Tag: History

Nov 12 2012

Book review: The Sleepwalkers: A History of Man’s Changing Vision of the Universe by Arthur Koestler

Sleepwalkers_ArthurKoestler.Another result of my plea for reading suggestions on twitter; this is a review and summary of Arthur Koestler’s book “The Sleepwalkers: A History of Man’s Changing Vision of the Universe”. The book is a history of cosmology running from Pythagoras, in the 6th century BC, to Galileo who spanned the end of the 16th century, just touching lightly on Newton. It traces a revolution from a time when the cosmos, beyond the earth, was considered different, stable and perfect, to a time when it was shown to be subject to earthly physics, be changeable and not perfect by any reasonable definition.

Kuhn’s language of paradigm shifts seems rather overused to me but here is an example of a true paradigm shift. The sleepwalkers in the title refers to the idea that the protagonists didn’t really know where they were headed with their ideas and quite often were lucky with errors which cancelled each other out.

The book starts with a cursory look at Babylonian and early Greek astronomy; despite considerable observational acumen their models of the universe were outright mythical. The Pythagoranean Brotherhood although in many senses still mystical started to think about the physics of the universe. I have a tendency to think of the ancient Greeks as one blob but as the book makes clear there is a huge span of time, and outlook, between Pythagoras, Aristotle and Plato and Ptolemy. Koestler is quite clearly disappointed with the Greeks: they make a promising start with Pythagoras, Aristarchus developed a heliocentric model for the solar system and then with Plato, Aristotle and Ptolemy they regress back to a geocentric model.

Following on from the Greeks the Middle Ages are covered, James Hannam in his book “God’s Philosophers” has covered why this period wasn’t all that bad in terms of intellectual development. Koestler is less sympathetic, his key accusations are that they philosophers of the middle ages were in thrall to the later Greeks and furthermore there were elements of Christian theology that abjured the pleasure of knowledge for knowledge’s sake.

After these preliminaries, Koestler turns to the core of his work: the cosmological developments of Copernicus, Tycho Brahe, Johannes Kepler and Galileo Galilei.

The model of the universe handed down from the ancient Greeks was one of circles (often referred to in this context as epicycles), they believed that motion in a circle was perfect, that the heavens were a separate, perfect realm and that therefore all motion in the heavens must be based on circular motion. Further, the model dominating at the end of their period, held that the earth lay at the centre of these circular motions. The only problem with this model is that it doesn’t fit well the observed motions of the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn – the observable solar system which lay against an unchanging starry background. Or rather you can get a rough fit at the expense of stacking together a great number of epicycles – something like 50.

Copernicus’ contribution, published on his death in 1543, was to put the sun back at the centre of the universe. Copernicus led a rather uneventful life, was no sort of astronomical observer and only published his thesis at the end of his life at the strong urging of Georg Joachim Rheticus. He’d discussed his model fairly freely during his life, and his reasons for not publishing were more to do with fear of ridicule from his contemporaries rather than theological pressure. After his death his work, with the exception of the astronomical tables, sank into obscurity partly because it was a difficult read and partly because he managed to ostracise his former cheerleader, Rheticus. Copernicus’ model still holds to the epicycles of the Greeks, and only marginally reduces the complexity of the model.

Next up comes Johannes Kepler, interspersed with Tycho Brahe. Brahe was an astronomical observer and nobleman, funded very well by the Danish king; given his own island Hveen where he built his observatory. As a keen astrologer he began his observation programme when he found a conjunction of Jupiter and Saturn was poorly predicted by current astronomical tables – how can you cast an accurate fortune under these circumstances?

Kepler was a theoretician rather than an observer but also a keen astrologer. I emphasise this because these days astrology is not held in high regard but it is the father of observational astronomy. He had started to develop a model of the solar system based on the Platonic solids – something of a mystical exercise but realised he needed better data to support his model. Brahe was the man with the data, Kepler was only just in time though – he travelled to work with Brahe when Brahe moved to Prague less than 2 years later Brahe was dead. Nowadays we know Kepler for his three laws of planetary motion – it’s worth noting that Kepler’s laws are labelled retrospectively.)

He left copious records of his progress which Koestler traces in great detail, Kepler’s struggle to recognise that planetary orbits were ellipses was heroic and has something of a pantomime air to it – “They’re right in front of you!”. His approach was unprecedented in the sense that he sought to accurately model the very best, most recent measurements. Kepler also made some attempts at a physical model to describe the motions but ultimately he is remembered for the detailed description of their motion. Since it is not central to his theme, Koestler makes only passing reference to Kepler’s work on optics.

The penultimate figure in the story is Galileo, despite Kepler’s best efforts Galileo pretty much ignored him. Galileo gets quite short shrift from Koestler who feels that he brought his troubles with the Catholic Church upon himself. Reading this account his position is not unreasonable. Galileo’s two big contributions to the story are his promotion and use of the telescope, and his work on the motion of terrestrial bodies, the generalisation of which and application to the solar system was Newton’s great triumph. Cosmologically he was only later in his life a supporter of the somewhat retro Copernican model which was a cul-de-sac in terms of theoretical developments. At the time the Catholic Church, particularly the Jesuits, were interested in astronomy and not particularly hardline about the interpretation of Scripture to fit observations. Galileo wound them up both by claiming all newly observed celestial phenomena as his own and by putting the words of the Pope in the mouth of an idiot in one of his Dialogues.

This highlights two of the wider themes that Koestler brings to his book. At one point he describes his cast of characters as “moral dwarves”, he states this is relative to their scientific achievements but returns to this theme in the epilogue where he feels that our scientific developments have not been matched by our spiritual development. The second is the schism between science and the Church that began in this period, Koestler seems to put much of the blame for this on Galileo’s head feeling that it is by no means inevitable. In the epilogue he also draws a comparison between biological evolution and scientific developments, highlighting specifically that there are long periods of not that much happening and many diversions from the “true” path.

The book finishes with a brief mention of Newton’s synthesis of Kepler’s laws and Galileo’s dynamics to produce a model of the solar system which is close to that which we hold today.

This really is a rollicking good read! This is a relatively old book, published in 1959 and one might anticipate that it has not fully caught up with modern historiography however a brief look around the internet suggests that he is not criticised in any great sense. Koestler does tend to focus on a limited number of “great” individuals and goes for “firsts” but this perhaps is what makes it a good read.

Footnotes

My Evernotes for the book are here, last page of the book at the top!

Oct 14 2012

Book Review: Alan Turing: The Enigma by Andrew Hodges

2012editionA brief panic over running out of things to read led me to poll my twitter followers for suggestions, Andrew Hodges’ biography of Alan Turing, Alan Turing: The Enigma  was one result of that poll. Turing is most famous for his cryptanalysis work at Bletchley Park during the Second World War. He was born 23rd June 1912, so this is his 100th anniversary year. He was the child of families in the Indian Civil Service, with a baronetcy in another branch of the family.

The attitude of his public school, Sherbourne, was very much classics first, this attitude seems to have been common and perhaps persists today. Turing was something of an erratic student, outstanding in the things that interested him (although not necessarily at all tidy) and very poor in those things that did not interest him.

After Sherbourne he went to King’s College, Cambridge University on a scholarship for which he had made several attempts (one for my old college, Pembroke). The value of the scholarship, £80 per annum, is quite striking: it is double the value of unemployment benefit and half that of a skilled worker. He started study in 1931, on the mathematics Tripos. His scholarship examination performance was not outstanding. Significant at this time is the death of his close school friend, Christopher Morcom in 1930.

King’s is a notorious hotbed of radicals, and at this time Communism was somewhat in vogue, a likely stimulus for this was the Great Depression: capitalism was seen to be failing and Communism offered, at the time, an attractive alternative. Turing does not appear to have been particularly politically active though.

During his undergraduate degree, in 1933, he provided a proof of the Central Limit Theorem – it turns out a proof had already been made but this was his first significant work. He then went on to answer Hilbert’s Entscheidungsproblem (German for “Decision Problem) in mathematics with his paper, “On computable numbers”1. This is the work in which he introduced the idea of a universal machine that could read symbols from a tape, adjust its internal state on the basis of those symbols and write symbols on the tape. The revelation for me in this work was that mathematicians of Turing’s era were considering numbers and the operations on numbers to have equivalent status. It opens the floodgates for a digital computer of the modern design: data and instructions that act on data are simply bits in memory there is nothing special about either of them. In the period towards the Second World War a variety of specialised electromechanical computing devices were built, analogue hardware which attacked just one problem. Turing’s universal machine, whilst proving that it could not solve every problem, highlighted the fact that an awful lot of problems could be solved with a general computing machine – to switch to a different problem, simply change the program.

Alonzo Church, at Princeton University, produced an answer for the Entscheidungsproblem  at the same time; Turing went to Princeton to study for his doctorate with Church as his supervisor.

Turing had been involved in a minor way in codebreaking before the outbreak of World War II and he was assigned to Bletchley Park immediately war started. His work on the “Turing machine” provides a clear background for attacking German codes based on the Enigma machine. This is not the place to relate in detail the work at Bletchley: Turing’s part in it was as something of a mathematical guru but also someone interested in producing practical solutions to problems. The triumph of Bletchley was not the breaking of individual messages but the systematic breaking of German systems of communication. Frequently, it was the breaking of a system which was critical in principle the Enigma machine (or variants of it) could offer practically unbreakable codes but in practice the way it was used offered a way in. Towards the end of the war Turing was no longer needed at Bletchley and he moved to a neighbouring establishment, Hanslope Park where he built a speech encrypting system, Delilah with Don Bayley – again a very practical activity.

Following the war Turing was seconded to the National Physical Laboratory where it was intended he would help build ACE (a general purpose computer), however this was not to be – in contrast to work during the war building ACE was a slow frustrating process and ultimately he left for Manchester University who were building their own computer. Again Turing shows a high degree of practicality: he worked out that an alcohol water mixture close to the composition of gin would be almost as good as mercury for delay line memory*. Philosophically Turing’s vision for ACE was different from the American vision for electronic computing led by Von Neumann: Turing sought the simplest possible computing machinery, relying on programming to carry out complex tasks – the American vision tended towards more complex hardware. Turing was thinking about software, a frustrating process in the absence of any but the most limited working hardware and also thinking more broadly about machine intelligence.

It was after the war that Turing also became interested in morphogenesis2 – how complex forms emerge from undifferentiated blobs in the natural world, based on the kinetics of chemical reactions. He used the early Manchester computer to carry out simulations in this area. This work harks back to some practical calculations on chemical kinetics which he did before going to university.

Turing’s suicide comes rather abruptly towards the end of the book. Turing had been convicted of indecency in 1952, and had undergone hormone therapy as an alternative to prison to “correct” his homosexuality. This treatment had ended a year before his suicide in 1954. By this time the UK government had tacitly moved to a position where no homosexual could work in sensitive government areas such as GCHQ. However, there is no direct evidence that this was putting pressure on Turing personally. Reading the book there is no sick feeling of inevitability as Turing approaches the end you know he has.

Currently there are calls for Turing to be formally pardoned for his 1952 indecency conviction, personally I’m ambivalent about this – a personal pardon for Turing is irrelevant: legal sanctions against homosexual men, in particular, were widespread at the time. An individual pardon for Turing seems to say, “all those other convictions were fine, but Turing did great things so should be pardoned”. Arnold Murray, the man with whom Turing was convicted was nineteen at the time, an age at which their activities were illegal in the UK until 2000.

What struck me most about Turing from this book was his willingness to engage with practical, engineering solutions to the results his mathematical studies produced.

Hodges’ book is excellent: it’s thorough, demonstrates deep knowledge of the areas in which Turing worked and draws on personal interviews with many of the people Turing worked with.

Footnotes

1. “On computable numbers, with an application to the Entscheidungsproblem”, A.M. Turing, Proceedings of the London Mathematical Society 42:230-265 (1936).

2. “The Chemical Basis of Morphogenesis”, A.M. Turing, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, Vol. 237, No. 641. (Aug. 14, 1952), pp. 37-72.

3. My Evernotes for the book

4. Andrew Hodges’ website to accompany the book (link)

Jul 25 2012

Book review: A computer called LEO by Georgina Ferry

AComputerCalledLEOThis is a review of “A Computer called LEO” by Georgina Ferry, recounting the story of the first computer developed for business use by J. Lyons & Co, the teashop and catering company.

Lyons formed in 1884, a spin-off from a family tobacconist company whose traveling salesman realised that there were few reliable teashops around the country, furthermore catering at large events such as the Great Exhibition was poor. Over the next 30 years or so the business grew, with a chain of teashops, and smarter establishments such as the Corner Houses and Trocadero. The teashops were supplied by Lyons own manufacturing and delivery service.

By the 1930s Lyons had approximately 30,000 workers, as such it was one of Britain’s larger employers. 300 clerks were used to tot up daily takings on mechanical calculators. Clerical work had risen in important during the second half of the 19th century with numbers rising from 70,000 in Britain in 1851 to 2 million in 1901. The company had a department of Systems Research led by a Cambridge mathematician, John Simmons, who the company had recruited in 1923, the hiring of such a graduate was a novelty at the time. The Systems Research department was interested in the efficient running of the business.

By this time various items of office machinery were commonplace, things such as filing cabinets, typewriters, mechanical calculators, and punch card readers. Telephone exchanges were in place, the electronic valve had been invented in the early years of the 20th century and magnetic storage devices were starting to become available. By the 1930s people such Oliver Standingford in Lyon’s Stock Department were talking about machines which would combine these elements, although he was not clear on the detail of how this would be done.

The Second World War then intervened, Lyons cut table service from its teashops as labour went short. Various people gained useful experience in electrical engineering through the wartime developments in radar, and possibly codebreaking. We now know that Colossus, a computer used for code breaking, was built at Bletchley Park during the war but it did not become public knowledge until 1974. In the US ENIAC had been developed at the Moore School in Philadelphia to do artillery range calculations. This was not a secret and immediately after the war, Oliver Standingford and Raymond Thompson visited from Lyons; they had a broad brief to investigate American business methods but it was ENIAC which really captivated them. Fortunately, their US trip put them in touch with more local expertise in the form of Douglas Hartree at Cambridge University who was building a computer, EDSAC, for the Mathematical Laboratory.

Lyons decided fairly quickly to construct their own computer, which was to be based on the EDSAC machine; US machines such as they were could not be purchased because of currency restrictions and there were no computer manufacturers in the UK. From the start LEO I (the first computer) was different, Simmons saw the computer fitting into a system of “scientific management” and as such LEO was crafted to exactly fit the role he foresaw for it based on detailed knowledge of the company’s processes. In some senses computing for business was more demanding than the computation done in the Mathematical Laboratory and other scientific laboratories: business computing had large demands for input and output (imagine a payroll system – it needs to read in details of each employee and print out the results), it had lower tolerance for failure (payroll failing to run has a serious impact on employees) and calculations could be more “complex” than mathematical ones in the sense that more steps in calculation and more conditionality was required. It was at Lyons that the art of flowcharting was developed. The first live duty that LEO carried out was in 1951, it was made public in 1955. It’s interesting to note that Charles Babbage had highlighted the potential for automation in both manufacturing and mathematical operations in his book “On the Economy of Machinery and Manufacturers”, published in 1832.

There were to be two further LEO computers, developed by a separate company, Leo Computers Ltd however things did not go well. The computers themselves were technically advanced, and the Leo Computers method of going into a business and closely examining their processes before writing programs and delivering a system combining both hardware and software usually had excellent results. However, this had the unfortunate side-effect of losing their best staff to their clients. Other problems were afoot: Leo Computers Ltd although nominally a separate company was under-resourced both financially and in personnel with development engineers also acting as salesman. The parent company, Lyons was struggling – victim of a family business mentality which put increasingly useless family members at the heads of divisions.

In 1964 Leo Computers Ltd was merged with English Electric, with Lyons divesting itself of any responsibility, following this union the LEO line died although the final computers in the series were installed by the Post Office, and continued to run there, in places, until 1981.

In contrast in the 1960s IBM were able to make an investment of $5billion on their System 360 computers – a compatible range designed to fit every need. They had a ready market in the US both of businesses willing to buy, unlike their British counterparts, and a government who bought locally first. Faced with this opposition, the British computer industry struggled to compete.

Focusing on the LEO computer makes this a human scale story with central cast of characters, but it also provides a wider view of the field in the years after the Second World War. The book makes clear how J. Lyons & Co had a system of management, and personnel in place which were ripe for computerisation; the developments in the 1930s made it clear that electronic computers were in the air. Large scale failures of computer systems in both public and private sectors are onging, John Simmons was rather insightful in his intimate coupling between business process and software system.

References

1. My Evernotes are here

2. The web page of the Leo Computer Society is http://www.leo-computers.org.uk/

May 12 2012

Book review: Measure of the Earth by Larrie D. Ferreiro

Measure-of-the-EarthThis post is a review and summary of Larrie D. Ferreiro’s book “Measure of the Earth” which describes the French Geodesic Mission to South America to measure the length of a degree of latitude at the equator. The action takes place in the 2nd quarter of the 18th century, the Mission left France in 1735 with the first of its members returning to Europe in 1744.

The book fits together with The Measure of All Things by Ken Alder, which is about the later French effort to measure a meridian through Paris at the turn of the Revolution in order to define the metre, The Great Arc by John Keay on the survey of India and Map of a Nation by Rachel Hewitt on the triangulation survey of the United Kingdom.

The significance of the measurement was that earlier triangulation surveys of France had indicated that the earth was not spherical, as had pendulum measurements made by Jean Richer in Guyana in 1671 which showed a pendulum there ran 2:28 slower there than in Paris. A Newtonian faction believed that the earth was flattened at the poles, its rotation having led to a bulging at the equator. A Cartesian school held that the earth was flattened around the equator and bulged at the poles, this was not a direct result of work by Rene Descartes but seems to have been more a result of scientific nationalism. Spoiler: the earth is flattened at the poles.

From a practical point of view a non-spherical earth has implications for navigation – ultimately it was found that polar flattening would lead to a navigational error of approximately 20 miles in a trans-Atlantic crossing although at the time of the Mission it was believed it could have been as much as 300 miles. Politically the Mission provided an opportunity for the French to form an alliance with the Spanish, and to get a close look at the Spanish colonies in South America which had provided huge wealth to Spain over the preceding 200 years. Ferreiro provides a nice overview of the L’Académie des Sciences under whose aegis the mission was conducted,and of the Comte de Maurepas, French minister of the navy and sponsor of the Mission.

The core members of the Geodesic Mission were Pierre Bouguer, Charles-Marie de La Condamine, and Louis Godin they were accompanied by Spanish Naval cadets Antonio de Ulloa y de la Torre-Guiral  and Jorge Juan y Santacilia. Other members were Joseph de Jussieu (doctor and botanist), Jean-Joseph Verguin (engineer and cartographer), Jean-Louis de Morainville (draftsman and artist), Theodore Hugo (instrument maker), Jean-Baptiste Godin des Odonais and Jacques Couplet-Viguier.

Louis Godin, an astronomer, was the senior academician and nominal leader of the mission. Pierre Bouguer, was a mathematician, astronomer and latterly geophysicist: as well as the measurement of the degree of latitude he also attempted to measure the deflection of a plumb-line by the mass of a mountain – an experiment which Nevile Maskelyne was to conclude successfully in 1775, I wrote about this here. Bouguer also wrote a treatise on ship building whilst away in South America. Charles-Marie de La Condamine could best be described as an adventurer although he was also a competent mathematician and geographer, it was his more lively writing on life in South America which would have a bigger impact on their return to Europe.

The scheme for the determination of the length of a degree is to measure the length of a meridian (a line of longitude) close to the equator by triangulation, making a ground measurement baseline to convert the angular measurements of the triangulation survey into distances and a second baseline to confirm your workings; the latitudes of the ends of the triangulation survey are determined astronomically by measuring the positions of stars. I’ve read of this process before, the new thing I learnt was the method for aligning up your zenith sector with the meridian – which I’m tempted to try at home.

These measurements were done in the area around Quito, in modern Ecuador (named after the equator), the endpoints of the survey were at Quito in the north, close to the equator and Cuenca approximately 200 miles south. During the survey, through the Andes, the team scaled peaks as high as Mont Blanc (and suffered altitude sickness for their troubles) which would not be climbed for another 50 years. The survey was repeated in the early years of the 20th century and even then it took 7 years – the same length of time as the original survey, due to the transport difficulties presented by the terrain.

The work of measuring the meridian was made more difficult by the journey to get there (which took the best part of a year), the terrain and conditions when they got there (mountainous and cloudy), the poor leadership of Godin, local political machinations and the mother country cutting them loose financially. Ferreiro makes a lot of Godin’s poor leadership, some of which is justified – he spent Mission money on prostitutes and regarded the Mission funds as his own purse. Frequently the Mission split into two groups, one containing Bouguer and La Condamine and the other Godin – sometimes this is quite appropriate, in duplicating measurements for consistency whilst on other occasions it is simply fractiousness.

To a degree the Mission was scooped by measurements made above the Arctic Circle in Lapland, this mission was also promoted by the L’Académie des Sciences, led by Pierre Maupertuis (a rival of Bouguer) and Anders Celsius. It completed its work in 6 months, well before the Geodesic Mission had finished their work, discovering that the poles of the earth were flattened. However, doubts remained over the results and the full determination required the data from the equator. Bouguer presented this on his return to France, to great acclaim, showing that the earth was flattened by 1 part in 179 (later measurements showed that the flattening is actually smaller at 1 part in 298).

The Mission spawned a wide range of publications by its members, covering not only the geodesic component of the work but also regarding life and nature in South America. Ferreiro credits La Condamine’s work in particular has setting the context of how South America was viewed for quite some time after the mission. The Spanish officers also made in impact an highlighting colonial misrule back to their home country. Arguably the international collaborative elements of the Mission set the scene for the measurements of the transit of Venus later in the 18th century.

Ferreiro makes a comparison between the French Geodesic Mission, which was centrally run by the state and the British Longitude Prize, which although state funded was privately executed, implying that the former was superior. It’s not clear to me whether he’s engaging in a degree of hyperbole here, since the Mission was to some degree an organisational car-crash and was in large part funded from La Condamine’s own purse at the time. Furthermore, L’Académie des Sciences also awarded prizes – having copied the British government in this and the Royal Society was from the outset a very internationally oriented organisation. So the picture as Ferreiro presents it is something of an over-simplification.

I found the book very readable, its clearly based on a large quantity of primary source material and covers a great deal beyond the simple mechanics of the Geodesic measurements.

Footnotes

My Evernotes on the book are here.

Apr 17 2012

Book Review: Stargazers by Fred Watson

41W3OswkqxL._SS500_This post is a review of “Stargazers:The Life and Times of the Telescope” by Fred Watson. It traces the history, and development of the telescope from a little before its invention in 1608 to the present day.

The book begins its historical path with Tycho Brahe, a Danish astronomer who lived 1546-1601. He built an observatory, Uraniborg, on the Danish island of Hven in view of his patron, King Frederick II of Denmark. Brahe’s contribution to astronomy were the data which were to lead to Johannes Kepler’s laws of planetary motion and ultimately Isaac Newton’s laws of gravitation. On the technical side his observatory represented the best astronomy of pre-telescope days with the use of viewing sights, his Great Armillary with it axis aligned with that of the earth and graduated scales to measure angles. Watson also cites him as a first instance of a research director running a research institute – alongside the observatory he ran a print works to disseminate his results.

The telescope was first recorded in September of 1608, when Hans Lipperhey presented one to Prince Maurice of Nassau in the Netherlands. Clearly it was a device of its time since in very short order several independent inventions appeared, Galileo constructed his own version which led to his publication of “The Starry Messenger” in 1610 which reports his observations using the device. The telescope grew out of the work of spectacle makers; there are some hints of the existence of telescope-like devices in the latter half of the 16th century but these are vague and unsubstantiated. Roger Bacon and Robert Grosseteste both conceived of a telescope-like device in the 13th century, around the time the first spectacles were appearing. Although there are a few lenses from antiquity there is no good evidence that they had been used in telescopes.

The stimulus for the creation of the first telescopes seems to have been a combination of high quality glass becoming available, and skilled lens grinders. The lens making requirements for telescopes are much more taxing than for spectacles. The technology required is not that advanced, if you look around the web you’ll find a community of amateur astronomers grinding their own lenses and mirrors now using fairly simple equipment, typically a turntable with a secondary wheel which produces linear motion for the polishing head back and forward across the turning lens blank. The most technologically advanced bit is probably captured in the first step: “acquire your glass blank”.

Through the 17th century refracting telescopes were built of ever greater length in an effort to defeat chromatic aberration which arises from the differential refraction of light as a function of wavelength (colour) – long focal length lenses suffered from less chromatic aberration than the shorter focal length ones which would allow a shorter telescope. Johannes Hevelius made telescopes of 46m focal length (physically the telescope would be a little shorter than this), mounted on a 27m mast; Christiaan Huygens dispensed with the “tube” of the telescope entirely and made “aerial telescopes” with even longer focal lengths, up to 64m.

It was known through the work of Alhazen in the 10-11th century, and others, that reflecting, curved-mirrors could be used in place of lenses. A telescope constructed with such mirrors would avoid the problem of chromatic aberration. However, the polishing tolerances for a reflecting telescope are four times higher than that of a lens. Newton built the first model reflecting telescope in 1668 but no-one was to repeat the feat until John Hadley in 1721.

Theoretical understanding of telescopes developed rapidly in the 17th century both for refracting and reflecting telescopes, indeed for reflecting telescopes there were no fundamental advices in the theory between 1672 and 1905. The problem was in successfully implementing theoretical proposals. Newton claimed that chromatic aberration could not be resolved in a refracting telescope, however he was proved wrong by Chester Hall Moor in 1729, and somewhat controversially by John Dollond in 1758 who was able to obtain a patent despite this earlier work (which was defended aggressively by his son) – the trick is to build compound lenses comprised of glass of different optical properties.

Also during the 18th century the construction of reflecting telescopes became more common, William Herschel started building his own reflecting telescopes in 1773 with the aid of Robert Smith’s “Compleat system of opticks”. Ultimately he was to build a 40ft (12m) telescope with a 48 inch (1.2m) mirror in 1789, supported by a grant from George III. During his lifetime Herschel was to discover the planet Uranus (nearly called George in honour of his patron), numerous comets and nebulae. At the time “official” astronomy was more interested in the precise measurement of the positions of stars for the purpose of navigation. Herschel was to be followed by Lord Rosse with his 1.8m diameter mirror telescope built in 1845 at Birr Castle, this has been recently restored (see here). He too was interested in nebula and discovered spiral galaxies.

During the 19th century there were substantial improvements in the telescope mounts, with engineers gaining either an amateur or professional interest (men such as James Nasmyth and Thomas Grubb). Towards the end of the century photography became important, which placed more exacting standards for telescope mounts because to gain maximum benefit from photography it was necessary to accurately track stars as they moved across the sky to enable long exposure times. This is also the century in which stellar spectrography became possible with William Huggins publishing the spectra of 50 stars in 1864. Léon Foucault invented the metal coated glass mirror in 1857 which were lighter and more reflective than the metal mirrors used to that point. As the century ended the largest feasible refracting telescopes with lens diameters of 1m were just around the corner, above this size a lens distorts under its own weight reducing the image quality.

In 1930 Bernhard Schmidt designed a reflecting telescope which avoided the problem of aberrations away from the centre of the field of view making large field of view “survey” telescopes practicable. As a youth in the 1970s I learnt of the 200-inch (5 metre) Hale telescope at Mount Palomar, since then space telescopes able to see in the infra-red and ultra-violet as well as the visible have escaped the distortion the atmosphere brings; adaptive optics are used to counteract atmospheric distortion for earthbound telescopes and there are “distributed” interferometric telescopes which combine signals from several telescopes to create a virtual one of unfeasible size.

Watson mentions briefly radio telescopes and in the final chapters speculates on developments for the future and gravitational lensing – natures own telescopes built from galaxies and spread over light years.

I enjoyed “Stargazers” as a readable account of the history of the telescope which left me with a clear understanding of its principles of operation and the technological developments that enabled its use, it also provides a good jumping off point for further study.

Footnotes

My Evernotes for the book are here, featuring more detailed but slightly cryptic notes and links to related work.