As Comet Hyakutake races away from Earth, elated stargazers are preparing for the second show in a celestial double bill -- the first total eclipse of the moon visible here in more than two years.
At sunset Wednesday, if skies are clear, the moon will rise in the east already draped in Earth's shadow. Some astronomers say enough sunlight will seep past the planet, through the halo of its atmosphere, to cast the moon in an eerie, coppery-red glow.
The show starts at 6: 26 p.m. The moon will begin to emerge from the shadow, or "umbra," and to reflect direct sunlight at 7: 53 p.m. It will be fully illuminated again by 8: 59 p.m.
Hyakutake, meanwhile, will provide eclipse-watchers with a rare twofer.
"The eclipse will darken the sky, so we will get a good view of the comet. That is extremely unusual," said David H. Levy, a comet hunter and astronomy writer at the University of Arizona. He is also co-discoverer of Comet Shoemaker-Levy 9, which smashed into Jupiter in July 1994.
Hyakutake should be visible Wednesday low in the northwest- ern sky, about halfway between the W-shaped constellation Cassiopeia in the north and the bright planet Venus in the west.
The comet has been crossing our night sky since mid-March, and is now receding from view as it races toward the sun. It was discovered in January by Japanese amateur astronomer Yuji Hyakutake and is said to be the brightest comet to buzz Earth in at least 20 years.
The lunar eclipse -- the first of two this year -- comes on the first night of Passover. That's not so unusual, Mr. Levy said, because Passover begins with the first full moon of the Jewish lunar year, and lunar eclipses can occur only with a full moon.
"I can remember five eclipses, partial or total, that have occurred on the first night of Passover," he said. "It's easy to remember because I've had to interrupt the Passover Seder to go out and look."
This week's eclipse also comes during the Christian Holy Week, which precedes Easter Sunday. Easter, too, is set according to a lunar schedule, falling on the first Sunday after the first full moon after March 21.
Millions of easterners will get an easy, dinner-time view of the eclipse, although the relatively bright sky at sunset, coupled with the dimness of the eclipsed moon, may make it hard to spot at first. The period of totality won't be visible in the nation's western half.
No dangers are associated with viewing a lunar eclipse directly. Unlike sunlight seen during an eclipse of the sun, the moon's reflected light is harmless.
The Maryland Science Center will sponsor a stargazing event beginning at 6 p.m. Wednesday in front of the center's main doors at the Inner Harbor. Telescopes will be provided, and science center staff members will be available to answer questions.
The scientific interest in this lunar eclipse is the moon's brightness during totality.
If Earth had no atmosphere, the moon would darken and disappear completely as it passed into the conical shadow that the planet casts into space. But Earth's thin envelope of air scatters the passing sunlight, throwing a dim glow across the moon's face even while it is in full shadow.
When the upper atmosphere is full of dust, more of that sunlight is absorbed by the dust and less reaches the moon. So the moon appears darker.
Lunar eclipses in recent years have appeared very dark. Scientists attribute that to volcanic ash thrown into the upper atmosphere by the 1991 eruption of Mount Pinatubo in the Philippines, but most of that dust has cleared.
Scientists can get a crude reading on the clarity of the atmosphere by observing the color of the eclipsed moon.
"It's more of a cultural event, and a beautiful event to watch, than LTC one that can teach us a lot," Mr. Levy said. "But that doesn't make it any less significant."
The next total eclipse of the moon is expected at 9: 12 p.m. EDT on Sept. 26. It should be visible throughout North America, weather permitting.
Pub Date: 3/31/96