What Causes Winter?

On December 21 North America will have fewer hours of sunlight than any other day in the year. That is the day of the winter solstice, the official first day of winter. If you ask people why it is warmer in the summer than in the winter, many people will guess that the earth is closer to the sun in the summer than in the winter. These people would be wrong. So what is going on here?

Think about it, if the summer is caused by the earth being closer to the sun, wouldn't all the earth have summer at the same time? But remember, the southern hemisphere (South America, Australia, Africa) has summer when North America has winter. So it must be something else. Besides, does the earth's orbit take it much closer to the sun in some parts of the year than at other times?

Most drawings show the orbits of planets around the sun are what's called a bird's eye view of the orbits. A bird's eye view looks from a little bit above, not from directly above. It makes it look like the planets spend part of the year much closer to the sun. Here is how to draw a model of the earth's orbit that shows its real shape. You will need a piece of cardboard about 12 inches long and 12 inches wide, a piece of string 12 inches long, 2 pins and a pencil. 

  1. Tie the string into a loop that is just 5 inches long.
  2. Push 2 pins into the cardboard near the center and about 2 inches apart.
  3. Put the string around the pins.
  4. Hook a pencil inside the loop of the string and hold it upright.
  5. Use the string loop to guide you in drawing a "circle" on the cardboard around the pins.
  6. The circle that you draw is not a perfect circle because of the 2 pins. This imperfect circle is called an ellipse. The shape of the ellipse is close to the shape of the earth's orbit.
So what does cause winter? NASA's Windows to the Universe has a pretty good explanation.

So you see, the real reason for seasons is the way sunlight hits the earth at different times of the year. This changes because the imaginary stick through the earth that it spins on (look at any globe to see a model) is tipped to the side instead of straight up and down. Remember, there is no stick through the real earth. In the warm parts of the year, sunlight hits the earth at a different angle than during the colder parts of the year. The earth's tilt also causes the number of hours of sunlight to change during different seasons.

You can model the strength of sunlight in different seasons using a ruler, some tape and a flashlight. (See Flashlight image 1 and 2 in the rollover image above.) Download graph paper and you can count the number of squares lighted in steps 2 and 3 to compare the size of the area that is lighted.

  1. Tape the flashlight onto the ruler so it shines down the length of the ruler.
  2. Stand the ruler up so the flashlight points to the floor or a desktop. Turn the flashlight on. Now tilt the ruler a little bit. Look at how bright the ellipse of light looks on the floor or desk.
  3. Tilt the yardstick more. Look at how bright the ellipse of light looks now.
  4. Do you see a difference in how bright the ellipse of light looks? Which one shows sunlight during the summer? Which one shows sunlight during winter? If you know how to use a protractor, you could stand the ruler at an angle of 70 degrees to show summer sunlight for New York State and at 23 degrees to show winter sunlight for our state.
What causes winter? Try explaining it to someone else, you might find out that you really do understand.
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