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What Causes the Seasons to Change?

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Author: Beth Touchette

What causes the seasons to change? The first answer many people give when asked why the Earth gets cold in the winter and warm in the summer is because the Earth’s distance from the sun can vary during its orbit. Astronomers have carefully measured the distance between the Earth and Sun and have discovered that the Earth is closest to the Sun in January, and farthest away in July.

But wait a second!  January is the peak of winter in the Northern hemisphere, and July is the peak of summer.  There is another problem with the distance/season misconception: Why is it that when it is summer in the Northern hemisphere, it is winter in the Southern hemisphere? Something else besides varying distances from the sun must be causing the seasons.

Eventually, humans figured out that the varying seasons were caused by the earth’s tilt in combination with its orbit around the sun. You might have learned that the Earth spins like a top. It does not spin straight up and down, though—it spins at a 23 ½ degree tilt. The imaginary line that the earth is spinning around on is called its axis. When the top of the Earth is tilted towards the sun, it is summer in the northern hemisphere. Since the bottom half of the Earth (the southern hemisphere) is tilted away from the sun, it’s winter there. 

Earth Tilt Seasons Diagram

On around June 21st, the northern hemisphere is at its max tilt towards the sun. This is called the summer solstice. This is also the longest period of daylight of the year in the northern hemisphere. On around December 21st, the Northern hemisphere is at its max tilt away from the Sun, which is known as the winter solstice and the shortest period of daylight. June 21st is the winter solstice for the southern hemisphere, and December 21st is its summer solstice. Just think about it: kids in Australia get to enjoy long summer days and winter holidays at the same time!

Solstices and Equinoxes Diagram

In addition to solstices, our planet also experiences equinoxes. During its journey around the sun, the Earth reaches two points in its orbit where the tilt isn’t towards or away from the sun. The length of day and night are equal. These are called the equinoxes. On about March 21st, it is the spring equinox in the northern hemisphere, and the fall equinox in the southern hemisphere. On about September 21st, it is the spring equinox in the southern hemisphere and fall equinox in the northern hemisphere.  

The tilt correctly predicts the seasons, but how does the tilt cause warmer or colder temperatures? You can see for yourself!

Problem: How does the tilt of the Earth cause the seasons?

Materials

  • Partner
  • Globe
  • Tape
  • Graph paper
  • Protractor
  • Flashlight
  • Room you can make dark
  • Pencil

Procedure

  1. Tape a piece of graph paper over the Northern hemisphere of your globe.
  2. Tape another piece of graph paper over the Southern hemisphere of your globe.
  3. Using the protractor, have you partner tilt the Northern hemisphere of your globe toward you 23 ½ degrees (Note: some globes might already be tilted on the correct axis).  
  4. Have your partner continue to hold the globe in position.
  5. Standing about one foot away from the globe, shine the flashlight at a point just above the equator. 
  6. Congratulations! You just modeled the way Earth and Sun are positioned during the summer solstice in the Northern hemisphere, which occurs on June 21st.
  7. Now, make the room dark.
  8. Ask your partner to trace the circle of light made by the flashlight.  He or she should be tracing on the paper, not the globe itself.
  9. Next, ask your partner to lower the tilted globe (without changing its tilt) so that circle of light is now on the middle of the Northern hemisphere of the globe. Keep the flashlight in the same position.
  10. Ask your partner to trace the circle of light made by the flashlight in this region. Do you notice anything about how the brightness and shape of the circle of light changes?
  11. Next, ask your partner to lower the tilted globe so that circle of light is now on the upper part of the northern hemisphere of the globe. Again, describe how the shape of the light circle has changed.
  12. Next, ask your partner to raise the tilted globe so that the circle of light is now just below the equator. Make sure that this hemisphere is still tilted away from the flashlight.
  13. Ask your partner to trace the circle of light made by the flashlight.
  14. Next, ask your partner to raise the tilted globe so that circle of light is now on the middle of the southern hemisphere of the globe. Keep the flashlight in the same position.
  15. Again, ask your partner to trace the circle made by the flashlight.
  16. Finally, ask your partner to raise the tilted globe so that the light is nearest to bottom of the globe.  What do you notice about the light circle?
  17. Have your partner do his or her best to draw the shape of light on the graph paper.
  18. Turn on the lights.  
  19. Compare the number of squares in the light tracings your partner drew.
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