Why Does a Solar Eclipse Move West to East?

By Stephanie Pappas

The sun rises in the east and sets in the west. A glimpse at the map of the April 2024 North American solar eclipse, however, shows a path from west to east. What gives?

The answer lies in the relationship between the rotation of Earth and the orbit of the moon, but the arrangement can be a bit tricky to visualize.

First, let’s look at the total solar eclipse path: on April 8 the moon’s shadow will fall on Earth, blocking out the sun along an approximately 100-mile-wide track that will cross through central Mexico, pass through Texas and cut diagonally across the continental U.S. It will then skim along the country’s border with Canada before it exits the U.S. near the northern tip of Maine and moves through parts of New Brunswick, Prince Edward Island, Newfoundland and Labrador.

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This path moves in the opposite direction of the sun’s apparent trek over Earth each day. But during an eclipse, the motion of the moon’s shadow—not the sun—is what matters. Earth rotates from west to east, says Michelle Nichols, director of public observing at the Adler Planetarium in Chicago. Imagine your vantage point on the planet rolling from west to east each morning to greet the sun, which thus appears to rise over the eastern horizon.

The moon also orbits Earth on a west-to-east path. If you were perched above the North Pole, looking down on the planet from space, Earth would seem to be turning counterclockwise, with the moon traveling in the same direction, just more slowly, Nichols says. Earth, of course, completes a rotation on its axis in about 24 hours, while the moon takes roughly a month to do a full orbital loop around the planet.

During a total solar eclipse, the moon comes between Earth and the sun, with all three bodies lined up in the same plane so that our natural satellite blocks the star’s light and casts a shadow on Earth’s surface. Suddenly, the west-to-east motion of the moon in the sky becomes far more obvious than it usually is. As the moon passes across the face of the sun, it casts its shadow in the same direction in which it is moving, from west to east.

“It’s a rising and a setting concept,” says Jackie Faherty, an astronomer and senior education manager at the American Museum of Natural History. “It has everything to do with the way the moon is moving.”

The lunar shadow moves at more than 1,000 miles per hour across Earth’s face during an eclipse, Nichols says, which is why any given location experiences totality—the moment the moon completely blocks the sun—for only a few minutes. During the 2024 eclipse, the longest duration of totality will be four minutes and 28 seconds near Torreón, Mexico, according to EarthSky.

Solar eclipses take different paths across Earth depending on which side of the planet is facing the moon when it passes in front of the sun. There are some patterns to these paths, though. Known as saros cycles, these patterns are determined by the path of the moon’s orbit around Earth and its distance from our planet at the time of an eclipse, Nichols says. They lead to eclipse paths with repeating shapes (albeit at different locations on Earth) approximately every 18 years and 11 days. But whatever its precise path, every eclipse travels from west to east.