Any fourth grader can tell you that the Earth is rotating. What a fourth grader probably can't tell you is why flying in the opposite direction that the Earth spins doesn't make for a faster plane trip. Heck, a college grad may not even know how to explain that one.
Let's break this down to really picture the scenario. You've got one Earth, which is rotating eastward. And you've got one airplane, flying just above the Earth's surface going westward. Put the two together, and the plane's destination would then, logically, be moving toward the plane as the Earth rotates. Ta-da! The result should be a super short flight from Chicago to San Francisco, with a little longer travel time on the return flight. Right? Well, no. That supposedly quick trip is complicated for a few reasons.
First, as the Earth itself rotates, it takes the air with it (thanks, gravity!). That includes the air through which planes fly. At the equator, the Earth spins about twice as fast as a commercial jet can fly. That rate slows the closer you get to the poles, but regardless, it's always going to be faster than a plane. Since it can't match the Earth's rotational speed, a westward plane technically travels east — just like the entire planet beneath it. It just has engines that help it travel east a little more slowly than everything else, making it move west relative to the ground.
Secondly, we've got jet streams yet to talk about. Jet streams are tunnel-like air currents in the atmosphere where hot and cold air meet and could either work with or against your plane. The rotation of the Earth pushes these winds into swirling patterns — the same type of action that takes place for hurricanes. In the Northern Hemisphere, the jet stream flows toward the east, giving a boost to eastward flights and working against westward flights. The speed of jet streams ranges from 80 to 140 miles per hour (130 to 225 kilometers per hour), and flying in one can save the airline time, fuel, and, duh, money. If you want to really take advantage of jet stream travel, fly from the west to east in the winter. That's when the difference between hot and cold air is most dramatic, making the jet stream even faster.
This article first appeared on Curiosity.com.