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What Makes an Eclipse Happen

A solar eclipse is the result of a combination of coincidence and clockwork.

The coincidence is that the Sun and Moon appear almost exactly the same size in Earth’s sky. The Sun is about 400 times wider than the Moon, but it also is about 400 times farther away. So when the new Moon passes directly between Earth and the Sun, it can cover the Sun’s disk, blocking it from view.

We don’t see an eclipse at every new Moon, however, because the Moon’s orbit around Earth is tilted a bit with respect to the Sun’s path across the sky, known as the ecliptic. Because of that angle, the Moon passes north or south of the Sun most months, so there’s no eclipse.

When the geometry is just right, though, the Moon casts its shadow on Earth’s surface, creating a solar eclipse, and that’s where the clockwork comes in.

The Moon goes through several cycles. The best known is its 29.5-day cycle of phases, from new through full and back again. Other cycles include its distance from Earth (which varies by almost 30,000 miles over 27.5 days) and its relationship to the ecliptic (27.2 days), among others. These three cycles overlap every 6,538.3 days, which is a little more than 18 years. Eclipses occur at the point of overlap.

This 18-year cycle is known as a Saros. The circumstances for each succeeding eclipse (solar or lunar) in a Saros are similar — the Moon is about the same distance from Earth, for example, and the eclipses occur at the same time of year. (Because of the extra one-third day, however, each eclipse occurs one-third of the way around Earth from the previous eclipse; the next eclipse in this Saros, in 2035, will be visible across portions of Asia and the Pacific Ocean.)

A cycle begins with a partial eclipse. A portion of the Moon just nips the northern edge of the Sun, for example, blocking only a fraction of the Sun’s light. With each succeeding eclipse in the cycle, the Moon covers a larger fraction of the solar disk, peaking with a series of dozens of total eclipses. Then the Moon slides out of alignment again, this time in the opposite direction, creating more partial eclipses. The series ends with a grazing partial eclipse on the opposite edge of the solar disk.

The August 21 event is eclipse number 22 of Saros 145, which began on January 4, 1639, and will end on April 17, 3009. The cycle will produce 77 eclipses, including 41 total eclipses.

Several Saros cycles are taking place at any given time, however, so we don’t have to wait 18 years between eclipses. Solar eclipses occur at intervals of one, five, or six months, with no relationship between consecutive eclipses.