Lunar Total Eclipse, 28 September 2015
The lunar total eclipse of 28 September 2015 was notable in that it occurred with the Moon at perigee (so appearing slightly larger than average). The Moon entered the umbral shadow at 01:07 UT and left at 04:27 UT. Several members of OASI observed the event; the best photos and video are below.
The Moon's appearance during a total lunar eclipse can vary enormously from one to the next. The geometry of the Moon's path through the umbra clearly plays an important role and, in fact, the Earth's atmosphere also has a major effect. Although the physical mass of the 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 (in the form of clouds, mist, precipitation) and solid particles (as dust, organic debris, volcanic ash). This material 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 (1890-1967) proposed a useful scale for evaluating the visual appearance and brightness of the Moon during a total lunar eclipse. "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 a lunar eclipse is best done with the naked eye, binoculars or a small telescope near mid-totality. It's also useful to examine the Moon's appearance just after the beginning and just before the end of totality, when the Moon is near the edge of the umbral shadow, and an "L" value can be assigned to the outer umbra. In recording an "L" value, instruments used and the date and time should also be noted. 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 invaluable in recalling details, events and impressions.
Members of OASI estimated the following "L" values for the eclipse of 28 September 2015:
|Paul Whiting||L0 during penumbral eclipse.
L2 at start of umbral eclipse with very bright lower edge.
L3 at mid-point of umbral eclipse.
|David Murton||L2, very dark with discernible white rim at totality.|