One consequence of the Moon's orbit about the Earth is that the Moon can shadow the Sun's light as viewed from the Earth, or the Moon can pass through the shadow cast by the Earth. The former is called a solar eclipse and the later is called a lunar eclipse. The small tilt of the Moon's orbit with respect to the plane of the ecliptic and the small eccentricity of the lunar orbit make such eclipses much less common than they would be otherwise, but partial or total eclipses are actually rather frequent.
For example there will be 18 solar eclipses from 1996-2020 for which the eclipse will be total on some part of the Earth's surface. The common perception that eclipses are infrequent is because the observation of a total eclipse from a given point on the surface of the Earth is not a common occurrence.
Date Areas of Eclipses
21 VI 2001 Angola, Zambia, Zimbabwe, Mozambique, Madagaskar 14 XII 2001 Costa Rica, Nicaragua 10 VI 2002 Pacific Islands: Sangihe, Talaud, Rota, Tinian 4 XII 2002 Angola, Botswana, Zimbabwe, Mozambique, Australia 31 V 2003 North Scotland, Iceland, Greenland 23 XI 2003 Antarctica 19 IV 2004 Antarctic, South Africa 14 X 2004 South-East Asia, Hawaje, Alaska 8 IV 2005 Panama, Columbia, Venezuela 3 X 2005 Portugal, Spain, Algeria, Tunisia, Libya, Sudan, Kenia, Somalia 29 III 2006 Ghana, Togo, Benin, Nigeria, Niger, Libya, Turkey, Gruzja, Russia 22 IX 2006 Gujana, Surinam, Gujana Fr. 19 III 2007 Asia, Alaska 11 IX 2007 South America, Antarctic 7 II 2008 Antarctica 1 VIII 2008 South Canada, Greenland, New Land, Syberia, Mongolia, China 26 I 2009 Sumatra, Java, Borneo 22 VII 2009 India, Nepal, Bhutan, China 15 I 2010 Zaire, Kenia, India, Ceylon, Burma, China 11 VII 2010 South Argentina, Chile
The preceding figure allows three general classes of solar eclipses (as
observed from any particular point on the Earth) to be defined:
Total Solar Eclipses occur when the umbra of the Moon's shadow touches a region on the surface of the Earth.
Partial Solar Eclipses occur when the penumbra of the Moon's shadow passes over a region on the Earth's surface.
Annular Solar Eclipses occur when a region on the Earth's surface is in line with the umbra, but the distances are such that the tip of the umbra does not reach the Earth's surface.
As illustrated in the figure, in a total eclipse the surface of the Sun is completely blocked by the Moon, in a partial eclipse it is only partially blocked, and in an annular eclipse the eclipse is partial, but such that the apparent diameter of the Moon can be seen completely against the (larger) apparent diameter of the Sun. A given solar eclipse may be all three of the above for different observers. For example, in the path of totality (the track of the umbra on the Earth's surface) the eclipse will be total, in a band on either side of the path of totality the shadow cast by the penumbra leads to a partial eclipse, and in some eclipses the path of totality extends into a path associated with an annular eclipse because for that part of the path the umbra does not reach the Earth's surface.
A total solar eclipse requires the umbra of the Moon's shadow to touch
the surface of the Earth. Because of the relative sizes of the Moon and Sun
and their relative distances from Earth, the path of totality is usually very
narrow (hundreds of kilometers across). The following figure illustrates the
path of totality produced by the umbra of the Moon's shadow. (We do not show the
penumbra, which will produce a partial eclipse in a much larger region on
either side of the path of totality; we also illustrate in this figure the
umbra of the Earth's shadow, which will be responsible for total lunar eclipses
to be discussed in the next section.)
If you are in the path of totality the eclipse begins with a partial phase in which the Moon gradually covers more and more of the Sun. This typically lasts for about an hour until the Moon completely covers the Sun and the total eclipse begins. The duration of totality can be as short as a few seconds, or as long as about 8 minutes, depending on the details.
As totality approaches the sky becomes dark and a twilight that can only be described as eerie begins to descend. Just before totality waves of shadow rushing rapidly from horizon to horizon may be visible. In the final instants before totality light shining through valleys in the Moon's surface gives the impression of beads on the periphery of the Moon (a phenomenon called Bailey's Beads). The last flash of light from the surface of the Sun as it disappears from view behind the Moon gives the appearance of a diamond ring and is called, appropriately, the diamond ring effect (image at right).
As totality begins , the solar corona (extended outer atmosphere of the Sun) blazes into view. The corona is a million times fainter than the surface of the Sun; thus only when the eclipse is total can it be seen; if even a tiny fraction of the solar surface is still visible it drowns out the light of the corona. At this point the sky is sufficiently dark that planets and brighter stars are visible, and if the Sun is active one can typically see solar prominences and flares around the limb of the Moon, even without a telescope (see image at left).
The period of totality ends when the motion of the Moon begins to uncover the surface of the Sun, and the eclipse proceeds through partial phases for approximately an hour until the Sun is once again completely uncovered. Here is a movie of the 1994 total solar eclipse (3.1 MB MPEG; Source; here is a QuickTime version, but note that it is 15 MB in length).
A partial solar eclipse is interesting; a total solar eclipse is awe-inspiring in the literal meaning of the phrase. If you have an opportunity to observe a total solar eclipse, don't miss it! It is an experience that you will never forget.
As we have noted in the preceding section, the Earth casts a shadow that the Moon can pass through. When this happens we say that a lunar eclipse occurs. Just as for solar eclipses, lunar eclipses can be partial or total, depending on whether the light of the Sun is partially or completely blocked from reaching the Moon. The following figure illustrates a total lunar eclipse with the Moon lying in the umbra of the Earth's shadow.
During a total lunar eclipse the Moon takes on a dark red color because it is being lighted slightly by sunlight passing through the Earth's atmosphere and this light has the blue component preferentially scattered out (this is also why the sky appears blue from the surface of the Earth), leaving faint reddish light to illuminate the Moon during the eclipse.