Stonehenge
Already in the 18th century the British antiquarian William Stukeley had noticed that the horseshoe of great trilithons and the horseshoe of 19 bluestones at Stonehenge opened up in the direction of the midsummer sunrise. It was quickly surmised that the monument must have been deliberately oriented and planned so that on midsummer's morning the sun rose directly over the "Heel Stone" and the first rays shone into the centre of the monument between the open arms of the horseshoe arrangement.
This discovery has had tremendous impact on how Stonehenge has been interpreted. For Stukeley in the 18th century and Sir Norman Lockyer in the first years of the 20th century, this alignment implied a ritualistic connection with sun worship and it was generally concluded that Stonehenge was constructed as a temple to the sun. More recently, though, the astronomer Gerald Hawkins has argued that Stonehenge is not merely aligned with solar and lunar astronomical events, but can be used to predict other events such as eclipses. In other words, Stonehenge was more than a temple, it was an astronomical calculator. The alignment also made it clear that whoever built Stonehenge had precise astronomical knowledge of the path of the sun and, moreover, must have known before construction began precisely where the sun rose at dawn on midsummer's morning while standing on the future site of the monument. This point needs to be made because, as I suspect, with Stonehenge and many other such monuments, it was the site, a particular place within the landscape, that was important; only later were these sites marked in some more permanent manner by the digging of ditches and banks and (or instead) the erection of wood or stone structures.
For reasons we shall never know, this particular spot in the landscape was so important that not only were ditches and banks dug and, later, stone circles and horseshoe arrangements const