Based partly on data taken from Pytheas, Hipparchus correlated cubits of the Sun's elevation at noon on the winter solstice, latitudes in hours of a day on the summer solstice, and distances between latitudes in stadia for some locations. Pytheas had proved that Marseille and Byzantium were on the same parallel (see above). Hipparchus, through Strabo, added that Byzantium and the mouth of the Borysthenes, today's Dnepr river, were on the same meridian and were separated by 3700 stadia, 5.3° at Strabo's 700 stadia per a degree of meridian arc. As the parallel through the river-mouth also crossed the coast of "Celtica", the distance due north from Marseille to Celtica was 3700 stadia, a baseline from which Pytheas seems to have calculated latitude and distance.