What Would Happen If the Earth Spun Backward?

Earlier this week, in honor of special guest Neil deGrasse Tyson, The Daily Show featured a new opening credit, fixing once (but not for all) the scientifically inaccurate globe that nightly graces the show's first few seconds. As Jon Stewart explains:

Tyson has good reason to be concerned: The planet's spin is not to be messed with if you like life on Earth pretty much the way it is.

"The Earth spins the way it does because it was basically born that way," Penn State astrophysicist Kevin Luhman explained to me. "When the sun was a newborn baby star, it had a whole bunch of gas and dust circling around it in a big disc-like structure." As that material clumped together to form the planets, they retained this spinning motion. Venus is the only planet that spins in the opposite direction, which is believed to be because of a major collision billions of years ago. (Uranus spins on an extreme angle, but not backwards.)

Though Luhman says that our planet has "almost certainly" spun in its current direction, the speed of the spin has changed dramatically over time. "When the Earth was born, it was probably spinning about once every six hours, so the length of a day would have been only about six hours back then," Luhman says.

The only real way the Earth could start spinning in the other direction would be if there were a massive collision, such as with another planet. "That event itself would disastrous," Luhman says.


But say you could by magic change the direction of the Earth without that sort of calamity, what would happen then? The BBC's meteorogist Peter Gibbs has worked through this thought experiment, and his answers, though very back-of-the-envelope, give you an idea of just how much of our climate depends on the planet's current rotational direction -- beyond the obvious shift to a sun rising in the west and setting in the east.

The more consequential result of a reverse spin, Gibbs explains, would be to upend the pattern of the Coriolis effect, which "transfers the spin of the earth into the motion of winds around a weather system." The Coriolis effect is why northern-hemisphere storms turn counterclockwise and cyclones, in the southern hemisphere, go clockwise. The jet stream too would reverse, and that would dramatically change weather patterns. He explains:

This river of high altitude, fast-moving air steers the mid-latitude depressions across the planet from west to east. Swirling masses of cloud and rain are pushed from Japan to the Pacific coast of America, and from Newfoundland to Cornwall. Reverse the flow and climate changes dramatically. The British Isles loses the moderating effect of weather from the Atlantic. A harsher continental climate becomes more likely, with a predominantly easterly flow bringing bitter Siberian winds in winter and hot, dry weather in summer. Goodbye green and pleasant land.

Tradewinds too would switch. What once flowed north and east would now flow north and west. "Patterns of human discovery, subsequent empire-building and the resulting political geography would all be different," Gibbs writes. Rains would no longer fall on the planet's lushest environments, and deserts could become jungles or forests. All told, he says, "It's reasonable to assume that a reversal would alter the pattern of habitable land."

But the really dangerous thing would be if the Earth stopped spinning all together. "That could be really bad," says Luhman. "The Earth is surrounded by a magnetic field, and that magnetic field is generated by the core of the Earth in its spin." No spinning liquid iron core, no magnetic field. "We need the magnetic field of the Earth to shield the Earth from the sun. If we didn't have the magnetic field, life on Earth would be in really bad shape."

The Daily Show's mistake is a reminder of the particular admixture of conditions that makes Earth habitable. But as Bill Bryson put it in A Short History of Nearly Everything:

A big part of the reason that Earth seems so miraculously accommodating is that we have evolved to suit its conditions. What we marvel at is not that it is suitable to life but that it is suitable to our life -- and hardly surprising, really. It may be the that most of the things that make it so splendid to us -- well-proportioned Sun, doting Moon, sociable carbon, more magma than you can shake a stick at, and all the rest -- seem splendid simply because they are what we were born to count on.

But splendid nevertheless.