Activity 3
Objective
To mark degrees on the compass rose and use it to measure and record the azimuth of the Sun over a period of time. Azimuth is the direction along the horizon measured in degrees clockwise from north. In our activities, we measure the Sun's azimuth by extending the shadow line through the center of the rose and out to the marked circle on the opposite side.
Time Allotment
At least 30 minutes will be needed to mark the degrees. Then short observations are made at scheduled intervals.
Procedure
- To convert the compass rose constructed in activity 2 into a device to measure azimuth, move the class outside and mark the rose at 10° intervals. To do this, label the north point of the compass rose "0°". Using a protractor placed at the center of the rose and a meterstick to extend the measurements to the circle, move clockwise around the compass from north and mark the rose with tick marks at intervals of 10° around the whole 360° circle (see figure 3). Figure 4. Marking the the azimuth of the sun.
- To find the azimuth of the Sun, mark the point on the circle where the shadow of the gnomon intersects it, remove the gnomon, and use a meterstick to draw a line connecting the point at which the shadow of the gnomon intersects the circle, the center of the circle, and the opposite point on the circle (see figure 4). This opposite point, recorded in degrees, is the azimuth reading for that time of day on that date of the year. Naturally this azimuth is only approximate because we are using rough instruments and a not-very-exact technique, but if we use the same procedure for all measurements, the changes in azimuth will be apparent.
- Determine the azimuth of the Sun on the compass rose at set hours (perhaps every hour) during one school day, and have the students record it on a table in their notebooks.
- Follow up these initial observations of azimuth by determining the azimuth of the Sun at a selected hour on a selected day every month (as nearly as weather conditions permit). It would be good to determine this on the day closest to the 22nd of each month in order to include the measurements at the time of the equinox.
Discussion
We are able to measure the azimuth, or horizontal position, of the Sun at different times during the day and at the same time on different days. Although the azimuth is always 180° from north at solar noon, it changes gradually throughout the year during the morning and afternoon hours. The azimuth readings for sunrise change from south of east in winter to north of east in summer; sunset readings for the same periods are south of west and north of west. Ask students who have a view that faces east at home to notice where the Sun comes up on a certain day of the month in relation to objects on the horizon that they can see. This is an informal way of observing the changing azimuths of sunrise. With a view facing west, the same observations can be made of the sunset.
As the students observe and record the changes in the Sun's azimuth during the same day, they are indirectly observing the rotation of the Earth. Changes in the Sun's azimuth at the same time on different days during the year are a part of the changes due to the revolution of the Earth around the Sun. Now that the students understand the concept of changing azimuth, they can progress to measuring the altitude of the Sun as it too changes with the seasons.
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