A week ago we walked home from the local restaurant. It was already dark and to Daughter’s surprise she could see the stars. We had seen them in France, while in the mountains, but at home she hasn’t had a proper chance to observe as many of them as that night. We saw one star shining very bright just above the trees. It wasn’t a star, it was a planet called Venus, the Man explained. Daughter was confused. Planets in her books are big and round. Not similar to a star. And didn’t mum and dad explain that planets were very far away? Then how was it possible to see one?

Daughter’s confusion was a cue for me to plan a visit to a place where we might be able to explain how this thing with the Sun and planets works. Of course fully aware that she is only three-and-a-half and the concept of space and time will remain fuzzy to her for some time still.

The plan got executed last Saturday. We drove to Franeker, a small town in Friesland. Many centuries ago, from 1585 until 1811, Franeker was a university city. For a short time, even Descartes was a student there. These days Franeker is mostly known for being one of the eleven cities of the Elfstedentocht. And for the planetarium Eise Eisinga built.

Eise Eisinga (1744-1828) was a smart man, but instead of going to school he had to work in the family business: carding wool. In his spare time he acquired knowledge on maths and astronomy. Once he got married he settled in Franeker with his family in 1768. In the house that we visited last Saturday. The reason it is open to the public is due to the amazing planetarium he built in his home to prove a point.

In spring 1774 turmoil in Friesland arose when a reverend from a small village about eighteen kilometers southeast from Franeker published an article in the Leeuwarder Courant announcing the end of earth. How he knew?

On May 8, 1774, a special constellation of planets formed. In the early morning, Mercury, Venus, Mars, Jupiter and the moon were positioned closely in the sky. It was claimed that the mutual forces of these celestial bodies would knock the earth off its path and cause it to be burned up by the sun.

From the website of Koninklijk Eise Eisinga Planetarium

Eisinga knew this was nonsense. He put all his knowledge of maths and astronomy into building a realtime moving model of the six planets then known to humans. This way he could show the general public how to planets moved around the sun and thus prove the reverend wrong.

It took him seven years to finish the model. He built it in his spare time, during the night, in his kitchen/diner/living room/bedroom. I can only imagine how annoyed his wife must have been with his project, but his endurance and vision resulted in something that is now the oldest still working planetarium in the world, 239 years after finishing it.

Earth roles around the sun in 356 1/4 days. Mars’s path around the sun is 687 days.

The biggest bonus of the Eisinga’s former house is that you can see the mechanism that keeps the planetarium moving. Basically it is a giant clock that uses weights to keep the wheels moving.

As a proper engineer, Eisinga left a handbook how to maintain the entire system. Those handbooks are still used today to keep everything working. For instance, what to do when the years run out.

Eisinga couldn’t have imagined the world in 2020, let alone know his masterpiece would be still functioning this year. His original board contained 22 years, more space was not available. His instructions for what to do afterwards are: remove the board, sand it, repaint with new numbers and put it back at starting position. They last did this in 2017. On top of the ceiling you can see the slot where the year board slides.

Daughter was a bit disappointed the spheres didn’t really move. Remember, Eisinga’s model is realtime. One can’t observe the earth move on such a small scale. Luckily elsewhere in the museum there were some models that did move at high speed.

What Daughter will take from this visit, I don’t know. However, it was one of the first things she explained to her carers at daycare this morning when the Man dropped her of. “And when it’s my birthday, they are in a different position.” Luckily the Man was there to provide the necessary context for the carers to understand her remark.