Modeling Day and Night

View from space shows the terminator on the Earth and moon.
Courtesy of NASA.

Grades:
Length: 60 Minutes

Overview

Earth rotates completely on its axis about every 24 hours. This rotation, in combination with Earth’s position relative to the sun, produces the cycles of day and night. In this lesson, students make a “mini-globe” to investigate the causes of day and night on our planet.

This activity is from The Science of Sleep and Daily Rhythms Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher
Background
Objectives and Standards
Materials and
Setup
Procedure and
Extensions
Handouts and
Downloads

Teacher Background

Our lives, and those of other organisms on Earth, are shaped in countless ways by the cycle of day and night. This repeating sequence of light and darkness is caused by the spinning of our planet and its position relative to the sun.

Earth, like other planets in our solar system, revolves around the sun in a slightly elliptical orbit. It takes about 365 days—one year—for Earth to go around the sun. Other planets require more or less time to complete their orbits, and their years are correspondingly longer or shorter than Earth’s. In any case, a year is defined as the amount of time it takes a planet to make one complete revolution around the sun.

As Earth orbits the sun, it also rotates, or spins, on its axis. It takes about 24 hours—one day—for Earth to complete a single rotation. As students will discover through the activities in this unit, the functions of living organisms on Earth are linked to this 24-hour cycle.

During each 24-hour period, most locations on Earth will experience several hours of sunlight (day) followed by a period of darkness (night). Solar noon is the moment at which the sun reaches its highest point in the sky in a given location. It rarely coincides with the “noon hour” on a clock. Many factors—location (longitude) on Earth, time of year, time zone, and whether daylight savings time is in effect—influence what “clock time” it will be when solar noon occurs. Midnight occurs 12 hours before and after noon.

As viewed from the North Pole, Earth spins counterclockwise. This is why the sun appears to rise in the east and set in the west. In reality, of course, the sun remains relatively stationary, while Earth rotates in its orbit. The following activity uses a simple model to help students visualize Earth’s rotation about its axis, the slight tilt in Earth’s axis, and the cycle that produces day and night.

Objectives and Standards

Earth and Space Science

Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the moon and eclipses.

Physical Science

The motion of an object can be described by its position, direction of motion and speed.

Science, Health and Math Skills

Observing
Measuring
Modeling
Mapping
Drawing conclusions

Materials and Setup

Materials per Group of Students

Large paper clip
Table tennis (ping-pong) ball or round foam ball (diameter of 1/2 in.)
Colored markers or pencils
Sheet of cardstock (8.5 in. x 11 in.)
2-3 strips of masking or clear tape
Copy of student sheets

Setup

Before the activity, follow the instructions on the “Earth Model” sheet to build a demonstration “mini-globe.” If using table tennis balls for the activity, use a pushpin to make a small hole in the bottom of each ball before distributing to students. For younger students, you also may want to straighten the paper clip, as directed.
Place the materials in a central location for materials managers to pick up.
Have students work in groups of four.

Safety

Always follow district and school laboratory safety procedures. It is a good idea for students to wash their hands with soap and water before and after any science activity.

Procedure and Extensions

Challenge students to think about what causes night and day on Earth. Conduct a discussion and list students’ ideas on the board. Ask, Is day always followed by night? Does the sun shine at night? Why does the sun appear in the east in the morning and disappear in the west in the evening? Do the combined hours of light and darkness in a day always equal 24? List any other questions posed by students.
Tell students they will be conducting an investigation that will help to answer many questions about day and night on Earth. Explain that each group will construct a model Earth and investigate what happens when light shines on the model.
Show students the globe model you made in advance. The model can be as simple or as elaborate as you choose, depending on grade level. Tell students they will create similar models for their investigations. Distribute the “Earth Model” sheets and ask materials managers to pick up their supplies.
Have groups follow the instructions and build their models.
Next, have students identify which end of the model Earth represents the North Pole. Then, have them determine the direction in which their model Earth must spin if it is to rotate counterclockwise when viewed from above the North Pole. (To review clockwise and counterclockwise, have the class stand and face the same direction, and then turn in place, first clockwise, then counterclockwise.)
Distribute copies of the “Rotation Observations” page and have each group work through the questions.
Point out how the Earth models appear slightly tilted. This tilt in Earth’s axis affects day length throughout the year and causes the seasons, which are explored in Activity Two.
Prompt students to reconsider the questions asked at the beginning of the activity, and to use their Earth models to obtain the answers. Ask, Is day always followed by night? [Yes, in most locations. However, during the summer at the North and South Poles, the sun is visible all day long.] Does the sun shine at night? [Yes, the sun always is shining, even when your part of the planet is in darkness. Similarly, one part of Earth always is facing away from the sun and in darkness.] Why does the sun appear in the east in the morning and disappear in the east in the evening? [The direction of Earth’s rotation creates the illusion that the sun rises in the east and sets in the west. In fact, the sun remains relatively still, while Earth rotates.] Do the combined hours of light and darkness in a day equal 24? [Yes, because the Earth rotates completely relative to the sun once every 24 hours.]

Extentions

Have students track the times of sunrise and sunset in their town for several weeks. This information is available from newspapers, weather broadcasts or the Internet. Have students compare these times to those for a city in the southern hemisphere at the same south latitude.
Have students find the latitude, longitude and time zone of the location in which they live. Then, lead a class discussion about the importance of standardized time zones. Students can find the times of solar noon and apparent sunrise and sunset in their location—and other places around the world—with the Sunrise/Sunset Calculator created by the National Oceanic and Atmospheric Administration (NOAA) (www.srrb.noaa.gov/highlights/sunrise/sunrise.html). This would be a good time to explain the difference between solar noon and chronological noon.
Astronauts on the space station orbit Earth every 90 minutes. Have students calculate the number of day/night cycles that a crew in orbit experiences during a 24-hour period.