Above the entrance of Cairn G is an opening or slot which seems to be an early version of a roofbox. The only other example currently known in Ireland is at
Newgrange in the Boyne Valley, which is famously aligned to the winter
solstice sunrise. Another somewhat unconvincing example was found at Crantit on the
Orkney Islands, where there are about 80 neolithic chambered cairns, several of which have proven astronomical alignments.
The Carrowkeel roofbox has several features in common with the one at Newgrange.
It is located over the entrance to the chamber; a stone door
is present; and the roofbox admits a beam of light from the sun
at sunset over several weeks on either side of midsummer.
However, there are also some features about Cairn G which are different
to Newgrange. The passage is much shorter than Newgrange, only
about two meters long, and so the roof box opens directly into
The short passage means that the sun can shine into the chamber
for a much longer period of time than Newgrange. The sun enters the chamber of Cairn G for approximately
a month before and after midsummer, though it only spends about
two weeks in the rear of the chamber.
Another factor to consider is that even though the sun has been displaced by 1.5° (three solar diameters) to
the west or left, it is impossible that the midsummer sunset could ever have shone
into the end recess of Cairn G. It currently shines into the left resess of the cruciform chamber.
But what about the setting moon, which always gets overlooked in preference to the easier to moniter
sun. At Newgrange virtually no research has been conducted into the rising moon at midsummer, which undoubtedly illuminate the chamber at specific points during the lunar cycle. Each month the full moons rises and sets approximately 180° opposite the sun's rising and setting positions.
The setting full moons on either side of the winter solstice will illuminate the chamber
of Cairn G in much the same fashion as the sun does each summer. I have been in Cairn G for several cold midwinter moonsets, but the moon has set into thick banks of
cloud over the Ox Mountains, too faint and obscure to capture in a photograph back in the late 1990's.
Some form of religious and ritual astronomy undoubtedly played a major role in the design and
location of the Irish passage-graves. Stone and chalk
balls are a kind of neolithic artifact commonly found in the chambers of these monuments. Quite possibly they may have been used as teaching or recording aids, placed in the roofbox to show
solar and lunar positions on the horizon. Some
of the examples found in Carrowkeel are on display in the National
Museum in Dublin.
The main axis of Cairn G is aligned to the left edge of Knocknarea mountain where Queen Maeve's cairn is clearly on the summit. This is a similar orientation to Cairn B, Cairn E and Cairn K, and is approximately the position of the northern extreme setting midwinter full moon.
The lunar standstill occurs every 18.6 years, when the moon reaches its maximum swing away
from the ecliptic. While plenty of research has been done on
lunar extremes at Stonehenge and Caillinish, relatively little
is known about how Irish monuments relate to the moon.
The lunar standstill is difficult to predict precisely and a distant foresight, usually a mountain was preferred for making observations. Knocknarea, which can be translated as the Hill of the Moon is the main foresight
for Carrowkeel, but it is too far to the north for the lunar standstill to reach.
The best candidate is the small cairn on Croghaun in the Ox mountains, which is visible through the roofbox from the back of the chamber. When viewed from within Cairn G, the cairn on Doomore marks the midsummer and Croghaun the lunar extreme or 'lunstice' setting positions. It has been suggested that the roofbox at Cairn G was used for predicting eclipses.
Knocknarea is located on the western end of the Cuil Iorra peninsula, which could possibly be translated as the Remote Angle. The lunar standstill was undoubtedly a great ritual event in a calendar of astronomical observations which were made by our neolithic ancestors. It defines an
axis between the central ridge of Carrowkeel and Knocknarea.
Knocknarea mountain was also used as a foresight from Moytura which is situated east of Carrowkeel across Lough Arrow.
The summer solstice sun drops behind Knocknarea when viewed from
Shee Lugh, the cairn on highest point of the ridge of Moytura, which waste be known as the Seat of the Sun God of the Túatha Dé Danann.
This cairn is where Lugh Lamh Fada, the young champion and grandson
of Balor of the Evil Eye, sat during the mythical
Battle of Maigh Tuireadh.
The physical astronomical features of the Sligo sites, coupled with their mythological associations opens the way for a broader interpretation of the meaning of these monuments.