The Old Observatory, tucked into a leafy patch of garden beside the Witte Singel, was built in 1860 and is the second-oldest functioning observatory on earth. A succession of giants of Dutch astronomy worked here — Jan Hendrik Oort, who mapped the structure of the Milky Way; Willem de Sitter; Jacobus Kapteyn.
Tours of the four working historical telescopes are run in the evenings several times a month, usually with an opportunity to observe the moon or planets if the weather cooperates. Book in advance through the observatory's website; English-language tours are regular.