Thomas Romney Robinson died on the 28 February 1882 at the age of 89 after occupying the position of Director of Armagh Observatory for 59 years. His work continued to within days of the end of his life as is witnessed by his correspondence. He was succeeded by J.L.E. Dreyer, a Dane, who had previously worked at Birr Castle and Dunsink Observatory Dublin.
When he came to Armagh he continued the fight which Robinson had begun for government assistance and eventually he was rewarded by a grant from the treasury of 2000 pounds, together with a clear indication that this was a once-only payment, not to be repeated. With this money, Dreyer was able to purchase a 10 inch refracting telescope, built by Howard Grubb, and with which he immediately set to work studying the many nebulae he had previously observed at Birr.
One of the problems he tackled was the question of whether the nebulae were inside our own galaxy (the Milky Way) or were indeed, as was suspected, island universes outside of our own galaxy. Some astronomers had previously suggested that the nebulae were variable in position, and therefore were likely to be nearby, however careful measurements with a micrometer on the 10 inch refractor, and later on photographic plates, showed that the earlier claims were false and that no movement could be detected. It was observations such as as these which eventually led to the realisation in the early twentieth century that the spiral galaxies were not truly nebulous but were, in fact, distant systems of stars similar to our own milky way.
Dreyer’s NGC catalogue, or the ‘New General Catalogue of Nebulae and Clusters of Stars’, to give it its full name, is probably the single most important contribution to science to have come from Armagh Observatory. Even though it was compiled over 100 years ago it remains to this day the principal catalogue of nebulae and galaxies used by astronomers around the world. In it Dreyer listed and classified all the nebulae and star clusters previously observed by Sir William and Sir John Herschel and subsequently by Lord Rosse and his associates in Birr and by himself in Armagh.
The government grant of 1883 had alleviated some of the immediate problems faced by the Observatory following the disestablishment of the Church of Ireland; however no on-going commitment could be extracted from the government of the day and, when the various Irish land reform bills were passed at the end of the nineteenth century, serious financial problems arose once more. As a result Dreyer was no longer able to afford an assistant to help with observations and was forced to give up this part of the work.
Dreyer, as a Dane, had a life-long interest in the great Danish astronomer Tycho Brahe, on whose work the great Kepler had based his laws of planetary motion. Tycho Brahe’s observations were the most comprehensive of any from the pre-telescopic era. When funds for observing ran out at Armagh, Dreyer decided to undertake a task that he had previously not had time to tackle; that of compiling and publishing the life’s work of Tycho Brahe.
It was remarkable omission, that for 300 years, the observations which had led Kepler to his famous laws of planetary motion, had lain unpublished in the Royal Library in Copenhagen. It was with this data that Kepler had proved the helio-centric theory of the solar system, (ie that the earth moves around the sun and not vice versa); probably the single most important discovery in the history of astronomy. One by one the manuscripts were sent from Copenhagen to Armagh where they were compiled for publication.
Dreyer became the most prominent historian of astronomy of his day and wrote ‘The History of the Planetary System from Thales to Kepler’, a classic work which has been frequently reprinted.
