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(originally published to Helium writing site, now gone)

A solar eclipse takes place when the moon is between the earth and the sun. Light from the sun is partly or fully blocked by the moon. Between two and five solar eclipses occur somewhere on earth each year. Up to two of them are total eclipses. During a total eclipse, the sky becomes quite dark and street lights come on.

The four types of solar eclipses are total, partial, annular and hybrid. A total eclipse is where the moon completely blocks the sun. At this time, the corona, which is the hottest part of the sun’s atmosphere, can still be seen. Any total eclipse is only seen along a narrow band of the earth’s surface. This is because the sun and the moon appear to be roughly the same size in the sky, as the sun is 400 times further away than the moon but is 400 times the moon’s diameter. A partial eclipse is where the moon only blocks out part of the sun. Any partial eclipse can be seen from a far greater area.

An annular eclipse is where the moon blocks all of the sun except for a narrow ring that forms a complete circle around the moon and looks like an illuminated hoop. Sometimes an effect called Baily’s beads is seen, because the lunar landscape features mountains and valleys, and the beads are the light shining through the valleys. The reason we can have both total and annular eclipses is because the moon’s orbit around the earth is elliptical or oval in shape and its distance from earth varies by up to 12 percent. The distance from earth to the sun varies about 3 percent, so this will have an impact too although smaller. A hybrid eclipse is where an eclipse is moving between an annular and total eclipse and is rarer.

A total eclipse occurs about every 18 months on average. However, a total eclipse at any particular point on the earth’s surface only happens once in about 370 years on average. The duration of a total eclipse can range up to about seven and a half minutes, although eclipses over seven minutes are rare, and they are usually much shorter. The longest total eclipse this century took place on 22 July 2009. It was seen from southern and eastern Asia and the Pacific Ocean and lasted about six and a half minutes. In 1973, scientists on a Concorde aircraft traveling at twice the speed of sound were able to keep up with a total eclipse for 74 minutes and make various observations.

Solar eclipses, when mentioned in old documents, can be useful to historians as it enables them to date certain events accurately. For example, Assyrian text mentions a revolt in the city of Ashur during the month of a solar eclipse. This eclipse has been dated to 15 June 763 BCE and is the oldest reliable dating of an event by means of a solar eclipse. An annular eclipse at Sardis when Xerxes was heading off to fight the Greeks dates this event to 17 February 478 BCE.

Attempts to date Good Friday and the so-called crucifixion eclipse to a known solar eclipse have so far been inconclusive. A full moon was also reported at this time and a solar eclipse can only occur at the time of a new moon as this is when the sun, moon and earth line up.

Watching an eclipse can present eye problems. Looking at the sun, including during any eclipse except a total eclipse, can damage the eye’s retina. This is because of the radiation from the sun’s surface, and not all the radiation is visible. It doesn’t matter that only a tiny proportion of the sun can be seen behind the moon. Using binoculars, a telescope or an optical camera viewfinder can be just as dangerous as looking at an eclipse with the naked eye. Nor do sunglasses make it safe.

A popular and safer way to see an eclipse is with a solar filter which only allows a small fraction of sunlight through. Even if only 0.01 per cent of the light is let through, you should still only look at the sun for a short time before looking away. There are other ways to view an eclipse, such as by using welder’s goggles.

The safest way to see an eclipse is to poke a hole in a thin piece of cardboard and hold it up to the sun. Hold a sheet of paper a few feet under or behind it. You will see the sun’s image on this sheet as it goes through the various stages of the eclipse. To reduce the amount of light, make a small hole in the side of a cardboard box and a larger hole at the bottom to put your head through. Put white paper opposite the small hole and close the lid of the box, and you will see an image of the eclipse. Similarly, this can be done in a darkened room.

A solar eclipse helped confirm Albert Einstein’s general relativity theory. Astrophysicist Arthur Eddington was on Principe Island off Africa to see the total solar eclipse of 29 May 1919. He noticed that stars just to the side of the sun would appear to shift slightly due to their light seemingly curving because of the sun’s gravitational field, confirming Einstein’s theory. At other times, the light from the sun is too strong to see this effect.

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