Ring of fire

Annular solar eclipse. Photographed from Scotland on May 31, 2003.-PICTURES: BY SPECIAL ARRANGEMENT

A SPECTACULAR sight awaits people residing at the southern tips of Kerala and Tamil Nadu on the afternoon of January 15, 2010. The moon will pass directly in front of the sun that day, leaving a ring of fire hanging in the afternoon sky. This rare astronomical event, an annular solar eclipse, was last visible from India on November 23, 1965 it was seen from Srinagar, Siliguri and Agartala and will not be seen again from India until 2019. Of course, the longest total solar eclipse of this century was already seen from many parts of India on July 22, 2009.

An eclipse of the sun is caused when the moon comes in between the earth and the sun so that the shadow of the moon sweeps across the surface of the earth. 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 is reached by the light from some parts of the suns luminous disk. To an observer within the umbra, the sun will appear completely covered by the moon, while to an observer within the penumbra, the suns disk will appear to be partially covered.

From the above, it will be understood that a solar eclipse can occur only at new moon when the moon is in conjunction with the sun. Had 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 have taken place at every new moon, at intervals of about 29 days. But this is not so. Actually, the moons orbit is inclined by about five degrees to the ecliptic, and it is only at those times when the moon happens to be at or near one of the nodes where the two orbits intersect that the sun, the moon and the earth are nearly in the same line and a solar eclipse can occur. At other times, the shadow of the moon just disappears into space.

A solar eclipse is not visible from most planets in the solar system. It so happens that the disk of the sun is 400 times larger than that of the moon and that coincidentally the distance of the sun from the earth is about 400 times the distance of the moon from the earth, so the sun and the moon have nearly equal angular diameters of 0.5 degree. Thus, our moon is just the right size to cover the bright disk of the sun and cause a solar eclipse.

The kind of eclipse that will result, total or annular, is governed by the fact that the orbit of the moon around the earth is not perfectly circular but is elliptical, with the result that the moon is sometimes nearer to the earth than at other times. When the moon in its orbit is nearest to the earth, this phenomenon is known as perigee, and when it is farthest from the earth, it is known as apogee. The distance of the moon from earth at perigee is 363,300 kilometres and at apogee is 405,500 km. As the distance from the earth to the moon varies, so does the apparent size of the moon in our skies. When it is closest, it appears larger in the sky than the sun and can completely obscure the solar disk, but when it is at its farthest point, its smallest apparent size is not enough to cover the disk of the sun.

The umbra of the shadow cannot reach the earth, instead a third portion of the shadow is born: the antumbral, or negative shadow. An annular eclipse is visible where the antumbral sweeps across the earths surface; this is known as the path of annularity. From within this region, the moons dark disk is centred on the sun but unable to obscure all its light, leaving a brilliant ring of fire, or annulus of light, around the edge of the moon in the sky. Unfortunately, the suns chromosphere and corona, which are visible during a total solar eclipse, are lost in the glare of an annular eclipse and as such no scientific investigational work, as is carried out during a total solar eclipse is possible during an annular eclipse.

The annular eclipse of the sun on January 15, 2010, will be visible as a partial eclipse from Africa, eastern Europe, Asia and the Indian Ocean. The partial phase will begin at 9-35 a.m. IST and end at 3-38 p.m. IST. The annular phase will start on the border between Chad and the Central African Republic. It will then cross the Democratic Republic of Congo (formerly Zaire), Uganda and Kenya, leaving land on the border between Kenya and Somalia. It will then cross the Indian Ocean, the southern tip of India, northern Sri Lanka and Myanmar, before ending in China, on the eastern coast. Annularity will begin at 10-48 a.m. IST and end at 2-25 p.m. IST. The instant of greatest eclipse will occur at 12-36.5 p.m. IST at a location (latitude 137N, longitude 6917E) where the duration of annularity will be 11 minutes and 4 seconds.

In India, the path of annular eclipse will pass over Thiruvananthapuram, Nagercoil, Kanyakumari (Cape Comorin), Tirunelveli, Rameswaram, Madurai, Thanjavur and, ultimately, Nagapattinam. Then it will cross the Bay of Bengal and reach Myanmar. Leaving aside the path of annularity, this eclipse will be visible as a partial solar eclipse from all parts of India. The times of the annular phase for the places in India are given in the table.

During the partial phase and the phase of annularity of eclipses, part of the sun remains visible, and it is hazardous to look at the eclipse without proper protection. Dense filters and exposed film do not necessarily provide protection because some do not block the invisible heat radiation (infrared) that can burn the retina. This has led officials to warn the public not to look at solar eclipses and has even frightened some people into locking themselves and their children into windowless rooms during eclipses. In fact, the sun is a bit less dangerous than usual during an eclipse because part of the bright surface is covered by the moon. But an eclipse is dangerous in that it can tempt one to look at the sun directly, leading to burnt eyes.

The safest and simplest way to observe the partial and annular phases of a solar eclipse is to use pinhole projection. Poke a small pinhole in a sheet of cardboard. Hold the sheet with the hole in the sunlight, and allow light to pass through the hole to a second sheet of cardboard. On a day when there is no eclipse, the result is a small, round spot of light that is an image of the sun. During the partial and annular phases of a solar eclipse, the image shows the dark silhouette of the moon obscuring part of the sun.

Another safe way to observe a solar eclipse is by projection on to a piece of card through a small refracting telescope (aperture two inches, or about five centimetres) or a pair of binoculars. Use the shadow of the telescope tube to align the telescope. When it is pointing directly at the sun it will cast the smallest shadow.

Place a piece of card around the eyepiece so that the imaging card is shielded from direct sunlight. Finally, fix the imaging card, which should be white or light grey, and adjust the focus. Never look through the eyepiece; it is a good idea to cap the telescope when it is not in use. Children should always be carefully supervised around the telescope.

As the date of the occurrence of the eclipse is in the middle of January, one may expect a cloud-free sky during the afternoon from at least a few locations of the southern tip of India. Observers are advised to consult local offices of the India Meteorological Department to select a location where a cloud-free sky is expected for two or three days in advance of the eclipse date.

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