Lunar Eclipses
Since the earth and moon shine only by the reflected light of the sun, each casts a shadow into space in the direction away from the sun. The shadow consists of a cone-shaped area of darkness called the umbra, where all light from the sun is cut off, and a larger area of partial darkness called the penumbra, which surrounds the umbra and receives light from a part of the sun's disk. Lunar eclipses can occur only when the moon is in its full phase, i.e., when the earth is between the sun and the moon. These eclipses may be total or partial, depending on whether the moon passes completely into the umbra of the earth's shadow or remains partly in the penumbra. Since the moon cuts the umbra close to the base, it can experience long periods of total eclipse ranging up to 1 hr, 42 min. A partial eclipse (when it passes through the penumbra) can last more than 2 hr, and the entire lunar eclipse may continue for as long as 4 hr. Some light is refracted, or bent, by the earth's atmosphere into the umbra, so that the moon at totality, instead of appearing black, ranges from a dull gray to a coppery color, depending on the amount of dust in the earth's atmosphere.
Solar Eclipses
A total solar eclipse can occur only when the moon is in its new phase. At this time the moon is between the sun and the earth and cannot be seen until it moves across the sun's disk. At the onset of totality, parts of the sun may be seen shining brightly between the high points of the moon's irregular edge, a phenomenon known as Baily's beads; the disk of the moon appears black and is surrounded by the sun's corona corona, luminous envelope surrounding the sun , outside the chromosphere. Its density is less than one billionth that of the earth's atmosphere. The corona is visible only at the time of totality during a total eclipse of the sun.
..... Click the link for more information. , out of which shoot immense, flamelike spurts called prominences. The sky darkens to twilight, the brightest stars become visible, and there is a noticeable drop in temperature. Baily's beads are seen again as the sun reappears and the sky grows lighter.
At apogee (when the moon is at its farthest point from the earth) the umbra of its shadow is too short to reach the earth's surface, causing the apparent diameter of the sun's disk to be larger than that of the moon. Where the moon would otherwise block the sun entirely, now the sun is seen as a bright ring completely surrounding the moon's disk; this eclipse is known as an annular, or ring, eclipse. The longest possible duration of totality for a solar eclipse is 7 min, 40 sec at or near the equator when the sun is directly overhead; the duration decreases with increasing latitude. The eclipse of June 20, 1955, lasted 7 min, 8 sec, which was the longest duration of totality in 1,238 years; an eclipse almost as long occurred on July 11, 1991.
Frequency and Prediction of Eclipses
If the plane of the moon's orbit about the earth coincided with that of the earth about the sun, a solar eclipse would be observed each month when the moon is new and a lunar eclipse when the moon is full. However, the moon's orbital plane is tilted at an angle of about 5°10' to the earth's orbital plane, making eclipses possible only when the three bodies are aligned (at new or full moon) and when the moon is crossing the earth's orbital plane (at a point called the node node, in astronomy, point at which the orbit of a body crosses a reference plane. One reference plane that is often used is the plane of the earth's orbit around the sun ( ecliptic ).
..... Click the link for more information. ). Within a given year, a maximum of seven eclipses can occur, either four solar and three lunar or five solar and two lunar. Despite the fact that there are more solar than lunar eclipses each year, over time many more lunar eclipses are seen at any single location on earth than solar eclipses. This occurs because a lunar eclipse can be seen from the entire half of the earth facing the moon at that time, while a solar eclipse is visible only along a narrow path on the earth's surface.
From their observations of eclipses the Chaldaeans (fl. 1000 B.C.–540 B.C.) discovered that similar eclipses of the sun recur in cycles of 18 years, 11 1-3 days; this cycle, called the saros, is an interval in which the sun, earth, and moon return to nearly identical relative positions. Since the orbits of the earth and moon are quite accurately known, eclipses can be predicted far in advance, both in time and location. Similar calculations can determine the time and place of past eclipses; this information is useful for dating historical events that are known to have occurred at the same time as an eclipse.
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Eclipse
(1) An open source Java-based platform for integrating software tools for application development. Running under Windows and Linux, it provides a universal platform for tools created as Eclipse plug-ins. IBM started the Eclipse consortium in late 2001 with $40 million and donated a large amount of code. In 2004, it was spun off as an independent foundation. For more information, visit www.eclipse.org. See NetBeans.
(2) (ECLIPSE) An early series of 32-bit minicomputers from Data General. The development of the initial 32-bit ECLIPSE MV/8000 was the subject of Tracy Kidder's best-selling book, "Soul of a New Machine" published in 1981 by Little, Brown and Company.
eclipse
eclipse [i′klips]
| ECLIPSE - A Prolog + CLP compiler from ECRC. |
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