About
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially covers the Sun as viewed from a location on Earth. This can only happen during a new moon, when the Sun and Moon are in conjunction as seen from Earth. At least two, and up to five, solar eclipses occur each year; no more than two can be total eclipses. Total solar eclipses are nevertheless rare at any particular location because totality exists only along a narrow path on the Earth's surface traced by the Moon's umbra.
Many people, sometimes referred to as "eclipse chasers" or "umbraphiles" , will travel to remote locations to observe or witness a predicted central solar eclipse. The solar eclipse of August 11, 1999, in Europe helped to increase public awareness of the phenomenon, which apparently led an unusually large number of journeys made specifically to witness the total solar eclipse of October 3, 2005, and of March 29, 2006.
The last total eclipse was the solar eclipse of July 11, 2010; the next will be the solar eclipse of November 13, 2012. The recent solar eclipse of January 15, 2010, was an annular eclipse; the next annular eclipse will occur on May 20, 2012.
A total solar eclipse is a natural phenomenon. Nevertheless, in ancient times, and in some cultures today, solar eclipses have been attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of their astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.
Solar Eclipse Phenomena
Totality at any particular solar eclipse can be seen only from a narrow belt on Earth, sometimes only 150 km (90 miles) wide. The various phases observable at a total solar eclipse are illustrated in the top portion of the figure. The designation “first contact” refers to the moment when the disk of the Moon, invisible against the bright sky background, first touches the disk of the Sun. The partial phase of the eclipse then begins as a small indentation in the western rim of the Sun becomes noticeable. The dark disk of the Moon now gradually moves across the Sun’s disk, and the bright area of the Sun is reduced to a crescent. On Earth the sunlight, shining through gaps in foliage and other small openings, is then seen to form little crescents of light that are images of the light source, the Sun. Toward the beginning of totality, the direct light from the Sun diminishes very quickly, and the colour changes. The sky near the zenith becomes dark, but along the horizon Earth’s atmosphere still appears bright because of the narrow extent of the umbra of the Moon’s shadow on Earth. The scattered light coming in from a distance beyond the umbral region produces the effect of twilight. Animals may react with fear, humans often with awe. Birds may go to roost as they do at sunset.
As the tiny, narrow crescent of sunlight disappears, little bright specks remain where depressions in the Moon’s edge, the limb, are last to obscure the Sun’s limb. These specks are known as Baily’s beads, named for the 19th-century English astronomer Francis Baily, who first drew attention to them. The beads vanish at the moment of second contact, when totality begins. This is the climax of the eclipse. The reddish prominences and chromosphere of the Sun, around the Moon’s limb, can now be seen. The brighter planets and stars become visible in the sky. White coronal streamers extend from the Sun to a distance of several solar radii. The air temperature on Earth in the path of totality falls by some degrees. The light of totality is much brighter than that of the full moon but is quite different in colour. The duration of totality is brief, typically lasting two to five minutes.
The moment of third contact occurs when the Moon’s west edge first reveals the Sun’s disk. Many of the phenomena of second contact appear again, in reverse order. Suddenly the first Baily’s bead appears. More beads of light follow, the Sun’s crescent grows again, the corona disappears, daylight brightens, and the stars and planets fade from view. The thin crescent of the Sun gradually widens, and about one and a quarter hours later the eclipse ends with fourth contact, when the last encroachment made by the Moon on the Sun’s rim disappears.
During the partial phase, both before and after totality, it is absolutely essential for an observer to protect the eyes against injury by the intense brilliance of the Sun. This phase should never be viewed directly except through strong filters, a dark smoked glass, or a heavily fogged photographic plate or film.
When totality is imminent and only a small crescent of the Sun remains, so-called shadow bands can often be seen on plain light-coloured surfaces, such as floors and walls. These are striations of light and shade, moving and undulating, several centimetres wide. Their speed and direction depend on air currents at various heights, because they are caused by refraction of sunlight by small inhomogeneities in Earth’s atmosphere. The phenomenon is similar to the images of water waves seen on the bottom of a sunlit swimming pool or bath.