Question

why is the axis of the earth tilted

Answer 1

Why is the earth tilted ? Are any other planets tilted ?

Hello!We are still not entirely sure why Earth has a slight tilt (23.5 deg.) but there is a popular theory explaining it.

The Earth and the solar system (including planets, asteroids, comets etc.) were formed as a by product of the formation of the Sun about 5 billion years ago. 

After the Sun was made, the left over material formed into the planets and everything else. The Earth was also formed in this manner. The early solar system as a very tumultuous place. Smaller particles of dust (silica) collided to form larger chunks of rocks, which further combined. This is what made the terrestrial planets like Earth, Mars etc. Even during and well after its formation, Earth was continually being bombarded with rocks and boulders of every size. It is believed that this is what caused the Earth to tilt on its axis. A glancing (tangential to the Earth) blow could have very well caused that. 

So, the tilt of the Earth is due to the formation of the Earth and happened very early in its lifetime.

This tilt is however stabilized by the pull of Sun and Moon.

 (copied)

Answer 2

One possibility is the Giant-impact hypothesis- the tilt relative to the ecliptic was caused by the off-axis impact of Theia.

Otherwise, since angular momentum is conserved, if the material from which the solar system was formed did not have a uniform angular momentum, once the material forming the proto-earth had coalesced, it would keep whatever angular momentum and hence tilt it had to start with.

Hope this will help u