Equinox at Loughcrew
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
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.
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.
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.
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.
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.