Encore eclipse -- This time it's the moon

May 23, 1994|By Frank D. Roylance | Frank D. Roylance,Sun Staff Writer

Nature will be putting on another sky show tomorrow night -- a partial eclipse of the moon. And it's no celestial accident that the eclipse comes just two weeks after the May 10 solar eclipse.

Weather permitting, Marylanders willing to stay up past 11:30 p.m. will be able to watch the bottom of the moon darken as it passes through the edge of the shadow that the Earth casts into space. Only 24 percent of the full moon will be obscured by the top of the Earth's circular shadow.

The entire eclipse will be visible wherever skies are clear in the eastern half of North America, all of Central and South America and in northwestern portions of Africa.

In Maryland, the moon will enter the Earth's shadow at 10:37 p.m. The eclipse will reach its maximum at 11:30 p.m. and end at 12:23 a.m. Wednesday.

It will be the last lunar eclipse visible here for nearly two years. The next one will be a total eclipse on April 4, 1996.

Aside from a possible sore neck, there is no danger associated with watching a lunar eclipse directly or through binoculars or a telescope. And if you don't have your own, the Maryland Science Center at Baltimore's Inner Harbor will let you use theirs.

"Well have our usual staffing out front with telescopes to let people know what's going on," said Joe Kelch, producer for the science center's Davis Planetarium.

Science Center staffers will set up their telescopes at 10:30 p.m. and remain there for visitors until about 12:30 a.m., "if we can stay awake that long," he said.

The planetarium is showing "Eclipse," a 25-minute program that explains the May 10 annular solar eclipse and tomorrow's lunar eclipse. It will run through June 12.

Mr. Kelch said it's not by chance that the lunar eclipse follows so closely on the heels of the May 10 solar eclipse.

"Generally, when you have one eclipse, within two weeks you will have another of the other type," he said.

Here's why:

Think of the moon's orbit as a hula hoop lying on a table. The Earth is on the table at the center of the hoop. The table top represents the plane of the Earth's orbit around the sun. You can think of the sun as a spotlight standing across the room at table-top height.

Now, tilt the hula hoop -- the moon's orbit -- 5 degrees, with half the hoop dropping magically just below the table top, and the rest rising just above it. There are just two spots -- on opposite sides of the Earth -- where the hoop passes through the table top.

"We call those nodes," Mr. Kelch said. "So, as the moon is going around in its orbit, it intersects the plane of the Earth's orbit with the sun twice."

The rest of the time, the moon is either just above the plane of the Earth's orbit, or just below it. And any time that's true, the sun, moon and Earth can't line up for an eclipse.

But the nodes themselves also rotate slowly around the planet. And about every six months they line up in a straight line with the Earth and sun.

When that happens, and the moon passes through the node between the Earth and sun, the moon's shadow sweeps across the Earth and there is a solar eclipse. Fourteen days later, the moon passes through the opposite node and through the Earth's shadow, creating a lunar eclipse.

Most of the solar eclipses are partial or visible far at sea. Likewise, most of the lunar eclipses are partial or "penumbral" -- with the moon passing through the faint outer rim of the Earth's shadow with no visible effect.

"So, when one of those happens, nobody cares, or notices it," Mr. Kelch said.

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