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Solar and lunar eclipses

Twice a year or so the Sun, Earth and Moon line up closely enough for one to shadow another. The geometry is simple; the coincidence behind it is remarkable.

Solar eclipses

A solar eclipse occurs when the Moon passes between Earth and the Sun, obscuring the Sun from a small part of Earth, totally or partially. There are four kinds. In a total eclipse the Moon covers the Sun completely; in an annular one the Moon is too far from Earth to cover it and leaves a bright ring; a partial eclipse covers only part of the disc; a hybrid shifts between total and annular along its track. Totality lasts at most a few minutes at any one place — the theoretical maximum is about 7½ minutes — and for a given spot on Earth a total eclipse comes round only about once every 360 to 410 years.

The 400-to-1 coincidence

Total eclipses are possible only because the Sun is about 400 times the Moon's diameter and about 400 times farther away, so the two look almost exactly the same size from Earth. Nothing requires this; it is luck.

Lunar eclipses

A lunar eclipse — the "blood moon" — occurs when the Moon orbits through Earth's shadow. It can be total (the Moon fully inside the dark umbra), partial, or penumbral (only the faint outer shadow). The red colour comes from sunlight bent through Earth's atmosphere on its way to the Moon: Rayleigh scattering removes the blues, leaving the reds, the same physics as a sunset. Unlike a solar eclipse, a lunar eclipse is visible from the whole night side of Earth, is safe to watch with the naked eye, and can last an hour or more — totality up to nearly 107 minutes. The twenty-first century has 228 of them, an average of 2.28 a year and at least two every year.

Why not every month?

New moon and full moon happen every month, but the Moon's orbit is tilted about 5° to Earth's orbit around the Sun, so its shadow usually passes above or below Earth and Earth's shadow usually misses the Moon. Only when a new or full moon falls near the points where the orbits cross — the eclipse seasons, roughly every six months — do the three bodies line up. That is why there are between two and five solar eclipses a year, not twelve.

Eclipses and the calendar

Eclipses run on the Moon's cycles; the civil year runs on the Sun and a Roman decision about 1 January. They have nothing to do with each other, which is why an eclipse can fall on any date. What they share is the reason this site exists: the Earth, Moon and Sun keep time far more precisely than anything people built until the atomic clock — the subject of UTC, GMT and leap seconds.

Sources

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