William Lassell (1799-1880) and the discovery of Triton, 1846

by Allan Chapman

William Lassell
William Lassell
Photo copyright © Liverpool Astronomical Society.

Liverpool Businessman

William Lassell was a Liverpool Businessman who had made his fortune in the brewing trade. This was a time, after all, when Liverpool was the fastest-growing port in Europe, if not the world, and a focus for ship building, import and export for Manchester and the industrial hinterland of South Lancashire and Yorkshire, and the focus of major projects of civil engineering. Britain's first steam-hauled passenger railway ran from Liverpool to Manchester in 1830, and the building of the Royal Albert and other major dock projects from the 1840s onwards made it a city of immense energy. Immigrants from Ireland, passengers bound for America and Australia, sailors, railway and dock navvies, and an army of miscellaneaus industrial workers flooded through the city. And what they all had in common was thirst. This was the golden age of the English beer-brewing industry, and the trade could make very handsome fortunes for those who developed it. By 1825, according to his notebooks preserved in the R.A.S. library, London, he was already in partnership in brewing, and the Lassell name was to be a leading one in Liverpool's beer-making partnerships for most of the 19th Century.

It was from the "demon drink", therefore, that the Nonconformist (he was a Congregationalist) business man financed one of the most extraordinary scientific enterprises of the Victorian Britain.


Lassell's real innovation did not simply lie in the near celestial objects that he discovered. It lay in his genius for instrument-making and adventurous engineering, for it was William Lassell who created the modern big reflecting telescope.

Though the Newtonian reflector had been around for 150 years when Lassell started to cast his first mirrors around 1820, and Sir William and Sir John Herchel and the young Lord Rosse had already built big- aperture reflectors in altazimuth mounts, their great wooden tubes, manhandled with ropes made the long-term tracking of an object impossible.

Yet in many ways, this was not too much of a disadvantage for these astronomers. When one is "sweeping" the sky, along and horizontal zones of declination for interesting deep-space objects, as did the Herschels, then one could almost leave the telescope fixed, and let the heavens do the moving. The British climate made deep space work, with low- reflectivity speculum metal mirrors, an essentially meridian activity anyway, so their apparently ponderous telescopes could do the task in hand with little difficulty.


But if it was the planets, and their satellites, that interested you, then altazimuth mounts of wood and rope were severely limiting.

Right from the beginning of his observing career in the 1820s, William Lassell was a planetary astronomer. Saturn was his first and enduring love, and to do serious planetary astronomy under the cloudy skies of Lancashire, one needed to grab whatever good observing conditions were available. To observe Saturn, and the other planets, one needed an instrument that was not only optically excellent and capable of rendering delicate surface detail and subtle colouring with exactitude, but which could do so in any part of the sky, and quickly. And when the telescope had been locked onto the object to be observed, it had to be rock-solid, steady, and capable of giving a continuous image to the dark-adapted eye. In short, Lassell's type of astronomical research demanded a reflecting telescope mounted in the equatorial plane, with smooth tracking.

9-inch equatorial

9-inch equatorial
Reproduction from the Memoirs of the R.A.S. xii (1842) plate V.
Reproduced courtesy of the Royal Astronomical Society.

In 1839, he described his home-made 9-inch equatorial reflector to the R.A.S. It was a revolutionary instrument. Though it's aperture was modest, even by the standards of 1839, its tube and accurate mount was built in cast iron. The tube, carrying a mirror of 112 inches focal length, was roller-bearing mounted within an iron box, which in turn was bolted upon a great iron cone, set to the latitude of Liverpool. Everything was set upon roller-bearings and precision geared, so that Lassell claimed the motions to be so perfect that nothing more than delicate finger pressure was necessary to start and stop the instrument. When compared alongside contemporary reflecting telescopes, Lassell's brilliantly-engineered instrument was immediately recognised as a thoroughbred amongst beast of burden. The R.A.S. requested Lassell to publish a nut-by-bolt description of his instrument, which he did in the Memoirs of 1842.

It was this 9-inch, iron equatorial, and the expert planetary work which he did with it in the early 1840s, that laid the foundations of Lassell's international reputation as a scientist.

Saturn Drawing
Saturn 1851 Sept 10/11. Observation with the 20 foot telescope.
Copyright © of the Royal Astronomical Society.
ref Lassell 16.1

"Grand Amateurs"

Yet how could a brewer, with no formal scientific qualifications ever win a front-rank reputation in international science? One should remember, however, that in Britain at this time virtually all of the leading astronomers were in a similar position to Lassell. The Royal Society and the R.A.S. was well-stocked with "Grand Amateurs": gentlemen of education, wealth and leisure who chose to spend their days in scientific research rather than in idleness. Lawyers, soldiers, clergymen, medical men, stockbrokers and industrialists, along with some aristocrats like Lord Rosse, contributed the creative heart of British astronomy and controlled its policies. Given a good education, a technical bent, and money -all of which Lassell enjoyed- then there was nothing to stop these "Grand Amateurs" running streets ahead of the professionals in terms of their discovery-making.

In 1840, England had only a tiny handful of formally-trained and employed astronomers. There was Sir George Airy at Greenwich, the Astronomer Royal, and the Directors-mostly Anglican clergymen-of the University Observatories of Oxford, Cambridge, Durham and Dublin. But most of these professionals were kept busy doing routine meridian or astrometric astronomy, on relatively modest salaries, or else were compelled to divide their time between observing and undergraduate teaching. In short, the professionals were the poor relations of the astronomical establishment, and only the Astronomer Royal, from his public position, was assured any pre-eminence.

So if one wanted to look at the planets, nebulae, or conduct astrophysical investigations, the last thing that one wanted was a salaried job! A personal fortune was much more important. William Lassell, therefore, far from being thought of as a part time amateur, very rapidly moved with, and was accepted by, the official establishment of British Astronomy.

24-inch telescope

As soon as his 9-inch had proved its worth, Lassell set about up- scaling the design, to carry a 24 inch diameter mirror inside a 20 foot tube. He never published any drawings of the 24-inch (which became the worlds first large equatorial reflector to be mounted in a prime-sky location, when Lassell took it to Malta in 1852-53), and claimed in his 1848 official description that it was an enlarged version of the 9-inch.

Polishing Machine
Polishing Machine.
Copyright © of the Royal Astronomical Society.
ref Lassell MS 14.2

Knowing that he would never be able to work the 24 inch speculum mirror-weighing nearly 500 lb., by hand, he devised a steam-driven grinding and polishing machine. This machine, which was built by Lassell's fellow amateur astronomer, and professional ironmaster, James Nasmyth of Patricroft, Manchester, is the ancestor of all subsequent large-scale optical polishing machines. It used steam power to figure a mirror which Lassell claimed was as near to optically perfect as a piece of speculum could ever become. Lassell's boast, more over, was not an empty one, for in 1995, the original mirror, which is preserved in Liverpool Museum, was optically tested, and found to be accurate to within a fraction of a wavelength of light.

Triton's discovery

When "Le Verrier's Planet", or Neptune, as it was to be officially named, was discovered in Berlin on September 23rd, 1846, the news spread rapidly to England. From the new planet's co-ordinates, published in The Times at the beginning of October, Lassell succeeded in seeing it over October 2nd and 3rd. Indeed, not only did Neptune reveal a distinct disk from the very outset when viewed through the 24-inch, but on the 3rd, he was convinced that the new planet had a ring, like that of Saturn. A week later, on October 10th, Lassell saw that Neptune also had a satellite, which was later to be named Triton. On October 14th, he announced these discoveries to The Times and on November 11th, his daughter Maria made an independent drawing at the eyepiece, of planet, ring and Triton.

Lassell continued to report the Neptunian ring to the astronomical community until 1852. But in that year, he improved the support system for the 24-inch mirror, and the ring vanished, a product of optical distortion. Yet Triton continued to shine, and Lassell determined its orbital period.

The discovery of Neptune was one of the great events of Nineteenth - century astronomy. Yet one notes how all of the events surrounding its discovery in 1846, including Lassell's own letters to The Times reporting Tritons discovery, refer exclusively to "Le Verrier's Planet". What about the English discoverer, John Couch Adams? For this side of the story, one should look at my companion piece, "Adams, Airy and the Discovery of Neptune in 1846", for until Sir John Herchel published Adams's prior claim to the Neptune discovery, in October 1846, everyone, on both sides of the Channel, was speaking of "Le Verrier's Planet"

One cannot help wandering what would have happened, however, if Adams, after making his original calculations for the position of a planet disturbing Uranus in the autumn of 1845 had sent them direct to Lassell? This was, after all, a year before the Berlin astronomers found Neptune from Le Verrier's co-ordinates. When Lassell did indeed see Neptune, on October 2nd 1846, we must remember his magnificent 24-inch reflector immediately revealed the planet as a distinct disk. Unlike the efforts of the Berlin, and Cambridge astronomers, working with smaller aperture refractors, no painstaking astrometric measurements were necessary. Similarly, the same 24-inch instrument picked up a satellite rotating around the disk, when its own position and the Liverpool skies became favourable a week later.


Lassell's 24-inch telescope produced a crop of planetary discoveries. In addition to Neptune's Triton, the same instrument revealed two new moons rotating around Uranus, (adding to the two discovered by Herschel), and facilitated a much more detailed understanding of the Saturnian belts and rings, not to mention work on Jupiter. His work on outer-planetary satellites enabled Lassell, furthermore, to establish the orbital periods of these bodies, which could be used in turn to extract the masses of their parent planets.

But in many of these researches, Lassell had a rival who was, in reality, a fellow Grand Amateur astronomer. This was William Cranch Bond, who was a Massachusetts businessman who held the unsalaried Directorship of the Harvard College Observatory in Boston, USA. Now Bond was fortunate in two respects. For one thing, he had the magnificent 15- inch aperture refractor, by Merz of Munich, at his disposal; and secondly, Harvard is 10 degrees closer to the earth's equator than Liverpool. This higher ecliptic latitude, plus cleaner air, was an immense advantage when it came to looking for dim planetary detail. Lassell, in fact, had already made neck-and-neck co-discoveries with Bond, for the discovery of Saturn's eighth satellite Hyperion in 1848, and Saturn's innermost Crape ring, in 1850. I strongly suspect that it was for this reason, and a wish to steal a temporary latitude and climate lead upon Bond, that Lassell took his great 24-inch equatorial to the island of Malta during the winter of 1852-53.


Lassell was enchanted by the high ecliptic latitude, mild climate, and unbroken succession of transparent night skies on this Mediterranean island, and one can understand how his English colleagues felt twinges of envy as they received his letters during that winter. Lassell's Malta expedition not only demonstrated the lavishness with which the Grand Amateurs went about their astronomy, but also opened the eyes of north European astronomers to what we now call "prime sky locations" for big telescopes.

Lassell's standing within the international scientific community was of the highest. Within Britain, his standing can be gauged by his election to the Fellowship of the Royal Society, in 1849, his 1858 Gold Medal from the Royal Society, the honorary degrees which he received, and his service as President of the R.A.S. between 1870 and 1872. He was presented to Queen Victoria, and by 1860, was one of Liverpool's most distinguished citizens.

Orion Nebula
Orion Nebula painting, from observations made at Valletta, Malta, with
the Twenty-foot equatorial. 1852/3. see note below
Copyright © of the Royal Astronomical Society.
Reproduction from the Memoirs of the R.A.S. xxiii (1842) plate I.

... In order to perpetuate as far as possible the results of these observations, I send herewith a painting in oil of this nebula, on the same scale as my original drawings, the acceptance of which I beg from the Astronomical Society. It is the work of my friend Mr. Hippisley, executed under my own superintendence and carefully compared with my original sketches. I consider it a very faithful picture of what I saw....

Lassell's contributions

But Lassell's enduring contributions to astronomy are greater than any of his particular discoveries. His real significance, indeed, stems from his approach to instrumentation. It was William Lassell who first overcome the problems involved in figuring large mirrors by means of steam-powered machinery, for while it is true that Lord Rosse had steam-powered a mirror-making machine before Lassell, it was the Lassell-Nasmyth arrangement which formed a prototype. And without doubt, it was to be the Liverpool brewer who first solved the engineering problems that were inherent in mounting a large speculum metal mirror upon a smooth roller-bearing controlled, iron equatorial mount. The modern equatorial tracking of objects across the sky, with a large optical aperture, therefore, is very much a Lassell contribution.

We must not forget, moreover, that the 24-inch was by no means Lassell's last word in telescope-making. So successful, in fact, was this instrument, that in 1858, he doubled its size, to produce a 48- inch speculum mirror which he successfully mounted upon a scaled-up version of the 9 and 24 inch mounts. But in addition to its size and excellence of design, the 48-inch telescope had another distinction: it was the first big telescope ever to be designed for use in a "prime sky" location. By 1858, Lassell had come to realise that it was useless setting-up this magnificent instrument under English skies, and took it to Malta, between 1861 and 1865. Under the same skies which had so inspired him eight years before, Lassell re-reviewed the solar system, and made fresh discoveries in the Trapezium of the Orion Nebula, which in England, always stays relatively low in the sky.

Whenever we think of modern observatories, located in remote prime-sky locations, we should think of the man who first showed what the big, industrial - machine- conceived telescope could do. We should think not of a professional, but of an English Grand Amateur astronomer who, had he received a letter from John Couch Adams in October 1845, could well have bagged the discovery of Neptune for England. On the other hand, he scored a superb first a year later when he discovered Triton encircling "Le Verrier's Planet".

And last, but by no means least, we should spare a thought for the anonymous thousands of immigrants and navvies that toiled in Liverpool, for ultimately, it was their unquenchable thirst which enabled William Lassell to do what he did.

Allan Chapman
Wadham College,
Oxford University
Copyright © 20th April 1996 Allan Chapman


Allan Chapman:
'William Lassell (1799-1880), Practitioner, Patron, and "Grand Amateur" of Victorian Astronomy' Vistas in Astronomy Vol. 32 (1988) 341-370. To be re-printed, July 1996, in A. Chapman's volume of Essays, Astronomical Instruments and their Users: Tycho Brahe to William Lassell. Variorum collected studies, Aldershot, England.

Gerard Gilligan:
'William Lassell', Liverpool Astronomical Society, (Liverpool, 1994)

Richard Baum:
'The Phantom Ring of Neptune', Journal of the British Astronomical Society, Vol. 99 p.77 April 1989.

William Lassell a brief history.
The Lassell Telescope Project.
The Lassell Telescope.

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