The University Record, October 15, 1997

 It's 'Moon over Aldebaran' on Sunday

When the moon moves in front of the bright star Aldebaran on Sunday morning (Oct. 19), astronomers across North America will be watching‹including U-M astronomer Richard Teske. The phenomenon, called an occultation, will be easily visible to early ris ers in Michigan when the moon and star are high overhead just before dawn.

"An occultation takes place when the moving moon overtakes a star and snuffs it out," Teske says. "Because the moon travels its own width on the sky in around an hour, the star soon reappears. Although the moon occults many stars during its month-lo ng orbit around Earth, Aldebaran‹in the winter constellation of Taurus, the Bull‹is the brightest one it can cover."

Patient observers can see both the disappearance and reappearance of Aldebaran on Sunday morning. "The exact moment depends on your viewing location in Michigan," Teske explained. "In Lansing, the moon will occult Aldebaran at 4:51 a.m. EDT; the sta r will reappear at 6:09 a.m. People watching from locations west of Lansing will see the disappearance and reappearance of the star a few minutes earlier. Observers on the eastern side of Michigan will see it a few minutes later."

Sometimes the occultation and reappearance of a bright star can be detected with one's naked eye, Teske says, although it's a good idea to use binoculars, if possible. Observers with or without optical assistance should see a good show if the sky is free of haze.

Because the moon is so much closer than the star, observers stationed at different locations on Earth "see" the star's light grazing slightly different locations at the moon's edge just before the star is extinguished. The time of the star's disappea rance depends on whether the light comes across a mountain top or through a low valley on the moon's advancing edge.

Timing records made by amateur astronomers from many observing locations are converted into maps of altitudes of mountains and craters on the moon's rim, according to Teske. "This is a cutting-edge scientific contribution that can still be made with relatively simple equipment," he says. "Many non-professional astronomers around the United States help make the required observations. The opportunity to make a valuable scientific contribution in this way will disappear shortly, however, when laser al timeter data obtained by a moon-orbiting spacecraft in 1992 are fully analyzed."

The sizes of stars also can be determined by recording how slowly or quickly a star winks out when overtaken by the moving moon. "While we usually think of stars as mere points of light, some are larger than just dots on the sky," Teske explains. "A t the moment of an occultation, the moon takes a few hundredths of a second to cover up the largest stars. Observing how their light changes during that very short time allows astronomers to estimate their diameter."

As it circles the Earth, the moon covers up about one-tenth of the stars in the sky at one time or another. Although that is a huge number of stars, only about 100 are near enough and bright enough to have their dimensions measured by observing an oc cultation by the moon.

"Using these occultation measurements, astronomers have found Aldebaran to be 46 times larger than our own sun," Teske says. "Were Aldebaran in the sun's place, we would need two hands to cover up its glare."