The Plane Astrolabe

Astrolabes are astronomical instruments that were used for several purposes such as timekeeping, celestial navigation, astrology, and surveying. Our plane astrolabe is a replica of an instrument that is depicted in the intarsia of the studiolo of Archduke Federico in Urbino. The original instrument was kept at the Musée Anne de Beaujeu and was stolen in 1977. The reconstruction was produced by Martin Brunold who works as a professional astrolabe maker since around 1995. The date of manufacturing (2004) and sign of the maker (MB) can be seen with the serial number "16" on the back side of the mater after removing the alidade and dismantling the instrument into pieces.

Our astrolabe is made of brass and has a diameter of 13 cm and a mass of 625 grams. On the back side of the mater, underneath the throne, one can find the signature of the maker of original instrument, which is "KP". Front side of the instrument includes a rule, which used to be a common feature in the European astrolabes to make some operations easier. On the rete, coordinates of 22 stars are shown with pointers. Ecliptic is divided into 1° intervals. The astrolabe has 3 plates for the latitudes whose front and back side are engraved for 34°, 38°, 42°, 46°, 50°, 54° geographical north. Flensburg being in 54° 47ʹ, only one of the plates could have been suitable with an expectancy of an error of half degrees. This 54° plate, which is one of two plates added to the replica during the reconstruction, would have worked more precisely in Neumünster though. According to our analysis, the astrolabe in HistoLab would have been more functional if we could have neglected the precession phenomenon[1].

Each of the latitude plates have almucantars with 5° intervals and azimuths with 10° intervals, celestial equator, tropics, unequal hour lines of the night according to Placidus House system accompanying with European numerals and Regiomontanus cusps with Roman numerals. Some of these elements also imply that the instrument might have been designed for astrological interpretations in addition to timekeeping operations. The throne contains leaf-like decorations and both the rete and alidade have dragon-head shaped figures on the corners. The limb scale is also divided into 1° intervals and labelled at 5° steps with European numerals both in the front and back sides. Typography of these numerals at the front side resemble some different Arabic numbers. On the back side of the mater, there is a zodiac scale with 1° intervals. On the upper half of it, each 5° steps are labelled with European numerals. Underneath, there is an unlabelled eccentric calendrical scale divided by 1°. Inside of the calendrical scale, two quadrants for universal unequal hours at the top and two quadrants for shadow square with 12 steps at the bottom are present.

Several operations could have been done with the astrolabe if it were reconstructed with the adaptations to 2023, Flensburg. To illustrate, the instrument could have been used for finding when and where Sun rises and sets or how long would a day and night take at a given date, what is the height of a building or width of a river etc. Even if our astrolabe may not be as functional as a historical one or a reconstructed one with the adaptations, it can still express the scientific status of the instrumentation in classical astronomy or the representative power of a replica.

If you are more interested in this instrument in particular, please read the longer excerpt that can be downloaded here.

[1] Coordinates of the stars have changed, and the Zodiac have shifted significantly since 1400.


Boxer, A. B. (n.d.). Alex Boxer's Interactive Astrolabe. Astrolabe. Retrieved April 27, 2023, from 

Morrison, J. E. (2009). The Astrolabe. Classical Science Press.

King, D. A. (2001). The Astrolabe Depicted in the Intarsia of the Studiolo of Archduke Federico in Urbino, in The Science of the Dukedom of Urbino, Urbino 2001. [Can be retrieved from

Stautz, B. (1999). Die Astrolabiensammlungen des deutschen Museums und des Bayerischen Nationalmuseums. Oldenbourg.