Leviathan of Parsonstown

In 1845, William Parsons, an Irish nobleman and the third Earl of Rosse, constructed a large telescope with an enormous 6-feet wide mirror on his estate Birr Castle, at Parsonstown in what is present day Birr, in Ireland. The telescope was so large that locals began to call it the “Leviathan of Parsonstown” — the leviathan is a sea monster or a whale in modern Hebrew. It was the largest telescope in the world at that time, a title it proudly held until the construction of the 100-inch Hooker telescope at the Mount Wilson Observatory, California, in 1917.

William Parsons built several telescopes in his lifetime, and the Leviathan of Parsonstown was the pinnacle of his achievements. With his great telescope, Parsons studied nebulas and discovered that these blurry objects are not not truly gaseous but contained fine stars that previous telescopes couldn't resolve individually. The Leviathan was the first to reveal the spiral structure of M51, a galaxy nicknamed later as the "Whirlpool Galaxy", and his drawings of it closely resemble modern photographs.

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Photo credit: National Library of Ireland/Wikimedia

To the build the Leviathan of Parsonstown, which had no precedent, William Parsons had to invent many techniques of telescope construction and perfect techniques of casting, grinding and polishing large telescope mirrors from speculum, an alloy of copper and tin. While earlier telescope builders guarded their secrets, William Parsons made his widely available. He published details of his techniques and presented them to the Belfast Natural History Society. The images he obtained using his telescope were widely circulated within the British commonwealth.

Parsons took five tries before he created a mirror that could be used, and 3 years to build the actual telescope. The mirror was 5 inches thick and weighed almost 3 tons. The telescope was mounted between two massive stone walls for support. A system of chains, pulleys and counterweights kept the telescope in balance. The supporting walls restricted the telescope’s azimuthal angle (left-right movement) but the telescope could point in any direction vertically up or down. As the Earth rotated, celestial objects would pass between the viewing gap of the walls, giving the observer, who stood precariously on an observing platform at the top of the telescope, no more than thirty minutes to observe. Because astronomical photography was in its infancy at the time, Parsons made sketches of what he saw instead. Some of these sketches can still be viewed today, and has to be admired for their accurate detail when compared to modern photographs.

After the death of William Parsons in 1867, his son Lawrence, the 4th Earl of Rosse, continued to operate the telescope until about 1890, after which it fell into disuse. When the First World War broke out in 1914, the telescope was taken apart and all metals were melted down to contribute to the war effort. Of the original two mirrors, one was lost while the other is preserved in London’s Science Museum.

Today, a replica of the Leviathan of Parsonstown stands at the site of the original telescope.

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