 By astronomical convention , the four seasons can be determined by the solstices — the points in the orbit of maximum axial tilt toward or away from the Sun — and the equinoxes , when the direction of the tilt and the direction to the Sun are perpendicular . In the northern hemisphere , winter solstice currently occurs around 21 December , summer solstice is near 21 June , spring equinox is around 20 March and autumnal equinox is about 22 or 23 September . In the southern hemisphere , the situation is reversed , with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped . 
