Banner: Knocknarea at Sunset.
Roofbxo, Cairn G, Carrowkeel.
The dramatic entry of the sun into Cairn G near the summer solstice. The distant hill of Doomore can be seen through the roofbox. This picture was taken in 1997.

Cairn G: astronomical observations

The entrance of Cairn G incorporates a structure known as a roofbox. The only other example currently known in Ireland is at Newgrange which is aligned to the winter solstice sunrise. Another was found in recent years Crantit on the Orkney Islands, where there are about 80 chambered cairns.

The roofbox at Cairn G in Carrowkeel.
The roofbox at Cairn G in Carrowkeel.

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 at midsummer.

However, there are also several 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 chamber.

Sunset through the roofbox.
Sunset through the roofbox. This picture is about 2 weeks before midsummer. Doomore, the solstice marker, is to the right.

This means that the sun can shine into the chamber for a much longer period of time than Newgrange - approximately a month on either side of midsummer, though it only spends about two weeks in the rear of the chamber.

Also, even though the sun has been displaced by 1.5 degrees (three solar diameters) to the west or left, it is impossible for the midsummer sunset to have shone into the end recess of Cairn G.

Sunset about one week from midsummer viewed from Carrowkeel.
Sunset about one week from midsummer viewed from Carrowkeel.

But what about the moon, which often gets overlooked in preferance to the sun, as at Newgrange. The full moons rise and set approximately 180 degrees 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 in to thick banks of cloud over the Ox Mountains, too faint and obscure to photograph.

I would imagine that after observing the moonset, the ancient skywatchers would have stood on top of Cairn K to watch the winter solstice sun rising over Doonaveeragh and Lough Key. Ritual astronomy undoubtedly played a major role in the design and location of these monuments. The small stone and chalk balls were may have been used as teaching aids, held up in the roofbox to show solar and lunar positions on the horizon to students or initiates. Some of the finds from Carrowkeel are in display in the National Museum in Dublin.

        on the summer solstice enters Cairn G through the roofbox. 
        The sun can shine into this monument for about 6 weeks on either side 
of midsummer.
Sunset on the summer solstice enters Cairn G through the roofbox. The sun can shine into this monument for about 6 weeks on either side of midsummer.

The main axis of the cairn and chamber is aligned to the left edge of Knocknarea with the Great Cairn on its summit. This is a similar axis to Cairns B, E and 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 of how Irish monuments relate to the moon.

Plan of Cairn G.
Plan of Cairn G.

The lunar standstill is difficult to predict precisely and a distant foresight, usually a mountain was preferred. Knocknarea, which could 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. So, 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.

            solstice sunbeam within the chamber of Cairn G.
Summer solstice sunbeam within the chamber of Cairn G.

Knocknarea is located on the western end of the Cuil Irra peninsula, which could possibly be translated as the 'Remote Angle of the Moon'. 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 was also used as a foresight from Moytura which is situated east of Carrowkeel across Lough Arrow.

Summer solstice sunset from the neolithic cairn called Shee Lugh on Moytura across Lough Arrow to the east.

The summer solstice sun drops behind Knocknarea when viewed from Shee Lugh, the cairn on highest point, which could be called 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 the Irish Passage Cairns.

The main group of cairns at Carrowkeel.
The main group of cairns at Carrowkeel. Cairn G with it's roofbox is in the foreground, Cairn H is to the right, and Cairn K is on the summit in the distance. At this time, about 1995 the entrance to the chamber was blocked.