If Stonehenge is a giant calendar, how did the ancient Britons measure the length of days precisely enough to put the Heel Stone in just the right place to mark the solstices?
Eric Kvaalen
Les Essarts-le-Roi, France
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They didn鈥檛 need to measure the length of the day. The solstices are the days when the sun rises the furthest north and the furthest south (in June and December respectively in the northern hemisphere). So they would simply observe where the furthest north or south sunrise happened.
Hillary Shaw
Newport, Shropshire, UK
This doesn鈥檛 require clocks, or any idea of what the sun is or why it appears to move. Farmers needed to know the day and the season to know when to plant, rather than hours or minutes. They would have observed that the sun (and moon) rose and set in different places each day, reaching different elevations. They could have then tracked and measured the minimum length of the sun鈥檚 shadow, cast by a fixed marker, and against what horizon landmarks it rose and set, from a fixed observation point. Members of the leisured priest-astronomer class would have measured and counted the days.
Over time, they would have seen, even with some observations missing due to clouds and some minor perturbations due to variable atmospheric refraction, a basic 182/183-day rhythm of sunset drifting east then west, plus the minimum-length shadow getting longer and shorter.
Cleverer priests would have noted that the moon was sometimes higher than the sun, sometimes lower, and it sometimes crossed the sun鈥檚 sky-path. Over time, it would have been possible to work out when a solar or lunar eclipse was due. Foretelling these events to uninformed peasant farmers would have proved their exclusive power of communing with the gods.
David Bortin
Whittier, California, US
The consensus seems to be that the most likely way of determining the solstices (literally 鈥渟un stands still鈥�) and equinoxes (鈥渆qual night鈥�) would have been to wedge a stick in the ground and then, every day for a year, place a pebble at the end of the stick鈥檚 shadow.
One problem is that it is hard to keep pebbles on the ground from being accidentally kicked or otherwise moved about. Solution: replace them with .
Nath Rao
Via email
I am reminded of a of how the Kalash Indigenous people of Pakistan determine the date of their winter solstice festival. The people in charge watch the sunrise each day, and when the point of sunrise is at a mark they have determined before, they declare the festival to be on.
David Perryman
Via Facebook
It would have been pretty obvious where to put the Heel Stone if they had been paying attention.
鈥淥i, Aglain, what鈥檚 the furthest right you said you saw the sun come up?鈥�
鈥淭hat tree over there, Terry.鈥�
鈥淕o over there a bit then and I鈥檒l tell you when you鈥檙e in front of it. That鈥檚 where we鈥檒l put that rock.鈥�
What is more interesting is why we think it is amazing. Just because we are distracted from marking where on the horizon the sun comes up, it doesn鈥檛 mean ancient people were.
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