Explaining the Seasons (astronomy)

[science_spot]

Most people know that autumn here in the United States coincides with spring in South America, but not everyone understands why.  The basic astronomy behind it really isn't that complicated, but instead of a basic verbal/written explanation, this post is designed to be interactive! Share this with your kids, or just do a quick activity to show yourself, it's your choice.  Or if you don't feel creative right now, jump to the video at the end of this post.

 

Items you will need:

A lamp with the lampshade removed

A paperclip

One sheet of paper or newspaper

Tape

Marker

 

Set up your Activity space:

1. Set up the Earth:

Straighten the paperclip.  Crumple the paper into a ball, with the paperclip inside, poking out of each end.  Crumple it tight, and tape it so it holds it's shape well.

Mark the equator around the widest part.  Mark an X in a position roughly showing where you live.

 

 

2. Set up the Sun:

Place the lamp on the floor or other convenient surface so that the lightbulb is about waist-high.  As you work through this activity, you will just hold the Earth by the paperclip as you move it around the sun, so make sure the area around your lamp is clear.

 

 

Start the movement

The Earth takes part in two different movements - rotation on its axis and revolution around the sun. 

1. Take the Earth in your hands and spin it using the paperclip to show rotation on the Earth's axis.  Using the X as a guide, notice that one complete rotation equals one day.  If you hold the Earth up a few feet from the sun and spin it, you can see it would be daytime when there is light on your X, and night when your X is being shaded from the sun.

An interesting thing about the Earth's axis is that it is tilted at a 23 degree angle.  Now hold your Earth up to the sun again and notice how that tilt affects where on Earth the light is falling.  Rotate your Earth on its axis. Can you see that one Pole has complete daylight and the other complete darkness?  See the first picture above for an idea of what the tilt looks like.

2. Take the Earth in your hands and walk around your sun.  This shows the revolution of the Earth around the sun.  Now pick a spot in orbit and stop.  Remember to hold your Earth at a tilt (towards the sun), and pay attention to only the shadow on the Earth and your X.  When the shadow is covering more of the upper Earth, it is winter there, and summer on the lower half (where there is bright light).  Now walk halfway through the orbit and you see the opposit effect - the bright light landing on the upper half of the Earth (summer) and less light on the lower half (winter).

So the seasons change as the Earth revolves around the sun.  With one revolution around the sun, one year has passed, with each season occurring one time.  The tilt of the Earth is important.  Without it, there would be no seasons - the light hitting the Earth would be constant all year long.  Try Step 2 again using an Earth without tilt and you will see no change in how the sunlight hits the Earth, no matter where in the revolution you place your Earth.

For a nice video on the subject see this Youtube post from ignitelearning:

 

The extra stuff

The Earth's orbit isn't a perfect circle, it's more like an oval.  When the Earth is at it's furthest point from the sun, it is about 94.5 million miles away.  At it's closest pass, it is 91 million miles away.  The average distance is about 93 million miles.  This variation in distance between the Earth and sun does have some effect on the seasons, mostly making them milder in the north and harsher in the south.  Seasons are dependent on the tilt of the Earth, not the distance between the sun and Earth.  You could use your Earth-Sun model to show this by walking your Earth around an oval orbit.

Actually there is more than one way to describe a season.  What we have discussed above is an astronomical season, as the seasons change with a change in position of the Earth around the sun.  Another common description of seasons is the Ecological Seasons.  An Ecological Season is a time of year when certain plant and animal events occur, such as plant growth in the rainy season and seed laying in the dry season.  You can read about the Ecological Seasons of a tree, to see how that describes the temperate forest.

 

Tonya

AKA sunmoonstars

The Science Spot

 

[aenima49]

Oct. 5, 2009 at 9:27 AM

I love the activities!

[mtnmama111]

Oct. 6, 2009 at 3:24 PM

[Hoomom]

Oct. 6, 2009 at 5:44 PM

Sweet!  My seven year old is fascinated with this.  He said it looked like night time was a "little winter".  He also wanted to know what would happen if Earth's wasn't tilted.  Thank you for this.  I have been trying to explain the seasons to my boys for some time now.