There are two solar eclipses every year — so why are they so rare?
With the great North American total solar eclipse rapidly approaching on April 8, 2024, it's being called an extremely rare astronomical event.
But is this true? After all, Carbondale, Illinois, will see two total solar eclipse (in 2017 and 2024) within the span of seven years.
A Question of Alignment
To tackle this assertion, we begin by considering the unique geometry that makes eclipses possible. Two conditions are needed for a solar eclipse to occur. The first is that the Moon must be in its new phase (passing between Earth and the Sun), during which its shadow points towards Earth.
The second condition requires a 3D view. The Moon’s orbit around Earth is tipped about 5° to the plane of Earth's orbit around the Sun. The two points where the Moon’s orbital plane crosses through the plane of Earth’s solar orbit are call the nodes. A solar eclipse can occur only when the Moon is close to one of the nodes so that its shadow falls on Earth. If the new Moon is too far from either node, the lunar shadow will miss Earth, passing either above or below our planet.
During each 29.5-day synodic month, as the Moon cycles through all its phases, the Moon passes once through each of its two nodes. Every five or six synodic months, the new Moon occurs with the Moon close enough to a node (within ~16.9°) to produce a solar eclipse. In other words, there are at least two solar eclipses every year — and there can be as many as five!
But a sizable fraction of solar eclipses are partial, meaning that the Sun is only partially covered when viewed from some part of Earth's surface. The new Moon must be even closer to a node (within ~10.9°) in order for the Moon’s central shadow, or umbra, to pass over Earth.
An additional complication to this geometry is that the Moon’s orbit around Earth has the shape of an ellipse. The Moon’s center-to-center distance from Earth changes by 14% between perigee (when the Moon is at its closest to Earth) and apogee (when the Moon is at its most distant). This variation in the Moon’s distance also means the Moon’s apparent size fluctuates by this same amount (see “Moon Perigee and Apogee” for even more on this topic).
If new Moon occurs near apogee, the Moon appears smaller than the Sun and can’t cover the Suns entire disk. The dark umbral shadow of the Moon doesn’t reach Earth’s surface and the resulting eclipse is annular, like the one happening on October 14, 2023. The Moon is then seen in silhouette against the Sun's blazingly bright disk. This so-called “ring of fire” is quite an amazing sight. However, this type of eclipse requires the use of a solar filter or some method of projection to watch, because the annular ring remains too dangerously bright to view with the unprotected eye.
When new Moon occurs near perigee, the Moon appears larger than the Sun and the solar disk is completely hidden from view. The Moon’s umbral shadow reaches Earth’s surface and produces an awe-inspiring total solar eclipse. It is only during such an event that observers are plunged into the eerie twilight of totality and the incomparably beautiful solar corona is revealed. This is exactly what will happen on April 8, 2024.
How Often Do Solar Eclipses Happen?
How often do these different types of solar eclipses happen? Using a catalog of eclipse predictions covering 2,000 years (from 1001 to 3000 AD), we find there are 4,773 solar eclipses, which averages to 2.38 eclipses per year. When organized by type, the eclipses break down as follows:
|Eclipse Type||Total number||Fraction of total|
Note that I’ve sneaked in another eclipse type in the table called hybrid. This unusual case happens when the Moon is just far enough away that the curvature of Earth’s surface will bring some locations into the umbra while others are too far away. So, the eclipse path is annular at some points and total at others. A quarter of all solar eclipses are total eclipses, and one occurs every 12 to 24 months — pretty often!
How Often Do Solar Eclipses Happen Here?
Total eclipses’ true rarity comes not from how often one takes place, but from the fact that each one is only visible from a tiny fraction of Earth's surface(less than 1%). The path of the 2024 total eclipse is 14,800 km (9,200 mi) long, with an average width of 186 km (116 mi), so it only covers 0.52% of Earth’s surface.
In fact, eclipse expert Jean Meeus has calculated that for any single location, the average time between two total solar eclipses is 375 years, on average (in spite of the seven-year gap between eclipses in Carbondale, which is exceptionally rare). The next time the path of a total solar eclipse crosses the U.S. from coast to coast will be on August 12, 2045, 22 years from now.
The need to be in the right place at the right time is what makes total eclipses so rare — and millions across Mexico, the U.S., and Canada will find themselves in the right place on April 8, 2024.
Retired NASA astronomer Fred Espenak has traveled to 30 total eclipses of the Sun. He has written many books on eclipses including the 2023 annular and 2024 total solar eclipses For more information, see eclipsewise.com.