What defines a planet’s north pole?

Uranus is often said to have a retrograde rotation with its axis tilted 98°. Why don’t we say it has a direct spin with the axis tilted 82°?

Uranus's rings feature in this 2005 shot from Hubble Space Telescope. NASA / ESA / and M. Showalter

Uranus's rings feature in this 2005 shot from Hubble Space Telescope.
NASA / ESA / and M. Showalter

Since 1982, the International Astronomical Union has defined the north pole of a planet to be the pole that lies north of the ecliptic plane (the plane of the solar system). Uranus’s “north star” happens to be Eta Ophiuchi, which is about 82° from the north ecliptic pole on the sky. To an observer looking down on this pole of Uranus, the planet’s rotation is clockwise, which is retrograde (backward) from the way most other planets turn.

By this definition, no planet’s north pole is ever tipped more than 90°. But you do have to mention, in addition, whether the rotation at that pole is direct or retrograde to specify it properly.

The 98° tilt, still quoted in many textbooks, comes from the fact that Uranus’s equator nearly coincides with the orbit planes of the main satellites, and the planet rotates in the same direction in which they move. As orbital elements are defined for planets, satellites, and comets, the inclination can have any value from 0° to 180°.

Either way, Uranus’s rotation is retrograde when you look down on the pole lying north of the ecliptic — the one now called (by convention) the north pole.

— Roger W. Sinnott