« A.U.C. 536 = 218 B.C. »
The first indication of the length of this year is the date of the battle of Trebia. This is less useful than it first appears. Polybius 3.72.3 tells us that Trebia fought around the winter solstice, c. 25 December 218. Since the number of intercalations between A.U.C. 537 = 217 and A.U.C. 564 = 190 can be fixed, we can estimate the equivalent Roman date as falling between c. a.d. IX Kal. Ian. and prid. Kal. Ian. if there was no intercalation in A.U.C. 536, and c. a.d. XVI - VII Kal. Feb. if there was one.
While the Roman civil date of Trebia is not known, Livy 21.57 states that the consul Ti. Sempronius briefly returned to Rome to conduct the consular elections after escaping from the battle. These were normally held at the end of Ianuarius, but could be held later if necessary. However, either of the date ranges above are compatible with elections being held after the battle, though the first is perhaps slightly more plausible. Further, Polybius 3.70.3 notes that one reason Sempronius was eager to bring Hannibal to battle is that he wanted to ensure that the glory of victory should be his, before the consuls-designate could enter into office. This has been taken to indicate that the elections had already been held before the battle. On the above analysis, this is unlikely. Further, Polybius does not actually say that the identities of the consuls-designate were known, and the political calculation he ascribes to Sempronius would be valid regardless of whether elections had actually been held yet. Therefore there is no reason to declare Polybius in conflict with Livy, and certainly no obvious reason to prefer the view that the election was held before Trebia. Hence the battle of Trebia is not a useful chronological datum.
The second indication is given by the date of the foundation of Placentia (Piacenza). Asconius, Comm. in Senatu contra L. Pisonem, dates this event to prid. Kal. Iun. A.U.C. 536. (N. Prack, Der römische Kalendar (264-168 v. Chr.) 75, notes that the MS source actually reads "Ian." not "Iun.", but the logic of events requires the correction.) If the number of intercalations after this year has been correctly determined and this year is not intercalary, this date falls in the range 8-15 June 218. If the year is intercalary, it falls in the range 18-24 May 218.
Polybius 3.40 notes that Placentia and Cremona were both founded after news had been received in Rome that Hannibal had crossed the Ebro. The Julian date of this event can be roughly estimated. Hannibal arrived in Italy around the setting of the Pleiades (Livy 21.35), i.e. c. 7 November 218. Livy 21.38 indicates that the total journey from New Carthage (Cartagena) took him around 5 months, which places his departure around the beginning of June. Since he will not immediately have crossed the Ebro, and since news of this event will have taken at least couple of weeks to reach Rome, we can estimate that the news reached Rome in late June.
Clearly, this is not compatible with either of the possible dates for the foundation of Placentia calculated above. The following solutions might be considered:
The number of intercalations after this year has been overestimated.
One fewer intercalation dates the foundation of Placentia to 1-7 July 218, and the winter solstice to prid Kal. - prid Non. Dec. A.U.C. 536. This (just) brings the foundation of Placentia into line with the reported length of Hannibal's expedition but reduces the pressure on Sempronius to rush to Rome to conduct an election considerably. It also creates conflicts with the data for A.U.C. 537 = 217, since it places the battle of Lake Trasimene too close to our best estimate of the date of the Nemean games.
Thus, this most obvious solution does not in fact help.
Asconius' date for the founding of Placentia as we have it is wrong. The logic of events appears to suggest a date in mid-late June, which could easily be met if "Iun." was an error for "Iul." Since Asconius lived in the first century A.D., such a correction is conceivable, however elsewhere in his commentaries (e.g. Comm. Pro. M. Scauro) Asconius refers to the seventh month as Quintilis, so here is no reason to suppose he was anachronistic here.
Livy's estimate of five months for Hannibal's expedition is wrong. Clearly ("some authorities") he had some reservations about it. But in Livy 21.21 and Livy 21.22 he describes events between Hannibal's breaking his winter quarters at New Carthage and his departure for Italy: a pilgrimage to Gades (Cadiz); organising the defence of Africa, Carthage and Spain; and requisition of the elephant corps and naval support for the Italian expedition. All this must have taken time, and 2 months (late March to late May) seems a very reasonable estimate. Therefore it is difficult to extend the time for his journey much beyond 5 months.
The report of Hannibal's movements that triggered the founding of Placentia was not actually based on his crossing of the Ebro but on knowledge of his intent to cross it. This seems to me to be the most likely solution. According to Livy's account in Livy 21.21 Hannibal decided on the plan over the winter and had briefed his troops on his general intent at that time. Thus, rumours that something was afoot must have been floating around Spain over winter, and some public knowledge of his intent must have followed his declaration of his vow at Gades, which we can estimate occurred in early April. Knowledge of the expedition first reached Rome through Massilian emissaries (Livy 21.25), and it was certainly in the interests of the Massilians to paint the picture as strongly as possible.
If we suppose that the Massilian report reflect Hannibal's plans rather than his actions, it is perfectly possible that it was received as soon as mid May. Even on this scenario, however, it is difficult (though not impossible) to credit that Placentia was founded only a few days later. The date of mid June is a much better match.
It follows that A.U.C. 536 = 218 was probably a regular year of 355 days.
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