Shadows

Where shadows come from Light travels in straight lines from the Sun. When an object gets in the way, a shadow is formed.

Truths about a shadow As the earth turns on its axis, the sun appears to move across the sky. The shadows the sun casts move in a clockwise direction for objects in the Northern Hemisphere. If the sun rose and set at the same time and spot on the horizon each day, shadow sticks would have been accurate clocks. However, the earth is always spinning like a top. It spins around an imaginary line called its axis. The axis runs through the center of the earth from the North Pole to the South Pole. The earth's axis is always tilted at the same angle. Every 24 hours the earth makes one complete turn, or rotation. The earth rotates on its axis from west to east. The earth's rotation causes day and night. As the earth rotates, the night side will move into the sunlight, and the day side will move into the dark.

Truths about a shadow On the earth's yearly trip around the sun the North Pole is tilted toward the sun for six months and away from the sun for six months. This means the shadows cast by the sun change from day to day. Because the earth is round, or curved, the ground at the base of a shadow stick will not be at the same angle to the sun's rays as at the equator. Because of this the shadow of the shadow stick will not move at a uniform rate during the day.

Lets Investigate! (OPTIONAL) Today, we are going to track the sun’s shadow and how it changes during the course of the day. http://www.schoolsobservatory.org.uk/astro/esm/shadows Remember, it is very dangerous to look at the Sun. So never look directly at the Sun!

Questions: Is there a pattern to following the track of the sun? How is a shadow used to tell time?

Materials: Plate Straw Pencil Ruler Scissors Tape

Directions: Distribute materials Students need to find the center of the paper plate and mark it with a dot. With a pencil, have students pace 4 marks along the edge of the paper plate. (These marks will help students reposition their sundial for taking measurements throughout the day.)

Directions Continued Make 4 half inch cuts on one end of the straw section. Flare out the cut portion of the straw and tape it onto the center of the paper plate. The straw should be perpendicular to the surface of the plate. Measure and cut the straw to a 2 inch length.

Directions Continued Mark the playground with chalk at the 4 registration points on the edge of their sundials. You may want to mark them with North, South, East, and West. Have students carefully trace the straw's shadow with a pencil.  Ask them to darken in the shadow with the pencil and write the time at the tip of the shadow. Ask students to note where the Sun is in the sky. Warn them NOT to look directly at it. After tracing ask students measure the shadow length and fill in their data sheets. Students will take more measurements later in the morning, at midday, and in the early afternoon. At the end of the day, ask students to predict where they think shadows will fall in the late afternoon. Ask them to look at their sundials and guess where the shadow would be at 2 PM (or any other afternoon time). Using a red pencil, have them outline the predicted shadow on their sundials. 

Conclusion What did you observe? What did the shadows do? When was the shadow the longest? Where was the Sun? When was the shadow the shortest? Why do you think the shadows change length? How can you explain what you are observing? How could you use a shadow to tell the time of day? How accurate were your predictions? What could you do to make them more accurate?