'If you blink, you miss it' Moon will eclipse bright star tonight

March 04, 1998|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

If you missed the boat to the Caribbean for last week's total eclipse of the sun, there's a consolation prize for you in the heavens.

Just after dinner time today, the first-quarter moon will eclipse a bright star called Aldebaran.

This "occultation," as astronomers call it, will begin at 7: 28 p.m. in Central Maryland.

That's when Aldebaran will abruptly wink out as it disappears behind the darkened upper left-hand edge of the moon.

Observers should go outside a few minutes earlier to be ready for it.

"I love watching those things, the sudden disappearance of that star as the moon pops over it," said Lucy Albert, president of the Harford County Astronomical Society. "It happens so incredibly fast. I found that amazing. If you blink, you miss it."

Aldebaran will pop back into view on the moon's bright side about an hour later, at 8: 29 p.m., as the moon moves east in its leisurely orbit of the Earth.

Occultations occur whenever one object passes directly in front of another as seen from the Earth, blocking it from view.

Astronomers use occultations to study small, dark objects such as asteroids as they pass in front of stars. Pluto's moon was discovered in 1978 when it occulted the tiny planet. Pluto's atmosphere was detected during its occultation of a star.

Aldebaran is the 13th-brightest star in the sky, and one of the few bright stars occulted by the moon. Although it might occur several times each year, the event is only rarely visible from a given spot on Earth, and at a convenient nighttime hour.

"Just stand outside on your back porch and watch it," Albert said. "A pair of binoculars would be fine. But even with the naked eye you should see it."

The occultation of Aldebaran will be visible nearly anywhere east of the Mississippi River where skies are clear.

The times and the location of Aldebaran with respect to the moon will vary depending on the observer's location. But all start times are after 7 p.m. EST.

Look for the moon high above the southwest horizon. Aldebaran will be to its upper left.

Astronomers say Aldebaran is a relatively close neighbor to our sun, 68 light-years away. (A light-year is the distance light travels in a year, about 5.9 trillion miles.)

It is an aging red giant, with a diameter 40 times that of our sun. Its rosy light caused the ancients to imagine it as the eye of the bull in the constellation Taurus.

It was an occultation of Aldebaran, recorded in Athens in 509, that set the stage for one of astronomy's most profound discoveries, more than 1,200 years later.

In 1735, the English astronomer Edmund Halley calculated the moon's position during the 509 occultation. He found that the occultation could not have been seen from Athens unless the star had been farther south in the sky than it was in 1735.

To his astonishment, Halley concluded that Aldebaran had moved north in the intervening 12 centuries. This marked the discovery of "proper motion" -- the slow movement of stars that, over any human lifetime, had always appeared fixed in the sky.

Amateur astronomers enjoy watching lunar occultations. Scientists study them as a way to make precise measurements of the moon's profile.

"What we're trying to do is to calibrate the lunar profile better than has been done before," said David Dunham. He is president of the International Occultation Timing Association (IOTA) and a spacecraft trajectory expert at the Johns Hopkins University's Applied Physics Lab.

By collecting data on the precise moments that a star disappears behind the moon as seen from many locations, IOTA members help to develop accurate maps of the mountains and craters on the edge of the moon where the star disappears.

"We have extremely accurate star positions. And the lunar orbit is well-known," Dunham said. "The only unknowns left are the lunar profiles."

Dunham plans to observe the occultation of Aldebaran from northern Michigan, where the star will appear to skim the moon's northern profile.

The lunar profile data are used during solar eclipses to derive precise measurements of the sun's diameter. With accurate measurements of the sun, Dunham said, scientists can learn to detect small changes that may signal increases or decreases in the sun's production of energy.

Knowing how much the sun's output is changing could help scientists sort out how much of the global warming they see is due to the burning of fossil fuels, and how much stems from solar changes.

IOTA has asked the public to help by recording Aldebaran's occultation with home video recorders.

Instructions can be found in the March issue of Sky & Telescope magazine, or on the Internet, at www.skypub.com/occults/occults.html. The Web page also offers a movie of a past occultation of Aldebaran.

Pub Date: 3/04/98

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