Orwell Astronomical Society (Ipswich)
Lunar Total Eclipse And The Danjon Scale, 03 March 2007
Event times (UT) for the lunar total eclipse of 03 March 2007 were as follows:
The idea to base an observing project around the eclipse came from Fred Espenak's eclipse page (see url below) in which he describes the Danjon Scale of lunar eclipse brightness as follows:
Danjon Scale of Lunar Eclipse Brightness
The Moon's appearance during a lunar total eclipse can vary enormously from one eclipse to the next. Obviously, the geometry of the Moon's path through the umbra plays an important role. Not so apparent is the effect that Earth's atmosphere has on total eclipses. Although the physical mass of Earth blocks all direct sunlight from the umbra, the planet's atmosphere refracts some of the Sun's rays into the shadow. Earth's atmosphere contains varying amounts of water (clouds, mist, precipitation) and solid particles (meteoric dust, organic debris, volcanic ash). This material significantly filters and attenuates the sunlight before it is refracted into the umbra. For instance, large or frequent volcanic eruptions dumping huge quantities of ash into the atmosphere are often followed by very dark, red eclipses for several years. Extensive cloud cover along Earth's limb also tends to darken the eclipse by blocking sunlight.
The French astronomer André-Louis Danjon proposed a useful five-point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. L values for various luminosities are defined as follows:
Very dark eclipse. (Moon almost invisible, especially at mid-totality.)
Dark eclipse, grey or brownish in coloration. (Details distinguishable only with difficulty.)
Deep red or rust-coloured eclipse. (Very dark central shadow, while outer umbra is relatively bright.)
Brick-red eclipse. (Umbral shadow usually has a bright or yellow rim.)
Very bright copper-red or orange eclipse. (Umbral shadow has a bluish, very bright rim.)
The assignment of an L value to lunar eclipses is best done with the naked eye, binoculars, or a small telescope near the time of mid-totality. It's also useful to examine the Moon's appearance just after the beginning and just before the end of totality. The Moon is then near the edge of the shadow, providing an opportunity to assign an L value to the outer umbra. In making any evaluations, the instrumentation used and the time should both be recorded. Also note any variations in colour and brightness in different parts of the umbra, as well as the apparent sharpness of the shadow's edge. Pay attention to the visibility of lunar features within the umbra. Notes and sketches made during the eclipse are often invaluable in recalling important details, events, and impressions.
Members of OASI reported the following observations.
Observed by naked eye from my home in East Ipswich. Sky reasonably transparent, very still.
The effects of irradiation were very noticeable during the eclipse. Irradiation is the phenomenon whereby a bright object, e.g. the lunar limb, tends to encroach upon a dark background to an extent that is proportional to the difference in intensity. It is a physiological effect caused by the spreading of excitation from the retinal area stimulated by the brighter object. During the central portion of the eclipse, the Moon appeared noticeably smaller than usual. Conversely, during the umbral partial egress phase, the bright portion of the limb appeared to bulge out beyond the circumference defined by the eclipsed portion of the Moon.
Estimated eclipse intensity was L=1 on Danjon scale (dark eclipse, grey or brownish in colouration).
In the past I've seen a lunar eclipse which corresponds to the description given for Danjon L0 (i.e. very dark) and another which was L4 (bright coppery red/orangey). I don't think this one was either of those but I couldn't match the appearance to any of the descriptions given in the TA circular passed on by Mike. It was medium brightness with a slight reddish hue and a bright edge to the umbral shadow.
I didn't watch the entire eclipse but what I did see was rather disappointing. The last eclipse I watched was I think the one before last which was very red. This time I have to agree with James and say that on the Danjon scale it was rather dull in colour giving it a value of about L=1. This was as seen from my back garden on both occasions.
Observed by naked eye from Northampton. Sky very transparent and still. Observation split between north and south lunar disk.
I hope to observe the next total lunar eclipse in August 2007 from Australia, and to estimate its brightness on the Danjon scale. It will be interesting to note any difference that may be caused by viewing from different hemispheres.
See Fred Espenak's eclipse web site eclipse.gsfc.nasa.gov/eclipse.html.
James Appleton, Martin Cook, Pete Richards, Eric Sims, Paul Whiting, FRAS