Total Lunar Eclipse - 10 December 11
- the second total lunar eclipse of 2011

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

On the night of 10th Dec 2011 more than half the world will have opportunity to watch the second total lunar eclipse of 2011. Though it will not be the darkest eclipse of the moon that took place about six months ago on 15th of Jun. The crisp winter night in India offers a very good chance across the subcontinent for a excellent chance of observing this event. The entire event will last just about 3minutes less than 5 hours.

This also happens to be the last chance for Indians to observe the total lunar eclipse for some time to come. The next total lunar eclipse will be on 7th Aug 2017, almost 6 years from now.

What would happen on the night of December 10, 2011
The Moon will in the penumbral shadow of the Earth at about 5:00 p.m. Indian time. At this time for most of the places in India the sun will be above the horizon. For Kolkata the sunset would be at 4:43 p.m. about 10 minutes after the moonrise.

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 6:16 p.m. the Moon will be in the umbra of the Earth's shadow.  The dark shadow of the Earth  will progress on the lunar disk, covering crater by crater. This progress of the shadow of the Earth will be quite noticeable to the naked eyes. 

In an hour and quarter's time the Moon will be totally 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 20:27:16. At this time the Moon will be very dark. The sky will be like what we have during the new moon day (or Amavasya). One can see lots of faint stars. The Moon will umbra till about 9:48 p.m. A by 10:00 p.m. the shadow play will be over and bright full moon will illuminate the Earth.

The circumstances of the total Lunar eclipse for India (IST) on 10 December 2011 

The eclipse geometry
A lunar eclipse takes place when the Moon enters the shadow of the Earth. On this night the Earth is directly between the Moon and the Sun, blocking the sunlight reaching the Moon. Or casting its shadow on the Moon.  The event obviously takes place on the full moon night. On this night the Moon and the Sun are in opposite side of the Earth.

The diagram below shows the geometry of total Lunar Eclipse or what is technically called   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 Umbra region. 

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

You can visualize the Umbra and Penumbra region by looking at a shadow of a disk (say 50p coin) kept far away from the sheet. You can see that the central part of the shadow is dark but towards the edge you can see a defuse shadow.

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 red 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 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.