Sky Talk August 2009: Penumbral Lunar Eclipse

Eclipses of the Moon have been featured in past installments of this column — but never one involving a penumbral eclipse due to the difficulty of viewing them. For those who enjoy challenges, this month offers observers an opportunity at a convenient time of evening to test their skills at seeing one of these events. But you’ll likely need your trusty Celestron SkyMaster binoculars from Edmund (#30311-02) to have a reasonable chance of success. And leave your telescope indoors!

Most readers have surely seen a total (or partial) lunar eclipse, where the Moon passes wholly (or partially) into and through the Earth’s dark inner shadow known as the umbra. This is a very beautiful and obvious celestial event, one readily visible to the unaided eye. But the Earth actually has another much fainter outer shadow surrounding the umbra known as the penumbra. If standing on the Moon, this is where you would see a partial solar eclipse — while inside the umbra itself you would witness a total solar eclipse.

While the umbra is unmistakable as it falls across the lunar surface, the penumbra is a very pale and ghostly-looking shading somewhat like viewing the Moon through a weak neutral density filter. Although it can be glimpsed with the unaided eye as a subtle drop in the intensity of the full Moon (which must always be in the full phase, opposite the Sun in the sky, to be eclipsed), it typically takes a trained eye to see the decrease in brightness. Binoculars are the best instrument with which to detect the penumbral shading, as opposed to most telescopes, which magnify the Moon too much even at their lowest power. This dilutes the image and reduces the contrast needed to see the penumbra. An exception is Edmund Scientifics’ popular Astroscan Plus wide-field telescope (#30050-01). Its 4.25-inch aperture, magnification of just 16X and 3-degree (six full-Moon diameters) of sky coverage is the perfect combination for seeing our satellite pass through this outer shadow. At around 9 p.m. EDT on Wednesday evening August 5th, if skies are clear, you will have an ideal opportunity to search for the penumbra. As the Moon moves slowly into this shadow in its eastward motion across the sky, look for a slight dimming of its brightness — and then as it moves out of it, watch for a slight brightening. This typically takes about an hour to unfold from beginning to end.

This is also “meteor month” — so named after the famed Perseid Meteor Shower that peaks each year during the second week of August. But viewing this time will be hampered by the presence of a waning gibbous Moon rising before midnight and lighting up the sky during the morning hours when these “shooting stars” are most numerous. (Last year it was a waxing gibbous Moon that spoiled viewing during the evening hours, which set just after midnight!) Peak activity of this year’s display happens on the evening August 11th and continues until dawn on the morning of the 12th, with the maximum itself occurring in the hours just before sunrise.

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


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