Astronomy at Cairns Hill
The large neolithic cairns of County Sligo make fine observation platforms with their flat summits and panoramic views across the horizon. The large cairns on Carns Hill are 8 km almost due east of Knocknarea and afford one of the best views to Maeve's mountain. Over the equinoxes the sunsets and full moonsets drop over and behind Knocknarea. On the equinoxes the sun, which is moving very swiftly across the horizon, sets over the ruined neolithic cairn known as Knocknarea South, which has a cruciform chamber within the low cairn.
The sun drops behind Queen Maeve's cairn approximately 8 days after the spring equinox and 8 days before the autumn equinox. The west cairn being slightly further to the north will have the sun drop behind Maeve closer to the equinox. When you consider that there are five or six monuments on a north south line running through Queen Maeve's cairn, the horizon profile would have had small bumps making notches or measurements on the horizon. This system of observation may have been more concerned with lunar observations, the full moons on or near the equinoxes will also set along this giant calibration.
What about looking back in the other direction? The platform summit of Knocknarea is too high to observe objects rising over Carns Hill. The best location for an equinox sunrise observation place is lower down on the shelf on the east side of Knocknarea, where Rathcarrick circle is located. This small boulder circle is a perfect place to watch the equinox sunrises, which come up between the two cairns on the summits of Carns Hill, with Lough Gill beyond.