Important points

Over the course of the year:

• The day length changes over the course of the year.
• The angle with which the sun hits the ground changes over the course of the year, making the sunlight more concentrated at certain times of the year than others.
• These two effects occur because the earth's axis is tilted relative to the plane of the earth's orbit.

Draw a line in your mind perpendicular of the plane of earth's orbit; the earth's axis of rotation is tilted at an angle of 23 1/2 degrees from this line. As you see on the diagram below, the result is that the Northern Hemisphere is tilted toward the sun for half the year and away from the sun for the other half.  When the Northern Hemisphere is tilted toward the sun, people in the North get more concentrated sunlight during the day; the days are also longer during this period; at the same time, people in the Southern Hemisphere experience short days and less concentrated sunlight.  Because there is a 'lag time' as parts of the earth heat up and cools down, our warmest weather is after the Summer Solstice (~June 21), and our coolest weather is after the Winter Solstice (~December 21).  The word "solstice" comes from: sol = sun, stice = stop, since the sun stops the trend in its apparent movement in the sky from day to day (most noticeable as a change in the position of the setting sun from day to day) and begins to move back.   Spring is said to begin at our Spring Equinox (~March 21), and fall is said to begin at the Fall Equinox (~September 21). The word "equinox is from: equi = equal, nox = night, since there are equal day and night lengths at these times.

Seasons are a popular topic in elementary school classrooms.  In addition to noticing the changes in the position of the sun during different times of the year, students can collect and graph temperature data, and they can observe changes in the world of living things around them.  For instance, plants an animals respond directly to the changes in light levels at different times of the year, and these are often the focus of the exploration of seasons in the classroom.  One popular topic is the color change in leaves in the autumn.  Check out an experiment you can do with children as well as a discussion of leaf color change in autumn at:  http://pm.appstate.edu/~goodmanj/leaves/leaves.htm.
 

For a great hands-on computer simulation of the seasons, use a computer in the 215, 216 of 218 labs in Edwin Duncan and Internet Explorer.  (There is special software on these machines to run the simulation.)  Go to: http://www.fd.appstate.edu/sas/sasinschool/Seasons/Seasons.htm and choose "Hands-On Activity".  To rotate the picture in the lower right, choose "Rotate", hold the mouse button down and drag (e.g. to look at the orbit from "the side" -- over the earth's equator).  To move through the months, drag the slider at the top. Click on the tilt icon to change the tilt of the earth to see the effect that would have.