Pinpointing totality
Eclipses are rare, so how do astronomers know when they’re coming?
Astronomers can pinpoint the time and location of an eclipse to the millisecond. “What you need is to have an accurate ephemeris, a computer program that can accurately predict where the Sun and Moon are, with respect to Earth,” says Fred Espenak, a NASA Scientist Emeritus best known for his work on eclipse predictions, who runs the website MrEclipse.com. That means knowing where all three celestial bodies are in three dimensions. “Then you can predict where the Moon shadow is, since it’s always pointing away from the Sun.”
Figuring out where that shadow is in space, where it’s going to be, when it’s going to intersect Earth’s surface, and how it moves across the surface – all in 3D – is then calculated using spherical trigonometry. “It’s all worked out by computer programs, but the techniques for calculating eclipses were worked out several hundred years ago,” says Espenak. “Back then they were doing calculations using tables of logs and trig functions. It was done by hand, so it took a huge amount of time to calculate timings for one location.” With the right type of software it now takes a fraction of a second.