The axis about which the
earth rotates is tilted
relative to the plane of
the earth’s orbit.  This tilt,
called obliquity, is the
reason why our planet
has seasons. Currently
the earth’s 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.