# Is there any simple way of using the Coriolis effect to determine what hemisphere you are in?

I have heard from many people that sinks do not empty in a particular pattern depending on what hemisphere you are in, but I have also heard from people who adamant that a sink of water would empty clockwise in one hemisphere and anti-clockwise in another.

While I acknowledge the above idea is probably a myth, is there any simple experiment that one can do do determine what hemisphere they are in, utilizing the Coriolis effect, or otherwise?

• +1 for accurately calling it the Coriolis effect, and not a force :) Apr 17, 2014 at 7:44
• @hugovdberg If the result of the effect is a force inside my (non-inertial) system, then it is a force. (However, that's more a philosophy than physics, which makes such a discussion completely pointless anyways.)
– yo'
Apr 17, 2014 at 11:24

You can use the Foucault pendulum to determine the hemisphere: Its plane of movement rotates:

• anti-clockwise in the southern hemisphere;
• clockwise in the northern hemisphere.

The rotation of the plane can be explained by the Coriolis force.

It's my understanding that the famous "sink swirl" example doesn't work simply because the Coriolis effect is too weak at that scale: it's insignificant compared to the motion of the water from residual momentum from when it entered the sink, and from vibration, air motion, and so on.

For long-range artillery, the Coriolis effect can become noticeable: it causes the projectile to deviate from the path that you would otherwise expect it to follow. The most famous example is probably the Paris gun. This probably doesn't constitute a practical experiment for most people, though.

To address the "or otherwise" part of your question, there are definitely simpler ways to determine your hemisphere: if the sun is to the south in the middle of the day, you're in the northern hemisphere, and vice versa. This assumes that you know where north is, but you can determine this by observing the stars. (This, of course, requires a clear and visible sky.)

If you have a way of measuring the vertical component of the Earth's magnetic field, you can also use the magnetic dip to determine your hemisphere. This won't work well if you're less than about 15° from the equator, since the magnetic equator doesn't correspond exactly to the geographical equator.

The Coriolis force does not impart any difference in the draining of your sink or toilet, that has much more to do with the construction and design of the basin. In fact, for Coriolis to do anything it needs hours to take effect. The Coriolis effect requires large spatial and time scales.

You can, however, look for its influence to determine which hemisphere you are in. The obvious way to do this is to look at weather. Mid-latitude cyclones and tropical cyclones center around low pressure, and the wind field around low pressure will swirl counter-clockwise in the North and clockwise in the South. If you watch a large long-lived storm swirl, you know where you are (note that the direction of rotation of supercell thunderstorms and tornadoes is not due to Coriolis and cannot be used for this purpose). Likewise with a high pressure system, the surface winds will swirl clockwise in the North and counter-clockwise in the south.

If you don't have access to weather to make this determination but do have access to geographical surveys, you can inspect wear patterns on long sections of riverbank or railroad track. These methods can be masked by other effects though, and are not as straightforward as using weather.

• I don't think watching large scale weather systems counts as a simple experiment as it requires a lot more data than, for example, the Foucault experiment. Apr 17, 2014 at 7:47
• +1 Looking at a radar or satellite image is probably the easiest way. Nov 12, 2014 at 15:05
• Yes. When a storm front/low is approaching/receding, put the wind at your back--In the northern hemisphere the low will be on your left, while in the southern hemisphere the low will be on your right. Oct 1, 2021 at 1:07

Another way of telling the hemisphere is with the phases of the moon.

In the northern hemisphere, the illuminated part of the moon "moves" from right to left, the opposite from the southern hemisphere, in which it moves from left to right.

For more details, check wikipedia: Lunar Phase

• But this has nothing to do with Coriolis, so it does not answer the question. Nov 12, 2014 at 4:03
• in the content of OP's question, at the end, he says ...utilizing the Coriolis effect, **or otherwise**?, well, this is otherwise. Nov 12, 2014 at 21:31
• Point well taken, although existing room for semantic nit-picking likely not intended by the OP. :) Nov 12, 2014 at 22:41