Does the rotation of the Earth affect toilets and baseball games?

By: Jennifer Horton

Would you believe that the direction of the water in this toilet bowl was influenced by the Earth's rotation? Didn't think so.
Ryan McVay/Getty Images

Philosophers, scientists and astronomers have been tackling life's most pressing questions since the beginning of time. They've convinced us that the Earth is indeed round, that it revolves around the sun and that it rotates on its axis once approximately every 24 hours. However, they can't seem to agree on the role that rotation plays in the way toilets flush and baseballs travel.

Popular culture has actually taken on the flushing toilet puzzle before. In a classic episode of "The Simpsons," Lisa convinces a skeptical Bart that drains in the Northern Hemisphere always empty to the left (counterclockwise) and those in the Southern Hemisphere empty to the right (clockwise).


The premise for this vexing question is a phenomenon known as the Coriolis effect. Basically, the Coriolis effect refers to the way that the Earth's eastward rotation influences how we see the direction of travel of certain moving objects. At the equator, where the Earth is about 24,900 miles (40,076 kilometers) in diameter, land moves at more than 1,000 miles per hour (1,609 kilometers per hour). As we move closer to the poles, land moves much slower. At 60 degrees north latitude, for instance, land moves at around half that speed [source: Plait].

In the Northern Hemisphere, this means that an object traveling from the equator toward the poles will appear to veer to the right because it maintains the greater momentum of its place of origin. On the return trip, the object would again turn to the right -- this time because it didn't have as much initial momentum. In the Southern Hemisphere, the effect is the opposite; objects would veer to the left.

Although Bart Simpson never got to the bottom of the Coriolis conundrum -- despite a $900 collect call to Australia -- this article will explain what's true about the Coriolis effect and what's all spin.

More Coriolis: What it is and isn't

This thing? No match for the Coriolis effect.
This thing? No match for the Coriolis effect.
Don Farrall/Getty Images

While some explanations of the Coriolis effect rely on complicated equations and confusing scientific jargon, there's a simpler way to visualize it: Picture yourself at the center of a merry-go-round (symbolizing the North Pole) spinning counterclockwise. If you throw a ball straight across to a person on the opposite side (the equator), the ball will appear to veer to the right because that person's moving faster than you are.

Sometimes the Coriolis effect is called the Coriolis force. The reason is simple: In causing an object to accelerate, it appears to change the direction of that object. However, the Coriolis force isn't a typical force like a push or a pull. The effect is actually based on the observer's perspective. A force isn't really acting on the object to make it go off course; it merely appears to curve because of the Earth's movement underneath it. To a person standing outside the rotating frame of reference, the object still moves in a straight line. Since the so-called Coriolis force doesn't actually act on the object to alter its course, some people argue that it's more accurate to continue calling it the Coriolis effect. Others differentiate between the Coriolis force and other (actual) forces by categorizing it as an inertial or fictitious force.


Now that you have a better understanding of what the Coriolis effect is, you should also know what it isn't. Most importantly, it isn't some all-powerful force that affects every moving object on the planet. Real forces, like gravity, can compete with -- or even overwhelm -- the Coriolis effect. This competition is more likely to occur with smaller objects that aren't traveling very fast or very far.

Let's reconsider the merry-go-round example. Unlike the Earth, that merry-go-round makes complete rotations several times a minute. Our planet, on the other hand, merely spins around once every 24 hours -- not enough to affect a game of catch or a flushing toilet. Even tornadoes are too small to be affected by the Coriolis "force." Find out exactly why on the next page.

The Coriolis Effect: Myths and Misconceptions

If the Coriolis effect makes some objects appear to change direction, why doesn't it have an impact on tornadoes or toiletwater?

While the premise makes sense -- that the earth's eastward spin would cause the water in a toilet bowl to spin as well -- in reality, the force and speed at which the water enters and leaves the receptacle is much too great to be influenced by something as miniscule as a single, 360-degree turn over the span of a day. When all is said and done, the Coriolis effect plays no larger role in toilet flushes and baseball games than it does in the revolution of CDs in your stereo. The things that really determine the direction in which water leaves your toilet or sink are the shape of the bowl and the angle at which the liquid initially enters that bowl.


The Coriolis effect may play a small role in the direction of a tornado's spin -- if the circumstances are right. More often than not, however, that direction is determined by the storm system that spawned the twister in the first place. These storm systems, or supercells, result when dry polar air clashes with moist tropical air and creates an updraft as the warmer air quickly rises. As the air flows upward, the increase in wind speed causes a tornado's characteristic rotation.

Likewise, the New York Yankees, the Los Angeles Dodgers and other elite baseball teams can't really credit scientific phenomena for their accomplishments. Things might be different if first base was on the equator and third was at the North Pole. But in the short distance between the pitcher's mound and the batter's box, the Coriolis effect doesn't gain a whole lot of traction. Even the most intimidating closer wouldn't see much difference in the placement of his fastball as a result of the Earth's rotation than he would from a gusty wind.

The Coriolis Effect in Action

Although the Coriolis effect doesn't determine the direction of toilet flushes or knuckle balls, it does have a significant impact on weather patterns. If you've ever watched The Weather Channel, you've probably noticed all those arrows swirling around the meteorologist's map to indicate wind direction. The direction those arrows are pointing toward is largely determined by the Coriolis effect.

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If the Earth didn't rotate, winds would travel either north or south due to differences in temperature and pressure at different latitudes. But since the Earth does rotate, the Coriolis force deflects these winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.


In the Northern Hemisphere, this deflection causes the wind flow around high pressure systems to go clockwise while flow around low-pressure systems travels counterclockwise. Imagine a low-pressure system as a vacuum that sucks all the surrounding air straight towards it, creating many vectors of wind that all focus on one spot. Because of the Coriolis effect, each of these vectors gets twisted to the right, which in turn creates a counterclockwise flow. With a high-pressure system, air gets forced outward and the Earth's eastward spin creates a clockwise flow. In the Southern Hemisphere, the opposite takes place: Wind around low-pressure systems circles clockwise while wind around high-pressure systems circles counterclockwise.

The swirling motions around low-pressure systems are actually the driving forces behind hurricanes. The air gets sucked in with such force and spins to such a degree that a potentially destructive storm develops. Warm ocean water fuels the system and if it gets a chance to grow over a period of time, powerful winds of more than 62 miles per hour (100 kilometers per hour) can form a storm strong enough to destroy anything in its path -- all just from the spin of our little planet.

The curvature of the winds created by the Coriolis effect also helps create surface ocean currents. The wind drags on the water's surface, creating spiral currents called gyres. As you may have guessed, the gyres in the Northern Hemisphere spin clockwise and the ones in the Southern Hemisphere spin counterclockwise.

Meteorologists and sailors aren't the only ones who have to contend with the Coriolis effect. Since aircraft cover large distances in a short period of time, pilots must also take its influence into account when charting the paths for their flights. For instance, a plane headed from Miami (where the Earth's rotation is more pronounced) to New York would end up in the Atlantic Ocean if the pilot ignored the effects of the Earth's rotation.

Observing the Coriolis Effect in Real Life

When he throws the ball, will the rotation of the Earth change where it goes?

If you have a lot of patience, you can see proof of the Coriolis effect on an object's movement using a device known as Foucault's pendulum. These pendulums can be found in several places around the world and are considered the best of their kind. Named after French scientist Léon Foucault, these massive experimental devices were designed to show how the Earth revolves on its axis. You can find them in universities and planetariums throughout the globe.

A Foucault's pendulum is anchored by a ball bearing, has an extra-large pendulum mass and a superlong string so it can swing slowly and withstand the effects of air resistance. Since the pendulum doesn't have outside forces influencing its movement, the only thing acting on it is the rotation of the Earth beneath it. As it swings, the Coriolis effect makes the pendulum veer off to the side and slowly change its swinging plane. After about 24 hours of painful waiting, you'll see that it has made one full rotation.


If you don't have that much time to kill, you can still observe the Coriolis effect in action. Although the Coriolis effect has a negligible impact on baseballs, it can affect the trajectory of very fast long-range projectiles like missiles and speeding bullets. During World War I, the Germans had to compensate for the Earth's movement as they fired shells at Paris with an extremely heavy howitzer that they called Big Bertha. If they hadn't taken the Coriolis effect into account, their shells, which were fired from 70 miles (112.6 km) away, would have gone astray by nearly a mile (1.6 km) [source: Veh].

So, while the Coriolis force might be called imaginary by some, its effects can be quite real. Just do everyone a favor and try not to leave your toilet unflushed for three weeks to prove that point.

Lots More Information

Related HowStuffWorks Articles

  • Boyd, Robynne. "Fact or Fiction?: South of the Equator Toilets Flush and Tornadoes Spin in the Opposite Direction." June 28, 2007. (January 27, 2009)
  • Department of Physics and Astronomy, University of Tennessee. "Consequences of Rotation for Weather." (January 27, 2009)
  • Fraser, Alistair B. "Bad Coriolis FAQ." (February 4, 2009)
  • National Oceanic and Atmospheric Administration. "Surface Ocean Currents." March 25, 2008. (January 27, 2009)
  • Plait, Philip C. "Bad Astronomy." John Wiley and Sons. 2002.
  • Scientific American. "Can somebody finally settle this question: Does water flowing down a drain spin in different directions depending on which hemisphere you're in? And if so, why?" January 28, 2001. (January 27, 2009)
  • Seligman, Courtney. "Online Astronomy eText: Background Physics: Motions and Forces: Coriolis Effects." (January 27, 2009)
  • Trampleasure, Lee. "The Coriolis Effect and Global Prevailing Winds." Oct. 29, 2005. (January 27, 2009)
  • Van Domelen, Dave. "A (Hopefully) Simple Explanation of the Coriolis Force." January 13, 2008. (January 27, 2009)
  • Veh, Andreas. "Very Important Topics: Gravitation and Motion in the Solar System." (January 27, 2009)