At the new moon phase, the Moon is located between the Sun and Earth, and its shadow falls toward Earth. If the three bodies align exactly, so that the Moon's shadow falls on Earth's surface, a solar eclipse is visible from Earth. During a solar eclipse, the shadow that the Moon casts consists of two parts: the umbra and the penumbra. The umbra is the dark center of the shadow where sunlight is completely blocked, and the penumbra is the outer part of the shadow where sunlight is only partially blocked.

People on the portion of Earth that passes through the Moon's penumbra see a partial solar eclipse — the Moon's disk blocking out a segment of the Sun's light. People in the narrow path of the umbra, however, see a total solar eclipse because the Moon crosses directly in front of the Sun and completely blocks its light. Partial solar eclipses are more commonly viewed solar eclipses because the path of the Moon's penumbra on Earth's surface is significantly larger than the path of the Moon's umbra, which can only be up to about 270 km (168 mi) wide.

During a total solar eclipse, the Sun and the Moon appear about the same size in the sky, so the Moon is able to completely obscure the Sun. The duration of time when sunlight is completely blocked by the Moon is called totality. Because orbits are not circular, variations exist in Earth's distance to the Moon and Sun, and as a result, the apparent sizes of the Moon and Sun, as seen from Earth, are not constant. When the three celestial bodies are in alignment, these variations in apparent size affect the length of totality of an eclipse, and they can also produce an annular eclipse — a special type of eclipse in which a ring of the Sun remains visible around the Moon. The total eclipse in July 1991 was an unusually long eclipse, with totality lasting six minutes and 53 seconds. In addition, the eclipse was visible from several astronomical observatories in Hawaiʻi and several major cities in Central and South America, making it an unusually well-observed event.

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