Self-orienting armillary dial of the professor Radovan Danić

Abstract

Prof. Radovan Danić, PhD (1893–1979), an honorary lifetime President of the Astronomical Society Ruđer Bošković in Belgrade, owned a brass universal equinoctial ring sundial (98 mm in diameter), preserved by his descendants, who continued his work on popularizing astronomy through the activities of the society. The sundial (ring dial) was measured, tested and compared to similar portable sundials (pocket sundials) exhibited in various European museums. In the classification scheme, along with the Parmenion’s and astronomical rings, it belongs to a group of pocket armillary sundials that do not require a compass. More precisely, it is a self-orienting armillary sundial whose rings are located under the circles of the celestial sphere of the same name at the moment of measurement. Therefore, when the apparent solar time is known, it turns into a solar compass. A corresponding sundial on the horizon to the self-orienting armillary sundial is the analemmatic sundial. The construction of a self-orienting armillary sundial was first described in the late 16th century by the English mathematician William Oughtred (1574-1660). In collaboration with the gnomonists from England and Austria, we determined where and when Professor Danić’s sundial was constructed: Vienna, second quarter of the 18th century. Originally, the sundial was adjusted for the latitude of Belgrade or Zemun (nowadays, a Belgrade municipality), which were under the Austrian rule for a long time during the 18th century. It is a beautiful, well-crafted, well-preserved, expensive sundial and astronomical instrument that should be kept in a museum, in the first place in the Museum of Astronomy of the Astronomical Observatory in Belgrade.