How a solar eclipse works

The coincidence that makes it possible

The Sun is about 400 times wider than the Moon, and it is also roughly 400 times farther away. From the Earth the two discs therefore look almost exactly the same size in the sky, about half a degree across. When the Moon lines up perfectly with the Sun, it can cover the bright disc completely and leave only the faint outer atmosphere, the corona, visible around it.

The match is not perfect, because the Moon’s orbit is an ellipse. When the Moon is near its closest point to the Earth it looks slightly larger than the Sun and can cause a total eclipse. Near its farthest point it looks smaller and leaves a ring of sunlight around its edge, which is an annular eclipse.

Why it doesn’t happen every month

The Moon passes between the Earth and the Sun at every new moon, roughly once a month. But its orbit is tilted by about 5 degrees to the plane of the Earth’s orbit, so most of the time it slips past a little above or below the Sun as seen from the Earth.

An eclipse can only happen when new moon falls close to one of the two points where the Moon’s orbit crosses the Earth’s, called nodes. The Sun lines up with the nodes twice a year, giving two eclipse seasons roughly six months apart. Each season lasts a little over a month, long enough for at least one solar eclipse and often a lunar eclipse a fortnight before or after it.

Umbra and penumbra

The Moon casts two shadows. The umbra is the dark inner cone, where the Sun is completely hidden. It is small when it reaches the Earth: on 2 August 2027 it is up to about 258 km wide. Everyone inside the track it draws across the planet, the path of totality, sees a total eclipse.

The penumbra is the much larger outer shadow, where only part of the Sun is covered. It spans thousands of kilometres, which is why far more people see a partial eclipse than a total one. In 2027, for example, Madrid sees about 86% of the Sun covered and Cairo about 95%, but neither gets totality.

How fast the shadow moves

The Moon moves along its orbit at about 1 km per second, while the Earth rotates underneath in the same direction, so the shadow sweeps across the ground from west to east. Its speed depends on the angle at which it meets the surface. Using the centre line of the 2027 eclipse, the shadow travels at roughly 4,700 km/h as it crosses southern Spain in the morning, then slows to about 2,400 km/h near greatest eclipse over Egypt, where the Sun is almost overhead. That slow, high shadow is the reason totality lasts longest there.

How eclipses repeat: the saros

Eclipses follow a cycle known since ancient times, the saros, which lasts about 18 years, 11 days and 8 hours. After one saros the Sun, the Moon and the nodes return to almost the same arrangement, so a very similar eclipse happens again. The extra 8 hours means the Earth has turned about a third of the way round, so the new eclipse falls roughly 120 degrees of longitude farther west.

The eclipse of 2 August 2027 is a direct descendant of the long total eclipse of 22 July 2009 over India and China. By our calculation the two moments of greatest eclipse are 6585.31 days apart, exactly one saros.

Eclipses will not last forever

The Moon is slowly moving away from the Earth, by about 3.8 cm a year. As it recedes it looks a little smaller, and NASA estimates that in roughly 650 million years it will no longer be able to cover the Sun completely. After that, the Earth will only have annular and partial eclipses.