Why do we have seasons?

Night and day are produced when the earth spins on its axis. The earth moves about the sun in an elliptical orbit that requires about 365 1/4 days to complete. The earth's spin axis is tilted with respect to its orbital plane and this is what causes the seasons. When the axis points away, winter can be expected. On the summer solstice, it points as close as it can to the sun.

So, the changes in the seasons are caused by Earth's movement around the sun. Earth makes one complete revolution about the sun each year. The tilt of the axis, called the obliquity of Earth's axis, determines the season.

Google search:
National Weather Service - NWS Flagstaff
http://www.wrh.noaa.gove/fgz/science/season.php

Thomson Gale search:
Seasons
Gale Encyclopedia of Scies, Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th ed. Detroit: Gale Group, 2008.
http//find.galegroup.com/srcx/retrieve.do

Print Source search:
Farndon, John; How the Earth Works, p. 19. Readers Digest Association, 1996