Predicting Today's Sunset

[from the planetarium show Stonehenge]

Materials:


The key activity in this program involves searching for horizon events, and then comparing those events with alignments of the stones at Stonehenge. Two basic tools are needed: horizon markers which students use to record rising and setting locations on the horizon, and indicators for the alignments Hawkins found at Stonehenge.

Students need to be able to stand near the horizon in your planetarium to use these markers. In a STARLAB or other small planetarium that may be easy, but it is more difficult in a bigger planetarium with two or more rows of concentric seats, or in a planetarium with unidirectional seating. In those cases, you can adapt the program by assigning a different small group of students to stand at the horizon and mark objects for each round of the experiment.

1. A Horizon Marker for Each Member of the Audience
Students are going to watch for horizon events, risings or settings they think may have been important to ancient people. They need to mark those events along the planetarium horizon.

Students need to be able to stand near the horizon in your planetarium to use these markers. In a STARLAB or other small planetarium that may be easy, but it is more difficult in a bigger planetarium with two or more rows of concentric seats, or in a planetarium with unidirectional seating. In those cases, you can adapt the program by assigning a different small group of students to stand at the horizon and mark objects for each round of the experiment.

Version I. A Marker That Can Hang from the Cove
Figure 1:
Horizon Marker

The marker can be a wood, cardboard, or plastic strip, about 5 centimeters wide and 40 centimeters tall. Fasten a hook on the back so that the marker can be hung on the cove to mark a position on the horizon. The markers should be numbered or lettered, so that participants can remember which one is their own. See Figure 1. The markers could also be attached by having small pieces of Velcro® attached to the back, and a band of the mating material all around the dome at the horizon. In a portable planetarium, it’s best to attach the “hook” component of the Velcro® on the markers, and the “loop” or fuzzy part on the dome.

Version II. Sticky Paper Dots or Squares
This fast, cheap alternative to a prepared marker is being used by many planetariums, particularly small units and portables. Your nearest stationery store will have rectangles in various sizes (3M Post-it® notes) as well as self-adhesive disks in many sizes and colors (Avery is a major brand). Fluorescent red dots in the 3/4 inch diameter size work well for portable planetariums. A few dollars will purchase a supply of a thousand dots.
The only trick is to choose material in a color and size that is easily visible in a dim planetarium, and has the right degree of stickiness. Too weak an adhesive can mean markers not sticking or falling off too soon, especially if the dome is dirty; too strong an adhesive might mean work for you cleaning them off after each program.


Presenter's Script:

To see what marking an astronomical event means, let’s try to record an astronomical event ourselves: where the sun rose on today’s date (give the current date), but 3550 years ago.
Even planetariums with precession settings cannot adjust for the small change in inclination of the Earth’s axis, which affects the position of sunrise and sunset. Thus we are taking artistic license here. In contrast to our simplified presentation in the planetarium, the computer programs used by Hawkins and other researchers must take into account many factors which affect observed rising and setting positions, including local terrain, atmospheric refraction, and the shift in inclination of the Earth’ axis. Lower daylight, diurnal motion on slow. Stop diurnal when the disk of the Sun is halfway above the horizon, and raise daylight. (Sunrise and sunset can also be defined with the disk standing tangent on the horizon, or tangent just below the horizon, but we will use halfway as the rise/set position for this program.)
Let’s mark the position of sunrise.
Demonstrate how to place a horizon marker at the sunrise position.
Now let’s predict where sunset happened on this same day, 3550 years ago. First I’ll speed up the planetarium, as if the Earth were spinning faster.
Turn on diurnal and let the audience watch the sun rise to noon. Stop diurnal.
It’s about noon on this day, 3550 years ago.
Have you decided where the Sun will set? Then take your pointer and go over and place it where you predict the Sun will set. Remember your pointer so you can tell how close your prediction is.
Point out wide range of guesses: due to our indoor life, we usually have a poor notion of sunset position. Many students will place their pointers exactly at West. Begin diurnal motion, continue to sunset (Sun halfway down). Congratulate the person coming closest. Have students retrieve their pointers, leaving only the two which mark the sunrise and sunset.

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