100 Days Until Totality Blog Series: 80 Days Left – How Do We Predict the Occurrences of Solar Eclipses?

Credit: NASA/Google

As we discussed in earlier blog posts, the Moon’s orbit is slightly tilted, preventing the Moon from blocking the Sun during every new Moon. The points where the Moon’s orbit intersects the plane of the Earth’s orbit around the Sun are called nodes. The position of the nodes change over time, when the nodes are in line with the Sun and the Earth eclipses are possible. When the Moon is nearby these nodes, we are in what’s known as eclipse season. Eclipse season occurs 19 days before and after the moon is at any particular node. Since astronomers have studied the orbital equations of motion for the Moon and the Earth’s orbits in great detail, they can use computers to calculate when the Moon will pass through a node. It’s also possible to calculate how much of the Sun it will appear to cover in the sky.

Even before the calculation power of the computer, humans were able to predict solar eclipses by noticing patterns in the records they kept. The Chaldean civilization was the first to discover that if a solar eclipse occurs then, another will happen in 6585.3211 days (18 years, 11 days, 8 hours). This periodicity in eclipse occurrences is known as the Saros cycle. Eclipses that are separated by one Saros cycle from each other will have very similar properties, but will occur over a different location on the Earth. For example, the eclipse this August is part of Saros cycle 145. All of the eclipses in this cycle are very similar. One cycle later on September 5th of 2035, another total solar eclipse will occur with a path shaped much like the path for the eclipse of 2017. However, this time the eclipse will occur over Asia and the Pacific Ocean.

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