Sky Talk February 2007: Watching the Moon Play “Tag” with the Pleiades Star Cluster

A spectacular encounter of the Moon with the bright naked-eye Pleiades Star Cluster (more popularly known as the “Seven Sisters”) is not to be missed this month. Depending on your exact geographical location, this will be seen as either a covering-and-then-uncovering of the cluster’s stellar diamonds – or as an exciting “near miss” – by our orbiting satellite!

Shortly after sunset on February 23rd, the First-Quarter Moon will glide by one of the brightest and most famous star clusters in the sky — the lovely Pleiades in the constellation of Taurus.

When alone in the sky on a dark night, this clan shows itself as a tight knot of stars arranged roughly in the shape of a small dipper. (Indeed, many people mistake it for the real Little Dipper located at the north pole of the sky, which is much larger and fainter.) But glare from the half-lit Moon nearby, combined with a bright twilight sky, will make it necessary to use your Edmund binoculars or a small, wide-field telescope like the Edmund Astroscan to make them out.

For some, depending on your latitude (the higher, the more likely), 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 unlit edge and then an hour later (depending on where they are along the lunar limb) uncover them. The disappearances will be easiest to view and most thrilling, for the stars suddenly “snap out” like someone turned off a light switch! The reappearances will be more difficult to see, due both to glare from the Moon’s lit half and also the uncertainty of just when and where the individual stars will reappear (which, unlike prior to their disappearance, can’t be seen until they actually do!).

For most observers (particularly those along the mid-Atlantic region) the Moon will pass north of the Pleiades by about one or two Moon-diameters, depending on your actual latitude. However, there will still be much to see! The sight of our satellite and the sparkling cluster of stars so near to each other in the darkening sky — especially as seen in binoculars or, again, in a low-power, wide-field telescope like the Astroscan that can encompass them both — will be truly stunning.

And there’s more. As for the actual occultation itself, the near-miss will also offer skywatchers 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 6 p.m. EST, after which the Moon will slowly pull away from the cluster.If you’re like most people and think heavenly bodies are fixed and don’t move in the sky (except for meteors and artificial satellites), here’s a wonderful chance to prove to yourself that indeed they really do move. All that’s needed are clear skies on that exciting evening!

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


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