Observational astronomy began to move in to the modern era in the 1840s, when the third Earl of Rosse (William Parsons) constructed the largest telescope in the world and used it to study objects that are now known to be other galaxies, beyond the Milky Way.
Rosse served as a Member of Parliament from 1822 to 1834, but resigned at the age of 34 to devote himself to astronomy. He was wealthy enough to fund the building of a series of telescopes at the family seat of Birr Castle, in Ireland, culminating in a reflecting telescope with a polished metal mirror (made from ‘speculum’, a mixture of roughly two-thirds copper and one-third tin) 72 inches across (6 foot, or 180 centimetres), dubbed the ‘Leviathan of Parsonstown’. The barrel of this huge instrument was mounted between two stone towers 15 metres high and 7 metres apart, and could be raised and lowered to point at different heights by a system of ropes and pulleys, but it could not be tracked across the sky and could observe only what swam into its field of view as the Earth rotated. The Earl had to develop the techniques used to build the Leviathan from scratch, both because of its unique size and design and because previous generations of telescope makers had often guarded the secrets of their trade. Unlike them, Rosse was open about his experiments, and presented details of the metal used in making the three-ton mirror – its casting, grinding, and polishing – to the Belfast Natural History Society in 1844, two years after construction of the instrument began. He started observing with the telescope in the following year, 1845, but because of the Irish famine he was preoccupied with his duties as a landowner over the next two years and only began a full programme of observations in 1847.
Rosse was particularly interested in the fuzzy patches of light on the sky known as nebulae (from the Latin for ‘cloud’). Using a 36-inch (90 centimetre) reflector, he had previously made drawings (this was before the advent of astronomical photography) of several nebulae, including one which he thought looked a bit like a crab, and named the Crab Nebula. With the 72-inch (180 centimetre) telescope, he was able to see more detail in the nebulae, and in particular he was the first person to see that one of these objects, known as M51, has a spiral structure, like the pattern made by cream being stirred into a cup of coffee. The object became known as he Whirlpool Nebula.
At the time, there were two schools of thought about the nature of these nebulae. One idea was that they were clouds of gas in the process of collapsing under the influence of gravity to make new stars and planets. The other suggestion was that the clouds were made of many stars, too far away and too faint to be picked out individually. Rosse was a firm adherent to the second school of thought, while John Herschel (the astronomer son of William Herschel; see here) was a leading light of the first school. It was only in the twentieth century that observations became detailed enough (using even bigger and better telescopes than the Leviathan, and aided by photography) to show that both men were right. Some nebulae are star-forming clouds of gas within our Milky Way Galaxy (some, such as the Crab, are debris from stellar explosions), but others, including the Whirlpool Nebula, are huge star systems similar to, but far beyond, the Milky Way. M51 eventually became the archetypal example of a so-called spiral galaxy.
Of course, the Leviathan could also be used to study objects closer to home, which made a big impression on non-astronomers. The Irish MP Thomas Lefroy said: ‘The planet Jupiter, which through an ordinary glass is no larger than a good star, is seen twice as large as the moon appears to the naked eye … But the genius displayed in all the contrivances for wielding this mighty monster even surpasses the design and execution of it. The telescope weighs sixteen tons, and yet Lord Rosse raised it single-handed off its resting place, and two men with ease raised it to any height.’19
The third Earl died in 1867, but his son Lawrence, the Fourth Earl, used the telescope until about 1890. Another son, Charles, is famous for inventing the steam turbine. The Leviathan fell into disrepair and was partially dismantled in 1908, but it was rebuilt in the 1990s with a mirror made of aluminium (the original mirror is in the Science Museum in London) and is now a visitor attraction. The first telescope with a larger mirror than the Leviathan, the 100-inch (2.5 metre) Hooker telescope at Mount Wilson in California, did not begin operating until 1918, sixty-three years after Rosse began his pioneering observations.