Effectiveness of Sun Shadows for Telling Time
Category: Astronomy—Celestial Motion
Project Idea by: Olivia Anderson
Since the earliest recorded history, people have used the movement of celestial bodies (the natural things in the sky, such as stars, suns, moons, and planets) to tell time. The Sun was used most often because it is the easiest to follow. Earth's rotation (turning on an axis—an imaginary line through the center of an object around which the object rotates) each day makes the Sun appear to move across the sky. As Earth rotates, the Sun appears to rise above the eastern horizon (where the sky appears to touch Earth) in the morning, travel across the sky, and set below the western horizon in the evening.
The Sun's apparent motion from east to west across the sky causes objects on Earth to cast shadows. The changing position of the shadow cast by an object during the day indicates a change in time. The sundial is one of the oldest, if not the oldest, known device for the measurement of time. It is made of an object called a gnomon, which casts a shadow on a scaled surface. The shadow of the gnomon falls on different points on the scale, telling you what time it is.
The scale of a sundial is designed so that the differences in the direction of shadows during the day indicate time. In the morning, shadows are long and point toward the west. As the day progresses, shadows shorten. At noon, when the Sun is at its highest point in the sky, shadows are shortest. In the afternoon, shadows are longer again and in the opposite direction, toward the east.
Not only does Earth rotate (turn on its axis), but it also revolves (to move in a curved path around another object) around the Sun. This motion changes the position of the Sun in the sky. In the Northern Hemisphere (the part of Earth north of the equator—the imaginary line dividing Earth into two parts), the Sun's highest noon position in the southern sky is on the first day of summer, called the summer solstice (the first day of summer on or about June 21/June 22). The Sun's lowest noon position during the year is on the first day of winter, called the winter solstice (the first day of winter on or about December 21/December 22). A project question might be, "How effective are Sun shadows for telling time?"
Clues for Your Investigation
Design a method for creating a time scale using shadows. One way is to use a watch to determine specific times during the day and to place objects such as stones on the ground to mark the position of shadows from a specific object at these times. Use the shadow of a stationary object such as a pole in an open area. Determine the time from the shadows from day to day for a specific time period such as each hour from 9 A.M. to 3 P.M. Check the accuracy against your watch. The longer the testing period, the more accurate the results.
Independent Variable: Changing position of the Sun's zenith in the sky
Dependent Variable: Time measured by shadows
Controlled Variables: Testing materials—objects casting the shadows, shadow markers (stones), and the watch you use
Control: Time for the shadows marked on the first day of the investigation
Other Questions to Explore
- What is latitude, and how does it affect the construction of a sundial?
- What is the declination of the Sun, and should it be considered when constructing a sundial?
- What is an analemmatic sundial, and how does its accuracy compare to a traditional sundial?
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