## Using a Sundial

• Length: Variable

### Overview

Students make a sundial (shadow clock) and use it to tell time.

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

For centuries, people relied on the position of the sun in the sky to estimate the time of day. Noon or midday was designated as the time when the sun was at its highest point in the sky. We now know, of course, that the sun does not move across the sky. Rather, Earth rotates as it revolves around the sun.

One of the earliest tools used to measure time was the movement of shadows on the ground over the course of a day. Since shadows move the same way each day, they can be used to estimate time. Simple sundials—perhaps only a stick placed vertically in the ground—used shadows to tell time in Egypt more than 3,000 years ago.

When humans first began keeping track of time, an hour was calculated as 1/12 of the period of daylight on any given day. Thus, the length of an hour varied with the seasons. Now, we divide each day into 24 equal hours, which remain unchanged regardless of the time of year. In modern society, time also is standardized from place to place. Before 1884, every town set its clocks to the highest position of the sun (noon). As a result, each town ran on a different time, and there was great confusion. Today, the world is sectioned into 24 standard, uniform time zones. Each time zone accounts for 15 degrees longitude and 60 minutes of Universal Time.

### Objectives and Standards

#### Earth and Space Science

• The sun, an average star, is the central and largest body in the solar system.

• Most objects in the solar system are in regular and predictable motion.

#### Physical Science

• The motion of an object can be described by its position, direction of motion and speed. That motion can be represented and measured on a graph.

• The sun is a major source of energy for changes on Earth’s surface. The sun’s energy arrives as light.

#### Science, Health and Math Skills

• Measuring

• Locating cardinal directions

• Making a model

• Observing

• Drawing conclusions

### Materials and Setup

#### Teacher Materials (see Setup)

• 24 sheets of white cardstock

• Flashlight

• Globe or map with latitude markings

• Sundial template

#### Materials per Group of Students

• Flashlight

• Hole punch

• Magnetic compass

#### Materials per Student

• Fine-tip ballpoint pen

• Opaque drinking straw

• Pencil

• Pair of scissors

• Ruler (12-in.)

• Several pieces of clear tape (each about 2–3 in. in length)

• Copy of student sheet

• Prepared copy of sundial template on cardstock

#### Setup

1. Make 24 copies of the sundial template on cardstock.

2. Place materials in a central area for materials managers to collect.

3. Have students work in groups of four to share materials—but each student should construct his/her own sundial.

### Procedure and Extensions

Time: Two 60 minute sessions

1. Have students observe while you darken the room and use a flashlight to make a shadow of your hand. Then turn flashlight off, and ask, Where is the shadow? Students should understand that an object in front of a light source blocks the light, causing an area of darkness—a shadow. Encourage students to create different shadows using their flashlights.

2. Challenge students to envision what happens to shadows outdoors as the position of the sun changes, relative to the Earth’s surface. Ask, Are shadows always the same size? Why or why not? When are the shadows we see outside smallest? When are they largest? Can we always see shadows outside? Ask students if there is a predictable pattern to the different sizes of shadows made by the sun (shadows are longest in the morning and evening, and shortest at midday). Using flashlights and pencils, have students make shadows of different lengths. Ask them to identify variables that affect shadow length (for example, height of pencil and angle of light).

3. Explain to students that they will be investigating and using patterns in shadow movement to solve a practical problem: how to tell time.

4. Distribute copies of the student sheet. Students will need to know the latitude of their location to build their sundials properly. Use a globe or map, or have students find this information on the Internet.

5. Have students test their sundials by placing the dials outside on the ground, with the gnomon (part of the sundial that casts the shadow) pointed directly north. To obtain a more accurate reading, students should use a magnetic compass to orient their sundials toward the north. They also can check and correct for the magnetic declination where they live, to ensure that their sundials are pointed toward “true north.".

6. Once the sundial’s gnomon is pointing in the correct direction, students should locate where the sun’s shadow crosses the dial and estimate the time, based on their reading. Then, have students compare the time indicated by their sundials to actual “clock time.” If daylight savings time is in effect, subtract one hour from the “sundial time.”

7. Conduct a class discussion or have students write about the accuracy of their sundials. Ask, How well did your sundial measure time, compared to a watch or clock? Why do you think your sundial did (or did not) measure time as accurately as other clocks do? Also, ask students to consider if there are times a sundial might not be useful. Examples: nighttime (too dark); or when skies are cloudy (no shadows).

#### Extensions

• Using a copy of the image above, have students draw the lines as shown. The mid-point is Bursa’s longitude line projected in the sky. Have students start from the “east,” and count the number of hours of daylight for each string. Ask, Which season does each string represent? During which season is the sun higher in the sky? Lower? What do these observations tell us about Earth’s seasons?

• Have students try using shadow length to tell the time.

• Have students think about why time zones, each of which accounts for a one-hour time difference, are separated by 15° of longitude.

• Have students figure out what time it is in other parts of the world when it is 8:00 am in their own location.

• ### Sleep and Daily Rhythms

Teacher Guide

Students explore the day/night cycle and seasonal cycles on Earth; create and use sundials; and investigate circadian rhythms, sleep patterns and factors affecting the quality of sleep. (8 activities)

### National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.