The Science Behind Solar Eclipses

If you haven’t been living under a rock for the past couple of months, you would’ve heard about the total solar eclipse that happened on April 8th this year. This particular eclipse passed over major population centers totaling over 30 million people and was only the third occurence since 1918 visible in the US. On the Deerfield campus, we were able to see 95% of the sun covered, with the path of totality (that is 100% of the sun being blocked) being three hours north by car in Vermont. Total solar eclipses are considered once-in-a-lifetime experiences, but to really understand how rare solar eclipses are, we need to first learn how an eclipse happens.

The fundamental premise of a solar eclipse is quite simple—it is an astronomical phenomenon where the sun, the moon, and the earth are perfectly aligned such that the sun is “blocked” by the moon for a brief period of time. While this definition of a solar eclipse is mostly accurate, it is, like most things on the cosmic scale, much more complicated than that.

Solar eclipses happen because, at some point during the moon’s orbit, it is directly between the Earth and the sun, blocking the light of the sun from the Earth’s perspective. This phenomenon is only possible because despite the moon being about 400 times smaller than the sun, it is also coincidentally 400 times closer to the Earth than it is to the sun. If the moon was any smaller, or any further away, it would not be able to fully cover the sun and total solar eclipses would not be physically possible. But the cosmic coincidence that allows for solar eclipses does not explain why they are so rare.

If the moon revolves around the earth every 27 days, then the position where the moon is between the sun and the earth, the conditions of a solar eclipse, should occur once every 27 days on the new moon. But the orbit of the moon is not perfectly level with the Earth’s orbit around the sun; it is tilted by a measly five degrees. The effect of this tilted orbit path, also known as the ecliptic, is that most of the time, the new moon is either slightly above or below the sun from the Earth’s point of view. So even if the moon was directly between the sun and the earth, it does not cover the sun completely, resulting in only partial eclipses at best.

So how often are the sun, moon, and earth perfectly aligned? The timeframe where the angle of the ecliptic is small enough for an eclipse to happen is called the eclipse season, which happens every six months. However, the interval of time it takes for the same alignment of these three celestial bodies, called a Saros cycle, has been calculated by astronomers to be exactly 6,585.322 days or 18 years, 11 days, and 8 hours. This means that an eclipse with the geometry that happened on April 8th, 2024 will happen again on April 20th, 2042. But similarly to how leap years happen every four years because of an extra 0.25 days in the Earth’s orbit, the extra 0.332 days in a Saros cycle means that the eclipse in 2042 will happen about eight hours later in the day, or a third of the earth’s rotation away from the path this eclipse took.

But even if the same solar eclipse happens once every Saros cycle, it doesn’t mean it is visible in the same place. Unlike lunar eclipses, in which the earth darkens the moon’s surface, which is visible around the globe, a solar eclipse is only visible from a fraction of the earth’s surface since the moon’s shadow can not fully cover the earth. This small strip of the earth’s surface is the path of totality, the only part of the earth that can witness the full solar eclipse. Even though solar eclipses could occur every six months, the chance of being able to see the eclipse, let alone be in the path of totality, is astronomically small; to have a total solar eclipse, like the one that happened this year, so close to our school is a great gift from the universe.