OBJECTIVES:

• Comparing night and day.
• Exploring the movement of the Earth.

VOCABULARY:

• axis
• day
• Earth
• night
• revolve
• rotate

MATERIALS:

• What makes Day and Night by F. Branley (Harper)

Students learn about night and day through reading a book.

BACKGROUND:

The Solar System and all its planets are in constant motion. Each of the planets revolves around the Sun. The planets also rotate, or spin, around an internal axis. One manifestation of rotation is the cycle of night and day. Day after day, month after month, year after year, the alternation of night and day continues. Sometimes it is sunny outside, and other times it is dark. In addition to rotation, day and night occur because the Earth is spherical. When a portion of the Earth faces the Sun, it is daytime. When the same are rotates away from the Sun, it is nighttime. The cycle of light and dark is continuous except near the North and South Pole. During the Northern Hemisphere summer, the North Pole always faces the Sun, so daytime is continuous for several weeks. At the same time, the South Pole faces away from the Sun (Southern Hemisphere winter) and is in continuous night.

The origin of night and day is very difficult for children to understand. The idea that the dark side is really a shadow of the planet as it rotates away from the Sun’s rays is difficult to imagine. The rotation of the Earth on its axis, is also not easy to understand, because we cannot feel this motion.

Rotation and revolution also cause the seasons. The Earth’s axis is tilted 23.5^o from vertical. This means that solar energy strikes the Earth unevenly. It is summer in the Northern Hemisphere when the North Pole tilts toward the Sun. This puts the Sun more overhead, so solar energy strikes the Earth most directly. In half of a revolution the North Pole tilts away from the Sun, making solar energy least direct, so it is winter.

PROCEDURE:

1. Read What makes Day and Night to the class. All of the students are of course familiar with day and night, but they may have never wondered why this cycle takes place. This book helps students understand that the rotation of our Earth on its axis causes night and day. Review the pictures and models in the book, so that they can begin understanding night and day. In the Lab, the students will do some of the experiments discussed in the book.
    
2. To illustrate rotation and revolution, draw the following pictures on the board. Explain that the Earth rotates on its axis, but revolves around the Sun at the same time. Explain that night and day is caused by rotation, and that the seasons are caused by the tilt of the axis and the revolution around the Sun.

3. Have the students stand up, and rotate or spin. Make sure they understand that this is rotation.

   Have students work in pairs. Have one student revolve around the other. Explain that this is revolution.
    

4. Tell the students that the Earth rotates as it revolves around the Sun. Ask them to figure out what these motions will look like, using one pair of students. Don’t let them do these activities for too long or else you will have many dizzy students!

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