Dunsink Time

Astronomical observations have always been important for time keeping. The time of day, originally measured by sundials, is a measurement of where the sun is in the sky. Of course, today we have mechanical, electronic and even atomic clocks, but where the sun is in the sky is still an important factor.

Our calendar aims to keep the seasons fixed, which involves tracking how the Earth’s axis is tilted relative to the sun. We want the long Summer days to be in June, the short Winter days in December and the Equinoxes, when day and night are equal length, to be in March and September. Our modern calendar, called the Gregorian calendar, is set up to keep the Spring Equinox near the 21st of March.

Knowing the time and finding related information like the latitude and longitude has been an important activity at Observatories for hundreds of years. For example, in 1787 the observatory’s first director, Henry Ussher, estimated the time difference between Greenwich and Dunsink to be 25m 7.4s. These measurements were improved by subsequent directors of the institute and were important for navigating at sea, where knowing the time and observing the stars would help ships know their location.

Providing the time to ships that were docked at a port was important for their safety. In the 1860s, a time ball was built at the corner of O’Connell Bridge. This was a large ball on top of a pole that would be dropped at midday to allow ships in the port to set their clocks. This ball was kept on time using observations from Dunsink. After telegraph lines became available, several clocks in Dublin were automatically synchronized with clocks in Dunsink to keep them on time.

In the 1880’s, Dublin Mean Time became the legal time for all of Ireland, at the same time as Greenwich Mean Time became the legal time in England. This meant that Dunsink Time was the legal time for the whole country. As this point, the offset with Greenwich time had been measured to be between 25min 21s and 25min 22s. Ireland kept its own time zone until 1916.

Early in 1916, Summer Time was introduced to save energy during World War I, and Ireland changed clocks by 1 hour on the 21st of May. People had suggested moving Ireland to Greenwich time but, because of the 1916 Easter Rising, this was put off until later in the year. When Summer Time ended on October 1st, 1916, clocks in Ireland were put back by 35 minutes, ending the official use of Dunsink time in Ireland.

By David Malone, NUI Maynooth