Sky surveys

According to Henrietta, an article Kapteyn read during one Christmas vacation changed the course of his professional life and provided his entree into a career of universe-charting in the tradition of William Herschel and Wilhelm Struve. The article, written by Kapteyn's acquaintance David Gill, described a new project to photograph the southern sky, frame by frame, to produce a catalog that would complement existing catalogs of the northern sky.

David Gill, a Scotsman, had taken charge of Britain's Royal Astronomical Observatory at the Cape of Good Hope in what is now South Africa. The British Board of Longitude, which ran the Royal Greenwich Observatory, had set about establishing the Cape observatory as a southern-hemisphere counterpart in 1820; it was built and equipped by 1828. Gill, a jovial and outspoken character who ran the observatory with an unprecedented degree of informality, took up his position there in 1879, when he was 36 years old.

Gill's self-imposed mission was to take advantage of recent improvements in photography. A large format photograph of a comet in 1882 showed a surprisingly vivid field of stars in the background, and Gill became convinced that photographic emulsions had become sensitive enough to record the positions of ordinary stars as well as capture the visual likenesses of bright or extended objects such as planets, comets, and nebulae. He proposed an international collaboration to photograph the stars of the entire sky, and began almost at once on his own photographic atlas and catalog of the southern sky.

In the article Kapteyn read in an astronomical journal, Gill described his project. He noted the immense volume of data that the project was generating—data that would require years of analysis, more data than Gill could hope to deal with on his own. The southern sky project, which came to be known as the Cape Photographic Durchmusterung or Cape sky survey, was meant to complement the Bonner Durchmusterung or Bonn sky survey. The Bonn survey was the most complete star-catalog to date, comprising 324 000 northern-hemisphere stars and indicating star brightness as well as position. But while Friedrich Argelander, compiler of the Bonn survey, had recorded star positions with a micrometer and had estimated the brightnesses by eye, Gill would record positions and brightnesses objectively and consistently on photographic plates. The hard part would be to measure the plates, that is, to extract the star coordinates and magnitudes from the photographs, and secondly to apply any necessary corrections to these raw data before assembling them into a catalog.

Kapteyn saw an opportunity. He dispatched a letter to Gill offering his help with the analysis. He had no illusions about the nature of the work; determining star positions and other information from the photographic plates would be tedious and repetitive, and he expected the work to take six or seven years. But the resulting data on the distribution of stars and their magnitudes would surely yield information on the architecture of the universe, a question of great importance to Kapteyn. From the beginning of his professorship, Kapteyn had envisioned studying the stellar system ''from the ground up'' by amassing large amounts of data and trying to infer the distribution and motions of the stars.

Kapteyn wrote to Gill, ''If you will confide to me one or two negatives, I will try my hand at them and, if the result proves as I expect, I would gladly devote some years of my life to this work, which would unburden you a little, as I hope, and by which I would gain the honor of associating my name with one of the grandest undertakings of our time.''8

Gill responded with warmth and enthusiasm. ''It is not easy to tell you what I feel at receiving such a proposal,'' he replied. ''I recognize in it the true brotherhood of science and in you a true brother.''9 And so, beginning in 1886 and continuing until 1892, Gill shipped negatives to Groningen and Kapteyn measured the star positions from them. Kapteyn, still strapped for resources, had no laboratory space of his own, but his friend professor Dirk Huizinga from the psychology department let him use two rooms in his building. At last Kapteyn was observing stars — if only on photographic plates.10

To obtain the star positions directly in right ascension and declination—rather than measuring them as x and y coordinates from a reference position on the photograph, and laboriously transforming those to coordinates on the celestial sphere — Kapteyn set the images on a stand and scrutinized them from a few feet away with a theodolite, a surveyor's instrument for the accurate measurement of small angles. This allowed him to progress faster than Gill had expected, although the project still took almost twice as long as Kapteyn had originally anticipated. Even when the right ascension and declination data were in hand, calculations were required, for example to correct for the effects of refraction and precession.

Kapteyn was equally creative when it came to finding an assistant, a ''computer'' like the women Pickering employed at Harvard. As Henrietta put it in her biography, ''Intelligent and talented men who wanted to work hard for little money were difficult to find.''11 Kapteyn turned to the director of a vocational school in Groningen, who recommended a 19-year-old student, T W De Vries. De Vries proved adept and had a long career at the university.12

By the time Kapteyn and De Vries finished measuring the photographic plates, the international astronomical community had begun to recognize Kapteyn's ability and diligence. No new, comprehensive models for the distribution of stars had yet emerged, but the compilation of the data itself was seen as extremely valuable. The French government bestowed on him the Legion d'honneur, and the Royal Astronomical Society in England elected him a foreign member. These accolades lifted his spirits and helped turn the tide in his efforts to garner funds and equipment at Groningen, but did nothing to relieve the physical and intellectual burden of the Cape survey work. Some 450 000 stars were cataloged in total, and calculations on the stellar data continued through the publication of the catalog between 1896 and 1900. Kapteyn wore himself out, and developed eye strain that would bother him the rest of his life.

Kapteyn found some respite from the grueling calculations in attending international meetings. It was at these meetings that he glimpsed the future direction of astronomical research, and began to see how he might forge a niche for himself once the Cape survey work was completed. In 1887, the Paris observatory, acting on Gill's earlier idea, proposed a scheme to photograph both northern and southern hemispheres of the sky. This Carte du Ciel, as the survey came to be known, would employ a standard plate scale (1 arcminute on the sky per millimeter on the plate) and two standard exposure times, one to record stars for a basic catalog and one to record stars as faint as the 14th magnitude. The observatory convened a series of annual meetings to discuss the plan and to apportion the work to participating observatories.

Kapteyn proposed some ideas of his own at the first Carte du Ciel meeting. He wanted to take advantage of the experimental set-up of the survey to measure photographically a large number of stellar parallaxes, at the same time. His idea was not adopted, but his commitment to the study of the ''sidereal problem,'' as he called it, of the distribution of stars in three-dimensional space, is reflected in the fact that he persuaded the director of the Helsingfors Observatory in Finland to carry out a limited version of the scheme. By 1900, already Kapteyn was able to add about 250 stars to the short list of those whose parallaxes had been determined.

Although his own parallax-measuring plan was not adopted generally, Kapteyn wholeheartedly supported the Carte du Ciel project. He and Gill attended the meetings and both played active roles in the project's planning and execution. Like the Cape survey, the Carte du Ciel took much longer to complete than the organizers had hoped.13 However, Kapteyn remained impressed with the possibilities he had seen at these meetings for international cooperation. In the original plan, observatories at 18 sites each accepted responsibility for photographing one zone of the sky. Ten of the observatories were in Europe (Greenwich, the Vatican observatory, San Fernando in Spain, Catania, Helsingfors, Potsdam, Oxford, Paris, Bordeaux, and Toulouse), one in North Africa (Algiers), one in Central America (Tacubaya, Mexico), three in South America (Santiago, Rio de Janeiro, and La Plata in Argentina), one in southern Africa (the Cape of Good Hope), and two in Australia (Sydney and Melbourne). The Carte du Ciel project would come as close to being a worldwide effort as one could imagine in the late 1800s.

The absence of American observatories in this list is noteworthy, because a number of them had acquired significant equipment and staff by the time of the fourth Carte du Ciel meeting in 1890. Representatives from the United States Naval Observatory and the observatories at Yale University and Hamilton College in Clinton, New York attended the first congress in 1887. But the American observatory director who was in the best position to participate in the data-taking, Edward Pickering of the Harvard College Observatory, rejected the French proposal in favor of his own sky-survey plan.

About the time Kapteyn finished measuring Gill's plates and began calculations for the associated Cape survey catalog, University of Groningen authorities finally responded to his pleas for support. They approved his request for instruments to measure data from photographic plates, and in 1896 found an entire building to put at his disposal.

In a speech at the building's dedication, Kapteyn explained the scope of the new Groningen Astronomical Laboratory (now called, in his honor, the Kapteyn Astronomical Laboratory) to his lay audience, who had never heard of an astronomical laboratory without a telescope. He foresaw that the work done there would eventually contribute not just to the Cape survey, the Carte du Ciel, and the analysis of parallax data from Helsingfors, but to subsequent research initiated by astronomers all over the world.

He explained, ''At each observatory much more is produced than can be analyzed, because the work force available for measurements and all the other work, while adequate for the photographic recordings, is insufficient for data reduction.'' Borrowing a phrase from Charles Darwin, he described his laboratory's purpose as ''the grinding of huge masses of fact into law.''14

As Kapteyn predicted, raw data have continued to accumulate faster than astronomers can analyze them, so that institutes similar to his are now commonplace. His idea was a sound one. An historian of astronomy has also noted that the concept of a low-cost astronomical laboratory performing high-value calculations fit perfectly with his temperament and talents. ''What satisfied him most about the project,'' the commentator wrote, was that ''it enabled him to be both humble and extremely ambitious at the same time.''15

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