What is Sidereal day?

A sidereal day is how long it takes for Earth to spin around once, as seen from the stars.

Imagine you're playing on a merry-go-round with your favorite toy. When you spin all the way around and face the same direction again, that’s one full turn, like a sidereal day.

On Earth, we usually count days by how long it takes for the Sun to come back to the same spot in the sky. That's called a solar day, and it's what most people use every day.

But if you look at the stars instead of the Sun, like when you're outside on a clear night, you’ll notice that after one full spin, Earth has turned just enough so that the same star is back in the sky. That’s the sidereal day, and it's about 4 minutes shorter than a solar day.

Think of it like this: if you're on a spinning ride at the fair, and your friend is holding a flashlight from far away, that’s like the stars. When Earth spins just enough so the same star comes back to where you see it, that's one sidereal day. It’s like finishing one lap around the ride slightly faster than when you match up with the Sun.