What in the world is a solar eclipse?

In case you’ve forgotten what you learnt in Solar Eclipse 101, the definition goes like this:

A Solar Eclipse occurs when the Moon passes between the Sun and the Earth so that the Sun is fully or partially obscured.

In English, the Moon’s path crosses in between us and the Sun. When this happens, its shadow falls onto the Earth’s surface. In turn, what we see is the spectacular sight of the dark silhouette of the moon blocking the blinding disc of the Sun.

11 AUG 1999

Photo: Luc Viatour (www.lucnix.be)

Solar eclipses happen between two to five times a year, but they are often not total eclipses. Even if they were, totality occurs only along a narrow corridor in the relatively tiny area of the Moon’s umbra (more on that later). For example, most of the total eclipse that happened in January 26, 2009, fell over the Indian Ocean. (Photo Credit: Luc Viatour)

This makes a total eclipse a rare and spectacular natural phenomenon and many people are willing to travel to remote locations to catch one. The August 11, 1999 solar eclipse that fell over Europe raised global awareness for the phenomenon.

There are four types of solar eclipse:

  • A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon, and a faint corona visible. During any one eclipse totality is visible only from at most a narrow track on the surface of the Earth.        


    Photo: NASA

  • An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon. (More about this later.) 
  • A hybrid eclipse (of annular or total eclipse) transitions between a total and annular eclipse. At some points on the surface of the Earth, it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are comparatively rare.
  • A partial eclipse occurs when the Sun and Moon are not exactly in line, and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside the track of an annular or total eclipse. However some eclipses can only be see as a partial eclipse, because the umbra never intersects the Earth’s surface.

The match between the apparent sizes if the Sun and Moon during a total eclipse is a coincidence.

eclipse diagramThe diagram here shows how a total solar eclipse works.

With the Sun, Moon and Earth aligned, the dark gray region below the Moon is the umbra, where the Sun is completely obscured by the Moon.

The small area where the umbra touches the Earth’s surface is where a total eclipse can be seen. The larger light gray area is the penumbra, in which only a partial eclipse can be seen.


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