The solar eclipse that began Sunday at dawn in North America and ends just before the eastern African sunset is a sort of eclipse that has not been seen on Earth for nearly 160 years and won't be seen again for nearly another 160 years – in 2172.
It is a "hybrid" eclipse, which means it begins as an annular eclipse and then becomes a total eclipse.
What does that mean?
Eclipses are different because the moon is not always a fixed distance from Earth; its distance varies during its orbit. When the moon is farther away, it will look slightly smaller, of course. When it's closer, it will look bigger. That difference in size is significant when it comes to solar eclipses.
If an eclipse occurs when the moon is farther away, the moon as viewed from Earth is not big enough to completely blot out the sun. That means a thin "ring of fire" is visible around the moon. This is an annular eclipse.
If an eclipse occurs when the moon is closer to Earth, then the disk of the moon is big enough to completely blot out the sun. All that can be seen is the sun's brilliant outer atmosphere, or corona. This is a total eclipse.
So how does a "hybrid" eclipse work? How can an annular eclipse become a total eclipse?
The answer lies in understanding how the moon's shadow hits Earth. The darkest part of the shadow, in which the sun is completely blotted out, is called the umbra. The umbra is shaped like an ice cream cone (see a good depiction here). The widest part of the cone is nearest the moon, because the moon blocks out more of the sky the closer you are to it.
The difference between annular and total eclipses depends on where the point of that umbral cone is.
When the moon is farther away, the point is actually above Earth's surface. That means there is no place in Earth that is directly in the umbra, and the result is an annular eclipse, where that thin ring of the sun can still be seen.
When the moon is closer, the point of the umbra hits Earth's surface, and every place where that point hits sees a total eclipse.
A hybrid eclipse happens when the distance between the Earth and moon is so finely balanced that the curvature of the Earth comes into play.
Sunday morning, when the moon's disk was beginning to slide in front of the sun in North America, the umbral point was not touching Earth, so it appeared as if the sun was heading toward an annular eclipse. And for 15 seconds somewhere out over the Atlantic ocean, an annular eclipse actually happened.
Then something spectacular followed. As the eclipse progressed, the curvature of the Earth actually reached up to intercept the point of the moon's umbra, making it a total eclipse. Joe Rao of Space.com describes the moment between annular and total eclipses this way:
The silhouette of the moon is not a perfect circle, but rather slightly prickly with mountains, so just before the transition from annular to total, the eclipse will become something neither annular nor total: for a few precious seconds it will be a broken annular.
As lunar mountains protrude onto the hairline-thin ring of the sun, it will be seen not as an unbroken ring but an irregular, changing, sparkling sequence of arcs, beads and diamonds very briefly encircling the moon: a "diamond necklace" effect!
This was visible only about 400 miles south of Bermuda, Mr. Rao said, and he noted that group of German eclipse enthusiasts hoped to fly at plane to the spot to see it.
For the rest of its track across the Atlantic and then into equatorial Africa, the eclipse will remain total. The last trace of the total eclipse will be visible from Somalia at 5:27 p.m. local time (9:27 a.m. Eastern time).