On harm to the eye due to observing a solar eclipse

An annular (not total) solar eclipse is due to occur in India on 26 Dec. 2019, after total ones on 11 Aug. 1999 amd 22 Jul. 2009.

1. Are piercing and dangerous rays or EM waves of a peculiar nature emitted during a solar eclipse?

A solar eclipse is an obscuring, interposing, occluding, or occultation, of the sun by another celestial body. The EM waves emitted then are exactly of the same nature as from the sun in plain sight, unobscured, or, even better, merely a portion of it. During a total (not annular) eclipse, the solar corona comes to view (as the ring), because the sun itself is obscured; but the EM waves emitted then are always there, only that they are more evident than with an unobscured sun.

2. How does observing the sun normally, i. e., unobscured, compare with observing it under (partial) eclipse?

It is exactly the same. Even in normal circumstances, one does not stare at the sun; and deliberately doing so contrary to natural instinct harms the eye. What is peculiar about observing the sun in eclipse is two-fold:

a. Exposure: When in eclipse, the sun appears less bright than normal, and thus the natural instinct in the observer of looking away from it after a minuscule amount of time is suppressed in the mind due to an error in cognition; thus the tendency to stare at it for longer times, normally impossible to tolerate. All the same, the EM waves emitted are just as dangerous as from the unobscured sun; partial obscuring or occultation of the sun does not reduce the harm of the rays emitted from its visible portion.

b. Aperture: Again, since the sun appears less bright than normal, the iris dilates the pupil to an uncustomary extent, and more of incident EM waves are let in to the retina than usual.

Both these causes result in greater irradiation of the retina and harm to the sensory cells (retinopathy). Some describe it as the burning of the retina by solar rays, but this is only a metaphor; no pain is felt, as testimonies show; only gradually is it realised by the observer that the sight has been impaired; and the harm comes more from the UV and higher frequencies above the visual spectrum than from the IR and lower frequencies which carry heat.

In my opinion, only these two things (and not any kind of peculiar rays or EM waves radiated just during an eclipse) give any particularity to watching the sun under eclipse; and these arise on the side of the observer, not on the side of the sun or the moon.

Of course, during totality (not partiality or annularity), which lasts at most a few minutes (the longest duration is about 7 1/2 minutes), it is completely secure and harmless to look at the sun, unaided. But one must be careful because, the immediately preceding and succeeding instants are particularly harmful for observation, when the phænomenon of the diamond ring occurs, the diamond being the minuscule portion of the sun emerging into visibility.

3. What are the ways to observe a solar eclipse?

Only through filters which are absolutely guaranteed to eliminate solar rays of harmful frequency and intensity.

Solar eclipses have been known since antiquity. Yet it must be remembered that classical man had extraordinary knowledge of astronomy, certainly better than the average or even `educated' man of today, and was able to predict rather than merely watch solar eclipses and other astronomical phænomena. Doubtless, initial observations led to damage to the eye, and further attempts must have been more careful.

Watching reflections on a mirror is not safe, as the attenuation is not usually sufficient to remove the danger. Watching reflections on water is also not absolutely safe, though the image has greater attenuation than a mirror. The pin-hole camera {Camera obscura} is the most appropriate one for home observations.
https://www.livescience.com/59721-solar-eclipse-viewer-photo-tutorial.html

4. A false story about Galileo Galilei turning blind due to solar observations persists.
https://aty.sdsu.edu/vision/Galileo.html

5. Antiquity also had many views of a different approach and nature on solar eclipses.
https://www.timeanddate.com/eclipse/solar-eclipse-myths.html

6. A battle was fought between the Lydians and the Medes at the banks of the River Halys in Anatolia/Asia Minor/Asiatic Greece on 28 May 585 B. C. A solar eclipse, predicted by Thales of Miletus, occurred, and the battle was called off. Herodotus writes: `Afterwards, on the refusal of Alyattes to give up his suppliants when Cyaxares sent to demand them of him, war broke out between the Lydians and the Medes, and continued for five years, with various success. In the course of it the Medes gained many victories over the Lydians, and the Lydians also gained many victories over the Medes. Among their other battles there was one night engagement. As, however, the balance had not inclined in favour of either nation, another combat took place in the sixth year, in the course of which, just as the battle was growing warm, day was on a sudden changed into night. This event had been foretold by Thales, the Milesian, who forewarned the Ionians of it, fixing for it the very year in which it actually took place. The Medes and Lydians, when they observed the change, ceased fighting, and were alike anxious to have terms of peace agreed on.' The River Halys was agreed upon as the border between the two nations, and Aryenis the daughter of Alyattes, King of Lydia, was married to Astyages the son of Cyaxares, King of Media. This may be the earliest recorded prediction of an eclipse, and in general, a scientific phænomenon, and also the earliest historical event known precisely by date.

7. A chronology of principal solar eclipses in India:

1868
18 Aug. -> Total solar eclipse, Central India (Belgaum, Kolhapur, Gulbarga, Hyderabad, Masulipatnam, Guntur, Vijayawada, Kakinada).

1871
12 Dec. -> Total solar eclipse, S. India (Mangalore, Coimbatore, Dindigul, Trichinopoly).

1872
6 Jun. -> Annular solar eclipse, S. India (Trichur, Mysore, Coimbatore, Salem, Madras).

1898
22 Jan. -> Total solar eclipse, N. India, Buxar.

1980
16 Feb. -> Total solar eclipse, Central India (Hubli, Raichur, Nalgonda, Konark). First total solar eclipse of the XXth century to be observed in India. https://www.iiap.res.in/?q=solareclipse

1995
24 Oct. -> Total solar eclipse, N. India (Rajasthan, Iradatganj, Calcutta). Deepavali.

1999
11 Aug. -> Total solar eclipse in Central India (Srikakulam).

2009
22 Jul. -> Total solar eclipse in N. India (Bhavnagar, Surat, Ujjain, Indore, Bhopal, Sagar, Jabalpur, Kashi/Benares, Allahabad, Gaya, Patna, Tawang, Guwahati, Dibrugarh). Longest total solar eclipse of the XXIst century. https://www.iiap.res.in/eclipse_varanasi

2010
15 Jan. -> Annular solar eclipse in S. India, Trivandrum. Longest annular solar eclipse of the IIIrd millennium.

2019
26 Dec. -> Annular solar eclipse in S. India, English Malabar (Cannanore, Ceylon).

2020
21 Jun. -> Annular solar eclipse in N. India, Uttarakhand. Summer solstice.

Further keywords and labels: Antumbra, Astronomy, Celestial mechanics, Eclipse, Electromagnetic waves, Eye, Light, Natural philosophy, Occultation, Optics, Physics, Rays, Sight, Solar eclipse, Solar rays, Thales of Miletus, Umbra, Waves, Solar system, Planets, Sun, Moon, Earth, Planetary motion.