Total Lunar Eclipse - 15 June 11
- Beautiful darkest night during a full moon

by Arvind Paranjpye (arp@iucaa.ernet.in)

On the night of 15th June, 2011 more than half the world will have the opportunity to watch one of the darkest Lunar eclipses.

People living in eastern Africa, the Middle East, central Asia and western Australia will have opportunity to observe the entire eclipse, from beginning to end. At mid-eclipse the Moon will be overhead at Mauritius. Observers in eastern Brazil, Uruguay and Argentina will witness totality. However, none of the eclipse will be visible from North America.

This will be the 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 47 years from now on on June 6th, 2058.

Other than the near perfect alignment of these solar system bodies, the atmosphere plays its role too in the darkenss of the eclipsed Moon. It may let some refracted stray light onto the Moon's surface (Earthshine), hence reducing the darkness. The eclipse of June 15th may also be one of the darkest due to the ashes thrown into the Earth's atmosphere by the recent eruption of Iceland's most active volcano, Grimsvotn.

The author 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 faint stars too. The Moon itself was difficult to spot. The 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)

This year in addition to the ashes from Grimsvotn, the Earth's  atmosphere is still contaminated with ashes of  last year's eruption of volcano Eyjafjallajokull.  It would really be interesting to see how dark the eclipse of Jun 15, 2011 would be.

The eclipse geometry
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 the the Sun, Earth and the Moon are nearly on one straight line. The Moon is in total shadow of the Earth or it is in the Umbra region. 

(The diagram above not to the scale and it is for the purpose of illustration only.)

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.

The circumstances of the total Lunar eclipse for India (IST) on 15/16 June 2011 

What would happen on the night of June 15, 2011
The Moon will in the penumbral shadow of the Earth at about 10:53 p.m. Nothing much will be noticeable to the untrained eyes for next 30 to 40 minutes.  After that one might notice gradual change in the brightness on the lunar disk.  By 11:53 the Moon will be in the umbra of the Earth's shadow.  The dark shadow will progress on the lunar disk. Covering crater by crater of the Moon. This will be quite noticeable to the naked eyes. 

In next one hour the Moon will be completely inside the shadow of the Earth. At this time the colour of the lunar disk will be red with it's many hues - crimson, brick red etc.

The time of the maximum eclipse is 1:42:24 a.m. of June 16, 2011. At this time the Moon is highly likely to be 'out of sight'. But soon its colour will start coming back.  It will be in Umbra till about 3:32 a.m. and will be back to its normal 'self' by 4:32 a.m.

The Moon eclipses a star
At about 11:30 p.m. of June 15, the Moon will hide behind it a star catalogued as 51 Ophiuchi in the constellation of Ophuuchus.  The star will reappear after about hour and half. This kind of event is called occultation. The exact time when the star disappears and reappears is different for different location. For occultation predictions for different cites in India, Nepal, Pakistan, Sri Lanka click here.

Why would the Moon look red in colour during the total phase
The Earth's atmosphere plays it's role here.  If the Earth had no atmosphere the 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 with the rising or setting Sun (or Moon).

As the sunlight passes through the atmosphere of the Earth, the light is scattered and bent slightly. The light is scattered by the constituent atmospheric particles.  The blue component of the sunlight is scattered most but 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. And the bending of like takes place as it goes from one medium to other, it 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.

As indicated above the Earth's atmosphere contaminated by the volcanic ash is likely to have its pronounced effect on how dark the eclipsed moon would be..    

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.  

How many eclipses in a year?
It can be calculated that maximum number of eclipses possible in a year is 7, of which 4 are solar and 3 lunar or 5 solar and 2 lunar. The least possible eclipses during a year is 2, and these are both solar eclipses..

The year 2011 has six eclipses - four solar eclipses and 2 lunar.

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.

Please visit NASA web site by Fred Espenak Danjon Scale and Crater timings.

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.