Dunsink and Timekeeping

On August 2nd 1880, the official time in Ireland was set to be Dublin Mean Time. This was probably administered from Dunsink Observatory to the northwest of Dublin city.

The offset between Dublin Mean Time and GMT would have largely been determined by the Longitude of the Observatory. I measured the location of the Observatory on 22/12/2002 with a hand-held GPS unit. There are two domes, one on the main building and a smaller one. Below are the measurements I took near each with a photo of where I was standing relative to the building.

Latitude Longitude  Elevation Offset        Near
53.38741 -6.33839   80m       0h25m21.21s   main
53.38756 -6.33856   73m       0h25m21.25s   main
53.38722 -6.33842   79m       0h25m21.22s   main
53.38712 -6.33769   80m       0h25m21.05s   small
53.38701 -6.33746   81m       0h25m20.99s   small
53.38718 -6.33738   79m       0h25m20.97s   small

Ian Elliott made some additional measurements with a camera GPS in 2013, and found the Meridian room to be at Meridian Room to be 53d 23' 13''.5 and 6d 20' 20''. Measurements for the South Dome game 53d 23' 13.5'' and 6d 20' 15''. In mid 2015, Tom Ray verified the coordinates of the telescope in the South Dome as 53d23' 13''.5 and 6d20'15''2, and height 75m.

All these would correspond to an offset of 25 minutes 21 seconds, to the nearest second. This agrees with the comments in the Unix zoneinfo file:

# Whitman says Dublin Mean Time was -0:25:21, which is more precise than Shanks.
# Perhaps this was Dunsink Observatory Time, as Dunsink Observatory
# (8 km NW of Dublin's center) seemingly was to Dublin as Greenwich was
# to London.  For example:
#
#   "Timeball on the ballast office is down.  Dunsink time."
#   -- James Joyce, Ulysses

Though I haven't checked Whitman or Shanks myself.

Note from JC: I would expect that the time standard was set from the transit circle instruments in the Meridian Room at the back of the observatory (about 20-30metres west of the first measurement position). That is where the dials of the clocks were originally installed to allow readings to be taken at the telescopes.

Historical Values

Patrick A. Wayman's book "Dunsink Observatory 1785-1985" gives some historical latitude and longitude measurements for Dunsink. The figures quoted for longitude are:

Rev. Dr. Henry Ussher (who worked at the observatory from about 1783 to 1790) had made various observations which give values about 25m48s, 25m18s and 25m07.4s The last is dated 1787.
Rev. Dr. John Brinkley (who worked from 1790-1826 and may have been Bishop of Cloyne) made a measurement reported by Hamilton in 1832 of 0h25m22s.
T. Romney Robinson of Armagh worked with Hamilton to make a detailed determinations for Dunsink and Armagh. His result was 0h25m21.08s, with an error of about 0.1s. This was about 1838.
The Nautical Almanac continued to use Brinkley's value until 1896, when it moved to Robinson's value of 25m21.1s.

And the figures quoted for latitude:

Brinkley gave values of 53o23'13''.5 or 53o23'14''.2, depending on the refraction tables he used. In a later paper he gives a value of 53o23'12''.6. Hamilton recommended a value of 53o23'13'' for the Nautical Almanac, which was used from 1835 to 1895.
In about 1874 Brünnow did further measurements, that gave a similar answer to Brinkley, 53o23'13''.0.
Rambaut verified these results in 1889, 53o23'13''.08 with an error bound of about 0''.04. This made it into the Nautical Almanac in 1896.

The 1935 Nautical Almanac (p. 684) uses these figures, which show that the altitude is pretty good too.

No	Place				Long		Lat		Alt
80	Dublin, Irish Free State	+0h25'21.1''	53o23'13.1''	86m	Dunsink Observatory, Trinity College

Interestingly, the values reported in the Astronomical Almanac from 1992 onwards seem to be incorrect, swapping the final digits of the latitude and longitude. This was pointed out to me by Ian Elliott in September 2013. Luke Drury, Tom Ray and I contacted the USNO in the summer of 2015, and in the 2017 Astronomical Almanac, the coordinates should be updated with those of the Grubb Telescope in the South Dome (determined using DGPS as N 53 23 13.5 W 06 20 15.2).

One of the other important observatories in Ireland is Armagh Observatory. A trip there with a hand-held GPS gives a location of around 54o21'10.2''N, 6o38'52.6''W, 60m. This seems to be in reasonable agreement with standard figures of around 54o21'12'' and 6o38'54.