WINDOWS TO THE STARS
They aren’t household names, but they should be. IVY SHIH and TONER STEVENSON introduce us to the forgotten women of Australian astronomy.
For seven decades, the world worked to map the heavens – a massive endeavour that ran from the 19th century to the era of manned space flight, taking in two world wars and vast social change. Here, we explore Australia’s role in the project and the lives of those who measured the stars: the “lost women” of Australian astronomy.
TO AN UNTRAINED EYE, the glass plate resembles a dirty tile covered with tiny black spots – as if an ink-filled paintbrush had been gently flicked across its surface. But in fact the plate is a piece of the night sky – the glass-plate negative from a photograph taken over a century ago. Each black speck – shown on these pages –is a single star.
There are thousands of such plates – the precious residue of one of the most ambitious international astronomy projects undertaken in the 19th century. The goal was to use photography to map the entire sky and create a complete record of the stars, known as the Astrographic Catalogue (AC), and a separate project of printed images called the Carte du Ciel.
Originally estimated to take only a few years, the project would span more than seven decades in Australia. Behind the scenes were female “human computers” who diligently measured and calculated the position of tens of thousands of stars, whose work has remained mostly unnamed and unrecognised for decades (see p 39).
Toner Stevenson was manager of Sydney Observatory in 2004 when she first saw the glass plates with observatory curator Nick Lomb. Each was 160mm2, with a curved corner at bottom left. She’d been reading about the AC for her doctoral thesis and had come across a sentence saying that the star measurements were why the project took so long.
The search for an answer led her to Macquarie University’s library and a basement lined with shelves stocked with thousands of cardboard boxes – each containing up to a dozen glass plates in paper covers, where they had rested for 20 years.
When Sydney Observatory stopped being a research station, Macquarie University astronomer Alan Vaughan volunteered to find a place to store the thousands of plates in its collection, and the University signed an agreement with the Museum of Applied Arts and Sciences, which currently operates Sydney’s Powerhouse Museum and Museums Discovery Centre, as well as the Sydney Observatory.
Each glass plate was wrapped in a paper cover with notes and observations; Vaughan was their caretaker from 1986. He recalls finding a 1910 cover with the notation “Halley’s Comet”. The glass plate inside had an image of the comet itself – a dark brushstroke across the surface rendered as it streaked across the sky.
The collection included the original logbooks and catalogues, which preserved all the data of exposure – including date, time, weather, observer, length of exposure, the relative ascension and declination (coordinates) and the plate number.
“I thought: This is an incredible story because this is the sky captured during a single epoch, because most of the photographs were taken over 20 years from 1895 to 1915,” says Stevenson, who now manages the University of Sydney’s School of Philosophical and Historical Inquiry. “So it was capturing something we could never capture again.”
THE GREAT STAR CATALOGUE
At Paris Observatory in 1887 a remarkable meeting of astronomers from around the world produced a startling idea – to create a map of the skies using the latest photography and measuring techniques. It would capture the position of the stars in both northern and southern hemispheres.
The task of compiling the work would fall to 18 observatories around the world. The sky was divided into zones, allocated to different observatories. In Australia, observatories in Melbourne and Sydney were the first to take part. Adelaide came on board shortly after to assist Melbourne; Perth joined the effort in 1900, taking over the zones originally assigned to Rio de Janiero Observatory. Together the Australian contingent would go on to map 43% of the southern hemisphere – one of the richest concentrations of stars because the Milky Way galaxy crossed right through it – and 17% of all the stars recorded.
The AC aimed to record the position of all the stars, and the Carte sought to be one of the most detailed photographic charts of the heavens. Each would require separate glass plates. The plan was to make photographic charts of all stars down to the 14th magnitude (some 40 million stars) and catalogues of the positions of stars down to the 11th magnitude (two to three million), based on accurate measurement of the photographs.
Coated with a special dry gelatine emulsion, the stellar photography plates were first exposed to a réseau plate – the same size as the photographic plate, but coated in silver and ruled into a grid of fine lines five centimetres apart. The réseau plate was placed in direct contact with the photographic plate and exposed for four to five minutes to imprint the réseau grid on the emulsion. The result was a photographic slide traversed by a grid of very fine lines – indispensable as reference marks in determining the placement of the stars. The plates were then ready for star photographs.
Male astronomers took the photographs using special “astrographic telescopes”. Every four-to-sixminute exposure was arranged so that a plate would show a number of standard stars, from which the rest of the stars on the plate – varying from 300 to 1000 – could be measured. In Australia, most of the photographs were taken by 1915; some were later retaken. Each contained hundreds, if not thousands, of stars.
Ensuring precise measurements was an intricate process. Originally the plates were to be measured at a central facility in Paris, but a change of plans saw the work transferred to individual observatories; Melbourne and Sydney shared a measuring bureau located in Melbourne.
The women who observed, catalogued and calculated the star positions on the AC had outstanding mathematical ability, with the analytical skill to measure and calculate the stars’ positions. They worked on purpose-built measuring machines called micrometers and used standard astrometry reduction formulas to determine the true star co-ordinates.
They worked in pairs, exchanging roles to measure each star on each photographic plate twice – once on the front face and again when the slide was turned 180 degrees. The photographic plate was placed in a micrometer and the star’s X and Y co-ordinates were measured and calculated. The measurer on the micrometer would read out the data, which was entered into logbooks by the second measurer. Afterwards the mean of the two measurements was calculated and also entered into the logbook.
Co-ordinate measurements were compared between measurers in an effort to eliminate human error. Given that the plates overlapped, each star could be measured three or four times. Given front, reverse and overlap measurements, each glass plate was handled multiple times. According to contemporary reports in the project’s early years, on average 170 stars could be measured in an hour.
THE WORTH OF STELLAR HISTORY
When the Sydney AC’S final volume was published in 1964, Australia’s involvement officially came to an end. The global endeavour to chart the heavens had lasted almost eight decades. Stevenson estimates that over that time Australia’s astronomers charted and measured the positions of more than 1.5 million stars. They photographed 142,021 glass plates and published over 50 catalogue volumes. Stevenson’s research indicates that Sydney and Melbourne completed their Carte photography; Perth exposed only 100 plates before abandoning the project after 1903.
The global astronomy and scientific community managed the immensity of the AC and Carte projects through the opening and closing of observatories and multiple funding and budget cuts. Work had waxed and waned but seemed to be a constant through a world changing around it. The project had continued through the Great Depression and two world wars. Even when Melbourne Observatory ceased research in 1945 and closed in 1948, Sydney Observatory decided to complete the unfinished measurements and take the projects through to completion.
Astronomy had also moved on. In the 1940s, Ruby Payne-scott’s work in radio physics and radio astronomy cemented her as an Australian pioneer – she is among the first female radio astronomers. In 1961, the Parkes radio telescope was opened. Just five years after Australia’s work on the AC finished, Apollo 11 landed on the moon.
Now it only takes months to image the night sky. The Cerro Pachón ridge in north-central Chile will soon host the new 8.4-metre Large Synoptic Survey Telescope at the newly named Vera C Rubin Observatory, which is capable of surveying the entire southern hemisphere sky in just three days.
The goal is to take 800 separate images of each region of the night sky over 10 years.
Given this, it seems easy to view the AC and Carte as historical remnants of humanity’s ambition and attempt to have some degree of mastery over the heavens. But the glass plates and catalogue have a legacy that’s still playing a part in modern astronomy.
Each glass plate’s smattering of black spots is the light of stars past – a unique window to the state of the night sky up to a century ago. The plates are effectively time capsules that allow astronomers to study the evolution of galaxies. Space is a constant roil of activity and motion of celestial bodies, dying, colliding and being born. Within the plate images are data that opens up new possibilities in astronomical research.
This is because of the speed at which light travels. It takes 1.25 seconds for moonlight to reach Earth. The Sun’s light takes more than eight minutes. Whenever we peer through a telescope at Proxima Centauri, Earth’s closest star other than the Sun, we’re looking at starlight that began its journey to Earth 4½ years ago. As Alan Vaughan’s research student 20 years ago, Lesa Moore painstakingly created a digital catalogue of the glass plates at Macquarie University. She spent months going through the plates, transcribing what they contained.
“The idea [was] to get… the variability and the movement in the stars so we can understand how little sections of space are evolving,” says Moore, who is now web editor at the Astronomical Society of New South Wales.
Keen to see the potential data locked in the glass plates, in 2006 Vaughan used a custom-built suitcase lined with rubber pillows to transport 650 plates to Cambridge University, where the Automatic Plate Measuring Machine at the National Astronomy Facility was capable of scanning each image in layers, thus ensuring every star was included.
When the images came back from Cambridge, Vaughan and Moore assessed the quality of each plate and, using software, extrapolated the stars’ positions and modern co-ordinates to determine how much they had moved in the time since the photograph was taken.
“To get a historic baseline and see things change in time is very difficult in astronomy,” Moore says. “We can’t wait around and see a star go supernova. We have to get all of the information from what we can see today and however far back our records go. So any kind of historic record, even sketches that date back before photography, are useful in understanding how celestial objects and systems evolve.”
The entire AC collection is now housed at the Powerhouse Museum. When Moore was cataloguing the fragile glass plates, she found that the years hadn’t been kind to them. Some were broken; the silver coating on some had oxidised to a solid black; mould had started to take hold on others.
On one glass plate, Moore could make out, among the smattering of dots, the curve of something that didn’t resemble any celestial body. Its identity remained a mystery until the plate was scanned at Cambridge. It was a partial fingerprint – a mark among the stars to remind us of those who captured and catalogued the sky.