Rare chance of observing annular eclipse and partial eclipse
 

On January 15, 2010 (tomorrow) an annular solar eclipse can be observed from Sri Lanka. This will be visible across Africa, the Indian Ocean, Maldives, Southern most part of India, Sri Lanka, and South-East Asia. The annular phase will start at 10:43:54 (Sri Lanka Standard time) in the Central African Republic and end at the Chinese Yellow Sea coast at 14:29:01 (Sri Lankan standard time). The greatest eclipse happened when the axis of moon’s shadow becomes shortest at 01 37’N latitude and 69 17E longitude at 12:36:31 (Sri Lankan time). While the annular eclipse path is 333 km wide at this occasion the annular phase will last just over 11 minutes (11 min 8s).

The annular eclipse path is then down to 328 km wide and the duration will be almost 11 minutes on the centerline when the path of the annular eclipse passes over the Maldive Island around 12:56. Although the centreline just misses the mainland of India, this should still be sight for people in the most Southeast parts of India.

Annular path

The annular path will be first touched Sri Lankan land at 13:17 around Kalpitiya area while the centre line of the annular path of the eclipse will be touched Kurichchikadu area in Jaffna at 13:18:56. The Penumbral (partial) shadow will be visible over eastern Africa, South-East Europe, the Middle East, Sri Lanka, India and South-East Asia between 09:35:26 and 15:37:33 (Sri Lanka standard time).

All solar eclipses involve when the Moon’s elliptical orbit which 5 inclined to the plane of the Earth’s orbit (ecliptic), passing between the Earth and the Sun.

When the Moon’s orbit crosses the ecliptic plane at the points referred as nodes, the Moon casts a shadow on the Earth. Depending on whether the Moon passes directly, or only partly, between the Earth and Sun, and the location on Earth you stand to observe it, the solar eclipse can be divided as Total, Partial, Annular and Hybrid. The other important factor which determines the type of the eclipse is the anomalistic month.

Solar eclipse

The moon takes 27.554549 days to travel from apogee to perigee and back again during the anomalistic month. The Apogee is the point in the Moon’s orbit that is farthest from the Earth and the Perigee is the point in the orbit of the Moon that is closest to the Earth. Therefore the types of the eclipse as well as the size of the eclipse depend on the distance from the Earth to the Moon during the eclipse; and this distance depends on the point during the Anomalistic Month when the eclipse occurs.

In a Total solar eclipse the Sun is completely covered by the Moon on New Moon Day where the moon is at a point relatively close to the Earth in its elliptical orbit. During a total eclipse the shadow known as umbra is casted by the moon totally obscured the Sun. People standing on those parts of the Earth, within the Umbra, observe the Sun’s face completely hidden by the Moon.

Outside the umbra where the Sun is only partly covered by the Moon; this is known as the penumbra, and it covers a much larger area of the Earth than the umbra. The area of the Earth which falls within the penumbra sees a partial eclipse of the Sun.

The eclipse on January 15, 2010 is called annular eclipse, occurred when the Moon is in its elliptical orbit farther from the Earth. This results that the umbra is too short to reach the Earth. However, the Moon is directly in front of the Sun, so the parts of the Earth right-underneath it, see the antumbra where the centre of the Sun is covered. This leaves a ring, an annulus, of the Sun visible round the edges of the Moon. Penumbra is also visible just outside the antumbra.

The path of this upcoming annular eclipse passes over the Northeast part of Sri Lanka and the centerline of the annular eclipse path traverses very close to the Delft Island and enters the land at Kuruchchikadu area in Punkuduthive (an Island of Jaffna) at 13:18:56 (Sri Lankan standard time). Then the centerline traverses along the Jaffna peninsula and it leaves the main land close to Valvatithurai area around 13.30:16 (Sri Lankan standard time).

Eastern horizon

The annular phase will last just over 10 minutes (10 min 09s) within the main land of Sri Lanka along the centre line. During the maximum annular phase along the centerline within the Sri Lanka main land, the moon will cover approximately 84 percent of the Sun.

At the time of the annularity in these areas the Sun will be at the altitude of 55.2 from the Eastern horizon.

Since the path of this annular eclipse is 323 km wide when it traverses Sri Lanka, the people in the area nearly 161 km South and North from the centre line can observe this annular phase of the eclipse. So that, the people live above (North ward) the line joining between Rajakadawa (near Arachchikattuwa) and Kumpurupiddi (near Kinniya) in the Sri Lankan map can observe the annular phase of this eclipse.

The rest of the people bellow this line can observe the partial phase of this solar eclipse. The penumbral (partial) shadow of the moon first hits (enters) the main land of Sri Lanka close to Ambalangoda area about 11.11 (Sri Lankan standard time). The people in Colombo can observe the partial eclipse from about 11.12 (Sri Lankan standard time). During the maximum partial phase which occurred at 13:19 the moon will cover nearly 82 percent of the Sun.

Another important phenomena, Bailey’s Beads, can be observed specially for the people living close to the Southern line of the annularity (the line joining between Arachchikattuwa (near) and Kinniya (near) in the Sri Lankan map).

Total eclipse

Baily’s Beads are the thin slice of the Sun visible appears broken up into beads of light when near the beginning and end of the total solar eclipse. The British astronomer Francis Baily discovered these beads. They occur because the edge of the Moon is not smooth but jagged with mountain peaks.

There are two basic things to be satisfied to occur an eclipse (i.e. either solar eclipse or lunar eclipse). One is, the Moon should be at one of the two nodes while the other is, the Moon also has to be aligned with the Earth and the Sun, i.e. at a New Moon (producing a solar eclipse) or Full Moon (for a lunar eclipse).

Draconic Month

The periods governing these two cases are determined by Draconic month and Synodic Month.

A Draconic Month is the time, 27.212220 days, taken for the Moon to return to the starting node. This time is shorter than its orbital period. This is because of the precession of the Moon’s orbit; the Moon’s orbital plane, and hence the nodes, rotate backwards around the Earth about once every 18.6 years.

The Synodic Month is the time, on average 29.530589 days, takes for the Moon to go from one New Moon to the next. This time is longer than the orbital period (sidereal month) of the moon because the Moon’s orbital period is the time it takes for the Moon to complete one orbit around the Earth. When these two cycles coincide, an eclipse occurs. Almost after 223 Synodic months and 242 Draconic months, 18 years, 10 days and 8 hours, this coincidence occurs and produce similar eclipses.

The period of 18 years, 10 days and eight hours is known as Saros. Saros cycle, means ‘repetition’, where similar eclipse happen every 18 years 10 or 11 days and eight hours. Therefore Saros series indicates that a series in which similar eclipses happen every 18 years 10 or 11 days and eight hours. About 70 to 85 eclipses occur during a Saro series over 1,200 to 1,500 years.

Each series starts with a small partial eclipse in either the North or South Polar Regions. Therefore first total eclipse will be seen near the Polar Regions. The eclipses of the series then move down or up the Earth until the diminishing partial eclipses occur at the opposite pole to where the series started; and then it ends. For example the annular eclipse occurred on January 15, 2010 is the 23rd eclipse of 141 Saros.

The Solar eclipses of Saros 141 series, all eclipses occur at the Moon’s ascending node and the Moon moves southward with each eclipse, began with a partial eclipse in the Northern hemisphere on May 19, 1613. The series will end with a partial eclipse in the Southern hemisphere on Jun 13, 2857.

The total duration of Saros series 141 is 1244.08 years. The first annular eclipse took place on August 4, 1739 and had a maximum duration just under 4 minutes. The annular eclipse of the 141 Saros series with longest duration of 12 minutes 9 seconds occurred on December 14 in 1955. The duration of annularity of each succeeding eclipse is now dropping and will dwindle to 1 minute 9 seconds when the last annular eclipse of the series occurs on October 14, 2460.

But, number of eclipses happens within 18 years 10 or 11 days and eight hours duration in different Saro series. At any one time, 42 Saros cycles running at once. This results in a little more than two eclipses per year.

Eye protection

Permanent eye damage can result from looking at the disk of the Sun directly, or through a camera viewfinder, or with binoculars or a telescope even when only a thin crescent of the Sun or Baily’s Beads remain. One percent of the Sun’s surface still visible is about 10,000 times brighter than the full moon. Staring at the Sun under such circumstances is like using a magnifying glass to focus sunlight onto tinder. The retina is delicate and irreplaceable. There is nothing a retinal surgeon will be able to do the help you. Never look at the Sun outside of the total phase of an eclipse unless you have adequate eye protection.

There are several safe methods to observe solar eclipse. One safe way of observing the Sun is to view a projected image of the Sun.

Sun image

A ‘pinhole camera’ is a one of such safe methods of observing the projected image of the Sun. Pinhole cameras you can make out of cardboard boxes, but a perfectly adequate (and portable) version can be made out of two thin but stiff pieces of white cardboard.

Punch a small clean pinhole in one piece of cardboard and let the sunlight fall through that hole onto the second piece of cardboard, which serves as a screen, held below it. An inverted image of the Sun is formed. To make the image larger, move the screen farther from the pinhole. By using a telescope or a binocular the Sun image can also be projected onto a screen.

The other technique for viewing the Sun safely is by looking at it directly through specially designed solar filters. Such filters permit only a minuscule fraction of the Sun’s light to pass through them. Such filters are made of aluminized polyester papers or black polymer sheets with optical density greater than 5.0. Welders’ goggles with a rating of 14 (#14) or higher are safe to use for looking directly at the Sun. They are also relatively inexpensive.

There are specially designed filters to observe the Sun directly through telescopes and binoculars. Many telescope and camera companies provide these metal-coated filters that are safe for viewing the Sun. They are more expensive than common filters.

- Space Applications Division

Arthur C. Clarke Institute