Observing and timing a lunar eclipse is a very subjective exercise. According to Westfall “the exact times you observe the four “eclipse contacts” depend on your eyesight, judgement, the characteristics of the umbra’s fuzzy edge and local sky conditions”. The historical application of such observations is at the heart of this exercise. In past centuries, naked eye timings of lunar eclipse contacts were one of the few ways that navigators and explorers could determine their longitude on earth.
In his Astronomical Calendar, Guy Ottewell presents an excellent treatise on observing and estimating the brightness or darkness of a lunar eclipse. Quoting Ottewell: “The penumbra is very faint. This is not surprising: from all parts of it, at least some part of the brilliant Sun would be visible, so it is essentially like early-morning daylight on the Earth. The outer part of the penumbra is imperceptible to the eye, so the beginning of penumbral eclipse is an academic event. The inner half or more of the penumbra is perceptible as a slight graying.”
For our eclipse timing exercise the first contact of the umbra is the first event timing of interest. Ottewell continues: “The umbra, by contrast, begins with an edge that is almost hard, and noticeably part of a circle (thus proving to the ancient Greeks that the Earth is a globe). Though the umbra is the “total shadow,” from within which no part of the Sun can be seen directly, it is generally not plain black, because sunlight is refracted into it through the atmosphere all around the Earth around the continuous ring of the sunrise-and-sunset line. The amount by which the light is reduced and reddened depends on clouds, volcanic ash, pollution, etc, all around this geographical band; so it varies from eclipse to eclipse and from part to part of the Moon. The standard way to record and compare these variations is to use this scale, proposed by Andre Danjon, and intended to apply if possible to the middle of the eclipse. L stands for luminosity”.
L=0: Very dark eclipse, Moon almost invisible, especially in mid-totality.
L=1: dark eclipse, gray or brownish coloration, details distinguishable only with difficulty.
L=2: deep red or rust colored eclipse, umbra usually having a very dark center and relatively bright outer rim.
L=3: brick-red eclipse, umbra usually having a yellow or bright gray rim.
L=4: strikingly bright copper-red or orange eclipse, with very bright bluish tint where umbra and penumbra meet.
Fractional estimates can be used, such as 1.3; and different parts of the Moon may have different values. Observers often make reports such as “Probably 2, but the Mare Crisium was invisble and there was a very bright patch along the southern edge”
Listen to our interview with Dr. John Westfall and learn how to time the four eclipse “contacts” and participate in the visual timing experiment. Click on the link above.
To learn more about determining one’s position on Earth read “Precision Geolocation Using Lunar Occultation”, Ingalls, Scientific American, January 1955. This article was re-published in 2001 in “Scientific American: The Amateur Astronomer”, by Shawn Carlson. Publisher: John Wiley & Sons. ISBN: 0-471-38282-5
Saturday, June 7th
