Star Cluster to Hide Behind Moon

If you live anywhere to the north of a line that runs across North America from roughly Queen Charlotte Island in British Columbia southeast to near Jacksonville, Florida and clear skies are forecast for Friday evening, Sept. 19, then be sure to be outside during the mid-evening hours and watch for the rising of the waning gibbous moon. If you have binoculars or a telescope you will also see the moon moving in front of the famous Pleiades star cluster.

This event is called an occultation, from the Latin occultatio, a hiding, or an "eclipse" of a star or planet. Put another way, it's when one celestial body — in this case, the moon — passes in front of and obscures another, here being the Pleiades. This is one in a series of Pleiades occultations that have been visible this year from various parts of the world. Last month, on Aug. 23, a Pleiades occultation was widely visible across Europe and Asia. Next month, on Oct. 17, eastern Asia and northwestern parts of North America will be favored.

For about the 60 to 90 minutes our natural satellite will slowly cover and uncover a number of the "Pleiads." The brightest stars of the cluster will appear to disappear along the bright side of the moon, reappearing later in dramatic fashion along the moon's unilluminated limb: seemingly "popping-on" suddenly as if someone threw a switch.

Much of the western half of the United States will miss out on the occultation, as the moon will have already moved past the Pleiades by the time it rises. Nonetheless, the view in binoculars of the moon sitting just below and the lower left of the Pleiades cluster as they come up over the east-northeast horizon late on that Friday evening should still make for an interesting sight.

For most locations within the prime viewing area, the occultation will already be in progress as the moon rises and will also be positioned at a rather low altitude. Therefore, a clear and unobstructed view toward the east-northeast is strongly recommended.

Prospective viewers will probably have to wait at least a half an hour after the moon rises for it to lift sufficiently up above the haze that normally hangs close to the horizon to get a good view. Interestingly, as seen from parts of Nova Scotia and northern Newfoundland, the brightest Pleiad (Alcyone) will appear to graze the moon's lower limb. Generally speaking, the sky beginning at about 10 p.m. local time onward will provide the best overall view for most locations. As the moon slowly climbs higher in the east-northeast sky, it will gradually uncover more and more of the cluster.

The best views will be over eastern Canada and the Northeast United States, where the rising moon should appear well above the horizon haze during the occultation. Places farther west will see the moon lower and closer to the horizon.

Interestingly, some European observers will be able to see the moon occult the northernmost members of the star cluster, notably Maia and Taygeta. Keep in mind that for this part of the viewing zone, the occultation will occur on Saturday morning, September 20 between about 2 and 3 hours Greenwich Time, with the moon high up in the southeast sky.

For a maps depicting the viewing zones for the six brightest Pleiads, as well as a complete listing of times for dozens of cities in North America and Europe, go to the International Occultation Timing Association (IOTA) web site.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.