Starry, starry night sky will give flickering performance


Marylanders missed Wednesday's total eclipse of the sun because the path of totality ran across Africa, Turkey and Central Asia. But we have a front-row seat for another celestial spectacle tomorrow evening.

Between dusk and 9 p.m., the slim crescent moon, traveling in its orbit around the Earth, will pass in front of a densely packed cluster of bright stars in the western sky called the Pleiades, or "Seven Sisters."

As it does, the "dark" side of the moon's disk will eclipse some of the Pleiades' brightest stars, one after the other. They will appear to wink out one by one, as suddenly as if someone pulled the plug.

Unlike a solar eclipse, this "occultation" of the Pleiades is safe to watch, and much more easily seen through binoculars or a small telescope under dark skies.

Occultations of the Pleiades by the moon occur in clusters separated by 18 years, according to David Dunham, an astronomer at the Johns Hopkins University Applied Physics Laboratory and president of the International Occultation Timing Association.

"During the next two years, the Pleiades will be occulted by the moon every month," he said. But many of the events occur in daylight, at an inconvenient hour, close to below the horizon, or with much more of the moon illuminated, producing a glare that makes the stars difficult to see. And often the moon covers only some of the cluster's brightest stars.

A rare sight

On all these counts, tomorrow's event will be a rare one, he said -- the best of the current cycle for Maryland, and the best since 1969. Only 19 percent of the moon will be illuminated, reducing glare and improving the view. Another -- almost as good with 23 percent of the moon in sunlight -- will occur after midnight July 20.

And, provided skies are clear tomorrow (the forecast is iffy) Marylanders are ideally situated to watch one of the event's most fascinating sights.

At least two dozen amateur astronomers, members of IOTA, are expected to disperse across Central Maryland and the Lower Eastern Shore to observe and record as the edge of the lunar disk is "grazed" by one of the brightest stars of the Pleiades -- called Maia.

As seen from inside a mile-wide band stretching from Bethesda and Takoma Park to Ocean City, Maia's light will appear to wink on and off as it is alternately blocked and revealed by mountains and valleys along the moon's profile.

North of the band, Maia will shine throughout the event. South of it, the star will disappear behind the moon for several minutes.

Scientists have long used such grazing occultations to precisely map lunar terrain not easily seen from Earth, and to track the moon's orbit.

"Eventually," Dunham said, "lunar orbiting satellites will map the moon's edge in more detail, but so far that hasn't been done as accurately as we can do it with lunar occultations measurements."

Astronomers can also identify double stars when they wink out in two steps, instead of one, as they are occulted. "When the moon is fainter, like this Saturday's event, the measurements of the occultations are easier and more accurate," he said.

The mythology

In Greek, Pleiades means "doves." It's a reference to a myth in which Zeus transforms the seven daughters of Atlas into a flock of doves and sends them into the heavens to escape the amorous advances of Orion (whose constellation hovers just east of the Pleiades).

Accordingly, the cluster is also known as the "Seven Sisters." Its seven-or-so brightest stars are visible to the naked eye. But with binoculars, dozens of stars become visible. A telescope reveals many more.

The cluster can be found in the constellation Taurus, which stands above the Western horizon after dusk at this time of year. But tomorrow night just look for the moon, which will be smack in the middle of the Pleiades by the time it's dark enough to see them.

Earth's neighbors

The Pleiades are pretty close by -- just 425 light years from Earth, within our own Milky Way galaxy. Astronomers believe they are all relatively hot, young stars that first ignited barely 100 million years ago. By contrast, our sun is believed to be 4.5 billion years old.

In addition to Maia, the cluster's brightest stars are Alcyone, Asterope, Celaeno, Electra, Merope and Taygeta. The sisters' mythological parents -- Atlas and Pleione -- are also represented by stars in the cluster.

The Japanese know the Pleiades as "Subaru," which means "unite." According to a company Web site, the name was adopted for a line of automobiles after Fuji Heavy Industries Ltd. was formed in 1953 from the merger of five smaller companies. And that explains the six-star cluster on the cars' emblem.

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