By Dr. S.A. Mohan Krishna, Amateur Astronomer

In the festive fervour, all shall be privileged to witness solar eclipse. Indians are really fortunate to witness the most astounding, anticipated and striking ‘Partial Solar Eclipse’ on Oct. 25 from all prominent parts of the country.

Though solar eclipse happens in regular intervals, annular/ partial solar eclipse is a sporadic phenomenon and it occurs when moon’s apparent diameter is smaller than sun’s, thus obscuring most of the sun’s light. On Oct. 25, the eclipse is visible in the region covering Europe, Middle East, Northern Africa, Western Asia, North Atlantic Ocean and North Indian Ocean.

In India, the eclipse will begin before sunset in the afternoon and will be seen from most of the places. However, the same cannot be seen from Andaman & Nicobar Islands and some parts of north-east India (name of few of such places are Aizawl, Dibrugarh, Imphal, Itanagar, Kohima, Sibsagar, Silchar, Tamenglong). Ending of the eclipse will not be visible from India it will be in progress after sunset. On that day, generally eclipse starts by 2.30 pm (IST) and ends by 6.32 pm (IST). But in India, it is partial solar eclipse and the eclipse begins at 5.15 pm and ending phase cannot be noticed. Moon can be noticed only after 5 pm in all parts of India.

From Mysuru, the type of eclipse is preferably partial, as we cannot completely notice obscurity. The moon covers a very small portion of the sun, so this is a decent sight. The obscurity of moon at Mysuru is about 9%. A pinhole camera is the simplest way to view an eclipse, but the solar image will be inverted.

During the eclipse, the sun is in the constellation Virgo. The Saros catalog describes the periodicity of eclipses. This Oct. 25 partial eclipse belongs to Saros 124. It is number 55 of 73 eclipses in the series. All eclipses in this series occur at the moon’s descending node. The moon moves northward in relation to the node with each succeeding eclipse in the series. Solar Saros 124, repeating every about 18 years and 11 days, contains 73 events. The series started with partial solar eclipse on March 6, 1049. It contains total eclipses from June 12, 1211, to September 22, 1968, and a hybrid solar eclipse on October 3, 1986. The series ends at member 73 as a partial eclipse on May 11, 2347. The longest total eclipse occurred on May 3, 1734, at 5 minutes and 46 seconds.

Observation and Safety

Eclipse photography is fascinating, but there is one point to be borne in mind. For telescopic use, filters made of chromium deposited on glass pass the requisite amount of photospheric light to photograph and view the partial phases while also giving images of excellent definition.

A very slight increase in transmission at the red end of the spectrum often gives a pleasant tint to the solar image. Welders’ glass of #14 or #15 grades is also safe for visual or photographic viewing, although the colour is usually less pleasing.

Filters that go on the eyepiece end are always liable to crack or be otherwise damaged under the concentrated solar radiation, so are much less safe than filters placed above the objective lens. But to observe a total or partial lunar eclipse, there is absolutely no harm in watching with unaided eye. There is absolutely no necessity of a telescope or binocular to observe total or partial lunar eclipses.

Brightness of the solar corona during annularity or totality is approximately that of the full moon, and the corona during totality is equally safe to watch. However, starting at crescent or other partial phases of eclipse can be hazardous to eyesight, so caution should be given about how to watch an eclipse safely.

In general, the partial phases are hazardous, but totality is both glorious and safe. In fact, we will not experience the full glory of totality unless we view it directly. To observe partial or total solar eclipse, it is safe to make use of solar filters. The key factor in safe solar filters, aside from their absorbing or reflecting sufficient levels of solar intensity, is that they do so evenly across the spectrum; that is, that they are of Neutral Density (ND). Solar Filters are made of aluminised Mylar, a coated plastic, are very popular and inexpensive. As long as these Mylar filters are undamaged, without creases or pinholes, they are safe to look through.

Observing directly at the photosphere of the sun (the bright disk of the sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the highly intense visible and indistinguishable radiation that the photosphere emits. This damage can result in the impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring.

Viewing the sun during partial and annular eclipses require special eye protection or indirect viewing methods if eye damage is to be avoided.

The sun’s disk can be viewed using appropriate filtration to block the detrimental part of the sun’s radiation. Sunglasses do not make viewing the sun safe. Only properly designed and certified solar filters should be used for direct viewing of the sun’s disk. Especially, self-made filters using common objects such as a floppy disk removed from its case, a compact disc, a black colour slide film, smoked glass, etc. must be avoided. Specially tested solar goggles are available at Mysuru, Bengaluru and other parts in Karnataka for very reasonable price, say Rs. 30 to 50, which are absolutely safe to watch any type of solar eclipse. 

From Mysuru, the best viewing location is Chamundi Hill. It is always recommended using binoculars or a small low-power telescope, since they both allow the entire moon to be viewed during the event. So, do not forget to witness the solar eclipse tomorrow.