The seasons are theresult of the tilt of theEarths axis. The Earthsaxis is tilted fromperpendicular to theplane of the ecliptic by23.45 .
This tilting is what givesus the four seasons ofthe year - spring,summer, autumn (fall)and winter.
Summer is warmer than winter(in each hemisphere) becausethe Suns rays hit the Earth ata more direct angle duringsummer than during winter andalso because the days aremuch longer than the nightsduring the summer.
During the winter, theSuns rays hit the Earth atan extreme angle, and thedays are very short. Theseeffects are due to the tilt ofthe Earths axis.
SolsticesThe solstices are dayswhen the Sun reachesits farthest northern andsouthern declinations.
The winter solsticeoccurs on December 21or 22 and marks thebeginning of winter (thisis the shortest day of theyear).
The summersolstice occurs onJune 21 and marks thebeginning of summer(this is the longest dayof the year).
• Equinoxes are days in which day and night are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator.
• The vernal equinox occurs in late March (this is the beginning of spring in the Northern Hemisphere and the beginning of fall in the Southern Hemisphere).
• The autumnal equinox occurs in late September (this is the beginning of fall in the Northern Hemisphere and the beginning of spring in the Southern Hemisphere).