Gamma (eclipse)

Gamma (denoted as γ) of an eclipse describes how centrally the shadow of the Moon or Earth strikes the other body. This distance, measured at the moment when the axis of the shadow cone passes closest to the center of the Earth or Moon, is stated as a fraction of the equatorial radius of the Earth or Moon.

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. 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).

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.

The absolute value of gamma allows us to distinguish different kinds of solar eclipses from the earth:

If the Earth were a sphere, the limit for a central eclipse would be 1.0, but because of the oblateness of the Earth (which causes the distance between the Earth's north and south poles to be slightly shorter than if the Earth were perfectly spherical), it is 0.9972.

The Solar eclipse of April 29, 2014, with a gamma of 1.0001, is an example of the special case of a non-central annular eclipse. The axis of the shadow cone barely missed Earth's south pole. Thus, no central line could be specified for the zone of annular visibility.

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