Why Does an Eclipse Sometimes Hide the Whole Sun and Sometimes Leave a Ring of Light?

There Are Four Kinds of Solar Eclipse, and the Total-or-Ring Question Comes Down to Distance

A solar eclipse happens when the Moon passes between the Sun and the Earth. Astronomers sort them by what the Moon's shadow does. In a total eclipse, the Moon covers the Sun completely. In an annular eclipse, the Moon is a little too far away to cover it, and a ring of sunlight is left around the Moon's dark disk; “annular” comes from the Latin word for ring [4] [5].

In a partial eclipse, only a piece of the Sun is covered. And a hybrid eclipse is one that appears annular along one part of its path and total along another [4].

This note chases one question: what decides between “total” and “ring”?

The Sun Is About 400 Times Wider Than the Moon, and About 400 Times Farther Away

The Sun is far larger than the Moon: about 400 times bigger across. But it is also about 400 times farther from us [1]. Something that is bigger and farther by the same amount looks the same size, the way a truck far down the road can look as small as a toy car held in your hand.

So from Earth, the Sun and the Moon look almost exactly the same size in the sky. That is the “cosmic coincidence” NASA points to as the reason total solar eclipses are possible at all [1].

The key word is almost. If the two were exactly equal, the Moon would only just cover the Sun, and the slightest change would tip the result one way or the other.

The Moon's Path Around the Earth Is a Slightly Squashed Oval

The Moon does not travel around the Earth in a perfect circle. Its orbit is an ellipse, a slightly squashed circle, so the Moon keeps getting a bit closer and then a bit farther. The closest point is called perigee and the farthest is apogee [2].

NASA's eclipse site gives the average distance, measured from the center of the Earth to the center of the Moon, as 363,396 kilometers (about 226,000 miles) at perigee and 405,504 kilometers (about 252,000 miles) at apogee [2]. These are averages; any single perigee or apogee can come out a little different. The difference is roughly a tenth of the Moon's distance [2] [5].

When the Moon Is Farther Away, It Looks Smaller and Cannot Cover the Whole Sun

A farther Moon looks smaller. The European Space Agency's explanation puts the condition simply: an annular eclipse happens when, besides the lining up, the Moon's apparent size is smaller than the Sun's [3]. At apogee, ESA says, the Moon looks about 14 percent smaller than it does at perigee [3].

NASA describes the same thing from the shadow's side. When the Moon is far enough along its orbit, the dark core of its shadow, called the umbra, does not reach the Earth, and a ring of sunlight shows around the Moon [1]. The part of the shadow beyond the umbra is the antumbra, and standing in it, the Sun is only partly blocked [4]. A total eclipse is the opposite case: the Moon is close enough that its umbra touches the ground [4].

The Sun's size changes too, but much less. Its apparent size varies by only about 3 percent over the year, because the Earth's orbit is also slightly oval [3]. The Moon's bigger swing is what usually settles the matter.

Why One Eclipse Can Be Total in One Place and a Ring in Another

Hybrid eclipses are the in-between case. NASA describes them as eclipses where the Moon's umbra and antumbra both touch the Earth along different sections of the path [4]. People on one stretch of the path see a total eclipse and people on another see a ring.

I did not find the reason spelled out in the English pages I read, so here is a way to picture it, not a quotation. The Moon's apparent size is only just close to the Sun's in these eclipses. The Earth is a big ball, and the Moon's shadow sweeps across it, so places along the path are not all the same distance from the Moon. A tiny difference in distance is enough to move a spot from “just covered” to “just not covered.”

Totality Is Seen Only From a Narrow Path, and the Ring Still Needs Eye Protection

The dark core of the shadow is small. NASA says a typical umbral path may be less than 50 miles (about 80 kilometers) wide [1]. That is why people travel to stand inside it: a few miles to the side, the Sun is only partly covered.

It is also where the safety rule comes from. Looking straight at the Sun is only safe during totality, when the Moon completely covers the Sun's bright face. During a partial or an annular eclipse, it is never safe to look directly at the Sun without proper eye protection [6]. A ring of light is still bright sunlight. NASA says safe solar viewers should meet the ISO 12312-2 standard, and ordinary sunglasses are not a substitute [6].

Try It at Home: Make a Ring Eclipse With Two Coins

You can copy the Sun-and-Moon trick with something safe: two coins of the same size, with no Sun involved.

1. Tape one coin flat to a wall at about eye height. This is your “Sun.”
2. Stand a few steps back. Hold the second coin (the same kind) at arm's length in front of one eye, and close the other eye. This is your “Moon.”
3. Slowly bring the coin in toward your eye. The close coin looks bigger than the one on the wall and hides it completely: a total eclipse.
4. Now push your arm out and move the coin away. The coin on the wall starts to peek out around the edge, a thin ring. That is an annular eclipse.
5. Find the spot in between where the two coins look exactly the same size.

Never try this with the real Sun. For the Sun, use eclipse glasses that meet the ISO 12312-2 standard, or watch a projected image of the Sun on a surface (for example, through a small hole in an index card), always with your back to the Sun and never looking at it through the hole [6].

If you want to read more, NASA's pages on eclipse geometry and eclipse safety are written for general readers, and ESA's page on the 2005 annular eclipse shows how the ring looked from Spain.

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