Budapest Régiségei 30. (1993)

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

fore, this belt is called "bilaterally shaded" (amphiskion). 2) Ecumene is a term used to describe the inhabited world, meaning the northern hemisphere. 3) Circles running parallel with the Equator were called clima-declinatio in astronomy and geography. Longitudinal, equal circles crossing both poles and another, concrete spot were called meridians. Circles in the sky and earth respectively correspond to each other. Time differences between two spots may be calculated from the distance of their respective meridians in accordance with the movement of the Sun. Differences in clima-declinatio define differences between the proportions of day and night between the summer and winter solstices. During Antiquity, geographic research usually considered latitudes as being marked by cities of outstanding importance as a reference point in the topographic identification of lesser known places. Strabo defined the following locations as fixed points marking a basic set of meridians: Meroe, Syene, Alexandria, Rhodos, Byzanthium, and the mouth of the river Borysthenes (Dniepr). (Map 1) On the basis of Strabo, the letter „D" written on side "B" of the fragment under discussion here may be instantly decoded: it is the abbreviation for DECLINATIO, that is the city names which stand for certain latitudes. The southernmost city in this list is Syene which is crossed by the Tropic of Cancer. Since these circles are not defined by the southernmost point of the ecumene one may conclude that the instrument made use of the sun dial principle. It is for this reason that the northernmost circle should be sought after within the temperate climate belt (clima heteroskion). The south-north line defined by Strabo, including Meroe, Syene, Alexandria, Rhodos, Byzanthium, and Borysthenes lay on the same meridian in classical geography. Syene, Alexandria and Rhodos also appear on the fragment which is the subject of this paper. The circle marked by the name of Athens, however, is of key importance since it falls west of the south-north meridian. In addition, Strabo placed Rhodos and Athens at the same latitude. The fact that they were assigned to different circles on the fragment under discussion here is indicative of a geographical knowledge that was more precise than Strabo's geography. The preparation of sun dials is described in chapter 10 of the book De Architecture written by Vitruvius , a contemporary of Strabo. His axiom is that the southern shadow of a standard size stick at the time of the equinoxes differs at various latitudes on the Earth. He illustrates this thesis by a number of examples. Of these, the proportion of the measuring stick (gnomon) is 5 to 3 in Alexandria, 7 to 5 in Rhodos, 4 to 3 in Athens, 11 to 9 in Tarentum and 9 to 8 in Rome. The latitude of a particular place may be calculated from the proportion between the gnomon and its equinox shadow. The same ratio also permits an estimate to be made of the local proportion between day and night at the time of the winter solstice (December 21) and the summer solstice (June 21). Furthermore, this information is instrumental in estimating the lengthening and shortening of days by months and days. These relationships could all be deduced from a geometric chart, the so-called analemma. When an analemma is constructed using the gnomon proportions calculated by Vitruvius, the geographical latitude of a place may be defined measuring the angle between a line drawn from the end point of the vernal equinox shadow to the tip of the gnomon and the vertically positioned gnomon itself. The latitudinal value thus obtained can be used in calculating the southern angle of the midday sun at the time of the winter solstice (x + 24 degrees) and the summer solstice (x - 24 degrees) (Figure 1). The seven inner half circles of the nine arches on the "B" side of this fragment are divided by small incisions. When a line is drawn between the first section point on each circle and the possible centre of these concentric circles and the angle between this and the straight line is measured the following values are obtained: SYENE: 2 4°-24,5° (24°5') ALEXANDRIA: 32°-31,5° (30°30') RHODOS: 36° (36°10') ATHENS: 38° (37°57')

10 5.... 42°- 41,5° 6.... 46°-45,5° 7.... 50"M9,5° Consequently, the first incision shows the geographical latitude of the place in question and thus the rest of the circles on this fragment may also be labelled with additional Antique city names. Circle 5 (42°) undoubtedly corresponds to Rome (41°57') while Circle 6 may equally well be assigned to Aquileia (45°47'), Mediolanum (45°28') or Lugdunum (45°45') as well as any other major city located at this latitude. Circle 7 (50°) corresponds to the latitude of Augusta Treveriorum (49°47'). Circle 8 crosses Britannia. If the 4 consistent shift observed north of Athens is taken into consideration, it also crosses Eburacum (53°54'). A variety of solutions may be proposed for Circle 9. The northern limes of Britannia alternates between the walls built by Antoninus and Hadrianus respectively. Since following the military campaign led by Agricola the line of the Antoninus wall became known concretely in Roman geography, and it had been previously recorded as a geographical concept, it did not necessarily have to fall within the area of the Roman Empire at the time this instrument was made. The other hypothetical possibility is that since the beginning of the clima declinatio was defined as the position of Syene and the Tropic of Cancer, logically, its end point may well be the Arctic Circle at 66°. 302