A solar eclipse is a sweet cosmic gift—a combination of orbital mechanics, random chance, and dazzling celestial aesthetics. They are also more common than most people would think; if you manage to live through the entire 21st century, you’ll be around for 224 solar eclipses. But only seven of those—or 3.1%—will be of the particularly rare variety known as a hybrid eclipse, one of which is occurring April 20. So what is a hybrid eclipse—and what makes it so special?
The best-known and most dramatic type of eclipse is the total eclipse, when the disk of the moon completely covers the sun—and a lot of things have to fall into place for that to happen. An eclipse occurs during that part of the moon’s orbit when it passes between the Earth and the sun. That happens once every 27.3 days, meaning an eclipse should be a monthly event. But the orbit of the moon is inclined about five degrees relative to the path of the Earth’s orbit around the sun, so most of the time the Moon passes above or below the solar face. It is only on those rare occasions that the moon’s orbit carries it directly in front of the sun that a total eclipse can occur.
Then too, there’s the relative size of the two bodies to consider. The moon is about 400 times smaller than the sun—but in a wonderful bit of cosmic serendipity, the moon is also 400 times closer to the Earth than the sun is, meaning that from our vantage point, the two disks appear to be about the same size in the sky. During a total eclipse, the moon thus covers the body of the sun entirely, leaving only the fiery solar corona visible.
Not every alignment in which the moon moves directly in front of the sun is equal, however—and that has to do with yet another variable of orbital mechanics. The moon’s orbit around the Earth is not a perfect circle; it is, rather, an ellipse ranging from a low, or perigee, of 363,300 km (226,000 mi) away from Earth to a high, or apogee, of 405,500 km (253,000 mi) away. Only when the moon is at its closest does it appear large enough to permit a true total eclipse. A moon at apogee appears smaller, and thus covers only part of the sun, producing a less breathtaking version of a total eclipse known as an annular eclipse.
In most cases, an eclipse is one type or the other, but the one coming April 20 will be both. That’s because of the path this particular eclipse will take across the sky, moving northeast across the South Pacific Ocean, crossing over the nations of East Timor, Australia, and Indonesia. Anyone in those countries will be in for a treat because it is at that point that the eclipse will reach totality.
At the beginning of its path in the southwest corner of the Pacific ocean south of Madagascar and its end in the northeast, equidistant between Indonesia and Hawaii, the eclipse will only be annular. That’s because the spherical shape of the Earth causes it to curve away from the moon at those two extremes, making the lunar disk appear to be smaller, and to cover only part of the sun. But in the middle, in the trajectory through northwest Australia, eastern Indonesia and East Timor, the eclipse will appear to be total. The annular-to-total-to-annular process will take nearly six hours to play out, before the moon at last moves out of the way of the sun and the rare phenomenon ends.
The next hybrid eclipse won’t occur until Nov. 14, 2031, but Americans don’t have to wait nearly that long for the next total eclipse. That will happen on April 8, 2024, with the path of totality passing over parts of Texas, Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire, and Maine, before passing into Canada, out over the Atlantic—and into the cosmic history books.