Slieve Guillion: Repairing a 5,000-year-old tomb

And whilst Slieve Gullion relates to the Solstice, Wednesday marks the Autumn Equinox – the halfway point between mid-summer and mid-winter.

That means there will be nearly equal amounts of daylight and darkness.

This arises as the earth’s axis is tilted to an angle that sees the sun shine directly onto the equator.

According to Dr Rok Nežič at Armagh Observatory and Planetarium the equinox occurs at a point when days are getting shorter by around four minutes each day.

“It is always around the 21st of the relevant month – so March and September for an Equinox and June and December for the Solstices (longest and shortest days),” he said. “But it isn’t quite on those dates and that is mostly to do with Leap Days so this year it is on 23 September, just after 1am.

“It’s the one time of the year when the sun does rise exactly in the east and set in the west. This far north it is actually more south-east for sunrise in the winter and north-east for sunrise in the summer. But for the two equinoxes it’s east for sunrise and west for sunset exactly.”

In the grounds of the Armagh observatory there is a modern interpretation of a stone circle.

According to Dr Rok, “we call this our stone calendar”.

“It shows the directions of sunset for the four notable astronomical times – the two equinoxes and the two solstices.

“Many of the stone circles around the country will focus on the solstices because they are more significant but some will also have equinoxes in their design.”