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. 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. Gamma changes monotonically throughout any single saros series. The change in gamma is larger when Earth is near its aphelion than when it is near perihelion. For odd numbered series, gamma decreases for solar eclipses and gamma increases for lunar eclipses, while for even numbered series, gamma increases for solar eclipses and gamma decreases for lunar eclipses. This simple rule describes the current behavior of gamma, but this has not always been the case. The eccentricity of Earth's orbit is presently 0.0167, and is slowly decreasing. It was 0.0181 in the year -2000 and will be 0.0163 in +3000. In the past, when the eccentricity was larger, there were saros series in which the trend in gamma reversed for one or more saros cycles before resuming its original direction. These instances occur near perihelion when the Sun's apparent motion is highest and may, in fact, overtake the eastward shift of the node. The resulting effect is a relative shift west of the node after one saros cycle instead of the usual eastward shift. Consequently, gamma reverses direction.
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, it is 0.9972.
If is 0, the axis of the shadow cone is exactly between the northern and southern halves of the sunlit side of the Earth when it passes over the center.
If 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 is between 0.9972 and 1.0260, the axis of the shadow cone misses Earth, but, because the umbra or antumbra has a nonzero width, part of the umbra or antumbra may touch down in the polar regions of the Earth. This is called a non-central total or annular eclipse.
If is between 0.9972 and 1.0260 and the special circumstances mentioned above do not occur, or if is greater than 1.0260 but less than approximately 1.55, the eclipse is partial; the Earth traverses only the penumbra.
If exceeds approximately 1.55, the shadow cone misses the Earth completely, and no eclipse occurs.
