Activity 1
Learning Outcomes
- Accurately record the movement of a shadow over a period of time.
- Accurately record the changing length and position of a shadow over a given period of time.
- Describe the movement of the Earth as it relates to the sun.
- Formulate a hypothesis for the movement and the changing length of the shadow.
- Accurately express the relationship between the sun and the Earth that causes the shadow of an object to move and change length.
Misconceptions
The following concepts may be difficult for students to grasp initially (misconceptions are italicized):
- The sun moves across the sky.
- The earth rotates in a clockwise manner.
- The earth is closer to the sun during summer and farther away during winter.
- The rotation of the earth on its axis every 24 hours produces the night and day cycle.
- The turning of the planet makes it seem as though the sun, moon, and star are orbiting around the earth once a day.
- The patterns of stars in the sky stay the same, although they appear to move across the sky nightly, and different stars can be seen in different seasons.
- The motion of an object is always judged with respect to some other object or point and so the idea of absolute motion or rest is misleading.
Time Allotment
- 45 Minutes prior to observation day (informal observations, creating the gnomon).
- 60 minutes on a sunny day for the observation. This will need to occur over solar noon, 11:30AM–12:30PM. If under daylight savings time the observation should happen between 12:30PM and 1:30PM.
- 60 post observation for discussion, recording, and graphing.
Safety Precaution
Emphasize that you will be making an indirect observation of the Sun. Caution the students that they must never look directly at the Sun's disk because even brief viewing can damage the retinas of their eyes.
Procedure
Prior to the day of the observation:
- Prepare a perpendicular gnomon. This can be constructed by inserting a 60-cm (24-in.) dowel, as near to vertical as possible, into a #10 can filled with wet plaster of Paris that has thickened. A good use of materials is to use the bucket that the plaster of Paris came in. Allow the plaster to harden before using. To make sure the gnomon is vertical a plum bob or bullet level work well. Be sure to make the gnomon at least a day ahead.
- Point out to the class that a perpendicular gnomon differs from a sundial because the gnomon of a sundial is set parallel to the Earth's axis of rotation. If an unattached sundial is available, bring it in for the class to examine.
- NOTE: This is a good opportunity to work in some physical science as well. When plaster of Paris hardens a chemical reaction is taking place. This reaction is an exothermic reaction; heat is released as the plaster dries.
- Tell the students that you will be observing how the relationship between the Sun and the Earth changes by marking shadows. Make a journal for recording these and later observations.
- Brainstorm possible variables that could affect the shadow thrown by the gnomon. These should include: moving the gnomon, clouds, wind, who and how the length and position of the shadow is marked, etc. With this list in mind, the students will decide how to control the independent variables listed.
- Writing Connection: Brainstorm a list of possible locations for your gnomon. Task the students to write a short persuasive essay with the goal of convincing you (the teacher) of the best location for the gnomon. A desirable location for the gnomon is paved, in a sunny location, level, away from busy areas of the school grounds, with adequate space for a class to spread out.
Day of the observation:
- Assemble the class outside at the observation site at least 20 minutes before your first scheduled observation. The students will set up the gnomon being mindful of controlling the independent variables from the brainstorming list. Once the gnomon is set up the observations can begin.
- At five-minute intervals mark the length and direction of the shadow on the pavement. Use the whistle to bring the students attention back to the observations.
Figure 1. Marking the shadows for direction and length.
- Mark the end point of the shadow of the gnomon with a dot of one color and the direction with a line of a separate color.
- Continue marking the intersection and the end of the shadow over a period at least 60 minutes. Students can take turns marking the shadow once they understand what is involved. Continue the observation until at least 12:30PM local standard time (see figure 1).
- Using their journals, students will record their observations including environmental observations such as the weather.
- Discuss with them whether it is the Sun or the Earth that is moving in the observations you are making, will the length or position of the shadow tell us anything, will the shadow continue to get longer, etc.
- At the conclusion of the observation time period use the string compass to draw a circle that intersects all of the shadow lines. This circle can be used for creating a compass rose on your playground as detailed later in this unit.
In the topmost tier of states in the US, if the students can stand the cold, this is very interesting to do in December. Because of the tilt of the Earth at this time of year there will be a great deal of movement in the shadow length and position.
Discussion/Conclusions
Upon returning to the classroom discuss the students' observations and list them. Using the observations bring the class to consensus regarding the following conclusions:
- The Sun's clockwise motion is an apparent motion caused by the rotation of the Earth.
- The counterclockwise rotation of the Earth in the Sun's light causes the shadow of the gnomon to move clockwise.
- As the Sun appears to move higher above the horizon before solar noon, the shadow grows shorter and shorter. After solar noon, the shadow grows longer and longer. The Sun is at its highest altitude above the horizon when the shadow is shortest. You can tell the students that at this instant, the Sun is on the meridian and is due south of the observer. NOTE: Depending on your location and the time of year, the length of the gnomon's shadow will vary. Therefore, your students may have to experiment to find a workable gnomon height and circle circumference in order to find the true North at Solar Noon.
Outcome/Assessment
- Create a line graph showing the relationship between time and the length of a shadow over solar noon.
- Write a persuasive essay (scored using district writing rubric/standards) regarding where to place the gnomon.
- Successfully predict what a shadow will look like in a variety of situations.
Web-based and Computer Resources
National Science Teachers Association
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www.nsta.org
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