Water goes down drains in different directions depending on which side of the equator you're on

No it doesn't.

The Coriolis effect
The 'force' that people talk about that apparently causes this to happen is called the Coriolis force. Strictly speaking, this isn't a force at all but an 'effect'. This is because Coriolis effects don't originate from a particular body, like a real force does (like gravity or magnetism), but instead are an artifact of being in a rotating frame of reference (such as being on a roundabout or a rotating planet (like the one you're probably sitting, standing or lying** on whilst reading this). The Coriolis 'force' is often described as a 'fictitious force' for this reason (as are centrifugal 'forces'). Here's an example of a fictitious force at work:
  • Imagine you're on a roundabout, facing inwards. A friend is sitting opposite you, on the other side of the roundabout, also facing inwards so that you're facing each other. Now imagine that the roundabout is rotating at a steady rate, in a clockwise direction.
  • Now try to think of it in a different way: As far as you're concerned, it could well be that you, your friend and the roundabout are perfectly still, and the rest of the world (or even universe) is turning around you. The only thing that makes you believe that it's your roundabout that's spinning, and not everything else is the force you feel pushing you backwards (the centrifugal force). This force isn't being generated by anything in particular; it's an effect due to your inertia.
  • Now imagine that you're holding a ball. Throw it across the roundabout, straight at your friend. What happens to the ball? If you tried this in real life you'd find that, to you on the roundabout, the ball appears to curve off to the left, rather than travelling straight to your friend as it should do. This is the Coriolis effect.
This video might help to get things straight (or un-straighten them, if that's the problem!) in your head:

I must state that it's not my video, but I did think it puts the point across quite nicely!

This thought experiment demonstrates three things:
  1. The idea of an inertial frame of reference- that is assuming the piece of space that you are dealing with is 'still', and that it is everything else that is moving instead (you're probably thinking 'what's the point?' Thinking in this way can help mathematicians and scientists to simplify a problem without having to cut out any important bits).
  2. The idea of a fictitious force: on the roundabout, you feel like you're being pushed backwards by something, but this is an effect of the rotation of the roundabout, rather than an actual force being applied to you.
  3. How the coriolis effect works on roundabouts.

What about planets?
On a planet, much the same thing happens, but on a larger scale. It is much easier to see the Earth as an inertial frame of reference as you can't feel the effects of the planet's rotation on a normal day**. If you stood still on the surface of Earth anywhere you liked for a full day you'd still actually have moved in a giant circle(ish) as the Earth rotates around its axis (much like sitting on the roundabout- you didn't move yourself, but you still went round in a circle).

Now, the Earth is so big and you're so small that if you throw a ball to your friend it pretty much goes in the direction you expect it to.
On a larger scale, however, the effect is noticed in a big way. Cyclones, large spiralling high speed wind systems, form when conditions are right. They spin because they are large enough to feel the Coriolis effect, or more accurately, differences in the Coriolis effect at different latitudes. The Coriolis effect gets stronger as you move further from the equator, so in the northern hemisphere, the air at higher latitudes is given more of a shove by the Earth's rotation than air at lower latitudes. This introduces a spin in the weather system that is always anticlockwise in the northern hemisphere. In the southern hemisphere, the opposite is true, and the cyclone ends up with a clockwise spin.

So what about baths, toilets and sinks?
In principle, the same effect could apply to water draining out of a bath, but much like throwing a ball, the effect is too weak and on too small a scale to be noticed. Numerous other factors, ranging from how the bath was made to how the plug is removed, have a much greater influence on the direction that the water spins as it drains away. If you could take your bath in, for example, Croydon and have it shipped to Ashburton, you'd most likely find that the water drained out in the same way in both locations.

Some interesting things
Well, I think they're interesting anyway. They might spark you into doing some further research, or asking more questions:
  • Jupiter's alternating bands of colour are an atmospheric consequence of Coriolis effects. Each band is called a Hadley cell (Jupiter has four of these in each hemisphere). The Earth has three Hadley cells per hemisphere. Venus has one Hadley cell per hemisphere. Mars has weird Hadley cells.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* Please let me know if you're doing anything more unique, like sky-diving or potholing, whilst reading this.
** It'd take a very abnormal day indeed to be able to feel them!


  1. Lovely post...extremely interesting, and a myth debunked - fab! So glad I asked the question...cant wait to be able to disillusion people myself with that info now!

  2. Great! You too will be almost as universally loved as me!



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