Astronomers theorize that over 4.5 billion years ago, Theia, an object about the size of Mars, collided with Earth to create a tilted axis. This is called "the giant impact theory." Prior to this event, the axis was straight at 180 degrees.
The theory holds that as Theia collided with the Earth, the mantle and elements flew into space, creating the materials for the moon. Scientists named Theia after the Greek god Titan.
The 23.5-degree tilt of the axis sustains life on Earth and produces seasons. Since the Earth rotates on an angle, no portion receives the brunt of the heat from the sun. Agriculture and many species rely upon the season changes from the tilted axis.