Why the Moon Can Almost Perfectly Cover the Sun

The Sun is about 400 times wider than the Moon but also about 400 times farther away, making total solar eclipses possible and revealing the Sun's faint outer atmosphere.

Astronomy

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A total solar eclipse depends on an unusual bit of celestial geometry. The Sun is roughly 400 times wider than the Moon, but it is also roughly 400 times farther from Earth, so the two objects can appear nearly the same size in our sky. When the Moon passes directly in front of the Sun and its apparent size is large enough to cover the bright solar disk, the faint outer atmosphere of the Sun, called the corona, becomes visible around the dark Moon. The match is not perfectly fixed because the Moon and Earth follow slightly oval orbits, so the apparent sizes change. When the Moon is farther away, it can leave a bright ring around itself instead, producing an annular eclipse. The geometry is also temporary on geological timescales. Tidal interactions between Earth and the Moon are gradually pushing the Moon farther away at about 3.8 centimeters per year. Eventually, the Moon will appear too small to completely cover the Sun, meaning future observers on Earth will no longer see total solar eclipses. NASA estimates that this transition is more than 600 million years away. The striking alignment is therefore a remarkable coincidence of size, distance, orbital position, and timing rather than a permanent feature of the sky.

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