Friday, June 10, 2011

Watch out! - Total Lunar Eclipse on 15-16 June 2011


Three of my earlier blog posts related to my personal experience of witnessing three great solar eclipses from widely different locations, both within and outside the country, over a span of three decades.  Though I have seen a fair number of lunar eclipses, the only reaon I have not written about any of them is that none of them is etched in my memory as strongly as the three solar eclipses did.  This time though, I am writing about a total lunar eclipse which is slated to happen in the immediate future, on the night of 15-16 this month (June 2011).  I am doing this by way of advance information to alert interested readers and invite them to share my experience if the rather unpromising monsoon skies permit viewers in any part of the country.

How Eclipses occur

As pointed out in an earlier post, solar and lunar eclipses as seen from the Earth would not have been possible at all but for a very fortuitous and remarkable circumstance relating to the three celestial bodies. The apparent sizes of the Sun and the Moon as seen by us on Earth just happen to be nearly the same, approximately half a degree in angular diameter. Let me first repeat my earlier explanation in lay terms of how lunar eclipses are caused.  At predictable times during the motion of the Moon around the Earth and of the two together around the Sun, the three objects can be found in a straight line for short durations of time. When the Earth comes between the Sun and the Moon as on a full moon day, the latter can be found in the shadow of the Earth and hence sunlight will not be falling on it directly.  This is when a lunar eclipse occurs.  As seen from any point on the Earth, the eclipse may be partial or total.  One might expect the Moon to be completely hidden from view during a total lunar eclipse.  However, the whole of the Moon can be seen clearly as a faint coppery red object because of sunlight scattered by the Earth’s atmosphere, predominantly in the red, falling on the lunar surface and thereby illuminating it.  It is a wonderfully beautiful sight to behold! Would it be fair to regard the Moon as truly eclipsed?

In contrast, when a total solar eclipse occurs, the Sun's disk is indeed fully hidden by the Moon for the duration of totality.  The spectacular sight we see then is that of the glowing solar corona, not of the Sun itself.

Total solar eclipses are relatively rare events; exceptionally rare from any specific location on Earth.  One may end up waiting a whole lifetime for it to be visible from ones' own backyard.  Also, the total phase lasts typically just a few minutes, the longest possible duration being about 7.5 minutes.  In contrast, total lunar eclipses are more frequent, visible over a very large area of the Earth's surface and with much longer duration of totality.   The reason is easy to understand.  The Earth is about eighty times as large as the Moon and hence the area of its umbral shadow at the Moon is very large, much larger than the visible surface of the Moon.  In contrast, the umbral shadow of the Moon at the Earth during a solar eclipse is only a small fraction of the visible surface of the Earth itself.

The following diagram shows how a lunar eclipse takes place.  Visualize the situation when the locations of the Earth and Moon are interchanged to depict a solar eclipse.

Forthcoming Eclipse of the Moon

The forthcoming total lunar eclipse of Jun 15-16 will occur when the Moon is situated in the southern constellation of Ophiuchus as shown misspelt in the following chart generated with the Starry Night Pro Plus 6 astronomy software package.

[Click to enlarge]

Apart from the constellation Ophiuchus, which is traditionally not regarded as a zodiacal constellation, the chart also shows the two neighbouring and very prominent southern  zodiacal constellations Scorpius and Sagittarius. The Moon passes well within the Earth's umbral shadow during a total phase lasting about 100 minutes, one of the longest in recent decades.  The Moon's contact times with the Earth's umbral and penumbral shadows are shown in the following figure adapted from the NASA ecipse website.  The times shown are in Universal Time (UT).  Add 5:30 to obtain the local time (IST) anywhere in India.  Thus, the onset of totality will start at approximately 11:52 PM on the night of June 15 and end a little after 3:02 AM on the morning of June 16.

Watching the Eclipse

Sunset on 15th eveneing should be greeted by a bright and brilliant full Moon growing more and more prominent in the sky as the night progresses.  As the path of the Moon enters the Earth's penumbral shadow around 10:54 PM it may be extremely difficult to notice even a slight decrease in the brightness of the Moon.  Only skilled observers may detect a faint shading across the lunar disk. This phase will last about an hour until the Moon starts entering the umbral shadow of the Earth around 11:52 PM.  This marks the beginning  of the partial phase of the eclipse.  Thereafter one should be able to see a progressive 'eating away' of the Moon's surface and a marked decline in brighness as the visible portion of the lunar surface reduces.  Around 00:52 AM on June 16, totality will set in when the whole of the lunar surface is inside the Earth's umbral shadow.  However, as already explained, the Moon continues to be visible as a faint copper red disk.  This phase lasts the next 100 minutes, with small but noticeable variations in the overall brighness.  This is the time when the viewer can soak in a wonderful view of the night sky with a faint reddish full Moon appearing to be suspended like a friendly ghost in the sky with rich star fields nearby.

The eclipse will be seen in a part of the southern sky which is very rich in stars, star clusters, nebulae and the densest part of our Milky Way galaxy.  Scorpius and Sagittarius are two of the most prominent and easy to identify constellations in the night sky. If the visibility is good, the patches of the Milky way shown in the above chart should be seen clearly during the total phase of the eclipse.

The total phase will end by 02:32 AM when the Moon should begin to shine in direct sunlight.  It should progressively grow brighter and restored very nearly to its full glory when the partial phase ends around 03:32 AM.  However, the Moon will remain in the penumbral shadow until around 04:30 AM.  This marks the end of what should be a memorable lunar eclipse.
Unlike a total solar eclipse, one need not be in any hurry to watch the lunar eclipse because of its very long duration.  While watching it, the reader is urged to explore the rich southern sky and try to locate most of the Messier and other stellar objects in Sagittarius and Scorpius, particularly the patches of the Milky Way in the direction pointing to the galactic centre in the former.  A pair of binocualrs would be the best means to do this and it can be used for a blown up view of the eclipsed Moon as well.  Unlike a solar eclipse where one has to take special precautions and safe filters to view the event, the Moon is a totally harmless object at any time, and especially so during an eclipse, no matter what the media may project.  The astrologers and soothsayers will no doubt have their field day with a highly gullible public willing to believe anything portentious and sensational and not caring for a scientific perspective of the event.

Eclipse Visibility

The following illustration, also from the NASA website,  shows the visibility of the eclipse from any place on the Earth.  As is evident, the event can be seen in its totality from most parts of Asia and Africa as well as many other regions.  For a solar eclipse one would have to show a narrow visibility band on a map of a small part of the Earth's surface (See my previous blog posts on solar eclipses).  Also, the contact times would be significantly different for different locations on the Earth, both inside and outside the totality band.  In contrast, one set of contact times will be sufficient for all observers of a lunar eclipse.

Dr Fred Espenk, popularly known as "Mr Eclipse" for his pioneering work on eclipses, recalls having seen thirty years ago a total lunar eclipse with the Moon located very nearly in the same part of the sky as the one expected on June 15 this year. He was "amazed at how brilliantly the summer Milky Way glowed since it was all but invisible during the partial phases." Observers will have a similar opportunity during the forthcoming eclipse. In this case, the totally eclipsed Moon will lie just 8° northwest of the brightest Sagittarian star clouds.

Here is a picture of the totally eclipsed Moon taken during an earlier occasion.

During the ensuing eclipse, as in the above picture, the northern regions of the Moon will probably appear brighter than the southern regions that lie deeper in the umbral shadow. Overall, the surface may appear darker than during a typical eclipse of this type. Since the Moon goes through a large range of umbral depths during totality, its appearance may change significantly with time. It is difficult to predict the exact brightness distribution in the umbra, so observers are encouraged to make their own comparative visual estimates at different times during totality using what is known as the Danjon Scale of Lunar Eclipse Brightness. Please refer to the NASA eclipse website for details.

The follwoing is a composite image of the Moon depicting its different phases during an earlier eclipse:

Photography of the lunar eclipse is relatively easy, especially during the partial phases.  Because of the long duration of totality, one can expect to capture excellent pictures after some initial trial and error with a good digital camera at high zoom levels.  Mounting the camera on a rigid tripod is strongly advised.


It is hoped that the prevailing cloudy monsoon weather will relent and provide a clear sky to view the special event on the night of June 15-16 everywhere within the country, particularly the southern region where the probability of clear skies may be rather discouragingly low.  I would love to hear from successful viewers by way of observations and comments posted here or mailed to me directly.  I will try to correlate these with my own observations and report in a later post before the end of the month.


A V G Rao said...

Thanks for the detailed information

manjunatha said...

Very good information -Manju BM

Dr. S. V. Narasimhan said...

Respected Sir,

But for your blog and also your talk, in Kannada, in ETV I would have remained least interested in this Lunar Eclipse. Thanks for that and I was awake whole night to have glimpses of moon peeping through the thick monsoon clouds in Coorg!



BM Hegde said...

You are a good writer, astronomer and an excellent photographer.