One of the most remarkable coincidences found in nature is the fact that the Moon and Sun both appear the same size as seen from Earth. The Moon, a small, cold, dark body, is only 3500 km in diameter while the Sun, a self luminous, gaseous giant, is 1,400,000 km across. The coincidence arises from the fact that although the Sun is 400 times larger than the Moon, it is also 400 times farther from Earth. A direct consequence of this fortuitous geometry is that during a total solar eclipse, the Moon occults the Sun with a nearly perfect fit.

The fundamental basis of the solar eclipse is the alignment of the Sun, Moon and Earth such that some region of Earth passes through the Moon's shadow. This shadow is composed of two parts: the outer or penumbral shadow and the inner or umbral shadow. From within the penumbra, only part of the Sun is obscured. In contrast, the dark, central umbra is the shadow of complete or total eclipse. During a total eclipse, the umbra sweeps across Earth from west to east and the course it travels is called the path of totality. Anyone standing within this zone will see the Sun completely obscured by the Moon for as long as 7 1/2 minutes. At this time, the solar corona is visible as a halo about the Moon and the landscape takes on the appearance of an eerie twilight. Outside the path of totality but still within the penumbra, a partial eclipse is seen. The path of the umbra is rarely more than 300 km wide while that of the penumbra is about 7000 km wide. Sometimes the umbral shadow misses Earth entirely and only a partial eclipse occurs.

Eclipse geometry is complicated by the fact that Earth's orbit around the Sun is elliptical. As a result, the Sun's apparent semi-diameter varies from 944 arc-seconds at aphelion to 976 arc-seconds at perihelion. This 3% range in apparent size is, of course, quite indistinguishable to the naked eye.

However, the orbit of the Moon about Earth is also elliptical. The Moon is 406,700 km from Earth's center at apogee and 356,400 km at perigee. This 12% range in distance causes the Moon's apparent semi-diameter to vary between 882 and 1006 arc-seconds. Thus, during a total solar eclipse, the Moon's apparent diameter can exceed the Sun's by as much as 7% or 2 arc-minutes. Conversely, it is also possible for the Moon to appear almost 10% or 3 arc-minutes smaller than the Sun.

Such a geometry results in the case of an annular solar eclipse. When the Moon is near apogee, its umbral shadow falls short of Earth and the Moon appears smaller than the Sun. An observer stationed in the path of totality would see the Moon completely silhouetted against the Sun's bright photosphere. This type of eclipse takes its name from the ring or annulus of sunlight which surrounds the Moon at maximum eclipse. Unfortunately, the blindingly bright photosphere hides the corona from view and precludes any measurements or photographs of the Sun's outer atmosphere.

The term "central eclipse" is reserved for any eclipse in which the axis of the Moon's shadow intersects Earth. From the previous discussion, it is obvious that a central eclipse can be either total or annular in nature. When the Moon is at perigee and Earth is at aphelion, the shadow extends 23,500 km beyond Earth's center. However, when the Moon is at apogee and Earth is at perihelion, the shadow falls 39,400 km short of the geocenter. These represent the extremes of the Moon's umbra. In the first case, the umbral cone can have a maximum diameter of 273 kilometers at Earth. If the surface of the shadow cone in the second case is extended, it generates a negative or anti-umbra. As it strikes Earth, the anti-umbra can have a maximum diameter of 313 km and an observer positioned there will see an annular eclipse.

A third type of central eclipse is the hybrid eclipse; it forms the transition between the annular and total eclipse. A hybrid eclipse occurs when the umbral shadow is just long enough to reach part of Earth's surface, but not Earth's center. In this case, the eclipse will be annular at either end of the eclipse path while it will be total along the middle section. The eclipses of 1986 Oct 3 and 1987 Mar 30 are two examples of the annular/total eclipse.

WebMaster: Fred Espenak Planetary Systems Branch - Code 693 e-mail: espenak@gsfc.nasa.gov NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 USA