Risings and Settings

Marking More Risings and Settings

[from the planetarium show Stonehenge]


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. See Horizon Marker in Predicting Sunset.

2. Stonehenge Alignment Indicators on the Dome

The alignment indicators are four simple rectangular archways, like the Stonehenge “trilithons,” projected or fabricated of cardboard. They are placed so that the planetarium’s Sun will rise and set in these openings on the solstices, the longest and shortest days of the year, at the latitude of Stonehenge, 51 degrees north. The solstices are usually June 21 and December 21, although variations in our calendar with respect to the actual motion of the Earth around the Sun vary the actual dates by a day or so from year to year.

Figure 1:

Version I: A Cardboard-drum Silhouette Projector

See Figure 1. A drum-shaped ice cream container, from your local ice cream shop, makes a fine structure for this projector, but any cylinder will do. You can cut the cylinder into two parts, along the axis, so that each half can project over half of the dome without the main star projector getting in the way.

The cutout Stonehenge outline does not have to be precise except in one feature: the larger archway openings must line up with the extreme northern and southern risings and settings of the Sun in your planetarium when set for the latitude of Stonehenge. The outline you will project cannot be a straightforward representation of Stonehenge in any event, since Stonehenge consists of several concentric rings of stones. Thus this projector is not intended to show a literal Stonehenge horizon, but rather to represent four of the key alignments which are discussed in the program.

You’ll need to experiment with your cutouts so that the archway openings are in the correct horizon positions, and then mount the projector securely so that the alignments are not changed by mistake. A check immediately before the program is advised.

A simple silhouette/shadow projector like this produces a very impressive result. Even the unevenness in brightness of the projected shapes serves to good effect, appearing to represent the worn, pitted surfaces of the stones.

Version II: A Photocopy Mini-silhouette Projector

Figure 2a:

This is the same basic idea as the cardboard drum, but in a miniature, one unit version (Figure 2a), particularly suited for a portable planetarium.

There are many ways to do this, but one simple way is essentially the versatile “mini-brute-force” horizon projector described in Interact PASS Astronomy of the Americas, adapted to project the Stonehenge alignments. Assemble the electrical parts as shown in Fig. 2a using a #605 light bulb, a Mini-mag lite® flashlight bulb, a STARLAB main star bulb, or other suitable light bulb with a very small filament as the light source. (The variable resistor is optional.) Photocopy the Stonehenge Mask Assembly onto a transparency and form the Stonehenge Mask Assembly as follows:

a. Cut out the six parts of the mask assembly: the Top, the Window Cylinder piece, and the four individual Trilithon Masks. Cut in slits (indicated by the word “cut”). Optional: with a hobby knife or small sharp scissors, cut out the four windows in the Window Cylinder piece. This will make the projected trilithon images brighter and clearer.

Figure 2b:

b. Fold the “tape” and “handle” tabs as shown in Fig. 2b. There are four on the Window Cylinder, and two on each of the Trilithon Masks. Also fold where indicated on the Top piece.

Figure 2c:

c. Roll the Window Cylinder piece into a cylinder so that the N, E, S, W, marks read backwards as seen from the outside (frontwards as seen from the viewpoint of the lightbulb; Fig.2c). Tape the seam without covering any windows or letters.

Figure 2d:

d. Position the Window Cylinder around the light bulb so that the light bulb is centered in the Cylinder. Tape the Cylinder tabs securely to the projector box top. (Fig. 2d.)

Figure 2e:

e. Lightly tape Trilithon Masks in front of the four clear windows of the Window Cylinder. Put a rubber band around the tops of them. (Fig. 2e.) The top tabs of the Trilithon Masks facilitate adjusting their positions during set-up for the Stonehenge program.

In setting up for the program, the Trilithon Masks need to be lightly taped in place, carefully adjusted so that they accurately mark the solstice sunrises and sunsets, and then taped more securely in place. This works fine, but you must be careful to check the position of the projection masks just before the program to see that all four archways show up in the correct position on the horizon. Moving the masks just a millimeter can throw your alignments off so that the Sun fails to appear in the proper archways.

Version III: A Regular Planetarium Horizon Panorama Projection System Showing the Alignment Archways

Figure 3a:

If your planetarium has a horizon projection system, and you can prepare slides for it, you can prepare simple artwork (like Figures 3a and 3b) for four archways and adjust the alignment positions as discussed above.

Figure 3b:

Version IV: Full-size Cardboard Archways to be Attached Directly to the Dome with Tape, Velcro® or Paper Clips (if you have a perforated dome)

Figure 4:

A low-tech alternative to the projectors above is to use four large cardboard pieces (Figure 4) attached to the cove. The only major disadvantage to this quick solution is that these alignments cannot be turned on and off with the flip of a switch, so they need to be installed during the program to create the dramatic moment when you compare horizon events discovered by your students with Hawkins’ Stonehenge alignments. The installation can be done if you have prepared in advance unobtrusive markers (like pieces of tape or thumbtacks) installed on the bottom of the cove. Students can use those markers to position the cardboard archways quickly.

Presenter's Script:

We can give our planetarium the same information that Dr. Hawkins gave his computer. Right now the stars, Sun, Moon, (and planets, if you are using them) are positioned as they would be on today’s date, but 3550 years ago, the time when the giant trilithons were erected.

The first stages of Stonehenge were built much earlier, 4800 years ago, in 2800 BC. While the stars change their rising and setting positions substantially, the Sun and Moon positions change only a little.

And here we have four of the major, repeated alignments as determined by Hawkins.

Turn on Stonehenge alignment indicator or install Stonehenge alignment arches.

Do any of these alignments match the sunrise and sunset we marked earlier? [No.]

Leave those markers in place. Turn off Stonehenge alignment indicator, or remove alignment arches.

At this point, Professor Hawkins made some more guesses, hypotheses, about what astronomical events might be marked by these special directions, and then tested them on his computer. We are going to do precisely the same thing here in the planetarium. We are going to experience a night at Stonehenge, 3550 years ago. I can speed up the rotation of the Earth, and make the night seem to go by in about three minutes.

You can each decide to mark the risings or settings of bright stars, planets, Sun, Moon, star groups, or whatever else you happen to see on the horizon where you are standing.

We are ready to begin. Remember what object you marked when you finish, and leave your marker in place when you sit down.

Turn on diurnal motion, slowly enough so that students will be able to mark risings and settings of their choice. Turn down daylight and fade in music. As you go through the night, ask what objects students are tracking. Some students will be watching bright stars in the north, such as the Big Dipper. When their stars fail to set, and start going up again, point out the existence of these “circumpolar” stars, stars which never go below the horizon.

As the Sun starts to rise, turn on sunrise and gradually, full daylight. Stop at mid-morning, fading out music and sunrise.

Good morning! We have lots of risings and settings marked, and now we can see which, if any, agree with the directions marked by Stonehenge.

Ask several students to describe what they marked, how it moved through the sky, and to point out their markers.

Turn on Stonehenge projector and reduce daylight.

Have we solved the mystery of Stonehenge?

As you might imagine, Professor Hawkins was also disappointed when his computer gave him this same result. There were one or two stars which agreed with one or two alignments, but that could have been just coincidence. To succeed, he needed something to work consistently with all his alignments.

Turn off Stonehenge alignment indicator (or remove arches).