On the Buildings and Location of Armagh Observatory

Abstract

We consider the buildings of Armagh Observatory and present their location to a few tens of centimetres or approximately 0.01 arcsecond in latitude and longitude, referred to the position of the Ordnance Survey (OS) benchmark (OSBM) inscribed on the historic eighteenth-century building. This has Irish Grid (IG) coordinates (287829.309BM, 345757.815BM) and a height above mean sea level of 61.08 m. It lies approximately 12.1 ± 0.15 m west and 7.9 ± 0.1 m south of the Observatory’s earliest transit telescopes and 1.5 ± 0.05 m east and 1.7 ± 0.1 m south of the 1795 Troughton telescope. Ground level at the Observatory has an orthometric height of 61.9 ± 0.1 m, approximately 2 metres below that given in the Astronomical Almanac. Accurate measurements of height have implications for values of atmospheric pressure reduced to mean sea level, for precise comparisons between weather stations, and for weather forecasting. Our position for the Observatory’s earliest transit telescopes lies within ~0.4 m of the coordinate origin of the 1835 OS 6-inch County Armagh maps, highlighting the link between astronomy and the first high-accuracy maps of Ireland. An 1834 plan and measurements of the surviving north meridian marks suggest that the transit room’s east windows were moved ∼0.8 m east when the sector tower was added around 1840. This resolves an unexplained discrepancy between the present distance (3.0 m) between the centres of the west and east windows of the transit room and the original distance (~2.2 m) inferred by other means. We suggest that we may have
found parts of early transit instruments believed lost and give improved coordinates for the meridian marks. This work has applications to the history of astronomy, geodesy, meteorology, and climate change.