On a map of Earth, coordinates zero-zero are in the eastern Atlantic Ocean, near the coast of Africa. All the other latitudes and longitudes are marked from that point.

On a map of the sky, those coordinates are in the constellation Pisces. And that’s just where the Sun appears early today – at the moment of the spring equinox. Confusingly, though, that spot is known as the First Point of Aries.

More than 2,000 years ago, the Sun appeared against the stars of Aries at the equinox. That spot was marked as the zero-point of the celestial coordinate system – the equivalent of latitude and longitude.

But Earth wobbles on its axis. As it wobbles, the Sun changes position relative to the background of stars. That causes the First Point of Aries to shift as well. And as the point moves, the rest of the celestial coordinate system moves with it. The constellations don’t change, but their coordinates do.

The First Point of Aries moves westward by one degree every 72 years. So today, it’s in Pisces, where it’s been for the past two millennia. Around the year 2600, it’ll shift into Aquarius, the next constellation over. So spring will arrive in the northern hemisphere when the Sun is in Aquarius.

The Sun will return to its current equinox location when Earth completes one full wobble on its axis – in 26,000 years.

Tomorrow: collisions heard across the universe.

Script by Damond Benningfield