The Barnhouse Stone, Stenness
The Barnhouse Stone is a solitary monolith, approximately half a mile (700m) to the south-east of the Stones o' Stenness.
Standing in a field near Maeshowe, the Barnhouse Stone is clearly visible from the main Kirkwall to Stromness road.
On first glance, the lichen-covered stone looks fairly insignificant - especially when compared to the Stenness giants a short distance away, to the north-west.
Appearances, however, can be deceptive.
The Barnhouse Stone is intriguing because it appears to be perfectly aligned to the entrance of the Maeshowe chambered cairn, approximately 700 metres to the north-east.
Local man Magnus Spence first recorded this alignment in 1893.
Then, in 1952, ex-provost Peter C. Flett OBE noted that:
"The alignment formed with this Barnhouse Stone and the long passage of Maeshowe seemed to have peculiar significance, and was too remarkable to be merely accidental."
Mr Flett also suggested an alignment between the Barnhouse Stone, the Watchstone and the Ring o' Brodgar.
He remarked that the two standing stones formed a straight line with the centre of the Brodgar ring, in a north-easterly, south-westerly direction.
This line, he suggested, pointed to the rising position of the sun at Beltane - May 1.
While the Beltane link is open to question, a definite alignment exists between the Barnhouse Stone and Maeshowe.
Standing just over three metres tall (10 ft), at the midwinter solstice, when the last rays of the setting sun shine through Maeshowe's entrance, the sun is directly over the top of the Barnhouse monolith.
The centre axis of Maeshowe's inner entrance passage is directly aligned with the centre of the Barnhouse Stone.
From Maeshowe, the line travels out to strike Hoy's Ward Hill at a place where the sun sets 22 days before and after the midwinter solstice.
This three-week period is referred to by archeoastronomers, such as Alexander Thom, as a megalithic month - a sixteenth of a year.
Whether this alignment meant that the stone was put in place at the same time Maeshowe was built or was a later addition, erected to mark the alignment, is not clear.
The discovery of a socket hole to the rear of Maeshowe, that would once had housed another stone, seems to indicate that the Barnhouse Stone did have some function in the ritual use of the chambered cairn.
What makes this phenomenon interesting is the fact that if, as has been suggested, the Barnhouse Stone was an outlier to the Stenness ring and a part of the ceremonial complex, we have a definite link between the ceremonies held at the stone rings and in Maeshowe.