Eclipses

When the Sun or Moon goes dark

Junior level — plain language, no maths

Every so often the sky does something dramatic: the Sun is blotted out in the middle of the day, or the full Moon turns a deep coppery red. These are eclipses, and they happen because three balls in space - the Sun, the Earth and the Moon - briefly line up in a row so that one casts its shadow on another. On 12 August 2026 a total solar eclipse will sweep across Iceland and northern Spain, the first visible from mainland Europe since 1999.

A solar eclipse happens at new moon, when the Moon slides directly between us and the Sun and drops its shadow onto the Earth. Stand inside that shadow and the Moon covers the Sun completely for a couple of minutes - day turns to twilight, stars come out, and the Sun's ghostly outer atmosphere blazes around a black disc. A lunar eclipse is the mirror image: at full moon the Earth slips between Sun and Moon, and our shadow falls across the Moon, staining it red.

So why isn't there an eclipse every single month, at every new and full moon? Because the Moon's path is tilted a few degrees, so most months its shadow sails just above or below the Earth and misses entirely. Only when the line-up is nearly perfect - a few times a year - does a shadow actually land. That near-miss the rest of the time is exactly what makes an eclipse feel special.

The geometry of eclipses, and why they're rare

Student level — the core equations

Eclipses are a shadow game with strict rules. A solar eclipse needs a new moon (Moon between Earth and Sun); a lunar eclipse needs a full moon (Earth between Sun and Moon). Shadows have two parts: the dark central umbra, where the light source is fully blocked, and the lighter penumbra around it, where it's only partly hidden. Stand in the Moon's umbra and you see a total solar eclipse; in the penumbra, only a partial one.

If the Moon orbited in the same plane that the Earth orbits the Sun - the ecliptic - we would get a solar and a lunar eclipse every single month. But the Moon's orbit is tilted about 5°, so it usually passes above or below the Sun-Earth line. Eclipses can only happen when the Moon is crossing that line - at one of its two nodes - at the same moment as new or full moon. Those alignments cluster into two "eclipse seasons" each year.

The Sun-Moon size coincidence makes Earth uniquely lucky. Because the Moon's apparent size so nearly matches the Sun's, it can just cover the disc and reveal the corona. When the Moon is a little farther out in its slightly elliptical orbit it falls short and leaves a bright ring - an annular eclipse. Totality is brief (a couple of minutes) because the umbra's tip is small and races across the ground at over 1,500 km/h.

Nodes, the Saros cycle, and eclipse prediction

Scholar level — full mathematical depth

01The line of nodes

The Moon's orbit intersects the ecliptic at two points, the nodes. An eclipse needs syzygy - Sun, Earth and Moon collinear - within a few degrees of a node, the "ecliptic limits". Because the line of nodes regresses (precesses) once every 18.6 years, the eclipse seasons drift about 19 days earlier each year rather than staying fixed on the calendar.

02Umbra, penumbra and antumbra

The Moon's umbral cone barely reaches the Earth; when the Moon is near apogee the tip falls short, the antumbra touches down instead, and the eclipse is annular. The path of totality is only ~100-270 km wide, and its duration - capped near 7.5 minutes - is set by the umbra's size against the relative motion of Moon and observer.

03The Saros

After 223 synodic months - 6,585.3 days, or 18 years 11 days 8 hours - the Sun, Moon and nodes return to almost the same geometry, so a nearly identical eclipse recurs. The extra 8 hours rotate the next event about 120° west in longitude; successive Saros members also creep in latitude, so a series is born near a pole, marches across the globe over ~1,200-1,500 years, and dies at the other.

04A coincidence with an expiry date

The angular diameters of Sun and Moon are both about 0.5°, which is what makes totality (rather than a permanent ring) possible. But the Moon recedes ~3.8 cm per year, so in roughly 600 million years its disc will always fall short and total solar eclipses will cease forever. We happen to live in the era when they occur.

05Eclipses as a laboratory

Totality has repeatedly advanced physics: the 1919 eclipse let Eddington measure starlight bending around the Sun and confirm general relativity; helium was first identified in the Sun's spectrum during the 1868 eclipse; and the corona and chromosphere are still best studied when the blinding photosphere is hidden. Recorded eclipses even anchor the chronology of the ancient world.