In the left part of the applet you can drag the earth on its orbit. On the right you can see how the axial tilt marks the seasons.[list][*]On the [b]21th of june[/b] the northern hemishpere is tilted towards the sun. It's summer. [br]The sun is perpendicular to the tropic of cancer.[/*][*]On the [b]21th of december[/b] the southern hemishpere is tilted towards the sun. It's winter on the northern hemisphere.[br]The sun is perpendicular to the tropic of capricorn.[/*][*]On the [b]21th of march[/b] and the [b]21th of september[/b] the sun is perpendicular to the equator. [br]Everywhere on earth the length of day and night are equal. [br]This point is called the [b]equinox[/b].[/*][/list]