We were winding up the mountain road when the disc of the sun broke on the horizon. We felt as if we were ten minutes late for an appointment made over 5,000 years ago. From the top of the road there would be a climb on foot to the mound perched on the summit of the mountain. The lock on the modern door leading to the passage had frozen during the night, and as we struggled with it the rising sun was already above the horizon. When we drew back the door a narrow chink of light streamed down the passage and flashed into the end recess of the chamber.
Martin Brennan, The Stones of Time
Outside Cairn T on equinox morning, taken from Cairn V.
American researcher Martin Brennan has made great advances in the understanding of the role of astronomical orientations of chambered cairns. During his work in Ireland in the eighties he rediscovered two major solar alignments at Loughcrew. His discoveries are detailed in his inspiring book, The Stones of Time. Brennan and his co-researcher Jack Roberts observed that the passage and chamber of Cairn T are oriented to the equinox sunrises.
Engraved stone in the passageway of Cairn T on an equinox morning, as the sun is streaming into the chamber. The art comes to life in the sunlight.
In general, on the equinox the sun will rise due east all over the world. On the horizon the sun is moving at its fastest ove the equinox, as opposed to the solstices when it slows to a standstill. Over the equinoxes the sun wheels around the sky, twelve hours of light and twelve hours of darkness. Full moons on equinoxes around the hills of Loughcrew are especially magical.
Waiting for the sun: view of the equinox sunrise as it enters the passage of Cairn T, taken from the rear recess.
At Cairn T, the sun does not enter the chamber until it has risen well over the horizon. This is because the monument is aligned 8° south of east - possibly to account for the different declinations of the sun at spring and autumn equinoxes. According to Brennan, the sun can enter the chamber for a maximum of six days over the equinox.
When the sun first enters the chamber, the whole top panel of the keystone is illuminated.
The passage orthostats, sillstones and roof all combine to shape the beam of sunlight into a large rectangle of light which forms on the backstone of the end recess. As the sun rises in the sky, the shape of the light beam in the passage changes, and the rectangle of light shrinks, moving down and right. Repeated observation has shown that the complex engravings of the backstone are carefully positioned markers used to calibrate the day of equinox. It is quite possible that the beam runs on a four year cycle used to calculate leap years.
View of the keystone from out in the passage, as the rising sun floods into the end recess. No flash was needed for any of these photos; the sunlight is very bright and warm on the stones, and illuminates the art beautifully.
The Cairn T equinox alignment is a fully functioning neolithic astronomical calendar, lit up in glorious golden sunlight twice each year, weather permitting. The moon shines in as well, though no one has researched it yet that I know of. The moon can enter any chamber that can be reached by the sun.
Looking south across the chamber of Cairn T. The entrance passage is on the left, and the end recess, which is illuminated by the equinox sunrise, is on the right. Photograph copyright OPW.
Equinox sunrise at Cairn T
A sequence of photographs showing how the sun moves across the keystone after sunrise. These pictures were taken in the mid 1990s. At the bottom of the page is a Youtube clip of the alignment.
At first the entire surface of the panel that the sun can reach is illuminated. Note the eightfold circular engraving on the sillstone in the foreground.
As the sun rises higher the beam contracts, moving across the stone to the right.
The beam continues to shrink. Note how it lines up with the engravings. Now that is art!
As the beam leaves the end recess it hits symbols carved on the other stones on the right side of the chamber.
A beautiful display of interactive art: the sunbeam can be seen to focus on the flower/clock design, and uses the calibration markings above as part of the process.
An exciting possibility yet to be explored is lunar observations at this monument. Full moons which fall on or near the equinoxes will also illuminate the backstone. The workings of the equinox lightbeams across the engravings on this stone provide a working example of neolithic thought, symbolism and technology. The highlighted symbols record the movement of the sun and moon over the period when day and night are of equil length. The main sunflower/clock symbol, the Neolithic equivelant of the Celtic cross, marks off the cardinal points and the solstice sun rise and set positions. At this latitude the sun travels approximately 90° between solstices, dividing the sky into four quarters.
Equinox in Cairn T by Michael Fox of Knowth.com.