Danish astronomer Ole Romer (1644–1710). (Photo by SSPL/Getty Images)


In his book, Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe, Professor Alan Hirshfeld reveals how the likes of a reverend, a construction worker and a young apprentice went on to transform our scientific understanding of the universe.

A professor of physics at the University of Massachusetts Dartmouth, Hirshfeld recounts how these British amateurs used innovative technology to usher astronomy into the modern age.

Here, writing for History Extra, Hirshfeld reveals 10 astronomers you’ve (probably) never heard of.

Aristarchus (c310 BC – c230 BC)

The astronomy of antiquity was nothing if not practical: telling the time of day, finding one's geographic location, establishing a calendar, and predicting the future. The latter motivated skywatchers as far back as the eighth century BC to keep close track of celestial movements and events: the discovery of solar, lunar, and planetary cycles prompted the ancient Greeks to speculate about the relative arrangement of the major bodies in the cosmos.

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Virtually every Greek philosopher from Pythagoras to Aristotle believed that Earth occupies the hub of the universe and that the Sun, Moon, planets, and the star-studded celestial sphere whirl around it. This geocentric mindset held sway until the mid-15th century when Copernicus, a reclusive administrator for the Catholic Church in Poland, proposed an alternative ordering.

Driven in part by the aesthetic imperative to centrally situate the Sun — “the lamp that illuminates the universe”, in his words — Copernicus bumped our Earth to the status of third planet out from the centre, after Mercury and Venus. Yet almost lost to history is the surprising fact that the Copernican – or heliocentric – cosmos was anticipated much earlier by a Greek geometer of the third century BC named Aristarchus.

What little we know about Aristarchus, other than he grew up on the Aegean island of Samos, comes from his lone surviving manuscript, plus a mention by his illustrious contemporary, Archimedes, architect of the principles of buoyancy and the lever. We know, for instance, that Aristarchus was the first mathematician to apply geometry to cosmic measurement: he estimated the Moon's distance by noting the width of Earth's shadow during a lunar eclipse, and the Sun's distance from the angle formed by the Earth, Moon, and Sun when a half-Moon appears in the sky.

Archimedes augmented his compatriot's otherwise humble portfolio with a revelatory bombshell: in a speculative treatise on how many sand grains it would take to fill the universe (a numerical entertainment for the king of his native Syracuse), Archimedes reported that Aristarchus had proposed the counterintuitive idea that Earth orbits a central, stationary Sun, in stark opposition to the entrenched geocentric model of the cosmos. Not only that, Archimedes added, but Aristarchus says the Earth is spinning, to account for the daily rising and setting of the Sun.

In the eyes of his fellow philosophers, Aristarchus was at best a radical thinker, at worst a heretic. At least one tract denouncing him for impiety was circulated during his lifetime. Although his Sun-centered model gained little traction compared to its geocentric alternative, history affirms that Aristarchus envisioned the true layout of our solar system some 1,800 years before Copernicus did.

Ole Roemer (1644–1710)

In 1638, while under house arrest for his heretical advocacy of the Copernican cosmos, a then-elderly, nearly blind Galileo directed an attempt to measure the speed of light, which was believed by many of his contemporaries to be infinite. However, his best efforts to time the passage of lamplight between adjacent hilltops in his native Tuscany yielded a null result. If not infinite, Galileo concluded, the speed of light was too fast to be measured in any earthbound experiment.

In 1672, some 30 years after Galileo's death, a 28-year-old Danish astronomer and instrument maker named Ole Roemer arrived at the Paris observatory, King Louis XIV's recently completed edifice to the astronomical sciences. While doubling as a construction supervisor for a number of state-sponsored projects, Roemer conducted regular observations of Jupiter's moons as they circled the giant planet.

Once every orbit, each moon was eclipsed by Jupiter’s disk. Roemer noticed that the interval between successive eclipses of the innermost moon, Io, grew steadily longer as Earth, in its orbit, swung away from Jupiter, and steadily shorter as Earth approached Jupiter. Roemer correctly surmised that this behavior arises, not from any speeding up or slowing down of Io itself, but from the greater or lesser time required for Io's light to traverse the changing gap between Jupiter and Earth. A straightforward calculation, Roemer realised, would yield the speed of light.

Roemer's observed 11-minute delay or advance of Io’s eclipses implied that light travels through space at about 140,000 miles per second. Although this result is a gross underestimate of the actual value — 186,000 miles per second — it was enough to convince contemporary scientists like Isaac Newton that the speed of light is not infinite.

Roemer returned to Denmark in 1681 to become director of the Copenhagen observatory. While continuing to conduct astronomical observations, he variously served as judicial magistrate, chief tax assessor, police chief, and mayor of the city of Copenhagen, as well as head of the Danish state council.

James Bradley (1693–1762)

In 1530 Nicolaus Copernicus formulated a model of the universe that placed the Sun, rather than the Earth, at the centre [De Revolutionibus Orbium Coelestium was published in early 1543]. However, observational proof of his proposed heliocentric system was lacking for a long time.

Adherents of the competing geocentric model were quick to point out that a moving Earth would cause stars to sway in the night sky, in reflection of our planet’s annual journey around the Sun. Copernicus countered that stars are incredibly far away, rendering their parallax oscillations too small to detect. But that didn't stop astronomers from seeking them out anyway.

Years later, in 1721, the unassuming Reverend James Bradley became Oxford's newly appointed Savilian Professor of Astronomy. For years he had quietly tended his flock of parishioners in Bridstow, Monmouthshire by day, and by night donned a warm coat to enjoy astronomical observation.

A staunch proponent of Copernicus's heliocentric model, he took up the search for the elusive stellar parallax. He installed a fixed, vertical telescope in his aunt's house in Wanstead, and every night noted precisely where the bright star Gamma Draconis traversed his telescope’s circular field of view. (To accommodate the instrument, holes had been sawed in the roof and floor of the single-story cottage, and the viewing eyepiece was relegated to the coal cellar.)

In 1729, after several years of observations, Bradley announced that Gamma Draconis indeed executes an annual wobble in the sky. Yet the size of the wobble and its seasonal variation were different to those expected. Bradley had inadvertently stumbled upon a different phenomenon, which he named stellar aberration, whereby a star's perceived location in the sky is altered, not by Earth's orbital position, but by Earth's orbital velocity. (An analogy: this is why raindrops streaking the window of a fast-moving train are angled instead of vertical – the speedier the train, the more oblique the angle.)

So nearly two centuries after its introduction, the ‘Copernican revolution’ was at last complete.

Friedrich Wilhelm Bessel (1784–1846)

As an apprentice in a Bremen import-export firm at the beginning of the 19th century, Friedrich Wilhelm Bessel daydreamed of abandoning his dreary ledgers and hopping aboard a cargo vessel bound for exotic ports. Each evening after work he labored over texts on celestial navigation and mapmaking, considering the methods of finding one’s place in the world, in preparation for these imagined journeys.

Soon his earthbound musings shifted toward the cosmic, and he began reading the densely mathematical works of Copernicus, Kepler, and Newton. By 1806, aged 22, he was a working astronomer at a private observatory in Lilienthal, and four years later was appointed director of Prussia's new state observatory in Königsberg. Over the succeeding decades, Bessel honed his telescopic skills by single-handedly measuring the precise sky coordinates of 32,000 stars.

By the mid-1830s, Bessel took on the challenge of measuring the distance to a star. The only way to accomplish this was to gauge the star's parallax – the tiny annual wobble in the star's position due to Earth's shifting viewpoint as it circled the Sun. Many others before him had tried and failed; evidently, as Copernicus himself had asserted, stars are so remote that their parallaxes shrink into insignificance.

Starting in 1837, using a specialised telescope crafted by German master optician Joseph Fraunhofer, Bessel painstakingly recorded the nightly sky coordinates of 61 Cygni, a moderately bright star in the constellation Cygnus. Just over a year later, in December 1838, he announced that he had detected the annual parallax angle of 61 Cygni: a mere nine hundred-thousandths of a degree, placing the star some 600,000 times the Sun's distance from Earth.

Awarding Friedrich Bessel the gold medal of the Royal Astronomical Society in 1841, English astronomer John Herschel called the achievement "the greatest and most glorious triumph which practical astronomy has ever witnessed."

The following year, aged 58, the now famous Friedrich Bessel embarked for England, fulfilling his long-ago dream to visit foreign ports.

Asaph Hall (1829–1907)

Speculation about the existence of Martian moons dates all the way back to Jonathan Swift's 1726 blockbuster, Gulliver's Travels, in which the fictional scientists of Laputa spy two satellites circling the Red Planet. Though the stuff of imagination, Swift's prediction would prove to be uncannily prophetic.

A one-time carpenter’s apprentice and aspiring architect, Asaph Hall discovered the joys of astronomy while an undergraduate at the University of Michigan. In 1857, now married to his former mathematics instructor, Angeline Stickney, Hall became an assistant at the Harvard College Observatory at a salary of three dollars per week. One astronomer advised him to pursue another line of work, lest he and Angeline starve.

Hall left Harvard in 1862 for a better-paid position at the US naval observatory in Washington DC, where he advanced to a senior post in mathematical astronomy. In 1875 he was assigned to the observatory’s new 26-inch refractor telescope. His charge: to track the movements of planetary satellites, whose orbital specifications revealed their host planet’s mass.

Hall was especially keen to wield the top-notch instrument in search of Martian satellites during the Red Planet’s close approach in August 1877. Swift's long-ago fabrication had escalated into a wishful expectation among astronomers, who, like Hall, sought the satellites' discovery.

In early August 1877, Hall commenced an intensive, but fruitless, search for the purported Martian satellites. Dejected, he arrived home, where his wife Angeline insisted that he try again. So instructed, he returned to the observatory.

In the pre-dawn hours of 12 August 1877, an asteroid-sized moon, to be named Deimos, swam into the field of view. Four nights later, it was joined by a larger mate, Phobos. Asaph Hall was acclaimed for his discovery worldwide.

Nearly a century afterward, the US Mariner 9 spacecraft captured the first close-up images of Mars's shrunken, oblong satellites. A pair of craters on Phobos now bear the names Hall and Stickney, in honor of the discoverer and his astute wife.

Henrietta Swan Leavitt (1868–1921)

Measuring a celestial object's sky coordinates — its astronomical latitude and longitude — is relatively straightforward. But gauging an object's distance from Earth has long posed a challenge for astronomers.

In regions of space relatively close to the solar system, the size of a star's annual parallax yields up its distance; the remoteness of more distant stars can be pegged by clues in their light spectrum. However, galaxies lie outside the borders of our Milky Way, largely beyond the range of such techniques. For these far-flung stellar vortices, new methods of distance measurement had to be developed.

Enter Henrietta Swan Leavitt, who took up the study of astronomy in 1892 during her senior undergraduate year at Radcliffe College in Cambridge, Massachusetts. Leavitt was doubly challenged, both by a progressive loss of hearing and by the lack of professional opportunities for women in astronomy. With no job prospects on the horizon, she engaged in volunteer work at the Harvard College observatory, tasked with measuring the brightness of thousands of stars on its burgeoning collection of glass photographic plates.

After a promotion in 1902 she began an investigation of variable stars — stars whose light emission rises and falls periodically. Leavitt maintained a blistering work pace: by 1905 she had added more than a thousand new entries to the roster of known variable stars. Princeton astronomer Charles Young dubbed her a "variable-star fiend" in a letter that year to Edward Pickering.

Leavitt focused her attention on a well-known category of variable stars called Cepheids: highly luminous objects whose brightness cycles in a distinctive fashion over periods ranging from about a day to seventy days. She identified hundreds of Cepheids in photographs of the Southern Hemisphere's famed Magellanic Clouds, satellite galaxies of our own Milky Way.

In compiling her results, she discovered what she termed a "remarkable relation": the period of a Cepheid's brightening-dimming cycle closely correlates with its average brightness; that is, the longer the period of the Cepheid, the more luminous it is.

Leavitt proposed to astronomers that, once calibrated against the properties of well-studied Cepheids, the period-luminosity law, as it is now known, would provide the means to infer any Cepheid's distance — and, by extension, that of its host star cluster or galaxy. She published her findings in a Harvard College observatory circular in 1912, including a now-iconic graphical rendering of her period-luminosity law.

Astronomers continue to use Leavitt's method to pinpoint the distances of star clusters and galaxies. With her groundbreaking work on Cepheid variable stars, she expanded the frontiers of cosmic measurement as well as the prospects of women researchers in the field of astronomy.

Annie Jump Cannon (1863–1941)

One of the great developments in stellar astronomy during the latter half of the 19th century was the ability to view, and later to photographically record, the spectra of the Sun and stars. The various patterns of dark lines that intrude upon these otherwise color-infused spectra were found to match those of chemical elements identified in terrestrial laboratories. Thus, by direct comparison, astronomers could deduce the elemental makeup of a star's atmosphere – a scientific feat that had been considered impossible only decades before.

Almost immediately, observers noticed spectral commonalities among stars of similar color: for example, a solar-type spectrum is characteristic of many yellowish, Sun-like stars. Several researchers proposed spectral classification schemes, but it was not until the 1890s that a viable system was created.

Annie Jump Cannon trained in the field of astronomical spectroscopy at Wellesley College, one of the premiere learning institutions for women in the US. In 1896, then nearly deaf from a severe case of scarlet fever, she joined the growing staff of women workers at Harvard College observatory. While her colleague Henrietta Leavitt had been tasked with the study of variable stars, Cannon was placed in charge of the Henry Draper catalogue, a massive effort to inspect and categorise the photographed spectrum of virtually every star visible through a small telescope — some 300,000 in number.

Cannon adopted the letter-classification scheme of a predecessor, whereby the relative prominence of particular spectral lines determined whether the star was designated type A, type B, type C, and so on. She came to realize that many of the letter designations were redundant, and should be combined. Furthermore, the alphabetical ordering of spectral types was arbitrary – her rearrangement O, B, A, F, G, K, M placed stars in the more logical sequence of decreasing surface temperature.

Cannon’s refined classification system was adopted by astrophysicists to develop the Hertzsprung-Russell diagram – a graph of stellar luminosity versus spectral type used to this day to analyze the physical properties and evolution of stars. And by the early 1900s, Cannon had so familiarised herself with photographs of stellar spectra that she could classify an astonishing 300 stars per hour and was recognised as the world’s leading authority on spectral classification.

Cannon became the first woman to receive an honorary doctorate from Oxford University, in 1925. She was also designated an honorary member of England's Royal Astronomical Society and voted the first female officer of the American Astronomical Society. Despite her scholarly reputation and accomplishments, only in 1938 did Harvard University appoint Annie Cannon, then aged 75, to an official professorial-level research position.

George Ellery Hale (1868–1938)

The introduction of the telescope in the early 1600s revolutionised astronomy by revealing a vast array of outer-space objects invisible to the naked eye. A similar leap in optical capability occurred during the late 1800s, when the camera and the spectroscope entered the observatory.

For the first time, celestial scenes could be chemically recorded, bypassing the physiological constraints of the human eye and the descriptive limitations of words or pencil sketches. But larger telescopes had to be built before these new technologies could be applied to more distant realms of the cosmos.

Raised in the Chicago suburb of Kenwood, George Ellery Hale’s scientific passions were amply indulged by his father, who had amassed a fortune in the elevator business. As a teenager, Hale transformed an upstairs room into a laboratory — complete with a Bunsen burner, batteries, chemicals, and a clamorous steam engine nicknamed “the demon” — then added a rooftop platform for his beloved telescope.

His interest in astronomy intensified during his undergraduate years at the Massachusetts Institute of Technology, and by the time he was 23, in 1891, Hale had assembled America's best-equipped private research observatory, in a lot next to his family home.

Hale’s invention of a solar camera, which imaged the Sun's surface more clearly than ever before, drew praise from astronomers worldwide, and garnered him a faculty appointment at the University of Chicago in 1892. And over the coming decades he spearheaded the construction of the world's largest telescope four times in succession.

In the mid-1890s, Hale extracted $300,000 from the larcenous streetcar magnate Charles Yerkes to emplace a 40-inch refractor telescope outside the city. This instrument was superseded in 1908 by a 60-inch reflector telescope at Mount Wilson observatory, a research facility in California founded by Hale four years earlier.

A decade on, the 60-inch was surpassed by a 100-inch reflector, which astronomer Edwin Hubble would subsequently use to prove the existence of galaxies outside the Milky Way and to detect the expansion of the universe.

Even in retirement, having suffered several crippling bouts of nervous exhaustion, Hale remained alert to professional opportunities. On a whim, in 1928, he mailed the Rockefeller Foundation a magazine piece he had written on next-generation telescopes. Shortly afterward, he informed a friend that his article “shot like an arrow into the blue, seems to have hit a 200-inch reflector."

Completed in 1947, nine years after Hale's death, and named in his honor, the immense telescope on Palomar Mountain remains one of the great engineering marvels of the age.

Milton Humason (1891–1972)

Perhaps no astronomer has had a career arc more remarkable than Milton Humason's. Coaxing a pack mule along the treacherous path up California’s Mount Wilson in 1910, Humason could hardly have suspected that one day he would plumb the cosmic depths with the world's largest telescope, much less co-author a landmark research paper in the Astrophysical Journal.

After desultory stints as a bellboy, handyman, and citrus farmer, he returned to Mount Wilson observatory in 1917 as a janitor. In his off-hours, he learned the rudiments of celestial photography from an undergraduate intern, and soon became so adept at it that he was appointed to the technical staff.

By the time famed astronomer Edwin Hubble came calling in 1928, Humason was Mount Wilson's foremost imaging expert and self-appointed guardian of the facility, instructing night assistants and astronomers alike on the proper use and care of the equipment.

Humason accepted Edwin Hubble's invitation to join him in an arduous project to obtain detailed photographs of galaxies and of their individual spectra. Features in each galaxy’s picture would allow an estimate of its distance, while line patterns in the galaxy’s spectrum would yield its speed of recession. (Astronomer Vesto Melvin Slipher at the Lowell Observatory in Flagstaff, Arizona, had previously shown that, with few exceptions, galaxies are receding from our Milky Way.)

Hubble sought to prove a lockstep mathematical relationship between galactic distance and galactic speed, a characteristic that implies an expanding universe — and would come to be known as Hubble's law.

Milton Humason's telescopic experience was pivotal in capturing the faint galactic spectra on photographic plates – a feat that required time exposures of many hours, even with the camera attached to Mount Wilson’s light-gulping one-hundred-inch reflector. Through herculean effort, Humason obtained spectra of forty galaxies over the span of two years.

So critical was Humason's contribution to the accumulating evidence of the expanding universe that Edwin Hubble listed the former janitor as co-author of his published paper in 1931.

Humason remained on the observatory staff until 1957, collaborating with a roster of notable astronomers to photograph some of the dimmest, most distant objects in the observable universe. In 1950, Milton Humason — a school dropout at age 14 — was awarded an honorary doctorate from Sweden's Lund University.

Arno Penzias and Robert Wilson

Edwin Hubble's confirmation that galaxies are receding from one another led to the concept of the expanding universe and to the Big Bang theory of its origin. As compelling as the Big Bang scenario was, scientists sought independent confirmation of its reality.

In 1948, physicists George Gamow, Ralph Alpher, and Robert Herman predicted that, if the universe had originated in a tiny, dense fireball, as per the Big Bang theory, then a much-diluted remnant of that hot primordial energy must infuse all of present-day space.

But having expanded for many billions of years, the universe would have cooled from its original trillions of degrees to just a few degrees above absolute zero. This remnant energy would comprise a veritable sea of short-wavelength radio waves, or "cosmic microwave background," that might, in principle, be detectable in every direction from Earth.

In 1963, research physicists Arno Penzias and Robert Wilson at Bell Labs in Holmdel, New Jersey, set out to repurpose the facility's satellite communications antenna to the study of cosmic radio waves. Although both had advanced training in radio astronomy, neither had employed a receiving antenna as odd-looking as the one at Bell Labs. Twenty-feet long and shaped like an outsized ear-trumpet, the steerable "horn antenna" was designed to capture and measure the microwave energy coming from any direction it was pointed.

However, before their instrument could deliver credible data about cosmic radio-wave emitters, Penzias and Wilson had to identify and mathematically nullify all sources of interference. These included terrestrial sources, such as microwave transmission towers and the radiation emanating from nearby Manhattan, plus celestial sources, such as the Sun and the energetic core of our galaxy.

After almost a year of intensive work, the horn antenna was still plagued by a persistent, low-level “noise” that appeared to arrive from all directions in the sky. Having ruled out every known terrestrial and celestial microwave source — even the heat emission from pigeon droppings on the antenna's interior — Penzias and Wilson were stumped.

They consulted scientists at nearby Princeton University, who realised that the two radio astronomers had stumbled upon the theorized cosmic microwave background. The long-sought confirmation of the Big Bang theory was unexpectedly at hand.

Penzias and Wilson's dogged persistence in the face of a seemingly insurmountable technical challenge resulted in one of the most significant astronomical discoveries in history. The pair received the Nobel Prize for their achievement in 1978.


Alan Hirshfeld is professor of physics at the University of Massachusetts Dartmouth. To find out more about his latest work, Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe, which is published by Bellevue Literary Press, click here.