Difference in Brightness of a Penumbral Eclipse
November 28, 2012
Today, November 28, the Moon will be partially eclipsed by the Earth’s shadow. A little project I’ve had on my mind for some time has involved an attempt to capture a (partial) penumbral eclipse of the Moon. Because of poor weather conditions, I had little success with this endeavor until the partial eclipse of August 6, 2009. Fortunately, at my location near Athens, Greece, I enjoyed pristine transparency (ideal viewing conditions) for this eclipse, where 40 percent of the lunar disk entered the penumbra. For today’s eclipse, 92 percent, virtually the complete lunar disk will immerse itself in the penumbra.
Partial penumbral eclipses are generally ignored since they’re considered unexciting and are hard to detect – the difference in brightness when the Moon is in and out of the penumbral shadow is rather minute. However, features not easily perceivable by the human eye can often be a trivial matter for sensitive cameras. The photo montage at top shows the full Moon on August 6, 2009, just before its entry into the penumbra (Pre-P1); the full Moon at its maximum penumbral position (Max); the difference between these two images (Diff1). This image is really the change in the Moon's brightness owing to its entry into the penumbra. Diff2 is an enhanced image that better shows the subtle differences in brightness. Looking at Diff2 it’s easier to now detect, for instance, the approximately one-third of the lunar disk that's entered the penumbra -- at lower left.
In a similar way, we’re able to detect exoplanet transits by measuring the intensity of the host star and continuing to gather intensity measurements during the transit. There’s only a tiny drop in brightness since the exoplanet blocks out a small fraction of the light being projected by the host star.
Click here to determine if you’re in a good area to see today’s partial lunar eclipse. The trick for observers is to see if you can visually detect a difference in the brightness of the Moon when it’s entered the penumbra. With a magnitude of 0.92 (92 percent of the lunar disk will enter the penumbra), it should be possible to detect some very slight reduction in the overall brightness of the full Moon during maximum eclipse.
Photo details: A non-linear adjustment was applied to "magnify" the effect in the first "difference" image as much as possible. However, in order not to bias this analysis in any way, the camera was set to RAW mode so that the images written to the memory card would be untouched by the camera's on-board software (typical of images saved in JPG format). Also, exposure settings such as sensitivity (ISO), exposure length, white balance etc were kept identical for the image at "P1" and "Max". Note that the "Pre-P1" image was taken four minutes prior to the predicted first contact of the Moon with the penumbra.