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.
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