Archaeo-astronomy at the Winter Solstice

Nature and ScienceFamily and KidsAdventure and Outdoors

17 Dec

sunset at Slieve Gullion passage tomb — *Sunset at Slieve Gullion passage tomb. Image © Padraig O'Cumasigh_*

Solstice means to ‘stand still’ and comes from the Latin ‘sol’ for sun and ‘sistere’ to stand still. The winter solstice is the time of year when the sun reaches its lowest point in the sky, appearing to stop before turning towards the north again. This is reflected in the Irish word for solstice ‘grianstad’ from ‘grian’ meaning sun and ‘stad’ to stop, and in the Welsh term ‘heuldro’ coming from ‘haul’ for sun and ‘dro’ from to turn.

The solstice and the equinox fall either on the 21 or 22 of the month in question. In Ireland, on the winter solstice, which is the shortest day of the year (21 December in 2021 and 2022, 22 December in 2023), the sun rises in the south-east at about 8.40 and sets in the south-west around 4.10 p.m. On the summer solstice, the longest day (21/22 June), the sun rises towards the north-east about 5 a.m. and sets towards the north-west around 10 p.m. On the equinoxes (21/22 March and September), the hours of night and day are equal with the sun rising in the east and setting in the west. The cross-quarter days fall roughly midpoint between the equinox and the solstice and mark the start of Spring at the start of February (Imbolc, St. Brigid's Day), Summer on 1 May (Bealtaine), Autumn on 1 August (Lughnasadh) and Winter in early November (Samhain).

The winter and summer solstices as well as the equinoxes and cross-quarter days were significant in prehistory. The archaeological site which most convincingly demonstrated an astronomical alignment, in a feat of engineering executed before the building of the pyramids at Giza, is Newgrange passage tomb. The rays of the midwinter rising sun enter the lightbox above the entrance passage at Newgrange to illuminate the carving of the triple spiral in the rear chamber.

Newgrange is not the only passage tomb to have an astronomical alignment but rather one of many. Knockroe passage tomb in Co. Kilkenny is also aligned on the winter solstice sunrise, Slieve Gullion passage tomb in Armagh aligned on the winter solstice sunset (see above image by Pádraig O’Cumasaigh). The entrance to the passage tomb at Ballycarty some 5km south-east of Tralee opens to the west, the passage possibly designed to catch the rays of the setting sun from the west during the equinox. Ballycarty is the only passage tomb to be recorded in Kerry but more tombs may remain undiscovered under the many cairns found throughout the county. A heritage guide published by Archaeology Ireland and written by Frank Prendergast includes a listing of 15 known astronomically aligned passage tombs in Ireland’s 32 counties (see below). In Wales, the Bryn Celli Ddu chambered tomb on the island of Anglesey is aligned with the rising sun on the Summer Solstice.

stone row alignments in Kerry — *Stone row alignments in Kerry*

In Ireland, monument classes other than passage tombs also have astronomical associations. Some are solar and others thought to be lunar, alignments with constellations also probable. The carvings on some rock art panels are best highlighted at certain times, particularly during the winter solstice and equinoxes. The rock art shown in the image below was only partially highlighted by the setting sun last May – a shadow cast by one of the stepped sections prevents a portion of the carvings from being highlighted. In fact, it is not possible to discern any rock art in the section under shadow. However, the whole of the carved upper surface of this rock art panel is highlighted at sunset only at the winter solstice. This may have been a consideration when, over four thousand years ago, the people who made the rock art decided to select this particular rock over many others for carving.

The long axis of many Standing Stones, Stone Pairs and Stone Rows is aligned NE/SW. The alignments of the Stone Rows of Kerry are illustrated below. The consistency in the alignments indicates that they are not predicated by the local topography but by something else. Could it be significant that the sun rises in the northeast on the longest day of the year and sets in the southwest on the shortest day, both the winter and summer solstices possibly simultaneously referenced in these monuments?

‘Five-stone’ Stone Circles - a distinctive form of stone circle thought to date to the Bronze Age (ca. 2400-500 BC) are found only in counties Cork and Kerry. The axial stone of both five-stone and multiple-stone Stone Circles is set consistently in the south-western part of the circle directly opposite two stones, usually the tallest, which mark the entrance to the circle.

You can also see any astronomical alignments on the days either side of the dates which are astronomically significant - useful to bear in mind especially with the Irish weather or if you’re planning on visiting more than one site!

rock art in shadow — *Rock art in shadow - this is all fully illuminated at sunset on the winter solstice - Photo: Aoibheann Lambe*

The December 2021 Dark Sky talk hosted by the LIVE Project includes a discussion of archaeo-astronomical associations and can be viewed on the project YouTube channel .

This winter solstice sunrise at Newgrange is live-streamed every year and can be viewed using the links on the social media pages of the Office of Public Works (https://www.facebook.com/opwireland).