mt. Hudson last year and Alaska’s Mt. Spurr in August--will distort its shadow.

In the past, dust and ash pushed into the atmosphere by volcanoes have been blamed for misshapen Earth shadows on the moon.

After a partial lunar eclipse in Canada last June, for instance, an amateur astronomer reported seeing a squared-off shadow on the moon. Sky and Telescope magazine recounted other odd Earth shadows as far back as 1886, each after a major volcanic eruption in which dust is distributed unevenly into the stratosphere.

At the very least, astronomers expect the volcanic dust to make the eclipse darker than normal. Total eclipses ordinarily shade the moon to a dark red disc because the only sunlight to hit it is filtered through and refracted by the Earth’s atmosphere. However, this eclipse is expected to almost blacken the moon because airborne ash and dust should block most sunlight.

Black would be all the more appropriate for a Florida astronomer seeking to name this celestial event the “Shame on You, Columbus” eclipse.

Jack Horkheimer, executive director of the Miami Space Transit Planetarium, wants to use this eclipse to wind up the quincentenary of Christopher Columbus’ arrival in North America, an event that critics blame for what has been described by some as the subsequent genocide of American Indians by Europeans.

The eclipse was chosen not only because of its occurrence at the end of the anniversary year, but because Columbus was believed to have used a lunar eclipse to extort food from frightened natives of what is now Jamaica during his fourth visit to the Caribbean in 1504.

Columbus, of course, was not alone in exploiting a fortuitously timed lunar eclipse. Turkish Sultan Mohammed II, for instance, conquered Constantinople in 1453 after an eclipse dispirited the city’s beleaguered defenders. And British Army Lt. Thomas E. Lawrence, “Lawrence of Arabia,” repeated the trick in the famous battle for Aqaba in 1917.

The worst consequence likely to emerge from this evening’s eclipse would be a freeway fender-bender as rush-hour drivers crane their necks to sneak a look at the partially obscured moon through their windshields or in their rear-view mirrors.

Such risks will hardly be worthwhile--North America will experience two more total lunar eclipses in 1993. The first, on June 4, will be the exact opposite of tonight’s event: The midsummer eclipse will only be visible on the West Coast and will occur at dawn instead of dusk.

The second eclipse next year will be plainly visible throughout the nation, directly overhead and in the middle of the night of Nov. 28 and early Nov. 29.

Three lunar eclipses so close in time is unusual. Eclipses usually occur on average every two or three years.

A total lunar eclipse occurs when Earth passes between the moon and the sun and casts a shadow on its natural satellite. The effect is enhanced because it can occur only when the moon is full, or directly opposite the sun as observed on Earth.

Because the sun is so much larger than a planet, Earth casts both an umbra, or full shadow, and a penumbra, or partial shadow.

On the East Coast of Canada and the United States, where the eclipse can be observed in its entirety, Earth’s shadow will appear to move slowly across the moon from west to east.

The Shadow of Things to Come

Weather permitting, Southern Californians will be able to watch the moon rise in partial eclipse tonight. A total eclipse will not be visible here because moonrise will occur too late. We will see only the late, partial phases.

As does a prism, Earth’s stratosphere receives sunlight and divides it into the colors of the visible spectrum. Red is the least distorted color and the only one to exit the atmosphere. In doing so, it reddens the Earth’s shadow and casts a red light on the lunar surface.

Penumbra: Lighter, outer part of Earth’s shadow

Umbra: Dark, inner part of shadow.

Umbra’s diameter: 3,728 miles

Where and When

* Moonrise will begin at 4:41 p.m.

* The entire moon will be visible around 5 p.m., just above the horizon. For the best viewing, find an unobstructed view to the northeast.

* The eclipse will end around 6:30 p.m.

Eclipse Talk

* Total lunar eclipse: Moon moves completely into the umbra, and may essentially disappear.

* Partial lunar eclipse: Moon is only partially into the umbra.

How to View

Binoculars will improve your viewing; a telescope will improve it even more.

Up in Smoke

Scientists will be measuring what effect the June, 1991, eruption of the Philippines’ Mt. Pinatubo is having. Because of the tons of volcanic debris blasted into the upper atmosphere, the moon may appear darker than normal as it passes through the Earth’s penumbra.

Mark Your Calendar

If you miss this one, make a note on your calendar: Next year two total lunar eclipses will be visible from Orange County--on June 4 and Nov. 28. After that, you’ll have to wait until 2000 to see a total eclipse.

Sources: Patrick So, Griffith Observatory; Astronomy magazine, December 1992; Audubon Society Field Guide to the Night Sky; 1992 Information Please Almanac; World Book Encyclopedia; NASA

Researched by STEVE LOPEZ / Los Angeles Times

Moon Measured (Orange County Edition, A30)

Age: About 4.5 billion years, same as Earth

Distance from Earth: Closest (called its perigee), 221,456 miles; farthest (apogee) 252,711

Diameter: About 2,160 miles, roughly the air distance from Anaheim to Washington

Circumference: About 6,790 miles, or about one-fourth of Earth’s

Elapsed rotation: 27 days, 7 hours, 43 minutes

Elapsed revolution around Earth: 27 days, 7 hours, 43 minutes

Average revolution speed: 2,300 m.p.h.

Temperature variation: 260 degrees to minus 280 degrees Fahrenheit at the Equator

Gravity: 1/6th of Earth’s

Surface area: 14.7 million square miles (about four times the United States)

Atmosphere: Little or none; no clouds, wind, or rain

Life: None

Number of craters at least one foot wide: about 30,000 billion

Largest crater: Imbrium Basin, 700 miles wide

Highest mountains: Leibnitz range (about 26,000 feet)

What the moon’s made of: Mostly calcium, iron, magnesium, silicon, titanium and aluminum

Why we only see one side: Because it rotates on its axis in the same time it takes to circle Earth.

Anomaly: The moon is the brightest object in the night sky, but does not emit light.

Another anomaly: Nothing lives on the moon, but some plants on Earth grow better when moon dust is added to their soil

Sources: 1992 Information Please Almanac; World Book Encyclopedia; National Aeronautics and Space Administration