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Waiting
for the sun: view of the equinox sunrise as it enters the passage of Cairn
T, taken from the rear recess. |
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Equinox at Cairn T 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 solar alignments at Loughcrew. He 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. 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. The sun can enter for a maximum of six days over the equinox.
When the sun first enters the chamber, the whole panel 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. |
| 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. |
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Equinox sunrise from outside Cairn T. 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 equnitocal 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. |
| Artwork
on the ceiling slab above the end recess; the designs are illuminated
by reflected light as the sun hits the key stone. |
