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Ephemeris: 03/17/2025 – When the largest telescope in the world was in Ireland

March 17, 2025

This is Ephemeris for St. Patrick’s Day, Monday, March 17th. Today the Sun will be up for 12 hours and 1 minute, setting at 7:51, and it will rise tomorrow at 7:49. The Moon, 3 days past full, will rise at 11:40 this evening.

In the 19th century Ireland laid claim to having the largest telescope in the world.  It was a reflecting telescope with a mirror diameter of 72 inches.  It was built by William Parsons the Third Earl of Rosse.  The base of the telescope tube rested in a pit between two massive walls and could only look to either side of a north-south direction.  It saw first usage in 1847.  The telescope was called the Leviathan of Parsonstown, and was in use until 1890.  Mirrors in those days were made of a silvery alloy called speculum.  Two mirrors were used alternately because speculum tarnished.  The mirror not in use would have to be re-polished and swapped in from time to time.  It was the largest telescope until the 100 inch at Mt. Wilson was put in service in 1917.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT – 5 hours). Times will be different for other locations.

Addendum

Leviathan of Parsonstown
The 72 inch Leviathan of Parsonstown. Source: http://www.klima-luft.de/steinicke/ngcic/persons/rosse3.htm
M51 drawing
A drawing of the Whirlpool Galaxy, M51 (NGC 5194 & 5195) by Lord Rosse with the 72 inch telescope. Public Domain.
M51, the Whirlpool Galaxy by Daniel Dall'Olmo
A modern digital color photograph of M51, the Whirlpool Galaxy by Daniel Dall’Olmo.
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