Sky Talk September 2007: Moon Occults Pleiades Star Cluster – Twice!

A pair of spectacular encounters of our Moon with the bright naked-eye Pleiades Star Cluster (more popularly known as the “Seven Sisters”) occurs this month and repeats again next month (which is being covered here, as the circumstances for observing both events are very similar — and since the October installment will be highlighting the Orionid Meteor Shower!) Depending on your exact geographical location, this will either be seen as a covering-and-then-uncovering of the cluster’s stellar diamonds — or as an exciting “near miss!”

During the early morning of September 3rd, the Moon will glide by one of the brightest and most famous star clusters 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 and fainter.) But glare from the nearly last-quarter Moon nearby will make it necessary to use your Edmund Scientifics’ binoculars or a low-power, wide-field telescope like the Edmund Astroscan to make out the cluster’s stars.

For some, depending on your actual latitude (the northernmost mid-western states are favored this time), this event will be an actual occultation, during which the Moon in its perpetual eastward orbital motion around the sky will first block out the cluster’s stars one-by-one at its bright limb and then uncover them roughly an hour later. 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 see happening due to the uncertainty of just when and where the individual stars will pop out along the Moon’s dark edge.

For most observers (especially here in the East) the Moon will pass northwest of the Pleiades itself. But, there will still be much to see! The sight of our satellite and the sparkling star cluster in close proximity is always stunning. And as is the case for the actual occultation itself, the near-miss will also offer stargazers an excellent opportunity to witness the slow orbital motion of the Moon in “real-time” as it moves roughly its own diameter each hour eastward across the sky.

Closest approach to the Pleiades in either case will occur about 2 a.m. EDT, after which time the Moon will slowly pull away from the cluster.The second occultation happens as the Moon rises on the evening of October 27th, with closest approach to the Pleiades occurring around 7 p.m. EDT. The Moon will be just past Full and about 95% illuminated, making it necessary to use binoculars or a telescope to see the cluster itself. And as for the September event, stars will disappear at the bright limb of the Moon and reappear at its dark edge. This time, observers in New England are favored to see an actual occultation, with another near-miss happening for those elsewhere.

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

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