How Scientists Predict the Path of the 2017 Total Solar Eclipse

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During a total solar eclipse, the moon completely obscures the sun for parts of the Earth. Careful calculations help researchers and the public determine where to go to see the total blackout.
Credit: Miloslav Druckmüller, Peter Aniol, Martin Dietzel, Vojtech Rusin
Millions of people intend to watch the 2017 total solar eclipse, which will cross the continental U.S. on Aug. 21. Here's how NASA scientists figure out exactly where the moon's shadow will fall on the surface of the Earth, down to the city block.


Space.com talked with NASA's Ernie Wright, who has been producing NASA's visualizations of the celestial event, to learn how satellites mapping the surface of the moon and advances in computing power have made it possible for scientists to predict precisely where on Earth the eclipse will be visible and for exactly how long — with a precision of about 100 meters (330 feet, or about the length of a city block).

Knowing where to watch the eclipse means the difference between seeing totality — when the sun is fully concealed by the moon — and just a partial eclipse, where the moon covers part of the sun but the sky doesn't fully darken. If you're outside the path of totality, the moon's crossing will be just a glancing blow. [Total Solar Eclipse 2017: When, Where and How to See It (Safely)]

A 19th century technique for predicting the eclipse has scientists using a coordinate system aligned with the shadow of the moon on the Earth, making it easy to determine whether a given observer on the ground was inside or outside the shadow's circle. But that method simplifies the sun-moon-Earth system, and so is accurate to only within a few miles, depending on the location, Wright said.

"That all assumes that the moon is perfectly smooth and that all the observers on the Earth are at sea level," Wright told Space.com. "These are simplifying assumptions; when you have to make these calculations with pencil and paper you need to simplify them a little bit."

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