The Reasons for the Seasons

Click on the photos to see larger images and additional information.

Background

If you try drawing the locations of sunrise and sunset near your house over the course of a year, you will see that the Sun rises and sets and takes different paths across the sky in different seasons.

Some people have the misconception that different seasons happen because sometimes the Earth is farther from the Sun and sometimes it is closer.  It makes sense -- when you are close to the Sun, it should be warm and summery, and when you are far away, it should be icy winter -- but this is not true. The Earth travels around the Sun in an almost perfect circle (it is only slightly eliptical -- the most accurate way to draw the earth's path around the Sun is as a circle).  So it really doesn't change by much how far away we are from the Sun.  Besides, we have summer in North America when it is winter in South America, so something else must cause the seasons.

 

The earth's axis is tilted about 23.5 degrees, so the Earth is never straight up and down compared to the Sun.  When it is summer in North America, that is because the North Pole and the rest of the Northern Hemisphere are pointing toward the Sun.  The Sun appears to stay in the sky longer each day (really, we are just pointed toward the Sun for longer each day).  Because we spend more time in the sunshine and less in the shadow of night, our part of the Earth gets warmer, and we experience summer.  

During the summer, the Sun is high in the sky.  Therefore, rays of light from the Sun hit us more directly during summer, making the sunlight more intense and warmer.  In winter, the Sun is low in the sky.  The rays of light are spread more thinly, and they do not warm us as much.  

         Summer light                   Winter light

You can observe this with a small flashlight: hold it straight above a piece of paper and trace the edges of the lighted area.  Then hold the light the same distance from the paper, but at an angle, as if it were the Sun low in the sky.  The lighted area is much bigger, even though the light itself has not changed!  A larger area of the paper is having to "share" the light and heat, so no part of the paper will get as hot as if the flashlight were overhead. 


Stars of the Changing Seasons

Even if you do not chart the locations of sunrise and sunset for a year, you can see an change in the sky during different seasons:  the constellations seem to move.  This is because we on Earth look out toward different part of the Universe as we orbit the Sun.  

The hunter, Orion, appears tall in the winter evening sky.  On summer evenings, Orion is hidden but the summer Triangle (the brightest stars in Cygnus, Lyra, and Aquila) shines above us.  The stars that make up these constellations haven't really moved -- we are just looking at them from a different place in our orbit around the Sun, so they appear in the sky at different times.  In July, Orion is high in the sky mid-day.  Then, the light from the Sun is so bright that we cannot see any of the stars in Orion. 

During school hours, viewing any star other than our Sun is impossible.  You can prepare students for nighttime observing by punching out or drawing the shapes of the constellations and telling the mythological stories from which the names of many stars and constellations are derived.

Familiarize older students with a star wheel, teaching them how to set the date and time to portray the sky above them.  Alan MacRobert's article on Star-Finding With a Planisphere, adapted from Sky & Telescope Magazine, will help you get started.  

These tips, adapted from NOVA, will also help young astronomers get started observing the night sky with the unaided eye: 


Links

www-spof.gsfc.nasa.gov/stargaze/Sintro.htm  A book-on-the-web about a wide range of advanced astronomy topics including stargazing, Earth's seasons, and the Moon.

www.skypub.com/tips/skycharts/planisphere.html  Familiarize students with a star wheel, teaching them how to set the date and time to portray the sky above them with Alan MacRobert's article on Star-Finding With a Planisphere, adapted from Sky & Telescope Magazine.  

www.windows.ucar.edu/cgi-bin/tour.cgi?link=/mythology/mythology.html  Mythology of celestial objects including constellations, planets, stars, and the Moon.

www.pbs.org/wgbh/nova/worlds/stargazer.html  Tips to help young astronomers start observing the night sky with the unaided eye. 


2001 Challe Hudson

This web page was produced for Morehead Planetarium during a North Carolina State University astronomy class taken in pursuit of a Masters in Science Education.