25 Ptolemy’s Data for the Latitudes of Alexandria, Syene and Meroë: Some Observations Irina Tupikova (Institute for Planetary Geodesy, Lohrmann Observatory, Technische Universität Dresden) Klaus Geus (Friedrich Meinecke Institute, Freie Universität Berlin) The Greeks and Romans used two methods to determine the latitude of a lo- cation: either observing and calculating the gnomon shadow-ratio, 1 or using the duration of the longest day of the year. 2 Evidence suggests that Ptolemy used both methods to determine geographical latitude. This paper focusses on three specifc locations – Alexandria, Syene, and Meroë – which Ptolemy discusses both in his earlier astronomical work, the Syntaxis mathematike or Almagest, and his later geographical treatise, the Geographike hyphegesis or Geographia. In the Almagest, Ptolemy provides two slightly diferent values for the obliquity of the ecliptic. At one point he ofers a value of 23° 51ʹ; 3 and another he claims that it is ‘approximately’ (ἔγγιστα) 23° 51ʹ 20ʺ. 4 In Book I of his Geographia, Ptolemy emphasises that Syene lies directly beneath the celestial summer tropic and its distance to the equator equals 23° 50ʹ. 5 In order to cal- culate the latitude of a location from the duration of the longest day of the year one also needs information about the obliquity of the ecliptic. Evidence suggests that the positions of the locations in the Geographia represent the results of adjusting a set of known mutual distances, rounded to the nearest 5ʹ-interval. The information given in the earlier text of the Almagest, however, is without such revisions. Instead, the data here refect either direct astro- 1 The mathematical relation between the length of the shadow and the length of a gnomon at equinox or solstices was already used in antiquity to determine the position of a location; see, e.g., Strabo, Geographika, II, v, 41, C 135, a passage often atributed to Pytheas (4th cen- tury BC). For another example, see Gysembergh 2012. 2 Due to the sphericity of the Earth, celestial phenomena are diferent for diferent latitudes. Already ancient astronomers like Eratosthenes used the duration of the longest day of the year for determining climates and positions of landmarks. 3 See Ptolemy, Almagest, II, vi, 7 (p. I 107 Heiberg = p. 85 Toomer). 4 See Ptolemy, Almagest, II, iv (p. I 97 Heiberg = p. 80 Toomer). See also Britton 1969 and Jones 2002. For a recent introduction to the works, see Evans 2018. For additional informa- tion on the Almagest, see Pedersen 1974 and Neugebauer 1975 (for this topic, see esp. I, 21– 44). 5 See Ptolemy, Geographia, I, xxiii, 7 (p. I 117 Stückelberger – Grasshoff).