The Earth over the
moon's north pole
during the
Moon Mission.

One of the most remarkable coincidences found in nature is the fact that the Moon and the Sun both appear the same size as seen from Earth. The Moon, a small, cold, and dark body is only 3,500 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 occludes 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 7,000 km wide. Sometimes the umbral shadow misses Earth entirely and only a partial eclipse occurs.


A series of scientific and mathematical tutorials
focusing on astronomy.


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