Annular Solar Eclipse

What you are actually looking at

An eclipse is three bodies falling into a straight line. In a solar eclipse the Moon passes between the Earth and the Sun and drops its shadow on us: the darkest part of that shadow is a strip rarely wider than 200 km, which is why a total solar eclipse is something you travel to rather than something that happens to you. In a lunar eclipse it is the Earth that stands in the middle and the Moon that enters our shadow — and because the Moon is the thing being lit, everyone on the night side of the planet sees it at the same instant.

That asymmetry explains a lot. You have probably seen several lunar eclipses without planning anything. Most people never see a total solar eclipse unless they deliberately go looking for one.

The four kinds of solar eclipse

Which one you get depends on how far the Moon happens to be from us that day. Its orbit is not a circle, so its apparent size changes by about a tenth over the month — just enough to decide whether it covers the Sun completely or leaves a ring.

Why there isn't one every month

The Moon goes round us every 29.5 days, so by rights there should be a solar eclipse at every new moon and a lunar one at every full moon. There isn't, because the Moon's orbit is tilted about five degrees against the plane of ours. Most months the shadow sails harmlessly above or below its target.

The alignment only works near the two points where the orbits cross, called nodes. Twice a year the Sun lines up with one of them and an eclipse season opens for about five weeks — which is why eclipses arrive in clusters a fortnight apart and then stop for months. Any given year holds between two and five solar eclipses; a year with seven eclipses of both kinds happens only a few times a century.

Common questions

How often is there a total solar eclipse?

Somewhere on Earth, roughly every 18 months. For one particular town the wait averages around 375 years, because the band of totality is narrow and the planet is mostly water.

Is it ever safe to look without a filter?

Only during the few minutes of totality in a total solar eclipse, and only while the disc is fully covered. At every other moment — including the whole of an annular eclipse, ring and all — you need glasses certified to ISO 12312-2. Sunglasses, smoked glass, exposed film and welding masks below shade 14 do not work, and the damage they allow is painless and permanent.

Why does the Moon turn red in a lunar eclipse?

Because the only light reaching it has been bent through our atmosphere, which scatters the blue away and lets the red through. You are watching every sunrise and every sunset happening on Earth at that moment, projected onto the Moon at once.

How long does totality last?

Seldom more than seven and a half minutes, and most run two to four. A total lunar eclipse is far more generous: the Earth's shadow is big enough to keep the Moon inside it for over an hour.

Can I photograph one with a phone?

A lunar eclipse, yes — it is about as bright as a dim full moon. For anything solar you need a filter over the lens, not just over your eyes: pointing an unprotected camera at the Sun can cook the sensor in seconds.

What is a saros cycle?

A rhythm of 6,585.3 days — about 18 years and 11 days — after which the Sun, Earth and Moon return to almost the same geometry and a very similar eclipse repeats. The leftover third of a day turns the Earth another 120 degrees, so the new track lands a third of the way round the world; it takes three saros, near enough 54 years, for a comparable eclipse to come back to the same region.