What is an eclipse?

A solar eclipse happens when the Moon comes between the Sun and the Earth and casts its shadow on the Earth's surface. It can only happen at new moon, the phase in which the Moon's lit face points away from us, into space. The Moon's shadow is small compared with the planet, so a solar eclipse — whether total or annular — is only visible from a narrow band, a few hundred kilometres wide, crossing one particular part of the globe.

A lunar eclipse works the other way round: it's the Earth that comes between the Sun and the Moon, casting its shadow on the Moon's surface. It can only happen at full moon. Because the Earth's shadow is far larger than the Moon, a lunar eclipse is visible from the planet's entire night side at once, and it lasts much longer than a solar one: up to nearly two hours of totality, against at most a few minutes for a total solar eclipse.

There's a third difference, and it's the one that shapes how each is observed: looking straight at a solar eclipse without proper protection can cause permanent eye damage, because you're still looking at the Sun. A lunar eclipse, by contrast, is entirely safe to watch with the naked eye at any stage: you're only looking at the Moon faintly reflecting sunlight that has been filtered through Earth's atmosphere.

New moon and full moon recur every 29.5 days — the synodic month — so if an eclipse depended only on the lunar phase, we would get one solar and one lunar eclipse every month. We don't, because the Moon's orbit around the Earth doesn't lie in the same plane as the Earth's orbit around the Sun (the ecliptic): it's tilted about 5.14°. In most months, at new or full moon, the Moon passes a few degrees above or below the line joining the Sun and the Earth, and its shadow — or the Earth's, for a lunar eclipse — simply misses the other body.

The Moon's orbit crosses the ecliptic plane at two opposite points, called the ascending node and the descending node. An eclipse can only happen when new or full moon coincides with the Moon being near one of those two nodes, within a window of about 18 days around each crossing. Those windows, which recur twice a year and occasionally three times, are called eclipse seasons, and they're the only time of year when an eclipse is geometrically possible. The nodes themselves aren't fixed either: they drift slowly westward under the Sun's gravitational pull, completing a full turn every 18.6 years.

The upshot is that a calendar year has at least two solar eclipses and up to five, and between zero and three lunar ones. That global count is misleading: most are partial, or only visible from the ocean or uninhabited regions, and a total solar eclipse is only visible from an extremely narrow band. For any fixed point on the planet, the average wait between two total solar eclipses is roughly 375 years.

Whether a solar eclipse turns out total or annular comes down to an almost whimsical detail: the Moon's distance from the Earth varies along its orbit — which isn't circular but slightly elliptical — between about 356,500 km at perigee and 406,700 km at apogee. That change makes the Moon's apparent size in the sky swing between slightly larger and slightly smaller than the Sun's, which itself varies very little over the year. By pure coincidence, the two discs are almost exactly the same apparent size as seen from Earth.

When the eclipse happens with the Moon relatively close, its disc covers the Sun's completely: that's a total eclipse. During the seconds or minutes of totality, the Sun's direct light disappears and its corona becomes visible — the faint outer atmosphere normally hidden by the glare of the solar disc. It's the only moment a solar eclipse can be viewed without any filter at all, and only from within the narrow path of totality.

When it happens with the Moon relatively far away, its disc appears slightly smaller than the Sun's and never quite covers it: a bright ring of photosphere remains visible around the lunar silhouette. That's an annular eclipse, and however thin that ring looks, it's still direct sunlight: during an annular eclipse the solar filter is never removed, not even at the moment of maximum coverage. The eclipse of 26 January 2028, the next one visible from Spain, is of this type: with a magnitude of 0.908 it leaves a ring equivalent to 17.6% of the Sun's usual light — far more than is safe to view with the naked eye.

When the observer is outside the central path but within the wider region reached by the Moon's partial shadow, they see a partial eclipse: the lunar disc covers only part of the Sun, never fully covering it or forming a complete ring. It's by far the most common type, since the region from which a partial eclipse is visible is much larger than the narrow central path. There's also a fourth, very rare type — under 4% of all solar eclipses — the hybrid, or annular-total eclipse, in which the Earth's curvature makes the same eclipse annular at the ends of its path and total in the middle stretch, because the tip of the Moon's shadow just barely grazes the Earth's surface.

The shadow the Earth casts into space has two parts: a dark core, the umbra, where the Earth blocks all direct sunlight, and a fainter outer ring, the penumbra, where it only blocks it partly. How much of the Moon enters each one determines the type of lunar eclipse. In a total lunar eclipse, the whole Moon passes through the umbra and the direct sunlight that lights it disappears; even so, it doesn't go completely dark, because some sunlight bends through Earth's atmosphere and reaches it tinted red — the same reason sunsets look reddish — which gives it its characteristic copper colour, popularly known as a 'blood moon'.

In a partial lunar eclipse, only part of the lunar disc enters the umbra, producing a dark bite that advances and recedes across the Moon's surface without ever covering it entirely. In a penumbral eclipse, the subtlest of the three, the Moon passes only through the penumbra: the dimming is so slight that it often goes unnoticed with the naked eye. Unlike a solar eclipse, any of the three types of lunar eclipse is entirely safe to watch without protection: you're never looking at the Sun, only at the Moon reflecting its light.