The Moon is a pebble; the Sun is a furnace more than a hundred times wider than the Earth. Yet a few times a year the pebble slides in front of the furnace and hides it completely, down to the last sliver. That it fits so exactly is one of the strangest accidents in the sky.

An almost impossible fit

Hold up a coin at arm's length and you can blot out the full Moon. Do the same trick on the Sun and — remarkably — the coin covers that too, because the Sun and the Moon look nearly the same size from Earth. Both span about half a degree of sky.

They have no business matching. The Sun is about 400 times wider than the Moon. But it also happens to sit about 400 times farther away, and those two enormous numbers cancel almost perfectly. Nothing in physics requires this; it's a coincidence of where we happen to be standing, in space and in time. It is also the reason a total eclipse looks the way it does. When the Moon's disk covers the blinding surface exactly, it uncovers the Sun's faint outer atmosphere — the pearly corona — which we can otherwise never see.

Why not every month?

If the Moon laps the Earth roughly once a month, why isn't there an eclipse at every new moon? Because the Moon's orbit is tilted — by about five degrees — relative to the plane in which the Earth circles the Sun. Most months the new Moon passes a little above or below the Sun in our sky, and its shadow misses the Earth entirely, sailing off into empty space. Only when a new moon happens to fall near one of the two points where the tilted orbits cross do the three bodies line up straight enough for a shadow to land. The same tidal geometry that raises the seas twice a day is the geometry that rations eclipses to a few a year.

Borrowed time

There's a second accident, this one in time. The Moon is slowly drifting away from us — about four centimetres a year, a figure we know because we bounce lasers off reflectors the Apollo astronauts left on the surface. It's the same tidal drag that long ago locked the Moon to show us only one face, and it is still gently shoving the Moon outward.

A total eclipse depends on a fit that is only temporary. The Moon once loomed larger; it will keep shrinking in our sky until, in a few hundred million years, it can no longer cover the Sun at all.

You can already catch the fit failing. The Moon's orbit isn't a perfect circle, so sometimes it rides a little farther off and looks a touch too small. When an eclipse falls in one of those weeks, the Moon can't quite cover the Sun and leaves a blazing ring around its rim — an annular eclipse instead of a total one. Same alignment, slightly worse fit.

So the next time a total eclipse tracks across a continent and the corona flares out around a black disk, it's worth knowing what you're seeing: not something the universe was built to do, but a passing alignment of one particular moon, one particular star, and one brief moment in the long life of a planet whose Sun will go on shining for billions of years after the show is over.