Sky Talk April 2008: Moon Encounters Pleiades Star Cluster

Skywatchers will have another wonderful opportunity early this month to witness the on-going celestial ballet that occurs as worlds silently move through space above our heads at night. This event will be visible along the entire Eastern United States and up into eastern Canada as well. And while the spectacle can be enjoyed using the unaided eye alone, binoculars and small telescopes will provide especially stunning views of the Moon and the Pleiades embracing each other.

As the sky darkens after sunset on the evening of April 8th, a slender three-day old crescent Moon will lie very close to the famed Pleiades Star Cluster in the constellation Taurus. Depending on your exact geographical location, their encounter will be seen as either a covering-and-then-uncovering (or occultation) of some of the stars on the cluster’s northern edge — or as an exciting “near miss” — as the Moon slowly glides past this glittering stellar commune. But in either case, it will be important to catch this event before the pair sets just after 10:00 p.m. EDT.

Observers in New England and Canada are favored to view multiple occultations, while those below about 35 degrees latitude will see the Moon poised just above the Pleiades missing it entirely. The same goes for the observers in the central and western parts of the country, where the Moon will have passed the cluster by the time darkness falls (but still remain strikingly close to it!). The sight of our lovely satellite and the sparkling star cluster in close proximity is always stunning. This is especially so when the Moon is a thin crescent — as it will be, with the unlit portion softly bathed in bluish light (known as “Earthshine”) reflected onto it from the Pacific Ocean where it will still be daylight. As is the case for actual occultations themselves, even the near-misses will 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.

A thin crescent also favors being able to see the cluster so close to the Moon with the naked eye, since glare from the lunar surface is much less than at other phases. Even so, your Edmund binoculars will make it much easier to pick out the individual stars of the Pleiades. Perhaps the very best view of events like this is to be had with a low-power telescope operating at a magnification of 4x to 5x per inch of aperture (i.e., 12x to 15x for a 3-inch glass), which yields a wide expanse of sky seen. Unfortunately, such low magnifications typically can’t be achieved with “normal” telescopes. But Edmund Scientifics’ famed Astroscan 4.25-inch aperture rich-field reflector is one that’s specifically designed to give a very low-power (16x), wide-field (3-degree) view of the sky. It will easily encompass both the Moon and the Pleiades in the same stunning eyepiece field during this and other such close celestial encounters.

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

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