Opinion: a square metre of ground in Cork is one of the most important places in Ireland to measure the earth's force of gravity
The coastlines of Cork and Waterford are slowly subsiding, though the northern parts of the country are rising, as Ireland slowly tilts about an East-West axis. We know this because of gravity measurements - and it turns out that a particular patch of ground on the UCC campus is one of the most important places in Ireland to measure the earth’s force of gravity.
The patch measures less than one square metre and is located in the Crawford Observatory, which dates from 1880 and was built during the golden age of Irish astronomy. The absolute value of the earth’s gravity was measured there 50 years ago. This measurement was made from a location fixed to the very stable foundations of the observatory, and hence one of the most reliable gravity base stations in Ireland.
Recently, an international project has measured absolute gravity multiple times and has seen the UCC campus recognised as part of a global network, namely the International Gravity Station Network (IGSN). This network consists of stations all over the world where absolute gravity has been measured accurately.
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Modern devices can measure the earth’s gravity to a precision of a few parts in a billion, a thousand times more accurately than possible 50 years ago. At this level of accuracy, the gravity of the Earth we experience can be significantly affected by several natural phenomena including the ocean tides and even the weather! In particular it is affected by any changes in elevation.
But why do we make such precise measurements of the earth’s gravity? First and foremost, the members of the IGSN are monitoring the shape of the earth, known as the "Geoid", which undergoes daily changes as well as long term changes seasonally, annually and over centuries. These measurements can be combined with satellite measurements, which cover a larger areas, but to less accuracy. Measuring the earth’s gravity from space has given us important data about melting ice and corresponding sea level increases.
Geophysicists regularly use ground based gravimeters to locate density anomalies beneath the surface, such as buried valleys or mineral resources, as well as assisting in engineering projects. These gravimeters measure gravity relative to an absolute base station, and the measurements from the Crawford Observatory can greatly assist these. Indeed, the crew of several survey ships have visited the Observatory in the past to calibrate their instruments before embarking on voyages exploring adjacent ocean basins.
Local measurements of gravity can tell us if our land surface is rising or falling over time
In addition, local measurements of gravity can tell us if our land surface is rising or falling over time. It will be possible to monitor this more accurately over the coming decades, if continued measurements are made in the Crawford Observatory. This is important, because the effect of the rise in global sea level as the climate warms is compounded by local land subsidence. We must add these two values together along the south coast of Ireland to get a proper estimate of the apparent sea-level rise, and its likely impact, for example, on coastal erosion and flooding.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ