On Aug. 21, a total solar eclipse will sweep across the U.S., starting in Oregon and ending in South Carolina. It’s been nearly a century since a total solar eclipse touched both U.S. coasts.
So how predictable are events like the Great American Eclipse? Phillip Nicholson, an astronomy professor at Cornell University who studies the solar system, says they’re very predictable – you can actually calculate future eclipse times within a few seconds. But millions of years from now, total solar eclipses will only be a thing of the past.
“While apparently random, solar and lunar eclipses in fact occur following a fairly complex pattern of cycles, in which the main 18-year period is known as the Saros cycle. The ancients knew of this cycle and used it to make predictions, but we understand now that this cycle simply reflects the orbital motion of the Moon about the Earth every 29.53 days combined with the Earth's motion about the sun every 365.35 days. After 18 years and 11 days, the pattern of New Moons and eclipses repeats itself almost exactly.
“We can predict future eclipse times within a few seconds for several centuries into the future, and the eclipse tracks within a few tens of kilometers. The limitation on our accuracy is set by the slightly irregular spin of the Earth on its axis, which changes exactly where the path of totality will fall on Earth. In fact, one reason we know the Earth's spin is irregular is by studying records of ancient eclipses and where they were seen.
“In the distant future, which is several millions of years from now, there will be no more total solar eclipses because the Moon will have moved too far away from the Earth to cover the sun's disk completely. We live in a lucky time!”