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 Mona Evans BellaOnline's Astronomy Editor

Teaching Why We Have Day and Night

We have alternating daytime and nighttime. Each cycle lasts twenty-four hours. It happens because the Earth rotates on its axis. As the Earth turns, the half of it facing the Sun has day and the other half has night.

If you understand this, it seems straightforward. However, in a 1999 Gallup Poll, nearly one in five Americans thought that the Sun revolved around the Earth. (By the way, it seems that the Germans and the British don't do any better.)

It looks to us as though the Sun is moving and it's against our intuition that instead it's the Earth spinning. Therefore we need to look for examples, experiences and models in order to help children (and adults) understand the scientific concepts.

In addition to its rotation, Earth orbits the Sun. Yet even if it were possible for it to stop somewhere in its orbit, day and night would continue as long as Earth kept spinning.

I have set out relevant concepts and some supporting evidence. Then I have listed some web materials that you may find useful.

Concepts

1. The rotation axis is an imaginary line through the center of the Earth from pole to pole.

Although the line is imaginary, the axis is real in that it acts as like the axle of a wheel.

2. The axis is tilted with respect to the path of Earth's orbit around the Sun (the ecliptic).

3. Earth spins in a counterclockwise direction, which is from west to east.

You can see the Earth's spin in this animation. The Sun isn't shown, but you can easily work out where it is.

>Since the Earth spins from west to east, the Sun seems to move in the opposite way, rising in the east and setting in the west. This is an example of relative motion.

Relative motion is a common experience. A carousel is an example. The riders go up and down while moving clockwise - as viewed from the sidelines. This family video taken on a carousel shows the riders moving up and down. Yet some shots that include the background make it looks as though it's the background moving counterclockwise, not the carousel moving clockwise.

Or think of being on a train when another train is alongside. You start to move. No! Whoops! It's the other train that's moving. This superb video shows relative motion. There are two trains, even though you only see one of them. Which one is moving and which one is stationary? And how does this change during the video? (I had to watch it several times.)

>At night the stars seem to move from east to west.

You can check this by stargazing, using a planetarium program or Heavens-above.com. In Heavens-above, go to "Astronomy" and then "Whole Sky Chart". By changing the time on the chart, you can see how star positions change from hour to hour. In the northern hemisphere, the pole star Polaris (in Ursa Minor) doesn't move in one evening, because it's just above the Earth's axis. The rest of the sky seems to turn around it.

If you aren't sure how to read the star map, there is some help in "Absolute Beginners - Start Observing". (There is a link below this article.)

>Star trails show that the Earth is moving.

In this picture taken by P-M Hedén you can see a beautiful set of star trails created by leaving the camera's shutter open for many hours. Polaris is near the center of the circular trails.

4. It takes the Earth 24 hours for one rotation, which is an Earth day.

Other planets have days of different lengths, depending on how long it takes them to rotate.

5. Light travels in straight lines.

Since light can't go around or through an opaque object, a shadow forms. Therefore the side of the Earth turned away from the Sun is in darkness.

Activities

#1. An animation showing two views of Earth's rotation - from the side and looking down at the North Pole. One arrow lets you watch it spin, but the other will let you move the Earth a bit at a time.

#2. Day and Night is a complete lesson plan with worksheets. It's aimed at pupils aged 10-12, but I think it could be used with younger ones, as it's pretty basic. This doesn't change the science, but it is aimed at the U.K.

#3. A helpful summary for students (aimed at age 11-13).

#4. What Makes Day and Night? The Earth's Rotation from the University of California. It's a detailed lesson plan, so it looks complicated. But it is a good approach. It requires a book, but there are reasonably-priced copies on Amazon and it looks like a good book. Everything else is here, with a summary, equipment list and objectives. Aimed at ages 6-8.

Last revised 2013.07.17

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