A rocket, or an air-mass, which is moving across the Earth, but not parallel to a line of latitude, seems to go off course when viewed from the ground: to Earthlings, it appears to veer to the right in the Northern hemisphere or to the left in the Southern. But looked at from space, every place on the rotating Earth’s surface, except the Poles, is moving eastwards at a speed which depends on latitude; so, when launched, say, due north, the rocket already has an additional eastward component of speed which stays with it (more or less) during its flight. In contrast, the speed of the Earth’s surface beneath it varies with latitude. The “Coriolis force” is a fictitious force which earthbound observers, on their rotating frame of reference, need to invent in order to account for this in their Newtonian equations of motion. (This poem is written for readers in the Northern hemisphere.)

A rocket, fired due north,
Veers off east as it flies forth –
Or so it seems to earthbound folk around.
But, prior to launch, the beast
Is moving to the east
As fast as is its launch-pad on the ground.

Its eastward motion stays
After launch: it still obeys
The law that says momentum is conserved.
But, Earth’s a ball and so, er,
Earth’s eastward speed gets slower
As your latitude increases, I’ve observed.

So the rocket’s eastward speed
While it travels north (agreed?)
Means it overtakes meridians below it.
That’s why someone on the ground
Thinks a force must be around.
Monsieur Coriolis was the first to show it.

[Image from]
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