The activities of Topic 2 have shown us that observing the shadows made by the Sun during the course of a day provides information about time: the times of Midday, sunrise, and sunset, for instance. This topic further explores the path of the Sun in the sky, and how we might use this information to determine the current time of day. New ideas to be understood include:

Benchmark:

How does the Sun appear to move across the sky? How could shadows tell us about the Sun's position?

Tracking the Sun's Path in the Sky

This activity allows us to track the apparent motion of the Sun in the sky on a model in which the plastic dome represents the sky. To do this, we mark the position on a clear dome which casts a shadow at the same central spot. As the Sun moves in the sky, so too does the position we must mark. For this activity, an outside spot must be picked which will not be shaded at any time during the day.

Materials: clear plastic dome (3" to 8" diameter or clear tops of pantyhose containers will work); grease pens or water-soluble markers; adhesive colored dots; (as an alternative to a plastic dome, a large kitchen strainer and sewing pins with large heads can be used); 9" x 12" oak tag or other stiff paper; compass.

1. Begin early on a sunny day. Place a clear plastic dome or a large kitchen strainer on a sheet of oak tag and trace the outline of the dome on the oak tag. Draw a mark on the edge of the dome and on the oak tag, so the alignment of the dome and oak tag can be checked before each observation. Remove the dome and mark the center of the circle just traced. (The center can be found by drawing two intersecting lines across the widest part of the circle, like

3.) The dome represents the sky to your horizon, the center mark represents your position on the ground.

2. Having carefully selected an unshaded spot for the observations, place the oak tag on level ground or asphalt. Trace the outline of the oak ag on the ground, so that its position can also be verified before each observation. Replace the dome on the oak tag outline aligning the marks on the dome and the oak tag. Using the compass, mark a North-South arrow on a corner of the oak tag.

3. Mark the spot on the dome whose shadow falls on the central dot. If using a strainer, large headed pins will work well. For a plastic dome, use an adhesive dot or a grease marker. (It may be easier to use the end of a pen to locate the position before marking it with an adhesive dot.) Number the marks and keep track of the times of the observations in a notebook. Continue to make observations at regular intervals throughout the day. At the end of the day, label the compass directions on the dome, date it, and keep the dome for comparison with later observations.

Analysis:

You should be able to convince yourself that the marks placed on the dome correspond to the position of the Sun in the sky a those times. If an observer (maybe an ant) were to stand at the central dot inside the dome, then each time a dot was marked eclipsing the Sun (casting a shadow on the observer), the dot would have to be in the direction of the Sun. The position of the dot, then, would have to be the same as the Sun's apparent position in the sky.

Discussion:

What sort of path does the Sun follow in the sky? Did the Sun travel directly over the top of the dome? Where did the Sun begin in the morning? Where was it in the afternoon? What about at noon? At midday?

Building and Using a Sundial

A sundial is a device to measure time by the sun. It is made of two parts: a gnomon (NO-men) and a base. The gnomon casts a shadow on the base, a flat surface with markings indicating each hour. On a properly constructed sundial, the shadow of the Sun moves equal distances each hour.

A sundial with a vertical gnomon will work perfectly at the North Pole because there the shadows cast will move equal distances each hour. But as one moves farther from the North Pole, the motion of the shadows varies more.

When you made the shadow stick measurements, you may have noticed that shadows separated by equal time intervals were rarely separated by the same distance. One solution to this problem is to tilt the gnomon so it is aligned as it would be if it were vertical at the North Pole (i.e. parallel with the Earth's axis). Knowing your latitude is all that is necessary to find the correct angle to tilt your gnomon. Since the latitude of the North Pole is 90° (N), just subtract your latitude from 90° to find the angle to tilt the gnomon (refer to the table of latitudes of major U.S. cities). The gnomon should point towards North .

You may want to repeat the shadow stick measurements in Topic 2 at the same time as the sundial measurements to see the difference of a vertical stick and one tilted to match your latitude. Instead of tilting a stick, we will use the "tilted" edge of a triangle for our sundial gnomon.

Materials: scissors, enclosed cut-outs, protractor, oak tag (or other heavy paper), popsicle sticks, graph paper.

1. Make a gnomon pattern like the example below:Refer to the latitude table to determine the correct angle to mark - 45° is marked as an example. Cut out the gnomon from this pattern.

2. Fold pattern along the dashed line so that the flaps A and B are on either side of the gnomon. These flaps will allow the gnomon to stand on its own.

3. Tape a sheet of graph paper over the oak tag.

4. Tape the gnomon to the middle of the sheet of graph paper on the oak tag. If the gnomon remains floppy, then tape a popsicle stick to it to provide support.

5. Start early in the morning. Place the sundial outdoors, with the gnomon pointing North. Record the outline of the gnomon's shadow and record the time next to it. Repeat this process each hour. See if the students notice a pattern in the movement of the shadow.

Discussion:

Count how many squares the shadow moves each hour. Compare how the triangle gnomon measures the hours compared to how the vertical shadow stick measures the hours. In which direction did the shadow appear to move? What if the sundial were in the Southern Hemisphere? Did you ever wonder why clocks run "clockwise"? Before mechanical clocks, people used sundials, which, as we have seen, run clockwise in the Northern hemisphere.

Before the establishment of the standard time zones we know (Eastern, Central, Mountain, and Pacific for the continental U.S.), each city kept its own time based on the observations of the Sun. We have seen how to find midday. Try keeping your own time based on your observations of the Sun by setting noon to midday. How close to the "standard" time are you? Where else on the globe should have the same "local" time as you? Are there any advantages to keeping your own solar time? What about disadvantages? Why might we have standardized on time zones?