It is believed that the Earth´s tilt originated 4.5 billion years ago. The Earth´s axis is tilted 23.5° with respect to the plane of the ecliptic. The Earth´s tilt and its orbit around the Sun is what produces the different seasons during the course of one year on Earth. In the early solar system, tiny pieces of debris accumulated through the process of gravitational collapse to form the planets. It is thought that a big impact could have originated the Earth´s tilt during the formation of the solar system 4.5 billion years ago.
The formation of the solar system took place 4.5 billion years ago, during which small particles of gas and dust, orbiting around the protosun coalesced to form planetesimals which later accumulated together to form the planets. It was during this period that the mass of objects increased, as did their gravitational fields. This increased in gravitational pull attracted more objects which made the developing object more gravitationally powerful. It is thought that when the Earth´s gravitational field increased enough, it attracted a big object which may have hit hard enough to cause it to tilt to one side. Subsequent impacts positioned the Earth´s tilt into its present position.
It is thought that an object the size of Mars may have caused the Earth´s 23.5° axial tilt. During this period, the Earth may have had a relatively slow spin, less mass and a lighter axial tilt than it is today. The impact caused the Earth to acquire a higher spin rate, and it also knocked the poles off by approximately 23.5°. The impact ejected enough material into space, which accumulated to eventually form the Moon. Shortly after the impact, one day on Earth was about 10-12 hours and the Moon was close enough to produce huge tides on the Earth´s oceans which eventually slowed down Earth´s rotational spin to a 24 hour day.
Another theory states that the tilt of the Earth may have had more than one origin. According to newton.dep.anl.gov, the tilt may have been caused by the formation of massive ice sheets during the ice ages. Massive ice sheets accumulated at the Northern Hemispheres during the ice ages may have changed the Earth´s tilt much the same way in which a ball with an object sticked on its top changes direction as it spins on the floor. This effect may have taken place over a large span of years to occur on Earth.
The Earth´s tilt affects the changing of the seasons on Earth. The tilt on the Earth´s axis influences the amount of heat received on a square meter of surface. The surface of the Earth receives most of the heating energy during those days when the Sun if higher up in the sky. The heat rate is highest in the summer, when the Earth´s Northern Hemisphere is tilted towards the Sun, whereas in the winter, when the Earth´s tilt points away from the Sun, the heat rates are lower. The Earth´s tilt determines the amount of hours the Sun is high above the horizon, heating the surface of the Earth, as well as well as the seasonal changes seen throughout the course of a year.
The fact that the Earth is tilted on its axis is what determines that the Northern Hemisphere is experiencing a summer season at the same time that the Southern Hemisphere is having a winter. If the Earth weren´t tilted, we wouldn´t see the features characteristic to the different latitudes on Earth. According to nasa.gov, the earthquake that hit Japan on March 11, 2011 may have shifted the position of Earth´s axis by approximately 17 cm. (6.5 inches) towards 133° east longitude, and caused the Earth to rotate a little bit faster, shortening the length of the day by approximately 1.8 microseconds.