The Coriolis Force
The Coriolis Effect is definitely real.
It drives ocean currents and weather patterns.
But does it make toilets flush and tubs drain the opposite direction
when you cross the equator?
No.
Let me lay some science on you.
And if that doesn't convince you, I'm the Toilet Guru and I have
experienced toilet flushes in both the Northern and Southern Hemispheres.
I can report that flush direction is a random mix everywhere.
Keep reading for the details and pictures.
First, though, you're exploring a web site full of travel stories
and toilet pictures.
So, let's check out the toilets of the other hemisphere.
Earth's population is overwhelmingly based in the northern hemisphere.
For most of us the Southern Hemisphere is the "other" one.
If you don't agree, Asia would like to have a word with you.
Traveling to Chile for the total solar eclipse
My first trip to the Southern Hemisphere was to
Chile
to experience the 2019
total solar eclipse.
It was a fantastic experience in a wonderful country!
With, well, pretty mundane plumbing.
Nice, but nothing exotic.
Certainly not backwards.
Here is my first Southern Hemisphere toilet,
at the Happy House hostel in the Barrio Brasil area of Santiago.
The only unusual thing about it was its tiny cubicle with the very close door.
As I said, the Coriolus effect is real.
Because of the Earth's rotation, objects in motion north of the equator
are deflected to the right of their direction of travel,
and objects south of the equator are deflected to the left.
This is why ocean currents move clockwise in the Northern Hemisphere,
and counter-clockwise in the Southern Hemisphere.
Ocean circulation, from the U.S. Government's
National Oceanic and Atmospheric Administration
However, ocean currents are huge in both time and space.
The water is there indefinitely, and the ocean floors are relatively
smooth in comparison to their width.
A Foucault pendulum can demonstrate the Coriolus force.
But it takes 24 hours to precess through 360°.
Your toilet or tub or sink, on the other hand, drains in seconds.
Yes, there is a Coriolus force working on the draining water.
But it is overwhelmed, tiny compared to other factors.
Toilets flush in a direction determined by how
the water is poured into the bowl.
For other vessels the pre-existing motion of the water,
the shape of the vessel,
and the shape of the drain dominate the direction.
Let's see another Southern Hemisphere toilet.
This is at the Neptuno Bar in the port district of Valparaiso, Chile.
There's a raised stool if you need it.
Or a washed-down tile wall and trough if you only need a urinal.
Pee on the wall and it drains straight down, and then to the left.
There's no mystical backwards swirling here.
I stayed at the Aji Verde hostel in La Serena before the eclipse.
La Serena was in the path of totality.
I joined some friends and we drove to the centerline to experience
the longest possible totality.
The shared bathrooms at the Aji Verde hostel were great.
The eclipse was at the beginning of July,
the middle of the South American winter.
At about 30° south latitude, it doesn't get terribly
cold there, maybe down to 2–3 °C at night.
But like most budget lodging in Chile, the hostel has
little to no heat, and sections of it are open to the sky.
So I didn't experience a shower in what seemed to me to
be an unusually short tub.
The Science
Isaac Newton figured out the physics back in the 1600s:
F = ma
where F is the vector of physical forces acting on an object,
m is its mass, and
a is the vector of acceleration.
That's in an inertial frame of reference,
where we aren't accelerating.
If you move in a circle, your direction of travel constantly changes,
meaning that there is acceleration.
So, things get more complicated.
In a non-inertial frame of reference like the surface of a
rotating planet, other forces appear to act on an object.
They're called imaginary forces because they only appear to
exist within your non-inertial frame of reference.
An observer coasting past in a spaceship would see that everything
on the rotating planet is really behaving according to Newton's laws.
The equation for the observer on the planet becomes the following,
where F' and a' are the vectors of
forces and accelerations, respectively, within the rotating reference frame.
We must subtract the Coriolis force, the centrifugal force,
and the Euler force.
Ω is the rotation vector, with magnitude ω,
relative to the inertial frame;
v' is the velocity relative to the rotating inertial frame;
and
r' is the position relative to the rotating inertial frame.
The Corliolis force is proportional to the cross product of two vectors.
It is perpendicular to both, the object's velocity and the reference
frame's rotation vector.
The History of the Science
The Italian scientist Giovanni Battista Riccioli and his assistant
Francesco Maria Grimaldi looked into this in the 17th century.
So did the French scientist Claude François Milliet Dechales.
All of them were offended by the new non-geocentric models of the universe.
Surely Man must be at the unmoving center of everything!
Image from Cursus seu Mundus Mathematicus (1674)
by Claude François Milliet Dechales,
showing how a cannonball should deflect to the right of its target
on a rotating Earth.
In Riccioli and Grimaldi's 1651 work Almagestum Novum,
and Dechales' 1674 Cursus seu Mundus Mathematicus,
they described how the Earth's rotation should deflect the trajectories
of falling bodies and projectiles sent north or south.
A cannonball shot north should be deflected to its right, to the east.
It is, but by a small amount.
Just about 3 centimeters over a one-kilometer trajectory
at the latitude of Italy or France.
They actually had the science correct,
but they couldn't detect that real but tiny deflection.
So, they happily concluded that they had proof that the
heliocentric model of Copernicus was wrong.
Leonhard Euler derived the equation for this deflection in 1749,
and Pierre-Simon Laplace included the effect in his tidal equations of 1778.
Then, in 1835, Gaspard-Gustave Coriolis
published a paper on the energy yield of machines with rotating parts,
such as waterwheels.
His analysis considered the additional forces detected within a
rotating frame of reference.
This was an extension to his 1829 textbook,
Calcul de l'Effet des Machines.
In 1856, William Ferrel proposed mid-latitude air circulation being
driven by the force described by Corliolis.
The effect Corliolis had described was called
"the acceleration of Coriolis", or the Corliolis force
by the early 1900s.
The Corliolis force or effect became a dominant part of the model for
large-scale dynamics of meteorology and oceanography.
Air flow around low-pressure centers moves counterclockwise in the
Northern Hemisphere, clockwise in the Southern Hemisphere.
Cyclones form at mid-latitudes; they rarely form near the equator because
the Corliolis effect weakens as you approach the equator.
Cyclones may be over 1,000 kilometers in diameter, and last for several days.
They're driven by the Corliolis effect, but your toilet isn't.
Next article:
Where Are the English-Speaking Bidets?
