The Astronomy of Birr Castle is one of its greatest attractions and, in many ways, one of its surprises. It is unusual for a Castle in the centre of Ireland to have become a great centre of astronomical discovery. However, in 1845, the 3rd Earl of Rosse built the biggest telescope in the world and it remained so for 75 years. Here you may see the great telescope – the Leviathan – still standing in the middle of the park with its huge 52’ ( metres) long tube with a diameter of 6’ ( 2 metres).
William, the 3rd Earl of Rosse was able to build this telescope with the help of his wife Mary’s fortune. It was a great engineering feat because he had to work out the ability to raise and lower the immense tube, and also to cast the mirror, or speculum, himself. It took many experiments to find the right combinations of metal to use for this. The metal was then heated in a furnace in the moat by turf (or peat) from the bogs which comprised much of his estate. The next problem he had was in cooling the speculum. The first two mirrors cracked as cooling happened too quickly. He eventually made two mirrors which were inter changed and removed for polishing as the damp caused them to mist over. This in itself was another engineering feat, and a small railway line was built to transport the massive mirrors to and from the workshops for polishing.
One of the major discoveries with the telescope was the spiral nature of the nebulae. The great mirror was the first to be able to see that many of these were spiral in shape. What Lord Rosse was seeing were in fact galaxies. The famous M.51 whirlpool galaxy is the classic example of this and we have commemorated it with a spiral of lime trees which visitors can walk around.
Astronomy at Birr continued with the 3rd Earls son, Laurence, the 4th Earl. He was especially interested in the moon and invented a machine called the lunar heat machine which measured the heat of the moon. The accuracy of this was proved to be correct with the landing on the moon in 1969. The machine is on display in the Science Centre.