The axis of the earth is not perpendicular to the plane through the elliptical earth's orbit. It's tilted by an angle of 23,5°. The consequence is that through the year a different part of the earth is tilted towards the sun, which explains the seasons. [br]If the north pole it tilted towards the sun, it's summer on the northern hemisphere and winter on the southern.[br]Drg the centre of the earth along its orbit around the sun and see how the axial tilt correspond with the succession of the seasons.

During a period of some 40 thousand years, the axial tilt varies between 22,1° and 24,5°. A greater angle means a greater contrast between the seasons on earth, warmer summers and colder winters. A smaller angle means colder summers and milder winters.[br]The axis of the earth describes a full circle within each 26 thousand years. Nowadays it's summer on the northers hemisphere when the earth is in the aphelion, so on its farthest point to the sun. For a summer in the perihelion we have to wait for another 13 thousand years. This effect is called precession.