An annular solar eclipse is…an interesting sight, to say the least.
(I sincerely hope the photographer didn’t fry his camera taking this picture. Enough light from the sun is still reaching us to fry your retina, or damage your optics…)
The annular eclipse is not to be confused with an annual eclipse. When my dad first got excited about it back in 2012, preparing us for the spectacular sight of a solar eclipse in May, I wondered why the heck we hadn’t done this every year before.
The fact is, I’d never heard the word “annular,” so I thought Dad was just wrong.
But in reality, “annular” means something very different from “annual.”
It means, in short, exactly what you see above. The sun forms a ring of light around the moon, known in fancy astronomy terms as an annulus. Annulus, annular…you get my drift.
But why? Why shouldn’t the eclipse above appear more like the famed total solar eclipse, which was all the rage just a week or so ago?
It all has to do with the moon’s orbit.
As you may know from my post on the moon’s orbit a few days ago, it’s not a perfect circle. It’s what we call an ellipse, which—as I’ve written before—I’ll explain in more depth in later posts.
This diagram is showing conditions that are very favorable for an annular eclipse.
Because of the moon’s elliptical orbit, it swings in closer to the Earth on one side of its orbit and farther away on the other side. When it’s closer, we say it’s at perigee, and when it’s farther, we say it’s at apogee.
Have you ever noticed that objects far in the background of your sight look smaller than closer objects, even if they’re much bigger?
Somehow, I doubt this guy is actually that much taller than those background trees. Or even as tall as they are.
The way we see the moon works the same way. It appears just a tiny bit bigger when it’s at perigee, and just a tiny bit smaller when it’s at apogee. That tiny bit, in fact, is about 6%.
That 6% is enough of a difference to turn a total solar eclipse into an annular one.
We get a total solar eclipse, or a “total” annular eclipse, when the moon’s orbit lines up precisely with the Earth’s. It’s tilted a bit, but it also wobbles like a spinning top, and every so often the moon’s shadow lines up with the Earth.
But if the moon is at apogee when the shadows line up, there’s a chance we’ll actually get an annular eclipse, rather than a total solar eclipse with “totality.”
I’ve written about the difference between the umbral shadow and the penumbral shadow before, but here’s a brief recap.
The umbra is simply the darker, inner part of any shadow when the light source is bigger than the object casting the shadow—or there are more light sources.
There’s an area that’s completely blocked from the light source and gets no light. That’s the umbra. But there’s parts outside of it that get some light from the light source, but have some light blocked as well. That’s the penumbra.
And any object can cast both a penumbral and umbral shadow, as long as it doesn’t completely block its light source…
Here, you can see that the bigger the light source compared to the object, the bigger the penumbral shadow will be and the smaller the umbral shadow will be.
Another way you can think of it is by imagining yourself standing between two street lamps. You cast two shadows, one for each light source. But each of the lamps shines on one of the shadows, making them a bit brighter—penumbral shadows. Any place where the two shadows cross, you’ll get an umbral shadow because no light is falling there.
So, here’s what happens during an annular solar eclipse.
An annular eclipse happens when the moon is too far away in its orbit for the umbral shadow to actually touch the Earth. Instead, we get an “antumbra,” because we never do get “totality.”
As this diagram demonstrates, though, we still call the path of the antumbra across the Earth’s surface the “path of totality.”
Anywhere outside the path of totality, we’ll get a sight much like we would in the penumbra of a total solar eclipse. If you’re standing in the penumbra, it doesn’t matter if the eclipse is total or annular—you’ll still just see a “partial” eclipse.
In May 2012, we saw a partial annular eclipse. We never saw the ring of the sun surround the disk of the moon, and I didn’t understand until much later that an annular solar eclipse isn’t the same as a partial solar eclipse.
During a partial eclipse, the moon’s shadow never lines up with the Earth exactly. No one on Earth sees totality—the penumbra sweeps over the Earth, but the umbra is cast on empty space.
Here’s what we saw…
For this annular eclipse, we weren’t standing in the “path of totality,” if you can call the most spectacular part of an annular eclipse a real “totality” phase. We watched from part of the penumbral shadow, so we never did see the moon get centered in the disk of the sun.
It was just as amazing to see, though. We set up telescopes on our neighbor’s front lawn—he had a better view of the eclipse than we did—and sat outside for a few hours as we watched the moon’s disk creep across the sun. It always surprises me how hot an event the solar eclipse is, even though it means standing under the sun for hours on end!
During a total solar eclipse, though, it certainly doesn’t stay hot. Next up is a description of what you’ll see and experience during a total solar eclipse—and how to protect your eyes!