was founded in 1642, and was, after the Vatican and Leiden Observatories, the third official observatory in the world. Initially the Observatory was situated on a tower, about 15 m high, built in 1145. In cases of alarm this tower had to be defended by the blacksmiths, and therefore was named the 'Smeetoren'. The Observatory was very simple and essentially had five floors, each of one room. To accommodate the tower to 'astronomical speculations' as it was officially stated, the top was removed and changed into a platform on which had been built an octagonal with glass windows, to serve as an observing dome. The platform was extended in 1725 and the observing dome was enlarged in 1818.
In the middle of the nineteenth century the Smeetoren gradually began to decay and was no longer suitable for astronomical observations. It was left, and badly enough, pulled down. In the meantime a new observatory was built at another of the fortifications surrounding the city of Utrecht, the fortress Sonnenborgh (meaning: 'Castle of the Sun'). What a suitable name for a place where later a solar observatory would be established! This fortress had been built in 1551-1552, at the same time as a few other bulwarks with astronomical names, like Manenborgh ('Castle of the Moon'), De Morgenstar ('The Morning Star') etc.
In 1853 the new Observatory was officially dedicated by H.M. King Willem III, grandfather of our present Queen. At that time the Observatory had to share Sonnenborgh with the Netherlands Meteorological Institute, which, however, was moved to De Bilt in 1896. This gave some room for a modest expansion.
Since the beginning of this century the research at the Observatory has been principally devoted to the observation of variable stars (A. A. Nijland and J. van der Bilt). Solar physics became the main theme of research when, after Nijland's death, M. Minnaert became director (193%63). His first action was to move the solar spectrograph to the Observatory. This spectrograph had been developed and built around 1920 by W. H. Julius at the Physics Laboratory and intensively used there by Minnaert and coworkers (Wanders, Mulders, Houtgast and many others). It was at the Physics Laboratory that so much fundamental solar research was carried out in the thirties: the introduction of notions like equivalent width and curve of growth, the theory of weak lines, the calibration of the Rowland scale (see review by Minnaert, 1965), and