Tracking the Migration of Sunset

[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. See Stonehenge Alignment Indicators on the Dome in Risings and Settings.

Presenter's Script:

Well, there we are. We know what doesn’t work. It may be discouraging, but scientists who have an idea usually keep trying until they have exhausted every possibility.
We have considered alignments on today’s date, but 3550 years ago.

If you have time, you can call for other suggestions and try them now, like the Moon in different phases, or the planets.

Would changing the date by a week or a month make any difference in where things rise or set?
Let’s find out if the Sun will set in a different place on a different day from today.
Which marker shows where it set last night? Which shows where it rose today?
Ask students to remove all other markers.
Now let’s try next week. ("Next month" if you have a projector like STARLAB which permits only monthly adjustments.)
I will run quickly through the day, but as I do I will also advance the date to one week (one month) from today.
Keeping daylight up, turn off the Sun and stars and advance annual motion in positive direction for however long it takes for your instrument to advance one week (this is done manually, by moving a plug from one hole to another, on a STARLAB). Turn on daily motion, then turn the Sun on again. During this operation, you can take a vote as follows:
Let’s take a vote. How many think the Sun will set in the same place next week as it did today? How many think it will set farther to the south? To the north?
Count and summarize vote. Stop diurnal motion when the Sun is at mid-afternoon, one week (one month) later.
We are close to sunset. Will someone please mark the position of the Sun when it sets? The date is now one week (one month) later.
Run diurnal motion until the Sun sets.
Who made the best prediction?
Note the direction the sunset position moved, north or south, on today’s date.
Now let’s see if that sunset is marked by Stonehenge.
Turn on, then off, Stonehenge indicator.
Too bad—we still don’t have it. Of course, the day we just tried is just (repeat date you used).
Is there anything special about that date? Can anyone suggest a date that is special in some astronomical way every year?
Get list of dates and repeat at the end. Included will generally be the holidays, birthdays, and shortest and longest days of the year. The latter two will generally be on the list, but include these yourself if nobody else does.
Which date from our list shall we try?
If time permits, try as many dates as possible, in the order they will come up as you advance annual motion (or uncover the Sun hole on a STARLAB). Let visitors mark sunrise and sunset positions. Point out the migration of the positions to the north or the south, depending on the season in which you started.
Between each date, turn the Sun off and stars off, and advance annual motion until you are at the next date you wish to try. Run daily motion until you are just before sunrise before turning the sun on.
Eventually you arrive at the next solstice. Adjust the order of seasons below according to your particular starting dates. The following assumes you started between June 21, and December, and are approaching winter. If you started January to May, make appropriate changes.

Happy holidays. Here we are at December 21, just before sunrise.
Please remove all the markers, and we will start fresh by marking where the Sun rises on this date, the morning before the shortest day of the year.
Today is the day called the “winter solstice” and it varies slightly on the calendar but always occurs within a day of December 21.
Turn diurnal motion on until sunrise (Sun’s disk halfway up), then stop.
Will whoever is closest please place your marker to indicate the position of sunrise?
Notice how far to the south the Sun is rising. Indeed, this is the farthest south the Sun will ever rise as seen from this place on Earth. From today on, the Sun will begin rising back toward the north, farther every day.
Start diurnal motion again, and continue through the day, stopping at sunset. Use daylight, sunrise and sunset lights as desired.
You may have noticed that the sun did not get very high in the sky at any time today. It is also up for a much shorter time than it has been on other days. Indeed, this is the most southerly rising, most southerly path, briefest journey of the Sun all year. And that is why this is officially the start of winter! The northern hemisphere receives less energy from the Sun today than it will any other day of the year.
It is interesting to note all the things that happen around December 21. Christmas occurs around then, and so do Chanukah and New Year’s Day, and numerous other religious and secular special days. Apparently many peoples have noted the extreme position of the Sun, the brief hours of sunlight on this day. Perhaps they wished to urge the Sun to come back north, eventually bringing longer days and warmer weather. By the way, our calendar does not track the Sun perfectly so some years the shortest day happens to be December 20 or December 22.
It is now sunset on the winter solstice, the shortest day of the year. Let’s mark the position of sunrise on this special day.
Will whoever is closest please place your marker to indicate the spot?
Shall we test our most recent hypothesis, that sunrise and sunset on astronomically significant days might be marked by Stonehenge’s alignments?
Turn on Stonehenge indicator and observe alignment.
Congratulations! Now we have learned that these stones could be used to mark the shortest day of the year by indicating the positions of sunrise and sunset on that one day.
Now that we have found the significance of two of the special alignments which Gerald Hawkins identified, would anyone like to suggest what we should do to find the other two major alignments? Any other dates on our list you want to try?
A good technique is to wait for several suggestions and vote to decide what to do. Since the longest day is now the most logical candidate, it is very rare that this will not work.
Turn off Stonehenge indicator. Turn on the sun, and adjust the planetarium to pre-dawn on June 21.
Begin daily motion to show sunrise on June 21.

We have now arrived on June 21, the longest day of the year.
Will whoever is closest please mark the point when the Sun is just half way up?
After marking, turn on diurnal and continue to sunset. During the minute or so necessary, you can invite questions or summarize proceedings so far.
This is the first day of summer, the longest day of the year. In contrast to December 21, notice how far north the Sun rose, how high in the sky it gets, and how long the path will be, giving us a long, hot day.