Post by Chicago Astronomer - Astro Joe on Dec 5, 2005 14:06:48 GMT -6
Moon will Occult The Pleaides frequently in coming months
During the next several years, skywatchers around the globe will periodically get a chance to watch the Moon pass in front of the most beautiful of all star clusters, the Pleiades. It will happen thirteen times in 2006 alone, beginning in January.
The last occultations of the Pleiades took place between 1986 and 1992. Since this cluster lies 4º north of the ecliptic, it can be occulted only when the ascending node of the Moon’s orbit is in the Pisces-to-Sagittarius part of the ecliptic. This situation repeats every 18.6 years, the time required for the ascending node to complete one circuit of the ecliptic. Hence, a new “season” of Pleiades occultations has begun that will continue until the year 2010.
Since the Pleiades are roughly twice as large as the Moon in apparent angular diameter only a part of the cluster can be hidden at any given moment; the Moon can never hide the entire cluster. The seven brightest Pleiads are (on average) around fourth magnitude. They are, in order of brightness: 1) Alcyone, magnitude +2.9; 2) Atlas, +3.6; 3) Electra, +3.6; 4) Maia, +3.8; 5) Merope, +4.2; 6) Taygeta, +4.3; 7) Pleione, +5.1.
The very best Pleiades occultations will be those involving a crescent Moon, where the Moon’s light is not overly bright. In addition, Earthshine may also be evident as a faint grayish-blue illumination of the dark part of the Moon’s disk. This dim light is produced by sunlight striking the “dark” side of the Moon after first being reflected from the Earth’s surface, clouds and atmosphere. This effect will give the Moon an almost 3-D quality and will only serve to enhance the overall view. The April and July occultations which favor eastern North America, should be especially striking and Night Sky will provide detailed outlooks as we draw nearer to both of these events.
In the table, above, is the complete list of Pleiades occultations through 2006, which will chiefly favor locations over the Northern Hemisphere. Note that each occultation will be visible only for a specific region of the Earth. In other areas, either the Moon will appear to miss the cluster or the event occurs during the daytime or when the Moon and Pleiades are below your local horizon. Even if your area is not in a favored position to see an occultation, you’ll still see the Moon and the Pleiades rather close to each other on the date in question. The date and universal time (U.T.) are given for the midpoint of each event. To convert to your time zone, keep in mind that U.T. that you will need to subtract five hours for Eastern Time, six hours for Central Time, seven hours for Mountain Time and eight hours for Pacific Time. When daylight saving time is in effect, subtract four hours for Eastern Time, five hours for Central Time, etc.
Full story here: www.space.com/spacewatch/051202_night_sky.html
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These many occultations will be a great way to introduce the public to Chicago Astronomy! An excellent wide-field event to catch...
