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SHARON SHANKS | The Cosmos Delight in the night sky



Published: Sun, November 18, 2001 @ 12:00 a.m.



Grab your pencils again to mark your celestial calendars -- this time for Friday.

On that evening the moon will creep across the sky and hide Saturn from our sight, a somewhat rare and interesting observing opportunity called an occultation.

In their orderly dance through the sky, it is inevitable that stars, planets, moons, and other bodies will pass and conceal each other from our line of sight here on Earth.

The passings are known as occultations (when a large body passes between us and a smaller body) and transits (when a small body moves in front of a larger object).

Definition: The word occult has its roots in Old English, Greek and Old German words that mean concealed, covered and secret. It shares some aspects of "hidden" and "secret" with the darker definition of occult and its association with the supernatural. The astronomical meaning of occult, however, simply means "covered."

Because most of the bodies we see in the heavens are the pinpoint bits of light that mark the stars, it makes sense that occultations are more common than transits.

The most common transits we can see from Earth are those of Mercury and Venus when they travel between our line of sight and the sun. The two planets, although huge in their own rights, appear as tiny spheres as they slowly move across the face of the much larger sun.

Occultations are more frequent. The most commonly observed are those by the moon.

As the moon moves, its edge seems to creep up to a star or planet and then suddenly cover it up. After about an hour, the star reappears just as suddenly on the other side of the moon.

The occultation Nov. 30 is a lunar occultation of Saturn.

When: From the Youngstown area, the occultation will start at 7:43 p.m. and end at 8:36. Saturn will be rising shortly after sunset, so viewers will need a fairly clear eastern horizon without nearby tall trees or buildings to block the view.

Because the moon will be full that evening, catching the exact moment when Saturn disappears and reappears from behind the moon will be difficult. A pair of binoculars or small telescope will help.

Moon phases: Occultations by a waxing moon are easier to watch because the dark edge of the moon does the initial covering. The contrast between the dark moon and the bright planet or star makes the sudden disappearance easier to spot and more interesting to watch.

For a waning moon, the bright edge does the occulting but makes a dramatic reappearance from behind the dark limb on the other side.

Any two objects that line up from our line of sight are candidates for occultations and transits.

Study: In addition to the moon, astronomers watch for planets that move between distant stars and us and for asteroids that move in front of or behind planets and pass in front of stars.

When carefully observed and timed, occultations can tell astronomers about the positions and sizes of stars and shapes of asteroids.

They have helped discover binary stars, revealed in that brief time span when one star is hidden and its companion is alone.

An occultation of a star by Uranus in 1977 led to the discovery of the planet's ring system -- the star unexpectedly disappeared and reappeared behind the rings.

There are two special types of occultations that most of us look forward to: solar and lunar eclipses. In the strict sense, a solar eclipse is nothing more than an occultation of the sun by the moon; a lunar eclipse is the moon being occulted by the Earth's shadow.

If you miss the occultation of Saturn on Nov. 30, you have another chance yet this year to watch the moon hide the ringed planet. The next date is Dec. 28, but be prepared for an early morning because the occultation starts at 3:58 a.m. and ends at 4:39 p.m.

While Saturn is hidden, you can enjoy the sight of bright Jupiter rising in the east. The two planets are in the midst of some of the brightest constellations of winter, a true sign of the changing seasons.

Saturn is squarely in the center of Taurus, the bull; look for the Pleiades to the right of and above the full moon and the planet.

Jupiter is in the constellation of Gemini, the twins. Below and between the two are the bright stars of the Orion constellation, the hunter.

XFor more information on occultations and how you can contribute to scientific study by recording their timings, check out the International Occultation Timing Association home page at www.lunar-occultations.com/iota/iotandx.htm.




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