The axis
about which the earth
rotates is tilted relative
to the plane of the
earthÕs orbit. This tilt, called obliquity, is the reason
why our planet has
seasons. Currently the
earthÕs obliquity is 23.5
degrees. If the obliquity
were 0 (i.e. no tilt),
the northern hemisphere
would get about
the same amount of
sunshine all year round.