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The Shadow of the Moon
As shown in the diagram below, two lines can be drawn from the bottom of the Sun, past the Moon, to define a region where the Moon blocks any view of the bottom of the Sun; and similarly, two lines can be drawn from the top of the Sun, past the Moon, to define a region where the Moon blocks any view of the top of the Sun. Where those overlap, on the night side of the Moon, is the Moon's umbra (Latin for shadow), a region where neither the top nor the bottom of the Sun can be seen (nor, of course, any part in between). Where they do not overlap, is the Moon's penumbra (Latin for partial shadow), a region where part of the Sun is visible, and part of it is blocked from view by the Moon. In the upper part of the penumbra, the Moon is in front of the bottom of the Sun, but an observer could look over the top of the Moon, and still see the top of the Sun; while in the lower part of the penumbra, the Moon is in front of the top of the Sun, but an observer could look under the bottom of the Moon, and still see the bottom of the Sun.
The shadow of the Moon
(not to scale; objects shown larger and closer than reality)
The umbra is a cone-shaped region, as large as the Moon at the center of the Moon, and gradually decreasing in size as you move away from the Moon, until -- at a distance approximately equal to the size of the Moon's orbit around the Earth -- you reach the tip of the cone. At that place, the Moon appears to be exactly the same size as the Sun, and in exactly the same direction as the Sun, and just barely covers it completely. Moving toward the Moon, the Moon looks larger and larger, and the region where the Sun is completely covered grows from a point, to a circle about one percent of the size of the distance from the tip. Thus, 100 miles in front of the tip of the umbra, the region where the Moon completely covers the Sun is about a mile in diameter, and 10,000 miles in front of the tip of the umbra, the region where the Moon completely covers the Sun is about a hundred miles in diameter. Anyone who is inside the umbra sees a total eclipse of the Sun, but since the tip of the umbra never reaches more than a few thousand miles past the surface of the Earth, the portion of the Earth's surface which is inside the circle of totality at any given time is never much more than a hundred miles across, and is usually considerably smaller.
Moving away from the tip of the umbra, the Moon looks smaller and smaller, and in fact, too small to completely cover the Sun, so that a ring of Sun is visible all around the Moon. This ring is called an annulus (Latin for ring), so an eclipse in which the tip of the Moon's umbra doesn't reach the Earth is called an annular eclipse.
Surrounding the umbra, and growing as you move away from the Moon at the same rate that the umbra is shrinking, is the penumbra. The top portion of the penumbra represents the region where the bottom of the Sun is blocked from view, but you can look over the top of the Moon, and see the top of the Sun. The central portion of the penumbra represents the area behind the umbra, where the Moon is directly in front of the Sun, but is too small to cover it completely, so that you see an annular eclipse. The bottom portion of the penumbra represents the region where the top of the Sun is blocked from view, but you can look under the bottom of the Moon, and see the bottom of the Sun. All three regions in the penumbra see a partial eclipse of the Sun, but annular eclipses are considered special cases, and are usually listed with total solar eclipses, for the reason discussed immediately below.
Sometimes, as the Moon sweeps past the Earth, the tip of the umbra reaches the surface of the Earth on the portion closest to the Moon (as shown in the diagram), but does not reach as far as the center of the Earth. When this happens, that part of the Earth which is close enough to the Moon to fall within the umbra sees a very short solar eclipse, while that part which is further away from the Moon sees a very short annular eclipse. Such an eclipse is called a total-annular or hybrid eclipse.
The Motion of the Moon Across the Sun
The total solar eclipse of March 29, 2006, as a composite series of exposures, showing the Moon gradually covering the Sun as they move eastward along the Ecliptic, while the rotation of the Earth carries both objects westward. Partial phases become deeper and deeper as totality approaches, and shallower and shallower after totality. The exposure times for partial phases are very short, while the image of the corona at totality requires a longer exposure. (Original image, including a gorgeous view of the Turkish coastline, copyright Stefan Seip, apod060404) |
The Motion of the Moon's Shadow Across the Earth
The path of the total solar eclipse of August 11, 1999. (Fred Espenak (GSFC), NASA, apod990810)
The region of totality, which is never more than about a hundred miles across, sweeps across the Earth from west to east, as the Moon moves around us, passing in front of the Sun. The map above shows the region of totality as a narrow band running from the area where the eclipse takes place at sunrise (on the left side of the map), to the area where the eclipse takes place at sunset (on the right side of the map). The regions above and below the region of totality see a partial eclipse. Above the region of totality, viewers see the Moon in front of the bottom of the Sun, but looking above the Moon, see the top of the Sun. Below the region of totality, viewers see the Moon in front of the top of the Sun, but looking below the Moon, see the bottom part of the Sun. Only in the region of totality is the Sun completely covered, top to bottom, by the Moon. |
 An animation of the movement of the Moon's shadow across the south Atlantic and Africa, during a June 2001 total solar eclipse. The darker area is the inner penumbra and umbra, while the lighter area is the outer penumbra. In this particular eclipse, the shadow moved across the ground at 1200 miles per hour, so each frame -- separated by about 20 minutes -- shows the shadow 400 miles further to the east. (EUMETSAT, apod021209) |
The August 11, 1999 eclipse shadow, from the Mir spacecraft. (Mir 27 Crew, CNES, apod990830)
The Appearance of the Sun During the Eclipse
Image of total solar eclipse of August 11, 1999
(Michael Kobusch, apod010620)
The solar corona, photographed during the eclipse of December 4, 2002.A specially designed radial density filter shows fainter outer regions as clearly as brighter inner regions. Prior to the space age, solar eclipses were the only way to observe the corona, as its fainter regions couldn't be seen amid the scattered sunlight still visible all around the Sun. (Wendy Carlos & Jonathan Kern, apod021213) |
 A close-up view of the October 3, 2005 annular eclipse of the Sun. Being further from the Earth than usual, the Moon's angular diameter was smaller than that of the Sun, so a 'ring' or annulus of sunlight was visible all around the Moon -- hence the term, 'annular' eclipse. (Stefan Seif, apod051005) |
 The "diamond ring" effect during the March 29, 2006 solar eclipse. At the start and end of totality, light may shine through valleys on the limb of the Moon, producing brilliant points of light (considerably overexposed here) which dramatically contrast with the black disk of the Moon.(Anthony Ayiomamitis, apod060330) |
A composite image of the Sun, during the solar eclipse of March 29, 2006. The central image, in orange, is a view of the Sun taken by the SOHO spacecraft, in a loop orbit a million miles in front of the Earth and Moon. The black and white donut is a view of the inner and middle corona, as seen during the eclipse, from the surface of the Earth. And the outer image, in orange, is a view of the outer corona, as taken by the SOHO spacecraft. Comparing the images allows the structure of the coronal features to be followed from the surface of the Sun, millions of miles outward into space. (2006 Team - Williams College Eclipse Expedition, NSF, National Geographic,
SOHO Consortium, ESA, NASA, apod060331) |
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