The geometry
The Sun is about 400 times wider than the Moon, but also about 400 times farther away. That coincidence makes the two look almost exactly the same size in our sky. When the Moon passes directly between the Sun and Earth at new Moon, and it is near enough to Earth to look slightly larger than the Sun, its dark central shadow, the umbra, reaches Earth’s surface.
As Earth turns and the Moon moves along its orbit, the umbra sweeps across the planet in a long, thin band called the path of totality. Only people inside this band see a total eclipse. Those outside it, in the much larger penumbra, see a partial solar eclipse.
If the Moon is too far away to cover the Sun completely, the result is an annular solar eclipse instead.
The stages of a total eclipse
| Stage | What happens |
|---|---|
| First contact | The Moon begins to cover the Sun’s edge |
| Partial phase | The Sun becomes a shrinking crescent over about an hour or more |
| Second contact | Totality begins |
| Totality | The Sun is fully covered; the corona appears |
| Third contact | Totality ends as sunlight bursts back |
| Partial phase | The crescent grows again |
| Fourth contact | The Moon leaves the Sun’s disk |
What you see during totality
The last minutes before totality are dramatic. The light becomes strangely flat and dim, shadows turn sharp, and the temperature drops. Just before second contact, a series of effects follows in quick succession:
- Shadow bands: faint, rippling lines of light and dark may move across light-colored ground.
- The approaching shadow: from a high vantage point, you may see darkness racing toward you from the west.
- Baily’s beads: the last sunlight shines through valleys on the Moon’s edge as a string of bright points.
- The diamond ring: a single bright bead remains, with the first glow of the corona around the Moon.
Then the Sun’s surface is fully hidden. The corona spreads out as a pearly white halo, often with long streamers. Pink prominences and the thin red chromosphere may peek out at the Moon’s edge. The sky darkens to deep twilight, bright planets and stars appear, and the horizon all around glows like a sunset.
How long totality lasts
At any spot, totality usually lasts a few minutes. The theoretical maximum is about 7 minutes 30 seconds, reached only when the Moon is near its closest to Earth, Earth is near its farthest from the Sun, and the observer is near the equator. The longest total eclipse of the 21st century, on 22 July 2009, reached 6 minutes 39 seconds. Many eclipses have totality of only one or two minutes.
How often it happens
Total solar eclipses happen somewhere on Earth roughly every 18 months on average. But because each path is so narrow, any given location sees one only rarely, on average about once every 375 years. That is why many people travel to see one. Upcoming eclipses and their paths are listed on the eclipse calendar.
Watching safely
Safety during a total eclipse depends entirely on the stage:
- During all partial phases, you must use certified solar eclipse glasses, a proper solar filter or an indirect method such as a pinhole projector. Even a thin crescent of the Sun can damage your eyes.
- During totality only, when the Sun’s surface is completely covered, it is safe to look with the naked eye and enjoy the corona.
- As soon as the first bead of sunlight reappears, the filter must go back on.
If you are outside the path of totality, there is no safe moment without protection. The full guidance is in solar eclipse eye safety.