A lunar eclipse takes place when the Moon enters the shadow of the Earth. On such a night, the Earth is directly between the Moon and the Sun, blocking the sunlight reaching the Moon. (otherwise put as “casting its shadow on the Moon”). Since the Moon and the Sun are on the opposite sides of the Earth, the event obviously is on a full moon night.
The diagram below shows the geometry of total Lunar Eclipse . In this case Sun, Earth and Moon are nearly on one straight line. Moon is in total shadow of Earth or it is in the Umbra region.
You can see the Umbra and Penumbra region by looking at a shadow of a disk (say a 50p coin) kept at a distance from a sheet of paper. You can see that the central part of the shadow to be dark but getting diffused and lighter towards the edge of the shadow.
What happens during a lunar eclipse: As Moon enters penumbral shadow of
Earth nothing much is noticeable to an untrained eyes first half an hour or so. After that one might notice gradual change in the brightness on the lunar disk. Umbral shadow of Earth on Moon is quite noticeable. This dark shadow can be seen covering Moon gradually.
If a lunar eclipse is total then in about one hour’s time Moon will be completely inside the shadow of Earth. At this time the colour of the lunar disk will be red with it’s many hues – crimson, brick red etc.
After that in reverse order Moon comes out of Earth’s shadow.
Why would Moon look red in colour during the total phase
Earth’s atmosphere plays it’s role here. If Earth had no atmosphere Moon would have ‘simply vanished’ from the sky at the total phase. However the Earth’s atmosphere plays a role – exactly the same way it does to rising or setting Sun (or Moon).
As sunlight passes through the atmosphere of Earth, light is scattered and bent slightly. The light is scattered by the constituent atmospheric particles. Blue component of the sunlight is scattered most and the red rays pass through. Absence of the blue rays and presence of the red rays make rising or setting Sun appear red in colour. Light rays bend as they travel from one medium to other in this case from vacuum to the Earth’s atmosphere.
Some of the red rays escape or come out of the Earth’s atmosphere reaching the Moon and giving the eclipsed Moon its red hue.
Red colour of Moon depends on how closely Sun – Earth and Moon are aligned. Total lunar eclipse of 15 June 2011 was darkest lunar eclipse in almost 100 years as the centres of the Sun, the Earth and the Moon would nearly be on one straight line. The earlier darkest lunar eclipse was observed on August 6, 1971 and the next one would be on June 6th, 2058.
Other than the near perfect alignment of these solar system bodies, the atmosphere too plays its role in darkness of eclipsed Moon. If Earth’s atmosphere is contaminated by the volcanic ash then it is likely to have pronounced effect on how dark the eclipsed moon would look.
I remembers the total lunar eclipse on the night of of December 9, 1992. It was such a dark lunar lunar eclipse that close to mid-eclipse the sky was completely dark and one could see many faint stars. And close to the mid eclipse, for a while, Moon itself was difficult to spot. The total phase of the eclipse was for about an hour. At that time Moon was to the north of Orion constellation. That year, on June 15, Mt. Pinatubo, Philippines, erupted for about 9 hours during which it discharged something like 15 million tons of sulfur dioxide into the atmosphere. (It was L 0 on the Danjon scale. See below for the explanation of Danjon.)
It may also happen that some refracted stray light fall onto the Moon’s surface (Earthshine), hence reducing the darkness.
Why we do not have a lunar eclipse every full moon?
This is because the plane in which the Moon orbits the Earth is slightly inclined to the plane in which the Earth orbits the Sun. The planes are inclined by 5.145 degrees.
The diagram below is nearly to the scale showing the relative sizes of the Earth (the blue dot) – the Moon (the red dot) and the distance between them (the black line). The area between the read lines the inclination zone of the lunar orbit. At any given time the moon will be between this limit at average distance of 384403 k.m. The blue lines shows the shadow zone of the Earth. Only when the Moon is in the shadow zone we observe the lunar eclipse.
When the Moon is between blue and red lines we see the full moon but no eclipse. As one can see form the above diagram The Earth Moon distance is very large and the orbital inclination is just about 5 degrees that on the night of full moon the the Moon and the Sun are in opposite side of the Earth and therefore the side of the Moon facing the Earth is seen fully illuminated by the sunlight.
Can I or my students do some astronomical observations?
Yes of course. With naked eyes, binoculars or low power telescope on can do exercises in crater timing and estimating the eclipse darkness on Danjon scale.
A quick note on Crater timings
It is interesting to note down how the shadow of the Earth progresses on the lunar surface. All one needs to do is to use a good pair of binoculars or a telescope and note the times as it touches various craters and then leave it.
For a lunar map with crater and crater time indicated – click here.
A quick note on Danjon scale
The French astronomer Andre-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:
L = 0 Very dark eclipse. Moon almost invisible, especially at mid-totality. L = 1 Dark Eclipse, gray or brownish in coloration. Details distinguishable only with difficulty. L = 2 Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright. L = 3 Brick-red eclipse. Umbral shadow usually has a bright or yellow rim. L = 4 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.