Solar spectacle

THE total eclipse of the sun that will occur on July 22, 2009, will be one of the most important astronomical events of this century for India as the path of totality will pass over a good number of cities and several densely populated areas of the countryside. Moreover, the next solar eclipse whose totality track will pass over the thickly populated areas of India is to occur in 2114, that is, after a gap of over 100 years. This eclipse is of special significance also because it will be the longest solar eclipse of this century, with totality lasting more than 6.6 minutes at maximum. This will make it the longest total solar eclipse until 2132.

Of the many astronomical phenomena of interest to the layman as well as the professional astronomer, and visible to the naked eye, a total solar eclipse is the most spectacular. Rarely does the track of totality pass over thickly populated regions, but if one is near the path, it is worth travelling to the area to witness the eclipse.

An eclipse of the sun is caused by the inter-position of the dark body of the moon between the earth and the sun so that the shadow of the moon sweeps over the face of the earth. It can occur only during the new moon, when the moon is in conjunction with the sun. In other words, an eclipse of the sun occurs when the moon passes so near the line between the earth and the sun as to cut off some or all of the suns light. This shadow consists of two parts: the umbra, or total shadow, a cone into which no direct sunlight penetrates; and the penumbra, or half shadow, which gets light from only a part of the suns disc as shown in Diagram 1.

To an observer who is within the umbra, the sun will appear completely covered by the moon, that is, a total solar eclipse (Diagram 1-b). If the observer is within the penumbra, the moons disc will appear projected onto the suns disc so as to cover it partly, that is, a partial solar eclipse (Diagram 1-c). An annular eclipse occurs when the moon is directly between the earth and the sun but the umbra does not reach the earth (Diagram 1-a).

If the plane of the moons orbit around the earth coincided with the ecliptic (that is, the plane of the earths orbit around the sun), an eclipse of the sun would take place at every new moon, that is, at intervals of about 29 days. But this is not so. The moons orbit is actually inclined about five degrees to the ecliptic, and it is only when the moon happens to be at or near one of the nodes where the orbits intersect that the three bodies are nearly in the same line and an eclipse can occur. At other times, the shadow of the moon just disappears into space.

In a year, there can be a minimum of two eclipses, both solar, and a maximum of seven eclipses, out of which four may be solar and three lunar or five solar and two lunar. The track of totality is narrow never more than 269 kilometres and as such the chances of seeing a total eclipse from any given locality are small. In the long run a total eclipse of the sun happens at a particular place only once in about 360 years.

The sun is 1,392,000 km in diameter and the moon is only 3,476 km in diameter. To us on the earth, the two bodies appear about the same size because while the diameter of the sun is about 400 times that of the moon, the sun is also about 400 times as far away. It is then to be regarded as a circumstance fortunate for the advancement of astronomical science that the spatial relationship of the sun, the moon and the earth are as they happen to be. In fact, a very small decrease in the angular diameter of the moon relative to that of the sun would have rendered improbable this most sublime spectacle of a total eclipse of the sun.

The Chaldean astronomers in about 400 B.C. discovered that eclipses occur in regular succession at an interval of 18 years and 111/3 days. This cycle is known as the Chaldean Saros. Saros comes from a Greek word for repetition. The exact interval is 223 lunations, or 6,585.3 days. The Saros cycle provides a simple method of predicting the eclipses. But though the eclipse repeats itself every Saros cycle, it is not visible from the same place on the earth. Since the cycle is one-third of a day longer than 18 years and 11 days, when the eclipse recurs, the earth will have spun one-third of a rotation farther east and the eclipse will occur to the west of where it did earlier.

As the moon moves eastward in its orbit with respect to the sun at an average speed of 3,400 km an hour, its shadow sweeps eastward across the earth at the same speed. The earth, however, is rotating towards the east in the same general direction. Since at the equator the earths surface moves at about 1,670 km an hour, the shadow moves relative to the earth at about 1,730 km an hour. In higher latitudes, where the velocity of the earths surface is less, the shadows relative speed is higher.

The tip of the truncated cone of the umbra of the moons shadow sweeps along a thin band across the earths surface and the phase of totality of the eclipse is observed successively along it. Because of the shadows speed, totality is brief. The path across the earth within which a total solar eclipse is visible is called the path of totality.

Within about 3,000 km on either side of the path of totality, a partial eclipse is visible, the observer being located in the penumbra of the shadow. An eclipse of the sun observed at a location near the equator under the most favourable conditions namely, when the moon is nearest to the earth and the sun is at its farthest from the earth may be a total one for 7 minutes, the maximum possible duration. Such an eclipse will be visible in India on July 5, 2168 the longest eclipse in human history.

The long totality phase of the July 22, 2009, eclipse will mainly be because the eclipse will start just a few hours after the moon reaches its perigee (closest approach to the earth). At such a close distance, the moon appears fully 8 per cent larger than the sun and casts a broader than usual shadow.

In the observation of a solar eclipse, four contacts are recognised: the first when the edge of the moon first touches the edge of the sun, the second when the eclipse becomes total or annular, the third at the cessation of the total or annular phase, and the fourth when the moon finally leaves the suns disc. From the first contact to the fourth, the time can even be a little over four hours.

The last total solar eclipse whose path of totality passed over India occurred on August 11, 1999, but this eclipse lost its importance as the phase of totality was visible just before sunset mainly from a few locations in Gujarat.

The total solar eclipse on February 16, 1980, was of particular interest to astronomers in India as there was a well-observable totality phase in India for the first time in the 20th century. The previous total solar eclipse seen over central India occurred on January 22, 1898 the solar eclipse of 1980 embraced the Indian subcontinent after a long gap of 82 years.

Of course, after a gap of only 15 years, on October 24, 1995, the path of totality of another solar eclipse swept over thickly populated areas of India, and a very large number of people and astronomers were able to witness it.

The path of totality of the July 22 eclipse will start just off the western coast of India and cross India, the extreme north of Myanmar and China to end in the Pacific Ocean among the Hawaiian islands. The totality phase will begin at 6-23 a.m. IST and end at 9-48 a.m. IST. This eclipse will be visible as a partial eclipse from the eastern part of Africa, Madagascar, Asia, part of Indonesia, the extreme north-east tip of Australia and the extreme northern part of North Island, New Zealand. The partial phase will begin at 5-28 a.m. and end at 10-42 a.m. IST.

The umbra of the moons shadow will first touch the earth off the western coast of India at sunrise at 6-23 a.m. Within seconds, the coastal city of Surat (Gujarat) will be plunged into darkness for three minutes and 17 seconds. Here the sun is only 3 above the eastern horizon, but the altitude of the eclipse will rapidly increase as the umbra rushes east. Vadodara (Gujarat) will be on the northern limit of totality and will experience darkness for one minute and 19 seconds.

The umbral shadow will take just eight minutes to cross India. The shadow will reach Indore (Madhya Pradesh), which will be plunged into totality for three minutes and 13 seconds; here the altitude of the sun is merely 6 above the horizon. Bhopal (Madhya Pradesh) lies 40 km north of the central line. Even at this distance, it will succumb to three minutes and 12 seconds of the total phase.

Approximately 400 km north of the path, the Taj Mahal in Agra (Uttar Pradesh) will experience a deep partial phase of magnitude 0.906 (magnitude is the fraction of the suns diameter obscured by the moon) at 6-26 a.m. IST. The width of the path of totality will be 218 km here. Allahabad (Uttar Pradesh) will just miss the totality track and experience a significant partial phase of magnitude 0.999.

Varanasi (Uttar Pradesh) and Patna (Bihar) both lie within the shadows path. About 500 km to the south-east, the populace of Kolkata (West Bengal) will be able to view a partial eclipse of magnitude 0.911. The track of the umbral shadow will sweep over Darjeeling and Siliguri, both in West Bengal; Gangtok in Sikkim; Thimphu in Bhutan (here the umbral path width will be 224 km); Dibrugarh in Assam; and Itanagar in Arunachal Pradesh and then across northern Myanmar before the entire shadow enters Chinas Yunnan province. Guwahati (Assam) will just miss the umbral track and experience a deep partial eclipse of magnitude 0.998.

The instant of greatest eclipse will occur at 8 hours 5 minutes 19 seconds IST at a location in the Pacific Ocean (latitude 24 13 N, longitude 144 07 E) where the duration of totality will be six minutes and 39 seconds; the suns altitude will be 86.

The countdown to the phase of totality is dramatic. Three minutes before totality, the sky darkens, some flowers fold up, and wildlife, especially birds, exhibit nocturnal behaviour. The landscape takes on unusual colour tones. Only a narrow crescent of the sun can be seen, sunlight filtering through the foliage of trees forms crescent-shaped images on the ground.

From atop a hill, if one looks towards the western horizon about two minutes before totality, one can see the umbral shadow of the moon approaching at about 3,000 km an hour. About one minute before totality, curious moving ripples of dark and light bands appear on any smooth white surface. These shadow bands (as they are called) are a curious atmospheric phenomenon. In the last few seconds, light falls rapidly, it becomes cooler and the wind tends to drop. And then the real drama begins.

In the last instant before totality, the only visible parts of the sun are those that shine through the lower valleys in the moons irregular profile and line up along the periphery of the advancing edge of the moon. This gives one the impression of watching a brilliant beaded necklace the phenomenon is known as Bailys beads. The final flash of sunlight through a lunar valley produces a brilliant flare known as the diamond ring effect. After the diamond ring disappears, if one looks at the crescent of the sun through a pair of binoculars, one can see the beautiful flash spectrum. The chromosphere (the suns lower atmosphere, which lies just above its visible photosphere) blazes away in full glory, an indication that totality has just commenced.

At this moment, one can see red and orange jets of fire shooting up. Known as prominences, they reach to heights of a few million kilometres above the solar surface. Prominences of various shapes and sizes are seen during totality as pink flame-like projections. The larger ones are visible throughout totality, while the smaller ones appear and disappear as the advancing moon uncovers or covers them.

After this phase, the bright disc of the sun is entirely hidden behind the moon, and the corona the suns outer atmosphere consisting of sparse gases that extend millions of kilometres in all directions from the apparent surface flashes into view. It is ordinarily not visible because the light from the corona is feeble compared with that from the underlying layers of the sun, but when the brilliant glare from the suns visible disc, or photosphere, is blotted out during an eclipse, the pearly-white corona becomes visible.

The corona is the most striking feature of a total eclipse. The bright inner corona contains elegantly shaped arches and loops that taper off into the fainter streamers of the outer corona. These various forms are created by the solar magnetic field. The shape of the corona varies with the 11-year solar cycle. Totality ends as abruptly as it begins. The corona vanishes, and the features of the partial eclipse now recur in reverse order. Finally, the moon leaves the suns disc. During a total eclipse, sunlight is greatly reduced and bright stars and the planets become visible to the naked eye, the planet Venus and Sirius, the brightest star, being the most prominent among these.

How must one witness a solar eclipse? Watching a partially eclipsed sun is as hazardous to the naked eye as the uneclipsed one. For safe direct viewing, the intensity of sunlight should be reduced at least 100,000 times and the ultraviolet and infrared part of solar radiation should effectively be cut off. The most effective filtering aid is a dark X-ray plate of sufficient thickness. Another safe device is the dark arc welders glass. The intensity of the uneclipsed portion, even when it becomes a thin crescent, remains high enough to cause permanent or partial blindness. During the phase of totality, one does not require any filters.

The progress of a solar eclipse can be observed safely by holding a piece of cardboard with a one-millimetre-diameter hole in it above a white surface, such as a concrete pavement. The hole in the cardboard produces a pinhole camera image of the sun. With a small telescope, the suns image can be projected onto a piece of paper held at the correct distance with the help of a fixed frame. It will help to keep this piece of paper within a small, empty, dark wooden box with no lid. This may well be the safest device with which to watch an eclipse; also many people can watch it simultaneously.

Why are expeditions from many countries sent out to observe a total solar eclipse in spite of the enormous expenditure involved? Because the sun is the one star whose atmosphere we can study in minute detail and whose visible activity provides us with valuable clues to the behaviour of the millions of other stars that populate our galaxy. Eclipses also offer opportunities to observe features near the suns edge that are ordinarily concealed by the glare. The corona can be studied both visually and spectroscopically only during a total eclipse.

Many important discoveries have been made in the past through total eclipse observations. The first attempt to photograph the corona for studies of its structure was made during the eclipse of July 28, 1851. During the eclipse of August 17, 1868, when the path of totality passed over India, the astronomer Pierre Jules Cesar Janssen of the famous Meudon Observatory of France camped in the tobacco fields of Guntur in Andhra Pradesh and detected the existence of an unknown element on the suns surface. Since it was first seen on the sun, it was named after Helios (the Greek word for the sun), and that is how the element helium came to be discovered. That is why it is said that solar physics was born in 1868 in the tobacco fields of Guntur.

Albert Einsteins theory of relativity was first tested during the total solar eclipse of May 29, 1919, when starlight was proved to be deflected by the suns gravitational field. Hitherto undetected comets may be found in the vicinity of the sun during totality.

In the third week of July, the monsoon season will be at its height in India, and as such, along the entire track of totality, the season will be humid with frequent clouds and rain. Satellite data reveal an especially high chance of cloud along the central line of totality in India, with average daytime amounts oscillating wildly around 75 per cent. But in spite of the gloomy cloud statistics, there is still a fighting chance of sunshine, and in some spots, the statistics tilt to the good side of 50/50.

The satellite observations of cloudiness compiled by the National Aeronautics and Space Administration (NASA) show a minimum in the central line cloud cover just east of Patna. This region, along the Ganga river, lies north of the 700-metre-high Chhotanagpur plateau.

The air descending from the plateau to the river will be warm and dry, slightly decreasing the cloudiness. This should lower the average cloud cover to a more tolerable 63 per cent in the vicinity of Patna. Dibrugarh, at the head of the Brahmaputra valley, will have an average cloudiness of 86 per cent according to surface-based cloud observations.

That the sun is the source of the energy that sustains life on the earth has been realised from the earliest times. As a result of the realisation of the essential part played by the sun in our lives, people have from ancient times feared a blacking out of the sun, perhaps more than any other natural phenomenon. The main point that should be realised is that all superstitions associated with solar eclipses became prevalent all over the world because of absolute ignorance of the science. It is only in comparatively recent times that people came to understand that eclipses do not mean the end of the world and that few scientific events can vie with them in terms of interest and beauty.

Professor Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.