This image taken inside Cairn T shows the shaped beam of equinox sunlight illuminating the back stone. The symbol on the top left of the illuminated area clearly represents the sun and the symbols with the horizontal markings may be measuring scales.

Tim O'Brien in his book Light Years Ago states "It would appear that the construction of Cairn T was specifically designed to cleverly fashion a beam of light from the rising and early risen sun around the time of the equinoxes to define the four year equinox or leap year cycle."

Light Years Ago: A Study of the Cairns of Newgrange and Cairn T Loughcrew, Co. Meath, Ireland by Tim O'Brien shows that the symbols of the sun engraved on the backstone of Cairn T at Loughcrew are precisely positioned to measure solar movement. A beam of light from the rising sun around the time of the March and September equinoxes is mapped on the backstone.

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Loughcrew Video - Spring Equinox 2005

The progress of the sunbeam on the backstone inside Cairn T at Loughcrew was video recorded at sunrise on the morning of March 23rd 2005. The 50 minute video has been compressed to 1 minute 46 seconds and included in the following YouTube Video.

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