Gamma (solar eclipse)
Encyclopedia
Gamma of an eclipse
describes how centrally the shadow
of the moon
or earth
strikes the other. The distance, when the axis of the shadow cone passes closest to earth or moon's center, is stated as fraction of the equatorial radius
of the earth. The sign of gamma defines, for a solar eclipse, if the axis of the shadow passes north or south of the center of the earth, a positive value means north. For a lunar eclipse it defines whether the axis of the earth's shadow passes north or south of the moon, a positive value means south. For solar eclipses the earth is defined as that half which is exposed to the sun (this changes with the seasons and is not related directly to the earth's poles or equator, thus the earth's center is wherever the Sun
is directly overhead).
Adjoining diagram illustrates solar eclipse gamma: The red line shows the least distance from center of the earth, in this case approximately 75% of radius of the earth. Because the umbra
passes north of the Earth's center, gamma in this example is +0.75.
The absolute value
of gamma allows to distinguish different kinds of solar eclipses:
Because of the ellipticity of the earth
the limit for a central eclipse doesn't match 1, but 0.9972.
The Solar eclipse of April 29, 2014
with gamma of 1.0001 constitutes the special case of an annular but not central eclipse. The axis of the shadow cone barely misses earth's south. Thus no central line can be specified for the zone of annular visibility.
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
describes how centrally the shadow
Shadow
A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the...
of the moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
or earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
strikes the other. The distance, when the axis of the shadow cone passes closest to earth or moon's center, is stated as fraction of the equatorial radius
Earth radius
Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...
of the earth. The sign of gamma defines, for a solar eclipse, if the axis of the shadow passes north or south of the center of the earth, a positive value means north. For a lunar eclipse it defines whether the axis of the earth's shadow passes north or south of the moon, a positive value means south. For solar eclipses the earth is defined as that half which is exposed to the sun (this changes with the seasons and is not related directly to the earth's poles or equator, thus the earth's center is wherever the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
is directly overhead).
Adjoining diagram illustrates solar eclipse gamma: The red line shows the least distance from center of the earth, in this case approximately 75% of radius of the earth. Because the umbra
Umbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
passes north of the Earth's center, gamma in this example is +0.75.
The absolute value
Absolute value
In mathematics, the absolute value |a| of a real number a is the numerical value of a without regard to its sign. So, for example, the absolute value of 3 is 3, and the absolute value of -3 is also 3...
of gamma allows to distinguish different kinds of solar eclipses:
- If gamma is 0 the axis of the shadow cone strikes the earth exactly between the northern and southern half of the earth. In the case of a lunar eclipse the axis of the earth's shadow cone strikes the center of the moon.
- If gamma is lower than 0.9972, the eclipse is central. The axis of the shadow cone strikes the earth and there are locations on earth, where the moon can be seen central in front of the sun. Central eclipses can be total or annular (if the tip of the umbra reaches surface of earth barely reaches earth the type can change during the eclipse from annular to total and vice versa, this is called a hybrid eclipse). In the case of a lunar eclipse a partial umbral eclipse will occur.
- If gamma constitutes between 0.9972 and 1.0260 the axis of the shadow cone misses earth, but because of the umbraUmbraThe umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
or antumbra has a certain width in some circumstances a part of the umbra or antumbra can touch earth in polar regions. The result is a non central total or annular eclipse.
- If gamma is between 0.9972 and approximately 1.55 and the special circumstances mentioned above do not occur the eclipse is partial, the earth traverses only the penumbra. In the case of a lunar eclipse either a total penumbral or partial penumbral lunar eclipse will occur.
Because of the ellipticity of the earth
Figure of the Earth
The expression figure of the Earth has various meanings in geodesy according to the way it is used and the precision with which the Earth's size and shape is to be defined. The actual topographic surface is most apparent with its variety of land forms and water areas. This is, in fact, the surface...
the limit for a central eclipse doesn't match 1, but 0.9972.
The Solar eclipse of April 29, 2014
Solar eclipse of April 29, 2014
An annular solar eclipse will occur on April 29, 2014. The center of the moons shadow misses the south pole of the earth, but the partial eclipse will be visible from parts of Antarctica and Australia.-References:...
with gamma of 1.0001 constitutes the special case of an annular but not central eclipse. The axis of the shadow cone barely misses earth's south. Thus no central line can be specified for the zone of annular visibility.