Neolithic astronomy - summer solstice - observatories
What it shows:
1:50 scale model of the Stonehenge site with the positions of Sun and Moon on important dates marked. It can be used with a light show to reproduce Sunrise on Midsummer's morning, June 21.
How it works:
The Stonehenge site consists of the sarsen circle of 30 megaliths capped with 30 lintels. Within this circle is a horseshoe pattern of five trilithons. 80m north-east of the circle's center is the Heel Stone; it is the alignment of this stone with the Sun that is relevant on Mid-summer's morning. The 1:50 scale model (1m = 2cm) is made of wood on a 90×90cm plywood base. The heel stone, 160cm from the center of the circle, sits on a separate 120×120cm base for ease of mobility. Figure 1 shows the layout of the Stonehenge site (accurate ground plans in references 1, 2); Figure 2 shows model measurements of the sarsen stones, trilithons and Heel Stone.
figure 1. Sarsen circle and trilithon horseshoe
figure 1 Labels:
1. Midsummer sunrise
2. Winter moonrise low point
3. Midwinter Sunrise
4. Southern moonrise (minimum)
5. Southern moonrise (maximum)
6. Midwinter sunset
7. Northern moonset (minimum)
8. Northern moonset (maximum)
9. Midsummer sunset
10. Winter moonrise high point
figure 2. Model stone dimensions
A slide projector on a mobile tripod (see details in Setting it up) can simulate the positions of the Sun at various times of the year, and especially at the Summer solstice. On the morning of June 21st, the Sun rises up behind the Heel Stone, casting a shadow through the sarsen circle to the center of the trilithon horseshoe. The projector bulb is fitted with a dimmer, so that the Sunrise begins a dull deep red, getting brighter as is rises into the morning sky.
Setting it up:
The slide projector is loaded with a masking slide with a half inch circular hole and mounted on a heavy wheeled tripod with elevating head. The bulb dimmer is a silicon controlled rectifier, a commercially available household dimmer switch that plugs straight into the projector. These are prone to blowing and should be tested before each show. The Stonehenge model itself sits on a cart, with the slide projector operating 2-3m distant. Owen Gingerich, creator and Master of Ceremonies, accompanies the midsummer event with Also Sprach Zarathustra by Richard Strauss.
The model represents the complete structure of Stonehenge III, constructed around 2100 B.C. Further details of the site, including the older Stonehenges, are available in the references. Due to precession, the Sun now rises slightly to the left of the Heel Stone, but due to its leaning, rises to pass over its tip. Wheeling the tripod, adjusting projector height and controlling the dimmer is a three handed operation; some practice is called for. Musical accompaniment to the dramatic sunrise has traditionally been Strauss' Also Sprach Zarathrustra, although Spinal Tap's Stonehenge would be a fitting alternative. Rating ****
1. Hawkes, J., Sci. Am. June 1953, pp.25-31
2. Brecher, K., Feirtag, M., Astronomy of the Ancients (MIT Press, 1979) pp.117-132
3. Daniel, G., Sci. Am. July 1980, pp.78-90
4. Heggie, D. C., Megalithic Science (Thames & Hudson, 1981) pp.195-206
5. Krupp, E. C., Echoes of Ancient Skies (Harper & Row, 1983) pp.214-223