GO IN DEPTH

#### Materials

Supplies to build shadow tracking devices:

• Foamcore poster board
• Round Tinkertoy connector and Tinkertoy stick
• Wrapping paper or newsprint
• Glue or rubber cement
• Scissors

Other supplies

• Magnetic compass (optional)
• Measuring tools
• Protractor
• Supplies to make models of the earth and sun (e.g., balloons, lamps)

### Purpose

To determine the pattern (length and direction) of shadows cast by sunlight during a several month period and to develop an interpretation of the daily and seasonal patterns and variations observed.

### Context

For the most part, change should not be taught as a separate topic. At every opportunity throughout the school year, the theme of change should be brought up in the context of the science, mathematics, or technology being studied. In the earliest grades, children should be encouraged to observe change and describe it. Once they have a variety of experiences with change, they are ready to start thinking more abstractly about patterns of change. (Benchmarks for Science Literacy, p. 271.)

In middle school, students can look for more sophisticated patterns, including rates of change and cyclic patterns. These activities have to be built into a framework of trying to understand the world around us, and realizing that some observations have to be taken over extended periods of time in order to see the patterns and to develop interpretations that explain them.

Activities associated with these objectives may occupy an entire school year, but, except for the discussions of results and interpretations, very little class time need be spent on any particular day. Students can be reminded that the study of planets and stars takes a lot of time because changes are gradual and not under human control.

### Motivation

Discuss as a class what students know about the directions that shadows fall at various times of the day; try to get them to recall that shadows generally fall only in a semicircle that always faces about the same direction. Perhaps they will recall that moss is said to grow on only one side of a tree, the shaded side.

Ask students to reflect in their notebooks around the following question: How do shadows cast by sunlight change during the day?

### Development

The hardest thing to find for this activity may be an open area that is sunlit all day long at all seasons of the year, that is easily accessible from the classroom, and where shadow tracking devices can be left undisturbed all day.

Shadow tracking devices, one for each cooperative group (if there is space enough for them), can be made from some stiff material like half-inch thick foamcore poster board. Pieces about 50 x 75 cm (2 x 3 feet) are a good size.

Use a round Tinkertoy connector and a Tinkertoy stick about 18-20 cm long (or some equivalent pair) as the object whose shadow will be cast. Insert the stick in the hole in the center of the connector and make sure it is firmly seated.

Mark a place on the poster board about 8 cm from the edge of one of the long sides and in the middle of the side where you will place the object. Use rubber cement or some other glue that will hold the connector to the poster board, but allow it to be removed between uses, so the apparatus is easier to store.

Cover the poster board with a sheet of newsprint, wrapping paper, or other paper that is large enough to fit; cut a slit or leave a space in the paper to go around the connector that is glued to the board. Use masking tape to attach the paper to the board and make marks on the board and paper to show the exact orientation and location of the paper.

Start making measurements as early in the day as possible. Set the board level in the open sunny area and find a way to mark its exact location and orientation, so it can be reproduced on subsequent days. (A magnetic compass can be used to orient the board in a particular direction, but this is not necessary. At some point, knowing the north-south orientation of the board may be useful, but this can be determined any time.)

The object whose shadow is going to be cast should be on the side from which the sun will be coming at about midday. Use a pencil to mark the tip of the shadow of the Tinkertoy stick and write the time of the measurement next to the mark. Repeat this marking procedure several times during the day, taking the last measurement as late in the afternoon as possible.

After the last measurement, take the apparatus indoors. Carefully remove the paper and store the board and shadow object. On the paper draw lines from the spot where the object was mounted to the tip of each shadow. Measure the length of each line and, with a protractor, the angle each line makes with a line perpendicular to the long side of the board and passing through the spot where the object was mounted.

Make a table of the times, lengths of lines, and angles for these data. Be sure to date both your data paper and your table.

Discuss the results as a class. If more than one shadow chart was made, discuss the similarities and differences. If the boards were not all oriented the same way with respect to the north-south direction, the differences in angle measurements should be an interesting topic for this discussion.

Repeat this activity several times through the year. Save all the data (the papers on which the shadow lengths and angles are measured) through the year. After each set of measurements, devote some class time to discussing them and making comparisons to previous results. Encourage students to come up with ways to present the results in formats that make the evolving patterns more obvious. An interesting discussion could take place as the data before and after the change from and/or to daylight savings time occurs.

While keeping shadow logs to determine other patterns in the apparent movement of the sun, a daily record of the time of sunrise and sunset should also be kept; these data are available in daily newspapers, some radio/TV weather forecasts, and at the U.S. Naval Observatory: Complete Sun and Moon Data for One Day.

### Assessment

In small cooperative groups, have students use balls, balloons, lamps, etc. to make and demonstrate models of the motion of the earth and sun that will explain the patterns in the shadows that they observed. Models should explain the changes in length and angle of the shadows through a day, and the changes in length and angle at the same time of day through the year.

In discussion, be sure that students come to realize the both heliocentric and geocentric models can explain these results. Ask students what other experiments would help to make a choice between the two models.

### Extensions

This lesson can lead to further explorations in many different content areas. It can lead to further study of the solar system or to further explorations of time-keeping devices such as sundials. These activities can also be connected to the concepts related to solar energy.

In small cooperative groups, have students design and build devices that will tell time (hours and quarter hours) through the sunlit part of the day and are based on their observations of shadows through the year. Their explanation of the principles behind the devices they construct should be directly related to their data, not to textbook explanations of sundials and similar devices.

The page Seeing Shadows from the @rt junction website presents an interesting activity in which children use observational skills to concentrate on shadows and apply their observations to photography.

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