Stukeley and the renowned astronomer Edmund Halley attempted what amounted to the first scientific attempt to date a prehistoric monument. Stukeley concluded the Stonehenge had been set up “by the use of a magnetic compass to lay out the works, the needle varying so much, at that time, from true north.” He attempted to calculate the change in magnetic variation between the observed and theoretical (ideal) Stonehenge sunrise, which he imagined would relate to the date of construction. Their calculations returned three dates, the earliest of which, 460 BC, was accepted by Stukeley. That was incorrect, but this early exercise in dating is a landmark in field archaeology.