Axial tilt
What is Earth’s axial tilt?
Earth’s spin axis is not upright. It leans about 23.4° from the vertical to its orbit. That lean is why we have tropics, polar circles, and seasons — and it barely changes from one year to the next.
A lean that holds still
Hold a tilted spinning top and walk it around a lamp without twisting the tilt to follow the lamp. For half the walk the top’s north end points toward the light. For the other half it points away. That is the year.
The size of the lean is about 23.4° right now. This page does not type that number. It takes the largest declination in a year of the solar series — the same series that places the yellow dot.
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It is slowly shrinking
The obliquity is 23.439° minus a tiny drift of 0.00000036° per day from the J2000 epoch. Over a human life you will not notice. Over millennia the tropics creep a few tenths of a degree. The four circles on the map would all move together.
What if the tilt were zero? Both tropics on the equator. Polar circles on the poles. The terminator always pole-to-pole. Every day the same length. No seasons. The lean is the whole of them.
See it live
On the hub: Day and night map