The basic idea
A solar eclipse is a lineup. The Sun, the Moon and Earth fall into a nearly straight line, with the Moon in the middle. From the ground, the Moon’s dark disk slides across the bright face of the Sun.
This can only happen at new moon, when the Moon is between Earth and the Sun. Most new moons pass without an eclipse, though. The Moon’s orbit is tilted by about 5 degrees relative to Earth’s orbit, so at most new moons the Moon passes a little above or below the Sun in our sky. An eclipse occurs only when new moon falls close to one of the two points where the Moon’s orbit crosses Earth’s orbital plane.
A cosmic coincidence
The Sun is about 400 times wider than the Moon, and it is also about 400 times farther away. The result is that the two look almost exactly the same size in our sky. That is why the Moon can cover the Sun’s bright surface so precisely, leaving the faint outer atmosphere, the corona, visible around it.
The match is not perfect. The Moon’s distance from Earth varies during its orbit, so sometimes it looks slightly larger than the Sun and sometimes slightly smaller. That variation decides which type of eclipse occurs.
The Moon’s two shadows
The Moon casts two kinds of shadow:
- The umbra is the dark inner cone. Inside it, the Sun is completely hidden. It touches Earth as a small spot, usually no more than a couple of hundred kilometers wide, which sweeps across the surface along a narrow track called the path of totality.
- The penumbra is the much larger outer shadow. Inside it, only part of the Sun is hidden. It can cover a region thousands of kilometers across.
When the Moon is too far away for the umbra to reach the ground, the extension of the umbra beyond its tip, called the antumbra, touches Earth instead.
The four types of solar eclipse
| Type | What happens | What you see |
|---|---|---|
| Total | The umbra reaches Earth | Inside the narrow path, the Sun is fully covered and the corona appears |
| Annular | The Moon looks smaller than the Sun | A bright ring of Sun remains around the Moon |
| Partial | Only the penumbra reaches Earth, or you are outside the central path | A bite out of the Sun, from a nibble to a thin crescent |
| Hybrid | The eclipse changes between annular and total along its path | Total in some places, annular in others |
Every total or annular eclipse is also seen as partial across a much wider area on either side of the central path.
What it looks like from the ground
During the partial phases, little seems to change at first. Even with half the Sun covered, daylight remains strong. As the crescent narrows, the light takes on an odd, flat quality, shadows sharpen and the temperature can drop.
In a total eclipse, the last moments bring dramatic effects: shadow bands rippling on the ground, the “diamond ring” as the last bead of sunlight disappears, and then totality. The sky darkens to a deep twilight, bright planets and stars appear, the pearly corona surrounds the black Moon, and a sunset-like glow lines the horizon all around. Then the Sun returns and the sequence plays in reverse.
Watching safely
Looking at any part of the uncovered Sun can damage your eyes, and eclipses are no exception. You need certified eclipse glasses or an indirect method such as a pinhole projector for every partial phase. The guide to solar eclipse eye safety explains what is safe and what is not.
For dates and maps of upcoming eclipses, see the eclipses page.