All About Solar Eclipses
The upcoming year is an exciting one for solar eclipses. For those of us living in the United States, we have not one but two upcoming eclipses to look forward to! On October 14, 2023, we’ll be able to view a partial annular eclipse from anywhere in the continental U.S., with full coverage visible along a path that stretches from Oregon to Texas. Six months later, a total eclipse will occur on April 8, 2024.
Many of us already know that a solar eclipse involves the Moon blocking the Sun, but how do these celestial events actually work? What are the differences between “total,” “annular,” and “hybrid” eclipses?
Above: The 2019 total solar eclipse in Chile. Image courtesy of CNN.
Solar eclipses occur when the Moon passes between the Earth and the Sun. This means that they can take place only during a new moon when the Moon and the Sun are positioned in the same direction relative to the Earth. Despite the ostensible rarity of these conditions overlapping, solar eclipses are actually quite common. In fact, an average of two solar eclipses a year can be seen from Earth. A new moon occurs every month, but solar eclipses only twice a year—why? The orbits of the Moon and the Sun aren’t perfectly aligned. Instead, the Moon’s orbit is tilted about five degrees relative to that of the Sun. Solar eclipses are only possible when a new moon occurs at one of the two nodes where the two orbits intersect.
Above: Simplified diagram (not to scale) showing the relative tilt between the two orbits. Eclipses occur only when the Moon and Sun both pass through one of the two nodes at roughly the same time. Image courtesy of Vanderbilt Dyer Observatory.
Purely by coincidence, the Sun’s diameter is 400 times that of the Moon, but the Sun is also 400 times farther from the Earth than the Moon is. As a result, they appear to be roughly the same size to viewers on Earth. Because the Moon’s orbit is gradually increasing, in around 600 million years, the Moon will no longer appear large enough to completely cover the surface of the Sun. Make sure to see a solar eclipse while you can—they won’t happen forever!
There are four types of solar eclipses: partial, total, annular, and hybrid. The Moon’s orbit is elliptical, and when the Moon is at a closer point of its orbit to Earth, the Sun is fully obscured and a total eclipse occurs. Total eclipses occur roughly every 18 months. However, the path of totality is narrow, and any given location experiences a total solar eclipse every 375 years on average. For U.S. viewers, the next total eclipse will take place on April 8, 2024. Totality was last visible in the United States in 2017. Unfortunately, these back-to-back total eclipses are far from common. Before 2017, the last total eclipse in the continental United States was in 1979, and after 2024 the next will occur in 2044.
Partial eclipses occur when the Moon and Sun aren’t quite aligned. The Moon doesn’t pass through the center of the Sun, and thus the Sun is only partially obscured. Partial eclipses are also visible during the other three types to anyone who is in the penumbra—the partially shaded outer region of the Moon’s shadow, as opposed to the fully shaded umbra.
Above: Diagram showing the narrow umbra, in which a total or annular eclipse is visible, as well as the much wider penumbra, in which the eclipse is only partial. Image courtesy of Vanderbilt Dyer Observatory.
When the Moon is at a farther point of its elliptical orbit from Earth, it does not fully block the Sun. This creates a “ring of fire” effect called an annular eclipse. The next annular eclipse will be visible in October of this year. The best viewing in North America will be in Oregon, Nevada, Utah, New Mexico, and Texas.
Above: Annular eclipse in April 2014. Image courtesy of NASA.
Hybrid eclipses are unique phenomena, as both total and annular eclipses occur during the same passage of the Moon across the Sun. For a hybrid eclipse to occur, the Moon must be at the boundary between creating a total or an annular eclipse. At this exact distance, slight changes in the Moon-Earth distance cause the eclipse to shift from annular to total and back. Because of the curvature of the Earth’s surface, its rotation brings any specific location on its surface first closer and then farther from the Moon. This requires such a particular arrangement of Earth, Moon, and Sun that it occurs very rarely—roughly once every decade. Unfortunately, the only points where both total and annular eclipses were visible during the April hybrid eclipse were in the middle of the ocean.
Planning to Watch
So how can you watch the upcoming solar eclipses? First, check the maps below to see if your area is in the path of the eclipse. Next, research ways to safely view a solar eclipse. Solar observation can be dangerous to view directly, so proper methods are essential. If you live outside of the penumbra of the upcoming eclipses, find other ways to experience these exciting events (hint: eclipse live streams are becoming more and more common by free sites like the Gravity Discovery Centre and Observatory and TimeAndDate.com). While videos aren’t a substitute for the experience of watching an eclipse in-person, they offer windows into these amazing phenomena that are available worldwide, 365 days a year.
Above: Map of the October 2023 annular eclipse path over the United States. While totality is limited to the narrow path highlighted in orange, some percentage of the partial eclipse will be visible from anywhere in the U.S. Image courtesy of NASA.
Above: Path of totality for the April 2024 eclipse. Image courtesy of NASA.