Interesting statement from "What is a Medicine Wheel?" at the Royal Alberta Museum:
"It is very difficult to confirm the astronomical hypothesis, and it is no longer as popular as it was a decade ago. A number of astronomers such as Steven Haak in Nebraska and David Vogt in Vancouver have critically evaluated the idea and have expressed severe reservations about the hypothesis. They note that simple familiarity with the night sky would likely produce an adequate estimate for timing ceremonies. Further, if great accuracy had been desired, it could have been attained better by using narrow poles as foresight and backsight than by using wider rock cairns. "
I would argue that a calendar can provide a go/no go decision, needed by a society for one reason or another. But otherwise it is a valid criticism: how can we want accuracy with such crude structures? Maybe because shadows are crude, or maybe because narrow poles would have to be erected, using something like a "wider rock cairn" to hold the pole and act as a base. Or better still: because the date only needs to be defined up to a unit that requires less precision, like a day rather than a second.