Sky Talk September 2008: Moon Occults Pleiades Star Cluster – Once Again!

For an unprecedented fourth time this year, the Moon will occult the bright naked-eye Pleiades Star Cluster, more popularly known as the “Seven Sisters.” Depending upon your actual geographic location, this will be seen as either a spectacular covering-and-then-uncovering of the cluster’s stars — or as a fascinating “close encounter” with them. In either case you won’t want to miss this event if skies are clear!

During the evening hours of September 19th, the Moon will again glide by one of the brightest and most famous clusters of stars in the heavens — the lovely Pleiades in the constellation Taurus. When seen alone in a dark sky, this stellar jewelbox shows itself as a tight knot of stars arranged in the shape of a small dipper. (Indeed, many people mistake it for the real Little Dipper, which is located at the north pole of the sky and is much larger in size.) Occultations are most easily seen (especially without optical aid) when the Moon is in its crescent phase. At such times, the disappearances take place along its dark limb and reappearances along its bright limb in the evening sky. (The sequence is reversed for a crescent in the morning sky.) But this time glare from a nearly three-quarter-full Moon will make it necessary to use your Celestron SkyMaster binoculars from Edmund Scientifics (#30311-02) or a low-power, wide-field telescope like the Edmund Astroscan (#30020-01) to make out the cluster’s stars.

Observers in the northeastern United States are favored to see an actual occultation of the cluster, beginning at moonrise just after 9:00 p.m. EDT. In its perpetual eastward orbital motion, our lovely satellite will first hide the cluster’s stars along its dark limb and then uncover them roughly an hour later at its bright one. The disappearances are the easiest and most thrilling to observe, for the stars suddenly “snap out” like someone turned off a light switch as you’re watching them! The reappearances are more difficult to catch happening due to the uncertainty of just when and where the individual stars will pop into view along the Moon’s edge. The occultation sequence will have completely ended by about 11:00 p.m. EDT. For those not experiencing an occultation, this event is still well worth viewing. The spectacle of seeing the Moon and the glittering star cluster in close proximity is always exciting. And as is the case for the actual occultation itself, the near-miss will also offer skywatchers an opportunity to see the slow orbital motion of the Moon as it serenely moves roughly its own diameter eastward each hour across the heavens.

Before taking leave of September’s sky mention must be made of radiant Jupiter hanging above the “handle” of the Teapot asterism of Sagittarius, located due south on early evenings this month. The nightly dance of its four bright satellites about the planet — disappearing into and reappearing from its shadow, casting shadows of their own onto the cloud-tops as they cross in front of it, or being occulted as they go behind it and later reappearing — are a never-ending source of fascination for users of even small telescopes like the Scientifics’ 60mm refractor (#30823-61).

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.

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