| Revision as of 05:01, 7 May 2006 view sourceFinell (talk | contribs)Extended confirmed users13,207 edits →Terminology: Copyedit par 2← Previous edit | Revision as of 05:13, 7 May 2006 view source Finell (talk | contribs)Extended confirmed users13,207 edits →Geometry of an eclipse: CopyeditNext edit → | ||
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| The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ]). Because of this, at the time of a New Moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the New Moon occurs close to one of the points (known as ]s) where the Moon's orbit crosses the ecliptic. | The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ]). Because of this, at the time of a New Moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the New Moon occurs close to one of the points (known as ]s) where the Moon's orbit crosses the ecliptic. | ||
| As noted above, the Moon's orbit is also ] |
As noted above, the Moon's orbit is also ]. The Moon's distance from the Earth can vary by about 6% from its average value. Therefore, the Moon's apparent size varies with its distance from the Earth, and it is this effect that leads to the difference between total and annular eclipses. The distance of the Earth from the Sun also varies during the year, but this is a smaller effect. On average, the Moon appears to be slightly smaller than the Sun, so the majority (about 60%) of central eclipses are annular.<!--This is a little redundant with previous material, isn't it?--YES, IT IS> It is only when the Moon is closer to the Earth than average (near its ]) that a total eclipse occurs.<ref>, R. Hipschman, The Exploratorium</ref><ref>, Earth View</ref> | ||
| The Moon orbits the Earth in approximately 27.3 days, relative to a fixed frame of reference. This is known as the ]. However, during one sidereal month, the Earth has rotated on its axis and |
The Moon orbits the Earth in approximately 27.3 days, relative to a fixed frame of reference. This is known as the ]. However, during one sidereal month, the Earth has rotated on its axis and revolved about the Sun. This means that the average time between one New Moon and the next is longer than the sidereal month: it is approximately 29.6 days. This is known as the ], and corresponds to what is commonly called the ]. | ||
| The Moon crosses from south to north of the ecliptic at its ]. However, the nodes of the Moon's orbit are gradually moving in a ] motion, due to the action of the Sun's gravity on the Moon's motion, and they make a complete circuit every 18.5 years. This means that the time between each passage of the Moon through the ascending node is slightly shorter than the sidereal month. This period is called the ]. | The Moon crosses from south to north of the ecliptic at its ]. However, the nodes of the Moon's orbit are gradually moving in a ] motion, due to the action of the Sun's gravity on the Moon's motion, and they make a complete circuit every 18.5 years. This means that the time between each passage of the Moon through the ascending node is slightly shorter than the sidereal month. This period is called the ]. | ||
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Revision as of 05:13, 7 May 2006
A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partially obscuring Earth's view of the Sun. This configuration can only occur during a New Moon, when the Sun and Moon are in conjunction as seen from the Earth. In ancient times, and in some cultures today, solar eclipses are attributed to mythical properties. Total solar eclipses can be frightening events for people unaware of their astronomical nature, as the Sun suddenly disappears in the middle of the day and the sky darkens in a matter of minutes. However, the spiritual attribution of solar eclipses is now largely disregarded.
Total solar eclipses are very rare events for any given place on Earth because totality is only seen where the Moon's umbra touches the Earth's surface. A total solar eclipse is a spectacular natural phenomenon and many people consider travel to remote locations in order to observe one.
The 1999 total eclipse in Europe, said by some to be the most-watched eclipse in human history, helped to increase public awareness of the phenomenon. This was illustrated by the number of people willing to make the trip to witness the 2005 annular eclipse and the 2006 total eclipse. The next total solar eclipse will occur on August 1, 2008.
Types of solar eclipses
There are four types of solar eclipses:
- 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 outline of the Moon, and the much fainter corona is visible (see image above). During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth.
- 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.
- A hybrid eclipse is intermediate 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 rather 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 of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra never intersects the Earth's surface.
The Earth's distance from the Sun is about 400 times the Earth's distance from the Moon. The Sun's diameter is about 400 times the diameter of the Moon. Because, by coincidence, these ratios are approximately the same, the sizes of the Sun and the Moon as seen from Earth appear to be approximately the same: about 0.5 degree of arc in angular measure.
Because the Moon's orbit around the Earth is an ellipse, the Moon's distance from the Earth varies, so the apparent sizes of the Sun and Moon likewise vary. The magnitude of an eclipse is the ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse. An eclipse when the Moon is near its closest distance from the Earth (i.e., near its perigee) will be a total eclipse because the Moon will appear to be large enough to cover completely the Sun's bright disk, or photosphere; a total eclipse has a magnitude greater than 1. Conversely, an eclipse when the Moon is near its farthest distance from the Earth (i.e., near its apogee), will be an annular eclipse because the Moon will appear to be slightly smaller than the Sun; the magnitude of an annular eclipse is less than 1. Slightly more solar eclipses are annular than total because, on average, the Moon lies too far away from Earth to cover the Sun completely. A hybrid eclipse occurs when the magnitude of an eclipse is very close to 1: the eclipse will appear to be total at some locations on Earth and annular at other locations.
The Earth's orbit around the Sun is also elliptical, so the Earth's distance from the Sun varies throughout the year. This also affects the apparent sizes of the Sun and Moon, but not so much as the Earth's varying distance from the Moon. When the Earth approaches its farthest distance from the Sun (the aphelion) in July, this tends to favor a total eclipse. As the Earth approaches its closest distance from the Sun (the perihelion) in January, this tends to favor an annular eclipse.
Terminology
Central eclipse is often used as a generic term for a total, annular or hybrid eclipse. This is, however, not completely correct: the definition of a central eclipse is an eclipse during which the central line of the umbra touches the Earth's surface. It is possible, though extremely rare, that part of the umbra intersects with Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse.
The term solar eclipse itself is technically a misnomer. The phenomenon of the Moon passing in front of the Sun is not an eclipse, but an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. For example, when the Moon disappears at Full Moon by passing into Earth's shadow, the event is properly called a lunar eclipse. Therefore, the proper, but rarely used, term for what is commonly called a solar eclipse is eclipse of the Earth.
Eclipse predictions
Geometry of an eclipse
The diagram to the right shows the alignment of the Sun, Moon and Earth during a solar eclipse. 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.
The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ecliptic). Because of this, at the time of a New Moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the New Moon occurs close to one of the points (known as nodes) where the Moon's orbit crosses the ecliptic.
As noted above, the Moon's orbit is also elliptical. The Moon's distance from the Earth can vary by about 6% from its average value. Therefore, the Moon's apparent size varies with its distance from the Earth, and it is this effect that leads to the difference between total and annular eclipses. The distance of the Earth from the Sun also varies during the year, but this is a smaller effect. On average, the Moon appears to be slightly smaller than the Sun, so the majority (about 60%) of central eclipses are annular.
- Solar Eclipses, University of Tennessee
- Types of Solar Eclipse, P. Tiedt
- Solar Eclipses for Beginners, O. Staiger
- Central Solar Eclipses, F. Espenak