Why is the earth tilted at 23.5 degrees?

Answer 1

At the moment the Earths tilt - or Axial tilt - is about 23.4o but it has not always been that value. Over a million years it has varied between 22o and 24o.

So why the tilt? It's not known for sure but a good suspect is the collision that occurred about 4.5 billion years ago, when Theia, a planet about the size of Mars, smashed into Earth. The impact, not only ejected material that created the Moon, but also could have destabilised the axis of the Earth.

Without the Moon, the Earth would have gone into axial freefall, tilting as much as 90o but because of the Moon, we have seasons, a stable tilt and a life.

The Moon is slowly receding away from the Earth and in about 2 billion years, Earths tilt will start to resonate.

Not to worry we will be long dead or at least pushing onwards to new sunnier climes.
We don't know why the Earth spins at an axial tilt of about 23.5 degrees. Some possibilities are...

1. That's the way the Earth formed naturally.

2. The collision with another planet (perhaps as large as Mars!) knocked the Earth a little sideways. We believe that this collision blasted enough mass off of the combined planet to create the Moon.