page 38 SEASON OF THE YEAR IS DETERMINED BY THE POSITION OF THE EARTH RELATIVE TO THE SUN (also determines photoperiod and day length) The earth's axis is tilted 23 1/2 degrees relative to the axis of the sun.  As the earth revolves around the sun, the axis remains pointed in the same direction in space. Notice: At the summer solstice The earth's axis is orientated 23 1/2 degrees towards the sun, thus the sun's rays strike the earth at a 90 degree angle at the Tropic of Cancer. At the winter solstice The situation is reversed, i.e. the earth's axis is orientated 23 1/2 degrees away from sun, thus the sun's rays strike the earth at a 90 degree angle at the Tropic of Capricorn. At the the autumnal and vernal equinox The earth's axis is neither orientated away from nor towards the sun (I made up the term obliquely parallel to describe this, or sometimes I call it cock-eyed parallel), thus the sun's rays strike the earth at a 90 degree angle at the equator. This unique orientation of the earth to the sun is what generates our seasons.  Later in the semester we will show you how this causes the differences in day length or photoperiod between the seasons.  This is one of nature's "biological clocks".