Solar and lunar eclipses are special shows for us on the earth of a remarkable circumstance involving the three bodies. The apparent sizes of the sun and the moon, seen from the earth, happen to be very nearly the same. In fact, you can look at the orange sun on the 13th or 14th day after new moon, and then just turn by 180 degrees to the east to see another exactly similar orange disc — the moon. For all practical purposes, their sizes are constant.
The periodic variations in the distances between any two of the three bodies make the moon appear sometimes slightly larger than the sun, resulting in the spectacular total solar eclipse when the former aligns in front of the latter for a viewer on earth. The other possibility is when the moon appears slightly smaller than the sun and aligns in front of it. This results in an annular eclipse, when the outer rim of the sun is seen as a thin annular ring.
Such an annular eclipse will be visible on December 26 (Thursday). A part of the path of annularity of the eclipse passes through the South Indian states of Kerala, Karnataka and Tamil Nadu. Observers located in this belt will be able to see the whole of the annular eclipse. Those outside the belt will be able to see only a partial eclipse. Mangaluru and Madikeri in Karnataka lie inside the belt. Many towns in Kerala and Tamil Nadu will offer the spectacle of annularity.
The time of start and end are only slightly different in different locations, but the duration of the annularity may be significantly so. For example, the duration is about 2 minutes 10 seconds in Mangaluru and about 1m 15s at Surathkal. At other places towards the east, there is an increase in the duration of annularity. At a place called Galang in the Indonesian island south of Singapore, it is going to be a maximum of about 3m 30s.
One may recall the eclipse of the afternoon of January 15, 2010. The duration of the annularity was more than 11 minutes, and it was the longest of the millennium! The size of the moon was at its minimum, while the sun was fairly large. The earth was the nearest to the sun on January 4. Thus, the eclipsed sun, referred to as the ‘ring of fire’, was large enough to brighten up the sky. The morning of 26 December 2019 is expected to be somewhat similar. Another annular eclipse is scheduled next June on summer solstice. On that occasion, a very thin ring of fire will be visible.
Eclipses are treated as unwelcome and evil events by some people. The fear is born of ignorance. A thousand years ago, when accurate prediction of eclipses was considered a challenge, astronomers were treated with gifts and grants. There is ample evidence that the eclipses were observed and recorded in stone inscriptions, of which there are many. While a few describe the details of the planetary positions, most of them give the genealogical details of the donors and the awardees.
A handful of records describing the event itself – as total or annular – have been useful in tracing the rate of change of rotation of the earth. For example, an inscription from Bagalkot described the event as total. This fixes the limit for the extreme positions of the width of the shadow. For either extremity, a specific value of the rotation speed matches. Quite interestingly, the speed differs from today’s value significantly. Thus, records of total and annular eclipses have helped us understand the variation in the rotation period of the earth.
Total eclipses provide opportunities to discover new ideas. For example, the corona and chromospheres were seen for the first time during eclipses. The element helium was discovered in the 1868 eclipse when the shadow swept across Karnataka and Andhra. Exactly 100 years ago, the theory of relativity was verified during an eclipse. In 1894, a team of scientists from Madras had declared that observing annular eclipses is useless. It is interesting that the chromospheric lines were indeed seen in an annular eclipse.
Now, all these types of experiments are done routinely from space-based laboratories. However, the shift is towards understanding the earth itself. There is a sudden change in the temperature and pressure during an eclipse and this generates waves in the atmosphere. The effect of these on the ionosphere generated great interest in the 1970s. Since the speed of the shadow (the moon) varies from eclipse to eclipse, wave propagation experiments are carried out. Thus, ‘eclipse meteorology’ has come into play. Radio observations are also carried out to quantify the behaviour of the ionosphere.
For researchers in the life sciences, these eclipses offer opportunities for rare experiments. For example, tree leaves fold exactly as they would do at sunset on a normal day. The late Prof M K Chandrasekharan, an authority on chronobiology and popularly known as “bat-man”, chanced upon the behaviour of bats during an eclipse. A young medical practitioner in Kerala monitored the functioning of her patients while keeping them uninformed about an eclipse. Their functioning was as on any other day. Another team of veterinary doctors experimented with hens, sheep and the like to see the effect on eggs and foetuses. There were none.
The shadow of the moon will sweep through the dense forests of Kodagu. It will provide a brief chance to study the effects on animals. The birds will return to the nests, monkeys will pretend to sleep by 9 am, only to wake up 10 or 15 minutes later, and some nocturnal animals may venture out, providing a chance for photographers.
The eclipse is a handy tool for education. A simple demo with a small handheld mirror, experiencing the darkness, the rings in the shadows of trees, and finally, the rarity of the event itself -- all lead to questions. Make sure that you or your children don’t view the sun directly, and the annular eclipse will make for an unforgettable experience.
(The writer is former Director, Jawaharlal Nehru Planetarium, Bengaluru)