How many degrees is Earth's axial tilt from vertical?

How to get close to this number

The tropics give you the answer. The Tropic of Cancer and the Tropic of Capricorn sit at the latitudes where the Sun can be directly overhead, and those latitudes are defined by the tilt — they are the same number. If you can recall roughly where the tropics sit on a globe, you already know the axial tilt without knowing you know it.

The misconception in the explanation is one of the most persistent in all of science education: seasons come from tilt, not from distance. Earth is actually closest to the Sun during the northern winter, and the decisive proof is that the two hemispheres have opposite seasons at the same moment — which distance could never produce, since both hemispheres are the same distance away.

The tilt is not perfectly constant either. It oscillates slowly over tens of thousands of years within a narrow range, and that variation is one of the cycles implicated in the timing of ice ages. On any human timescale the figure is fixed, which is what makes it a fair thing to ask, but on a geological one it is a slow-moving variable.