The Leviathan: the telescope that made Parsonstown famous

The reason to come to Birr is a machine. In 1845 William Parsons, Third Earl of Rosse, finished a reflecting telescope with a 72-inch mirror and a focal length of 16.5 metres, slung between two masonry walls in the middle of his own parkland in the Irish midlands. It was called the Leviathan of Parsonstown. For decades after, if you wanted to look through the largest telescope in the world, you came to a small town in Co. Offaly.

The claim, and the exact years

This is the most repeated sentence about Birr and it is worth getting right, because the sources differ. The UNESCO and IAU Portal to the Heritage of Astronomy records the Leviathan as the largest telescope in the world from 1845 until the 100-inch Hooker Telescope was built at Mount Wilson in California in 1917. Birr Castle's own account of the instrument dates the record from 1845 to 1914, which is the year the original speculum mirror went to the Science Museum in London, where it remains. Either way the record ran from 1845 into the twentieth century, and neither figure is a round number worth rounding. The honest version names the years.

How it was built, and why that is the interesting part

Rosse had no optical industry to buy from. He cast his own mirrors from speculum metal, an alloy of copper and tin that tarnishes and has to be repolished, and it took six casts between 1842 and 1845 to get two usable ones. Having two mattered: he could observe with one while the other was being repolished. The mirror weighed about three tons. The mounting was finished in 1845 and was an altazimuth, so the tube swung between the two walls and could only follow a narrow band of sky. The whole thing was built in the castle workshops by Rosse and the people of Birr.

Before the Leviathan he had built a three-foot reflector in 1839, which is a substantial instrument in its own right and which his son later used to measure the heat of the Moon.

What he saw

In 1845, with the new telescope, Rosse drew the spiral structure of M51. We call it the Whirlpool Galaxy now. Nobody had seen spiral structure in a nebula before, because nobody had had enough light-gathering power to resolve it. He went on to find that many of these spiral nebulae resolved into stars, though he still read them as star clusters within our own galaxy rather than as separate galaxies, which is what they turned out to be. The drawing is the point: it is a hand-made record of something the human eye had never had access to, made in Offaly.

People came from across Europe and beyond, some to observe with Rosse and some simply to look at the thing. The estate makes the point that the telescope was also used to map the surface of the Moon.

Not a one-off family

The Parsons record is broader than one instrument. Mary, the Third Countess of Rosse, was a serious early photographer, and Ireland's Historic Science Centre in the demesne holds what the estate describes as the oldest intact darkroom in the world, with a hologram of Mary Rosse inside it. Laurence, the Fourth Earl, measured the radiant heat of the Moon in 1868 using thermopiles and exhibited the work at the Franco-British Exhibition in 1908; the machine he used is in Gallery 5. Charles Parsons, the Third Earl's youngest son, invented the compound steam turbine in 1884, the one that took his name to sea in Turbinia ten years later, which changed shipping and made electricity cheaper, and the same principle turns in jet engines today.

Seeing it now

The Great Telescope stands outdoors in the middle of the demesne and is included in general admission. It was restored in the late 1990s with a new modern mirror fitted in 1999, and the estate says the last time it was used was in the early 2000s. Do the Science Centre first. The galleries explain the casting, the polishing and the workshop construction, and the telescope reads completely differently once you know how it was made.

Astronomy at Birr is not finished. I-LOFAR, a radio telescope run by Trinity College Dublin with a 1.4 million euro grant from Science Foundation Ireland, sits in the same parkland and connects Birr to the European LOFAR network. Birr, Armagh and Dunsink, Ireland's three historic observatories, have jointly begun seeking UNESCO World Heritage listing on the strength of that continuity.

One correction to a common assumption. The original 1845 mirror is not in Birr. It has been in the Science Museum in London since 1914.