Ask Marilyn: How Is the Direction of North Decided?

James Barr of Birmingham, Alabama, writes:

Marilyn: How do they decide which way is north on a planet?

Marilyn responds:

The convention is a system called right-handed angular coordinates. To envision it, hold out your right arm and give yourself a thumbs-up. Your fingers are curled with the tips heading rightward; this represents the direction of a planet’s rotation on its axis. Your thumb points north. You can do this while looking at a globe to see how it works with Earth.

Envision a planet from a point in space where it looks to you that it’s rotating rightward. The ends of the axis are the two poles. Hold out your thumbs-up fist. The direction of your fingertips will match the way the planet spins, and your thumb will point to its north pole.