Part of the design for the mural by Fynn van der Ziel

A mural for J.C. Kapteyn

Astronomer without a telescope

Part of the design for the mural by Fynn van der Ziel
He was the astronomer who didn’t have a telescope. But even without looking at the stars, Jacobus Kapteyn still managed to become one of the world’s most influential astronomers. And now, he’s getting his very own mural.
14 June om 10:01 uur.
Laatst gewijzigd op 14 June 2022
om 10:03 uur.
June 14 at 10:01 AM.
Last modified on June 14, 2022
at 10:03 AM.
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Door Christien Boomsma

14 June om 10:01 uur.
Laatst gewijzigd op 14 June 2022
om 10:03 uur.
Avatar photo

By Christien Boomsma

June 14 at 10:01 AM.
Last modified on June 14, 2022
at 10:03 AM.

He must’ve been so upset. At twenty-seven years old, astronomer Jacobus Kapteyn had only just started his job as an observer in Leiden, when he was made a professor in Groningen. It allowed him the opportunity to create an astronomy department basically from scratch, since the Dutch government had decided in 1876 that all universities should be providing the same curriculum. That meant the UG would have to start working on creating a course that had until then not enjoyed a stellar reputation: astronomy.

But Kapteyn quickly discovered the catch: Groningen did not have an observatory, or even a telescope. In spite of repeated and urgent requests from the university’s curators, the government adamantly refused to provide any funds.

‘It was Leiden who advised them against it’, says retired astronomy professor Piet van der Kruit. ‘They were worried they’d have to share and lose funds.’

Kapteyn had to make do with a simplistic demonstration telescope that he’d bought with some money he’d managed to collect. He would occasionally take this telescope up to the roof of the Academy building, where he and his students observed the night sky. It wasn’t exactly the career he’d always dreamed of. 


It could have spelled the end of serious astronomical research in Groningen before it had even properly started. ‘But the whole thing turned out to be a blessing in disguise’, says Van der Kruit. It forced Kapteyn to find a new way of doing research. His new method made him a celebrity, but that wasn’t all. ‘The important position of Dutch astronomy in the world is all thanks to him’, says Van der Kruit.

Leiden was worried they’d have to share funds

Now, J.C. Kapteyn – one of the most important scientists at the university and after whom the Groningen Kapteyn Institute is named – is the subject of the second ‘academic’ mural, after Aletta Jacobs. The enormous artwork has been painted on the wall of the alley leading to the University Museum and will be unveiled on Friday.  Van der Kruit will also be presenting his book Kapteyn en zijn Sterrenkundig Laboratorium een eeuw later. 

It will be his third work on the famous astronomer: he’s already written a long biography on the man, as well as a thinner one for the general public. He also wrote about Kapteyn’s student Van Oort, after whom the Van Oort cloud is named.

‘Everything was different then’, says Van der Kruit. ‘Observatories had a lot of photographic material of the stars, but no staff to look at it all. So he figured he would go do that.’

Photographic plates

Kapteyn decided to focus on the theoretical side of astronomy: calculating the stars’ position in the sky, as well as the distances between them. This led him to Scottish astronomer David Gill, who’d used the then-new technology of photographic plates to capture the sky of the southern hemisphere in Cape Town. ‘Gill endlessly complained to Kapteyn in his letters to him’, says Van der Kruit. ‘He was so busy, he didn’t have time to analyse it all, woe was him, etc. He was baiting Kapteyn, of course, trying to get him to analyse the plates.’ 

Kapteyn refused to commit for a long time, but in the end, he finally relented. Between 1885 and 1889, dozens of wooden boxes containing a total of 2,400 glass plates arrived in Groningen. Today, they can be found at the University Museum. 

The important position of Dutch astronomy in the world is all thanks to him

In the end, it would take Kapteyn and his assistants twelve years to analyse the 454,875 stars that Gill had photographed, using a home-made parallactic instrument. ‘He put a small telescope at the same distance to the plate as the focal length of the telescope that was used to make the photographs’, Van der Kruit explains. ‘By measuring the position of the stars on the photographic plate like this, he was able to calculate the correct values.’ 

It was an amazing feat, no doubt about it. ‘But this was just the beginning.’ What ultimately mattered was creating a catalogue. Kapteyn wanted to know what the Milky Way looked like and how the stars moved. They were grand plans, but hard to realise. ‘Fortunately, he was stubborn’, says Van der Kruit. ‘So he did it.’

Stellar streams

After studying Gill’s data as well as that of others, Kapteyn thought he’d found two stellar streams that were counter-orbiting each other. But whether this was actually true or whether there was a different explanation for what he’d seen required further research. Not even a gifted mathematician like Kapteyn could do that all on his own.

During a conference in Saint Louis in 1904, he announced a revolutionary plan. What if astronomers at observatories all around the world worked together? What if they were to study 206 selected areas of the sky, which could then be subjected to the necessary statistics? What if Groningen coordinated the entire project? 

And so for a little while, Groningen became the centre of the world of astronomy. 

The Americans say that Kapteyn was wrong

The project showed, among other things, that the stellar streams Kapteyn thought he’d observed didn’t exist. His observations were correct, ‘but the random movements in one direction turned out to be bigger than the random movements in another direction’. One of Kapteyn’s colleagues had already suggested this, but now there was proof. 

This also meant that Kapteyn was finally free to start doing his own observations. He made friends with George Ellery Hale, who was in the process of building the largest reflecting telescope on Mount Wilson near Pasadena in California. Kapteyn was made a research associate at the Mount Wilson Observatory. He was given his own house, the Kapteyn Cottage, where he would stay every year during summer and early autumn.

Milky Way model

But his biggest dream, creating a model of the Milky Way, remained out of reach for the longest time. He did make an attempt in 1921, when he was nearly seventy years old. Using the data available at the time, he and his student Van Rijn created the ‘Kapteyn universe’. 

‘It was a sort of flattened contraption’, says Van der Kruit. ‘The sun was quite close to the centre of it.’ The model also explained its own stability. Unfortunately, it was incorrect. While it did represent reality in a vertical direction, the Milky Way system was much more expansive horizontally than Kapteyn had ever imagined, mainly because of the presence of large dust clouds. Kapteyn hadn’t taken those into account.

That is perhaps the reason America seems to have forgotten Kapteyn, Van der Kruit thinks. After all, it was the American astronomer Shapley who corrected the Kapteyn universe. ‘And Americans turn everything into a competition. They’re like: Kapteyn was wrong!’

He had no way of knowing whether the stars were just red on their own

When the Kapteyn Institute used Kapteyn’s likeness on the cover of concept publications, American colleagues said this was inappropriate. After all, Kapteyn had been wrong, while Shapley had been right. 

‘Unfortunately, we acquiesced in this issue’, says Van der Kruit. ‘I remember I was quite upset about it.’ In the end, he says, it’s about what scientists do with the knowledge they have at any given moment. Kapteyn did know there was dust in the space between stars. But no one knew at the time that the dust in the Milky Way was more concentrated. 


‘He was even the first person to say that if there was dust, blue light would probably diffuse it more than red light. But he also said that he had no way of knowing whether the stars were just red on their own. He kept his options open.’

His vision, revolutionary ideas, and the countless students that followed his work and together put Dutch astronomy on the map; all these things combine to form a beautiful legacy. ‘He was a truly inspiring person’, says Van der Kruit. ‘A real go-getter.’

Unfortunately, he never got his own observatory. That this bothered him is shown in a letter he wrote later in life: ‘There is a sort of fate’, he said, ‘which makes me do all my life long just what I want to do least of all.’

The academic mural depicting J.C. Kapteyn was created by Fynn van der Ziel and will be unveiled on Friday in the alley next to the University Museum.