Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
The Armies at Pharsalus
Caesar lay to the south of Larisa in the plain--which extends between the hill-country of Cynoscephalae and the chain of Othrys and is intersected by a tributary of the Peneius, the Enipeus-- on the left bank of the latter stream near the town of Pharsalus; Pompeius pitched his camp opposite to him on the right bank of the Enipeus along the slope of the heights of Cynoscephalae.(30)
30. The exact determination of the field of battle is difficult. Appian (ii. 75) expressly places it between (New) Pharsalus (now Fersala) and the Enipeus. Of the two streams, which alone are of any importance in the question, and are undoubtedly the Apidanus and Enipeus of the ancients--the Sofadhitiko and the Fersaliti--the former has its sources in the mountains of Thaumaci (Dhomoko) and the Dolopian heights, the latter in mount Othrys, and the Fersaliti alone flows past Pharsalus; now as the Enipeus according to Strabo (ix. p. 432) springs from mount Othrys and flows past Pharsalus, the Fersaliti has been most justly pronounced by Leake (Northern Greece, iv. 320) to be the Enipeus, and the hypothesis followed by Goler that the Fersaliti is the Apidanus is untenable. With this all the other statements of the ancients as to the two rivers agree. Only we must doubtless assume with Leake, that the river of Vlokho formed by the union of the Fersaliti and the Sofadhitiko and going to the Peneius was called by the ancients Apidanus as well as the Sofadhitiko; which, however, is the more natural, as while the Sofadhitiko probably has, the Fersaliti has not, constantly water (Leake, iv. 321).
Old Pharsalus, from which the battle takes its name, must therefore have been situated between Fersala and the Fersaliti. Accordingly the battle was fought on the left bank of the Fersaliti, and in such a way that the Pompeians, standing with their faces towards Pharsalus, leaned their right wing on the river (Caesar, B. C. iii. 83; Frontinus, Strat. ii. 3, 22). The camp of the Pompeians, however, cannot have stood here, but only on the slope of the heights of Cynoscephalae, on the right bank of the Enipeus, partly because they barred the route of Caesar to Scotussa, partly because their line of retreat evidently went over the mountains that were to be found above the camp towards Larisa; if they had, according to Leake's hypothesis (iv. 482), encamped to the east of Pharsalus on the left bank of the Enipeus, they could never have got to the northward through this stream, which at this very point has a deeply cut bed (Leake, iv. 469), and Pompeius must have fled to Lamia instead of Larisa. Probably therefore the Pompeians pitched their camp on the right bank of the Fersaliti, and passed the river both in order to fight and in order, after the battle, to regain their camp, whence they then moved up the slopes of Crannon and Scotussa, which culminate above the latter place in the heights of Cynoscephalae. This was not impossible.
the Enipeus is a narrow slow-flowing rivulet, which Leake found two feet deep in November, and which in the hot season often lies quite dry (Leake, i. 448, and iv. 472; comp. Lucan, vi. 373), and the battle was fought in the height of summer. Further the armies before the battle lay three miles and a half from each other (Appian, B. C. ii. 65), so that the Pompeians could make all preparations and also properly secure the communication with their camp by bridges. Had the battle terminated in a complete rout, no doubt the retreat to and over the river could not have been executed, and doubtless for this reason Pompeius only reluctantly agreed to fight here. The left wing of the Pompeians which was the most remote from the base of retreat felt this; but the retreat at least of their centre and their right wing was not accomplished in such haste as to be impracticable under the given conditions. Caesar and his copyists are silent as to the crossing of the river, because this would place in too clear a light the eagerness for battle of the Pompeians apparent otherwise from the whole narrative, and they are also silent as to the conditions of retreat favourable for these.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=82