Budapest Régiségei 30. (1993)

TÁRGYI EMLÉKEK ÉS LELETEK = DENKMÄLER UND FUNDE - Madarassy Orsolya: Tabula gromatici az aquincumi canabaeból 297-315

The interpretation of the fragmented circles remains uncertain. No unambiguous meaning can be attributed to these arches, chiefly due to their incomplete state. It is for this reason that no attempts were made to identify these markings. It is worth returning to the interpretation of side "B". The incised letter D was decoded as the abbreviation for DECLINATIO, and the first intersection points of the half circles corresponds to the geographical latitude of cities marked down on the base line, the following conclusions may be drawn on the basis of these observations: Since this instrument was designed for use in the temperate zone (clima heteroskion) where the noon shadow always points to the north, the angle characteristic of geographical latitude is shown by the eastern and western inclination of shadow at the time of equinoxes from the geographically determined east-west direction. Following this logic, the base line on the fragment under discussion here can be used in marking the east-west direction. Consequently, another line, perendicular to this, marks north and south. The interpretations of the inner incisions is hampered by the schematic nature of the design. Although the circles are equidistant the cities assigned to the various arches are located at different distances from each other. If the most generally used 4 is taken as a basic unit, these distances are as follows: Syene-Alexandria 2 units Alexandria-Rhodos 1 unit Rhodos-Athens 0,5 unit Athens/(Rome) 1 unit (Rome/Mediolanum) 1 unit (Mediolanum/Aug. Trev.) 1 unit (Aug. Trev./Eburacum) 1 unit (Eburacum/Thule) 3? units This arrangement makes it understandable why Alexandria was placed at a latitude of 32° instead of 30,5-31°. Consequently, however, incisions on each circle must have been defined by reconstructing a basic scheme in the case of each individual arch. Their data, therefore, became to some extent generalized. It seems certain that the internal incisions are arranged at varying distances on each arch while the solar hours 18 do not seemed to have been marked down. On the sun dials of both Vitruvius and Ptolemaios, the orbit of the shadow between sunrise and sunset is subdivided by twelve even notches: six of these are located east and west from the south to the north shadow respectively, each one of them corresponding to one hour of a given day. There was, however, a distinction made between Sun times and astronomic times during Antiquity. These data were tabulated by Ptolemaios for the basic median taking each latitude separately. It was not only local time and latitude which could be determined with the help of this information. Measuring the angle between the zenith and disappearance of certain constellations, time differences could be calculated along the same latitude, which lead to the measurement of meridian differences. Syene and Rhodos are both included in the table by Ptolemaios and the list of cities on the fragment under discussion here. The angles measured between the line defined by the inner incisions and the hypothetical centre of the instrument and the eastwest base line, almost completely correspond to the equinox data listed by Ptolemaios. These observations suggest that side "B" of the instrument could be used to convert astronomic hours into solar hours. It may thus be concluded that side "B" of the Aquincum marble fragment was used as a compass. At the time of the equinox (March 21 and September 23), it showed: - the cardinal points of the compass - the geographical latitude of a given location on the basis of the angle between the first incision and the east-west base line - the geographical longitude of a given location on the basis of the time difference determined 22 by the inner incisions and the basic meridian. Once local data were identified using side "B", the auxiliary design on side "A" could be used in constructing the local horologium and the location itself could be mapped. When the gnomon on side "A" was replaced by a groma, the instrument could be directly used for mapping purposes. Since the Aquincum find is only a fragment of a complete instrument, the original artifact undoubtedly served to function in even more ways and could have been used in a variety of additional calculations. O. MADARASSY Budapest History Museum Museum Aquincum Notes AMR. 91.121/12.02 Régészeti Füzetek Ser. 1. No. 43.1991.47/15. Henszlmann-lapok Nr. 4 (in press). Since the centre of these concentric circles could be determined by geometrical construction an 0,5° theoretical error margin was reckoned with. 3 Strabo: Georaphika (J. Földy), Budapest (1977) 4 Strabo: C. 96. 5 Latituds coud be identified by geographers in Antiquity significantly more precisely than meridians. It is for this reason that when data are 304