Curiosities and future eclipses

The coincidence that makes total eclipses possible — that the Sun and the Moon have almost the same apparent size in the sky, explained in more detail in 'The science behind eclipses' — isn't permanent. The Moon is drifting away from the Earth at a rate measured with millimetre precision thanks to the reflectors the Apollo missions left on its surface, which astronomers still fire lasers at from Earth: about 3.8 centimetres a year, roughly how much a person's fingernails grow in the same time. It's a tiny rate on a human scale, but given enough time it changes the sky's geometry: if that rate holds steady, in about 600 million years the Moon will appear too small to fully cover the solar disc, and total eclipses will stop being possible from Earth — though annular ones will keep happening. In that sense, we live in a geologically privileged era to witness them.

The saros cycle, the roughly 18-year-and-11-day interval that makes an eclipse recur with very similar geometry, hides a curious detail in that leftover fraction of a day: since it isn't a whole number of days, each following eclipse in the same saros series happens with the Earth having turned an extra third of a rotation, so it's seen from a band roughly 120° further west in longitude. Only after three saros cycles — about 54 years and 34 days, a period with its own name, the exeligmos — does that drift cancel out and the eclipse is seen again, roughly, from longitudes similar to where it started.

A single saros series can produce between 69 and 87 eclipses over 12 to 16 centuries before running out, always starting and ending as very faint partial eclipses near the pole, and passing through its central phase — with the series' longest total or annular eclipses — somewhere in the middle of that thousand-year-plus life cycle.

The theoretical maximum duration of totality for a solar eclipse, under the most favourable conditions imaginable, is around 7 minutes 32 seconds; in practice, real eclipses rarely come close to that limit. The one on 2 August 2027 will be among the closest: its maximum duration, in Egypt, will exceed 6 minutes — the longest totality of the century — thanks to a combination of favourable circumstances, a Moon relatively close to Earth and an Earth relatively far from the Sun on those dates, which makes the lunar disc cover the solar one with more margin than usual.

The total eclipse of 12 August 2026 has already happened: it was visible as total from 138 Spanish cities, where the Sun — very low above the horizon, between 1.7° and 12° depending on the location — stayed above the real terrain relief at the crucial moment.

The next eclipse visible from Spain is total, on 2 August 2027. This time the Sun will be much higher, between 37° and 41°, and the path of totality — 258 km wide — crosses the south of the peninsula with a much more favourable geometry than 2026's: of the 38 Spanish cities that fall within that path, totality is visible from every single one. The point of longest duration along the eclipse's entire track is in Egypt, where totality exceeds 6 minutes, the longest of the century, as explained in the previous block.

After that comes the annular eclipse of 26 January 2028. Because it's annular, not total, the solar filter is never removed, not even at the moment of maximum coverage: with a magnitude of 0.908, the ring that remains visible corresponds to 17.6% of the Sun's usual light, far more than is safe to view with the naked eye. Terrain relief matters even more here than in 2027: of the Spanish cities within the path, 102 see the annularity with the Sun above the real horizon, 63 are in a marginal situation, and 124 have the Sun blocked by relief at the crucial moment — a good reminder that being within the theoretical path doesn't, on its own, guarantee seeing the full eclipse.

The path of totality of a solar eclipse is, at most, a few hundred kilometres wide, against the planet's surface of over 510 million square kilometres. In that quite literal sense, each total eclipse lights up a tiny sliver of the Earth, and that sliver shifts from one eclipse to the next: as explained above, the very geometry of the saros cycle shifts each new eclipse in the same series about 120° of longitude westward, so not even two 'sibling' eclipses from the same cycle cross the same spot, unless three full saros cycles have passed — an exeligmos — and even then only approximately.

On top of that, no fixed point on the planet has a guaranteed privileged position: the average wait between two total eclipses at the same place is around 375 years, as explained in 'What is an eclipse?', though the real figure varies hugely depending on latitude and each specific point's geometric luck. That's why eclipse chasers — the so-called umbraphiles mentioned in 'Eclipses in art and mythology' — travel the entire world instead of waiting at home: for most people, seeing a total eclipse without ever leaving their hometown simply isn't something that's going to happen to them twice in a lifetime.