Stjerneborg - An astronomical observatory underground

Painting of Stjerneborg during the renaisance

To further improve the accuracy of the position measurements, Tycho found that it was necessary to shield the instruments from disturbing wind gusts, and to have the instruments on solid fundaments. This led him to design a new observatory in 1584, Stjerneborg. Located partly underground, and with hatches to open in the observation direction only, a new level of accuracy was obtained. At the same time temperature fluctuations were suppressed, further improving the accuracy.

The excavations in the 1950

The five crypts of Stjerneborg housed each one a permanently installed instrument. The central square room was the hypocaustum, i.e. a heated room where Tycho and his assistants could prepare observations, study results or simply wait for the clouds to break up. The entrance was from north, i.e. from right on the plan. The fence had the same shape as the outer walls of Uraniborg.

The biggest crypt housed the great equatorial armillary sphere. It was a very big one, made of steel and brass. The declination circle visible on the picture had a diameter of 2.72 meter, and the equatorial circle to the left of it had a diameter of 3.50 meter. Thanks to a clever bearing arrangement it could be pointed towards all directions of the sky. Tycho's aiming devices and transversal scales permitted a resolution of a quarter of an arc minute, i.e. 15 arc seconds. This made it, next to the mural quadrant, to Tycho's most precise instrument.

When Tycho went into exile, he brought his instruments along. Since Stjerneborg was of no use to anyone else, it rapidly decayed, and was later covered by soil. During the last century several excavations have been made. The one on the picture shows the site in 1951.