People in parts of the U.S., Mexico, and Central and South America are in for a rare celestial treat in October: an annular eclipse of the sun.
An annular eclipse is similar to a total solar eclipse, in which the moon completely covers the sun’s face, except at the former’s peak, the moon is too small in the sky to fully blot out our home star. This creates a bright ring of sunlight around a dark lunar silhouette—thus the name “annular,” which means “ring-shaped.” (Some people prefer to simply call an annular eclipse a “ring of fire” instead.)
Solar eclipses happen when the moon passes directly in front of the sun—essentially, our natural satellite’s shadow sweeps across the surface of Earth. If the moon orbited in the same plane as Earth orbits the sun, we’d get a solar eclipse every 29 days—the length of time it takes the moon to move around our planet, relative to the sun. But the moon’s orbit is actually tipped, inclined by about five degrees to that of Earth.
Like two Hula-Hoops, one inside the other and tipped, the path the sun appears to take around the sky once per year and the moon’s monthly orbit intersect at two points, called nodes. We only get a solar eclipse when both the sun and moon are at a node at the same time; otherwise the moon “misses” the sun in the sky, passing it above or below. Most of the time the two don’t line up perfectly, resulting in a partial solar eclipse, in which the moon blocks only a fraction of the sun’s face.
If they line up just right, the entirety of the sun’s Earth-facing hemisphere is blocked, and we get the glory of a total solar eclipse. The sky gets dark, and the sun’s ethereal outer atmosphere, called its corona, pops into prominence. I was in Wyoming for the 2017 total solar eclipse, and it was, without exaggeration, one of the most beautiful and moving events I have ever witnessed. The U.S. will be treated to another one of these incredible phenomena on April 8, 2024.
But sooner than that—on October 14, to be specific—yet another factor will come into play: the moon’s distance from Earth.
By an amazing coincidence, on average, the sun is 400 times farther away from Earth and 400 times physically bigger than the moon. These two effects cancel out, so the sun and moon are the same apparent size in the sky: about half a degree, or half the width of your extended pinky seen at arm’s length.
The moon doesn’t orbit Earth in a circle, however, but in an ellipse. At its closest, the point in its orbit called perigee, the moon is roughly 355,000 kilometers from the surface of Earth. At its farthest—apogee—it’s about 397,000 km. That change of about 10 percent in distance means that the moon’s apparent size in the sky changes by 10 percent over the course of half an orbit. So at its apogee, the moon can appear to be smaller than the sun.
The moon reaches apogee on October 9, just five days before this upcoming eclipse. It will be about 391,000 km from Earth—more or less. The exact figure depends on other factors, such as the latitude and longitude of the observer, the time of day, and so on. At that distance, it will appear to be 0.51 degree across. At the same time, Earth and the sun will be at almost exactly their average separation, 149 million km, so the sun will appear as a disk about 0.54 degree in size.
And that makes all the difference in the worlds. The moon will be too small to completely cover the sun. Instead it will leave a ring—an annulus—around it as it passes, creating this annular eclipse. At most, 91 percent of the sun’s surface will be covered by the moon, so technically this will be a partial eclipse—but a very special, perfectly centered one.
The eclipse will have three main stages: first contact, annularity and last contact. First contact will be when the moon’s edge first appear to touch the edge of the sun. Over time, as the moon moves, it will appear to eat more and more of our star’s disk. About 70 to 90 minutes after first contact, depending on your location, the annularity will occur. It will last from one to five minutes, also depending on location. Then the moon will start to leave the sun’s face. Our natural satellite will take another 70 to 90 minutes to completely move off the star (last contact).
That’s how the eclipse will work. So how can you see it?
First, there is no safe time to watch this with your naked eye! Throughout the eclipse’s duration, the sun will be visible, so you will need adequate eye protection. (Do not just use sunglasses, exposed film or welder’s glasses.) A lot of companies sell “eclipse glasses”—usually a paper frame with heavily filtered plastic film to look through—but not all of these are safe. The American Astronomical Society has a list of trusted vendors that have glasses that are compliant with the ISO 12312-2, the International Organization for Standardization’s (ISO’s) safety standard for directly viewing the sun. I’ll add that Astronomy for Equity sells them in bulk, and the money they organization makes from those sales goes toward excellent causes.
A fun way to observe this event is with a pinhole projector. A small hole poked into a piece of cardboard will allow sunlight through as parallel rays—that is, as focused light suitable for imaging. You can then hold it up and project the resulting rays onto a sheet of white paper, a sidewalk or even the bare ground. You’ll see a lovely perfect little image of the sun as the moon eats away at it. Foliage can produce a similar effect as well (overlapping leaves make lots of little holes), so you can see dozens or even hundreds of images of the sun on the ground below most trees.
On the morning of October 14, the path of the moon’s shadow will start over the Pacific Ocean. The exact time it will reach any given spot will depend on the location and the time zone. For example, Eugene, Ore., the first big city to see annularity, will get the first contact at 8:06 A.M. PDT, the annularity for four minutes starting at 9:16 A.M. and then the last contact at 10:39 A.M. The moon’s shadow will move southeast, passing over extremely northeastern California, Nevada, Utah, extremely northeastern Arizona, New Mexico and then Texas. After that it will pass over the Yucatán peninsula in Mexico, followed by southern Central America and then northern South America before moving off onto the Atlantic Ocean.
Astronomer and long-time umbraphile (literally, “shadow lover”) Fred Espenak has created an interactive map for the eclipse path. Just zoom in and click anywhere to get detailed information on timing.
I’ll add that the entire continental U.S. will see at least a partial eclipse, but only those along the narrow shadow path will be able to view the ring of fire. NASA has an excellent website with loads of information about the eclipse. SciStarter, a terrific program that promotes “citizen science” projects, has a list of scientific projects you can participate in as well—excellent prep for next year’s total solar eclipse. The American Astronomical Society also has an app (available for iOS and Android devices) called Totality 3.0, which has a ton of information about this eclipse and next year’s total one, too.
I’ve never seen an annular eclipse, and from my location in Virginia, only be about one third of the sun will be covered. But if you’re in the path of annularity, and you’re able to do so, go out and take a look (safely, please)! Although it’s perhaps not as grand as a total solar eclipse, it’s still a fascinating and rare astronomical phenomenon and one well worth your time to observe.