Note 3554 — THE JULIAN CALENDAR AND THE CHRONOLOGY OF CAESAR’S INVASIONS OF BRITAIN

Napoleon’s reasoning is based upon assumptions, one of which is certainly incorrect, while all are doubtful. We know that Caesar started on his return voyage soon after midnight (B. G., iv, 36, § 3). ‘If,’ says Napoleon (Hist. de Jules César, ii, 180, note), ‘we assume that he had a favourable wind, as he had on his return from the second expedition, and that his voyage lasted nine hours, Caesar would have reached Boulogne about nine o’clock in the morning. As the fleet could only enter the harbour on a rising tide, all that we need do in order to ascertain approximately the date of his return, is to find out on what day of September, 699, there was high tide at that hour at Boulogne. Now in that harbour there is always a high tide about nine o’clock in the morning two or three days before full moon and before new moon. Therefore, as the full moon of September, 699, took place on the 14th of the month, Caesar must have returned to Gaul about the 11th or 12th of September.’

There is no fault to be found with the conclusion (except that it is uncertain), but much with the argument. To begin with, as there had been a full moon on the 31st of August, it is obvious that not the full moon but the new moon of September took place on the 14th of the month. This error, indeed, is immaterial; but Napoleon has no right to assume that Caesar reached Boulogne about nine o’clock in the morning, for the circumstances of his return voyage in the second expedition were totally different from those of the preceding year. In 54 B.C. there was a dead calm (summa tranquillitate, B. G., v, 23, § 6), and the ships were rowed: in 55 they sailed. Moreover, it is untrue that the fleet could only enter the harbour of Boulogne at high tide (see p. 586, supra).