Special notion: the Visitor Center of the Old Observatory is open on weekends from 10 a.m. to 6 p.m. (in winter to 4 p.m.). The Visitor Center can only be visited with a ticket from the Hortus; this does not apply to the Old Observatory itself.
Guided tours are at 11.30 am, 1.00 pm and 3.00 pm. For youngsters aged up to 18 years these tours are free of charge, for students the additional fee is €2 and for adults €4. Click here for more information about the Old Observatory.