Birr, Co. Offaly

(formerly Parsonstown, King's County), Ireland, the site of Birr Castle, seat of the Parsons family, the Earls of Rosse. This unlikely site was an astronomical centre of the world for 75 years. William Parsons (1800-1867), third Earl of Rosse from 1841, built here several telescopes, including a 72 inch diameter reflecting telescope, the Leviathan, the largest in the world until that at Mount Wilson, California, in 1919. While Lord Oxmantown (his courtesy title as heir apparent to an Earldom), William Parsons developed techniques for casting, grinding and polishing mirrors in the 1830s, training his own workmen. He completed a 36 inch reflecting telescope in 1839. This gave a magnification of 900. Parsons invited several competent observers to use it--one opined that it was better than Herschel's 49 inch telescope. In 1842, Parsons, now the third Earl, cast a 72 inch mirror, weighing over four tons, using three crucibles. This broke during grinding, but it was recast and a second mirror was produced. The mirror was mounted in a 58 foot tube, but there was no way to put this in any kind of universal mounting, so it was mounted between two massive walls and could only view a star for about an hour as it crossed the meridian. The tube was 8 feet in diameter in the middle, tapering to 7 feet at the ends, and was made of inch thick wood with iron rings and strengtheners. The telescope was completed in 1845, but only a few observations could be made before the Famine struck and Rosse devoted most of his efforts to relief for several years, so it was not until 1848 that systematic observations really started. It has often been stated that the telescope was not as good as expected, but this was because the mirror, being outside and uncovered, tarnished quickly and had to be repolished regularly. When not in good repair, vision was indeed not good, but when it was clean and atmospheric disturbance was absent, it certainly was the best telescope in the world, as attested by numerous users. It reached magnifications of 650 on good nights and could see ten million lightyears away. Many of the observations could not be confirmed elsewhere for 50 years. Sadly, it was never adapted for photography, partly because the tracking was not smooth enough for long exposures. Some of the drawings made are quite exquisite. The main work of both the 36 inch and the 72 inch was the observation of clusters and nebulae. The 36 inch indicated that some nebulae might be resolvable into stars and the 72 inch made it clear that many of the nebulae were galaxies and first revealed that some had spiral structure.

A succession of observers were employed over the next 50 years. Robert S. Ball (1840-1913)--later Sir Robert, Astronomer Royal for Ireland--was observer and tutor in 1865-1866. Ralph Copeland, later Astronomer Royal for Scotland, was here in 1871-1874. John Louis Emil Dreyer, compiler of the New General Catalogue of clusters and nebulae in 1888, was here in 1874-1878.

William Parsons had three sons born at Birr. Laurence Parsons (1840-1908), later fourth Earl of Rosse from 1867, took an active interest in astronomy from his teens. Laurence made various improvements to the telescopes, fitting clock drives. His remounting of the 36 inch introduced the fork mounting, rotating head and moving observing platform, now standard ideas. He introduced spectroscopy, confirming Huggins' work on spectra of nebulae. His most notable work was determining the temperature of the moon's surface, obtaining results which were not fully accepted until the middle of the 20th Century. On 17 Sep 1877, Rosse confirmed the existence of Phobos and Deimos, the two dwarf moons of Mars discovered by Asaph Hall the previous month.

The youngest son, Charles Parsons (1854-1931)--later Sir Charles--achieved fame as an engineer, inventor of the steam turbine and creator of Grubb Parsons, telescope makers.

After Laurence's death in 1908, the 72 inch was dismantled and its mirror was sent to the Science Museum in London. The first World War and the Troubles meant that the rest of the structure decayed and most of the wooden parts were demolished in 1925. The 36 inch was reported as nearly intact in 1927, but has vanished. Most of the masonry structure and the tube of the 72 inch remain and there are remains of the forge where casting of mirrors was carried out. A model of the 72 inch telescope, its mirror and some eyepieces are in the Science Museum, London, but the Astronomy Gallery has been closed for some time and these items were not on view in 1993.

A museum was being built at Birr Castle in 1971. There is a statue of the third Earl in the town. A 1982 booklet says the tube can be seen between its supporting walls, that there is a tape of Patrick Moore narrating the story of the telescope, and that there is a display area at the base of the Leviathan and an exhibition room, initially commemorating the work of Sir Charles.

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An extract from The Mathematical Gazetteer of the British Isles created by David Singmaster

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