Debunked: The Equator Drain Trick (direction of water flow)


Senior Member
On our honeymoon my wife and I went to Kenya "on safari"

this often meant long drives between game reserves

on one such drive we came across a gaggle of Kenyans and Tourists huddling around a painted line in the road

the line signified the Equator - the northern hemisphere on one side the southern on the other

quite awe-inspiring if you accepted it as true

anyway a young enterprising Kenyan was proving it (for a small consideration, business is business after all)

he had a bucket full of water with a hole in the bottom and a matchstick floating on the surface

in essence he showed, as the bucket drained the matchstick rotated (on the surface of the rotating water) one way then the other depending on whether he had walked (10mtrs) either side of the line

the coup de grâce was when he placed the bucket directly over the equator and the matchstick stayed still as the water drained out - no swirl/rotation at all

was this a clever trick?, or would the physics of rotation show up over such small variation in position

I had no reason to suspect a trick at the time and simply took it as face value

but is/was it bunk?

Something like this?

Watch the guy working the ruse carefully. First he goes through a show of finding north, and locating the position of the Equator. This is all simply for show. As you’ll see in a second, this exact trick will work anywhere, not just on the Equator. I could do it in Boulder, at a latitude of 40° north!

He goes first to a basin on the Equator. It’s already filled, and the video jumped right before that, so I don’t know if he filled it right before draining it, or if it was sitting there for a while already filled. My guess is it had the water sitting for awhile — you’ll see why in a second. When he pulls out the plug the water drains straight down. OK.

He then grabs the basin and bucket and moves a few meters south. Watch very carefully what happens then: he pours the water from a bucket, making sure the water is flowing in to the left of the drain hole
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Position isn't the deciding factor, though it makes it even more unlikely. Time is. The Coriolis Force only shows up over extended periods of time. Under about three hours it is fairly negligible. The direction of drain vortices is determined by initial rotation instead of Coriolis forces.

It's a good trick, but it's probably sleight of hand, imparting opposite rotations to the water while moving or pouring it.

Ah! Beat to it!
ahah, excellent, thanks everyone

so I have been the victim of "bunk", -- I would recount this story (it was in 1996) to people down the years,

Although I haven't for a few years partly coz I suspected it was a trick - I must have been less sceptical then

but if it allows a enterprising lad to make a few bucks of passing tourists then fair doos

although I would like to think the line in the road was the actual equator and not, well

a line painted in the road!!!!!
Unfortunately the line they do these tricks on is old, and not quite on the exact equator:

"The exact equator" is not as easy as that. It's more a matter of definition. GPS receivers give the co-ordinates based on an mathematical model of the Earth, which is only an approximation. And there are several possible "geodetic datums" that you can use, all of which will have the zero-degree line in a slightly different place.

(For the same reason, a GPS receiver placed directly on the Greenwich meridian line in London does not read zero: most GPS receivers use the WGS84 datum, and the WGS84 0º longitude line is about 100 metres from the "official" Greenwich meridian defined by the telescope at the obervatory.)

And then there is the fact that the equator is not fixed. The earth's axis of rotation wobbles slightly, which means that the equator also moves by a few metres over the course of a year. So, it's clear that just moving a metre or two either side of the line would not show an effect, because depending on when you carried out the experiment, the equator might not be on the line any more!

(When I visited Kenya, I was told by those in the know that the effect was created by the person turning in the appropriate direction while holding the basin of water before putting it down, to impart a rotation in the water.)
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so is the often repeated tale of water draining from a bathtub in the southern hemisphere, in say in Cape Town, rotating in a the opposite direction bathtub in the UK
The last video is extra funny when you realize that he has mixed up the directions in which the water is supposed to swirl due to the Coriolis effect.