The Great Cairn of Knocknarea is the focal point of the extensive network of ancient sites in Sligo. It is visible from almost every ancient monument in the region, and some, such as Cairns E and K at Carrowkeel are directly oriented on it. Several astronomical alignments can be identified at Queen Maeve's Cairn, even though the passage and chamber are hidden from view. My own guess would be that there is an east and a west facing passage within the cairn, and that it is aligned to the equinoxes similar to Knowth in the Boyne Valley.
Two stones, a large boulder and a flat slab mark a north/south line through the cairn; another great boulder lies 0.5 km north of the site. Several of the smaller megaliths - two ruined monuments and a hut site also lie on this cardinal line. This linear arrangement of monuments has been compared by some archaeologists, such as Bergh, to the layout at Newgrange.
When you stand on top of the Cairn on Knocknarea a vast panoramic vista opens before you. From this platform, at sunrise on the equinoxes, the sun comes up over Lough Gill in the east, whos name means 'The Lake of Brightness' in Irish. At sunset on the equinoxes, the observer standing on the western cairn at Carns Hill, just south of Sligo Town, can watch the sun setting over Knocknarea, as shown above. The sun does not set directly behind the cairn on the equinoxes, but the ruined sites arranged in a north/south line would have created a series of bumps and notches which could be used for surveying the sunsets/moonsets. This certainly hints at a kind of ritual astronomy associated with these monuments. This will hold true of any full moon near the equinoxes: they will rise over Lough Gill and set behind Knocknarea.
From Queen Maeve's Cairn, the winter solstice the sun rises in the Lough Arrow region over the legendary ridge of Moytura with it's Cairn, Shee Lugh. It is possible that the alignment is to the Hill of Sheemor, which lies beyond Shee Lugh and is visible from Knocknarea on a good clear day, peeping out from the edge of the Arigna Mountains.
Of course, the reverse holds true, and the summer solstice sun sets behind Knocknarea when viewed from Shee Lugh, as illustrated in the photo above. This is quite a spectacular landscape alignment - the sun dropping behind the cliffs at Strandhill on the left side of Knocknarea. The sun no longer drops behind the cairn - the obliquity of the ecliptic, the wobble as the Earth spins through space - has offset the alignment by 1.5 degrees, or three solar diameters.
The lunar standstill is probably one of the most important Knocknarea alignments. The Moon's cycle takes 18.6 years to complete as it moves from it's most northerly to it's most southerly positions. At the southern summer rising position when viewed from Queen Maeve's Cairn, the Moon rises over Lough Arrow and crosses the Carrowkeel sites in the Bricklieve Mountains, to set behind the cairn topped hill of Knocknashee. This is the shortest time that the moon spends in the sky during its cycle. A similar alignment is known at the neolithic circle at Callinish on the Isle of Lewis.
At the other end of the cycle, near the winter solstice when the Moon drops into the sea in the region behind Croughan, it will illuminate the chambers of four of the Carrowkeel Cairns.
The Great Cairn on Knocknarea is the most important piece of hardware remaining from the Irish neolithic in terms of its perfect horizon and observation platform. The cairn is pretty much in its original shape and form, minus its sheath of white quartz. The flat summit of the cairn has a mighty view of the surrounding horizon and is a perfect dialling location; by dialling I mean observing the basic clock of the Planet, the movements of the sun and moon as they work their way across the horizon. Knocknarea has a wonderful diaroma with big views in all directions and many important sightlines delineated by other sites and monuments.