The science behind eclipses

The Sun's diameter is roughly 400 times the Moon's, but it's also, on average, roughly 400 times farther from the Earth. The result of that double coincidence is that, seen from the Earth's surface, the Sun and the Moon have almost exactly the same apparent size in the sky — to the point that, depending on the Moon's exact distance along its elliptical orbit at each eclipse, it sometimes covers the Sun completely and sometimes falls a little short, leaving the ring of an annular eclipse. As far as is known, this is an extremely uncommon coincidence in the solar system: no other planet with known moons has such a close match between a moon's and the Sun's apparent size.

That coincidence is what makes it possible to observe the solar corona naturally. Astronomers also use coronagraphs — instruments with an opaque disc that artificially blocks the Sun's light so the surroundings can be studied — but that disc has to be somewhat larger than the Sun itself to also block light scattered inside the instrument's optics, and that size difference hides the innermost corona, the part right up against the Sun's edge. During a total eclipse, the Moon occupies exactly the Sun's spot, no more and no less, which is why it reveals a band of corona that no ground-based coronagraph can show that close to the edge.

The Sun's visible surface, the photosphere, has a temperature of about 5,500°C and is by far the brightest part of the star. Above it lies a thin, fainter layer, the chromosphere, which only becomes visible as a thin reddish rim right at the start and end of totality, coloured by hydrogen's characteristic emission. Beyond that stretches the corona, the Sun's outer atmosphere, which extends millions of kilometres into space — far beyond what's visible to the naked eye during an eclipse — and which, surprisingly, reaches temperatures of one to several million degrees, far higher than the photosphere beneath it.

That figure isn't a mistake: the corona is hundreds of times hotter than the surface it comes from, which at first glance contradicts the intuition that heat should decrease with distance from the source. It's called the coronal heating problem, and although the general mechanisms behind it are now fairly well understood — magnetic turbulence and waves that carry energy from the Sun's interior out into the corona — it remains an active area of research, and eclipses still contribute data because they allow direct observation, without a coronagraph's filter, of how that energy is distributed near the solar edge.

Despite those extreme temperatures, the corona is so tenuous — it has very little matter per unit of volume — that its total brightness is only a millionth of the photosphere's. That's the plain physical reason, no mystery involved, why the corona is only visible during totality: the rest of the time, the Sun's direct light is a million times more intense and completely drowns it out, the same way a lit candle can't be seen next to a floodlight.

The historical examples are the best known — the discovery of helium in 1868, made possible by a spectral analysis only feasible during totality, or the 1919 confirmation that the Sun's gravity bends starlight, exactly as Einstein's general relativity predicted, both detailed in 'Eclipses in history' — but an eclipse's scientific usefulness didn't stop there. Eclipses remain, even today, the only moment ground-based instruments can observe the corona closest to the solar edge without a coronagraph's interference, which makes them an opportunity to calibrate and cross-check the data that does arrive continuously from space missions dedicated to observing the Sun.

Eclipses have also given rise to large-scale citizen science projects: by coordinating hundreds of volunteer observers spread along the entire path of totality, each photographing the eclipse from a different point during the minutes it passes over their location, those images can be strung together to reconstruct a sequence far longer than the totality seen from any single point — minutes instead of seconds — tracking how the corona evolves along the Moon's shadow's whole path.

There's also a less obvious, almost circular contribution: the eclipse records left centuries ago, the very same ones ancient civilisations kept for religious or political reasons and that are explained in 'Eclipses in history', are used today to precisely measure how the Earth's rotation speed has very gradually changed over the centuries. Calculating where and when a particular eclipse would have been seen 2,000 years ago, and comparing that calculation with where and when the chronicles say it was actually seen, reveals tiny accumulated discrepancies that show how Earth's rotation has slowed over time — the same tidal-friction effect that, as explained in 'Curiosities and future eclipses', is also slowly pushing the Moon away from the Earth. A phenomenon studied by astronomers more than three millennia ago is still, without them ever knowing it, feeding data into today's astronomy.