Monsoon Eclipse: Aug 1 2008

In recent times most of the eclipses have been visible with a very small fraction of the Sun being seen as eclipsed, from India. The last total solar eclipse from India had not been too happy an experience, with most of the totality belt having been clouded out during the eclipse. We now have the eclipses of 2008 and 2009, also falling in the monsoon season, and will therefore need to gear up and scan Indian Geography and its monsoon vagaries very carefully, to decide where we are going to be, to observe the 2009 Total Solar Eclipse.

Photo Courtesy: Ajay Talwar

The August 1st 2008 Total Solar Eclipse will be seen as a partial eclipse from India, so perhaps people might not travel to different locations in India, for this eclipse, and will more likely want to know what are the chances from their home location, of viewing this eclipse. Before we discuss anything else, it is very important to emphasise that viewing the eclipse with naked eyes would be very dangerous for the eyes. Viewing the Sun through a telescope or a binoculars without a proper filter is many times more dangerous – do not ever do that, it could destroy your eyesight. The safest way of viewing a partial solar eclipse is through the method of projection. Let us now, look at the circumstances of the August 1st 2008 eclipse, for different locations in India. We may be missing the totality of this eclipse, but, the northern parts of India do get to see a large fraction of the disc of the Sun eclipsed.

In this figure, adapted from
The path of totality is marked in dark blue, and passes well to the North of India. The dark green lines indicate regions where the eclipse will start at the times indicated. These timings are in Universal Time (UT) to which we need to add 5.5 hours to obtain the Indian Standard Time. The light blue lines are contours of constant Eclipse Magnitude.

The Eclipse Fraction is defined as the fractional diameter of the Sun eclipsed, at the maximum of eclipse at any given point. The southern parts of the country will see between 20 -40 % eclipse fraction, the central regions between 40-60 % while the Northern parts of the country see between 60-70 eclipse fraction, at maximum, during the 2008 eclipse. There is another way of looking at a partial eclipse – through a quantity known as the Obscuration Fraction. The obscuration fraction is the fractional area of the disk of the Sun covered by the disk of the Moon. For Delhi, for instance, the eclipse magnitude is 62.26 % while the maximum obscuration fraction is 53.7 %. Using the box projection apparatus mentioned above, and by projecting an image of the Sun on to a graph sheet and photographing that image, one can make some estimates of the obscuration fraction as the eclipse progresses from beginning to end and check them against known theoretical values. Try it, it is great fun!
Here is a graph of the measured obscuration fraction of the eclipse of March 2006, compared with theoretical values. The measurement and comparisons were done by members of the Amateur Astronomers Association, Delhi, at an eclipse skywatch organized at the Nehru Planetarium, New Delhi.

The binocular/pinhole box projection apparatus can also be used to measure the relative angular diameter of the Sun and the Moon, using simple school geometry. To learn how to make a simple pinhole camera click here This is an exciting activity to do, if we are located in the partial zones of a total or an annular eclipse. With such a measurement we can discern whether the eclipse is total or annular, in the central belt, even if we are situated far from the central belt. A kind of a celestial ventriloquism that we can practice 🙂

We might also be able to measure the position angle of the first contact point, with such an apparatus, if there is a sunspot visible on the day of the eclipse. The First contact is the very beginning of the eclipse as the first dent or a miniscule bite appears to have been taken out of the Sun – the first external tangency between the Moon and the Sun, as seen from a given location. The position angle of this contact point is defined as the contact angle measured counter-clockwise from the north point of the Sun’s disk.

There, we are all set, if we manage to do this carefully and obtain our East-West and North-South axis on our projected image that we capture just at first contact, we should be able to pull out the position angle of the first contact reasonably accurately.
So, go ahead and make your binocular box projection apparatus, or a simple projection apparatus for a telescope with a stand, and enjoy watching the eclipse safely. Be sure to do a little something quantitative, with measurement of some eclipse related parameters, to enhance your enjoyment of the eclipse! And look ahead, to the July 22nd 2009 Total Solar eclipse in India.

Some Tips on Solar Safety can be found on this page

Penned by Dr N Rathnasree, This article also appears in a Forthcoming issue of Krittika

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