Gaius Julius Caesar. National Archaeological Museum, Naples.
I: 55BC, The First Expedition
In fact, for his first expedition, both Caesar's port of departure in Gaul and his landing-place in Britain are uncertain. Caesar says that he set out from “Portus Itius” on his second expedition. It is often assumed that he set out from there on the first too, but Caesar gives no indication that this was the case. The location of Portus Itius is itself a subject of debate – Wissant is a strong contender, but Boulogne is the definite favourite. Actually, the general consensus seems to be that Caesar's first expedition set out from Boulogne, regardless of whether it was Portus Itius or not. The eighteen delayed transport vessels were in a harbour “eight miles off” from Caesar's point of departure. This “upper port” may, then, have been Ambleteuse, to the north of Boulogne. At any rate, when favourable weather allowed Caesar to set sail for Britain, he despatched the cavalry to this “further port”, with instructions to “embark there, and follow him”. It seems indisputable that Caesar first anchored off Dover. Various textual details strongly indicate that it was near Deal, to the north-east of Dover, that he landed, but there is no archaeological evidence to support this – indeed, it is, perhaps, surprising that no direct evidence at all has been found of Caesar's campaigns in Britain – and there is a snag.
Caesar says he “reached Britain about the fourth hour”. Daytime was divided into twelve hours – the ‘first hour’ beginning at sunrise, the ‘twelfth hour’ ending at sunset – so, the length of an hour depends on the latitude and time of year. At the time of year in question, at Dover, dawn is about 5am (GMT) and daytime is about fourteen hours, so the fourth hour was approx. 8.30–9.40am. He says he waited “till the ninth hour”, i.e. 2.20–3.30pm, for the rest of the fleet to join him, and then sailed, and this is the crucial point, with the tide to his landing-place. Now, Caesar reports that:
“... four days after the expedition reached Britain, the eighteen ships ... which had taken the cavalry on board, sailed from the upper port with a light breeze. They were getting close to Britain and were seen from the camp, when such a violent storm arose that none of them could keep their course...
The same night it happened to be a full moon ...”
‘The Gallic War’ Book IV Chapters 28 & 29
Because of Caesar's various references to the time of year, the full moon in question has to have been on the night of 30th/31st August. Depending how the four day interval before the cavalry set off is counted, his arrival at Dover would have been on 26th or 27th August. Unfortunately, on those dates, between 2.20pm and 3.30pm, the tidal stream would have been running westwards i.e. away from Deal. Proposals for landing sites to the west of Dover have been made, but they are difficult to reconcile with Caesar's text. One simple solution to this conundrum would be that Caesar didn't sail until a short while after the ninth hour (presumably, his times were only estimated from the sun's position anyway), when the tide had changed in his favour. According to data published by T. Rice Holmes, in ‘Ancient Britain and the Invasions of Julius Caesar’ (1907), the tide turned at 3.54pm on the 26th (he suggests that the wind may have caused it to turn earlier – perhaps bringing it within the ninth hour). On the same basis, however, the tide wouldn't turn until 5.15pm on the 27th, which, even if the wind advanced it a little, would, surely, be too late in the day for all that was yet to occur:
“But, it will be objected, Caesar may have landed on the 27th of August; and in that case the stream could not have turned east­ward before the close of the ninth hour. Certainly it could not have done so unless it had turned westward unusually early, or unless its westward duration had but little exceeded four hours; and although this has been shown to be within the bounds of possibility, it is to the last degree improbable. But if he landed on the 26th of August, an assumption which has been proved to be not inconsistent with his narrative, it is not improbable that the stream may have turned eastward in the ninth hour; and this is all that am concerned at present to show.”
T. Rice Holmes ‘Ancient Britain and the Invasions of Julius Caesar’
T. Rice Holmes' exhaustive analysis of Caesar's campaigns in Britain still carries great weight. He was an ardent advocate of Caesar having sailed in the direction of Deal:
“There is but one conclusion to which we can come, and that conclusion is absolutely certain: when Caesar weighed anchor off the Kentish cliffs, he sailed towards the north-east.”. 
He examined the various solutions to the riddle of Caesar's landing-place:
“I began this inquiry early in 1900 with a mind absolutely un­ biassed, resolved to do one of two things – either to solve the problem, or, if that could not be done, to show, once for all, that it was insoluble. The reader knows that I have not neglected any means of ascertaining the truth; and I have provided him with the means of controlling every statement which I have made. I have set down fully and fairly the arguments of those from whom I differ; and I have kept back nothing, I have called attention to everything. that might appear to tell against the conclusion to which the evidence inevitably led. I need not say anything by way of recapitulation, for no man who has read this article attentively can be lacking either in patience or in intelligence; and I am sure that the reader is by this time convinced of these things:– that it has been demonstrated that Caesar did not land at Pevensey, or anywhere in Sussex; that it has been demonstrated that he did not land at Hythe, or anywhere in Romney Marsh; and that it has been demonstrated that he did land both in 55 and in 54 B.C. in East Kent – in the former year between Walmer Castle and Deal Castle, in the latter north of Deal Castle. That some will still for a time dispute these conclusions is likely enough; but not those whose judgements count. For them the problem is solved.”
In ‘Britannia: A History of Roman Britain’ (Third Edition, 1987), S.S. Frere writes: “Short of actual archaeological identification of Caesarian earthworks, no better identification of the landing-place is likely to be available than that offered by Rice Holmes's masterly and very full examination of the evidence.”
There is, however, another, perhaps more elegant, solution to the nagging ‘tide problem’ than that espoused by Rice Holmes. Possibly, an error in regard to the “four days” has crept into Caesar's text. In a footnote to R.G Collingwood's ‘Roman Britain and the English Settlements’ (Second Edition, 1937) is the comment:
“I follow Rice Holmes in making Caesar land east of Dover ... I suspect something has gone wrong with the numeral quartum in iv. 28 [of ‘The Gallic War’]; if the interval was more like a week, the stream would be running north-eastwards when he weighed.”
Roman numerals are, indeed, notoriously fragile – easy to make errors when reading and easy to make errors when writing. Perhaps, then, Caesar simply misread the Roman numeral VII or VIII as IIII when transcribing his field-notes.
II: 54BC, The Second Expedition
To generate a meaningful chronology of Caesar's second expedition, the date references in Cicero's letters have to be converted into modern usage. Cicero expresses years in terms of ‘ab urbe condita’ i.e. ‘from the founding of the city [of Rome]’ (usually abbreviated to AUC). Caesar's second expedition took place in our year 54BC, but for Cicero it was 700AUC. The months of July (after Julius Caesar) and August (after Augustus, the first emperor) were, of course, not used by Cicero – Quintilis was renamed July following Caesar's assassination in 44BC, and Sextilis was renamed August in 8BC. The days are numbered relative to the next of three fixed monthly points: the Nones, the Ides, and the Kalends of the following month. The Kalends is the 1st, the Nones is the 5th and the Ides the 13th, except in March, May, Quintilis/July and October, when the Nones are the 7th and the Ides are the 15th. For example:
a.d. v Non. Quint. a.u.c. 700
This is the fifth day before (a.d. = ‘ante diem’ = ‘days before’) the Nones of Quintilis 700AUC. In Quintilis/July the Nones is the 7th, so, counting inclusively, the fifth before is the 3rd. The date, then, is 3rd July – except that Caesar's calendrical reform (the ‘Julian Calendar’) was introduced on 1st January 709AUC (1st January 45BC). Applying the appropriate correction produces the final result: 8th June, in the Julian Calendar, of the year 54BC.
In his ‘Ancient Britain and the Invasions of Julius Caesar’ (1907), T. Rice Holmes outlined the process by which he calculated the, often quoted, date of Caesar's arrival in Britain:
“On the 27th of July, that is to say, on the 2nd of July of the Julian calendar, Cicero wrote to Atticus, “Judging from my brother Quintus's letters, I imagine that by this time he is in Britain” [‘Letters to Atticus’ Book IV no15] ... It would be very rash, however, to infer from this that Caesar landed in Britain before, or even as early as, the 2nd of July; for, as we have seen, his embarkation was delayed by the long continuance of adverse winds. The first letter which announced the arrival of the expeditionary force in Britain was referred to by Cicero in a letter to Quintus, in which he says, “How I rejoiced at your letter from Britain! I was nervous about the sea and the coast of that island” [‘Letters to his Brother’ Book II no15] ... This letter is undated; but it must have been written some time after the one which Cicero wrote to Atticus on the 27th of July; for in the letter to Atticus we find the words, “I have undertaken to defend Messius ... After that I have to prepare myself for Drusus, and then for Scaurus” ... while in the letter to Quintus Cicero wrote, “The day I write this Drusus has been acquitted ... The comitia have been put off to September. Scaurus's trial will take place immediately” ... Asconius tells us that the last day of Scaurus's trial was the 2nd of September; and Cicero's remark that “the comitia have been put off to September” makes it evident that he wrote in August; while from his saying that “Scaurus's trial will take place immediately” we should naturally infer that when he wrote the 2nd of September was not far off. Letters from Britain generally reached Rome in about 27 days; and accordingly we may conclude that the letter in which Quintus Cicero announced his arrival in Britain was written about the end of July; that is to say, about the 6th of July of the Julian calendar.
In a footnote, T. Rice Holmes comments: “There can, I think, be little doubt that Quintus wrote and dispatched this letter on the very day of his arrival, or, at the latest, before the storm which totally wrecked 40 of Caesar's ships on the next night but one after his arrival. If the storm had occurred when he wrote, he would surely have mentioned it; and there is not a word in Marcus Cicero's reply which would lead us to suppose that he had done so. Moreover, Quintus knew that Marcus was waiting impatiently for news; and Caesar would naturally have desired to communicate at once with Labienus whom he had left in command in Gaul.”
Now Caesar says that the tide [in the Straits of Dover] turned westward soon after daybreak on the morning of his arrival in Britain [‘The Gallic War’ Book V Chapter 8]; and this statement proves that he landed either about the time of full moon or about the time of new moon. There was a full moon on the 21st of July, 54 B.C.; and the previous new moon occurred on the 7th of July. [Here, T. Rice Holmes refutes the theory that “the landing must have taken place on the day of full moon”, proposed by Napoleon III]... If, then, we decide that Ceasar landed in Britain on the day of new moon, the 7th of July, 54 B.C., we shall not be more than one day wrong; but to fix the date with absolute precision is impossible.
As Caesar landed about the 7th of July, it follows that he had reached the Portus Itius, where he was delayed about 25 days, about the 11th of June.”
Concerning the date of Caesar's departure from Britain, T. Rice Holmes says:
“We have seen that on the 25th of September (the 29th of August of the Julian calendar) he [Caesar] wrote to Cicero, saying that he was on the point of bringing back the army [‘Letters to Atticus’ Book IV no18]... On the whole, it appears to me that all we can say for certain regarding the date of Caesar's return is this. It cannot be fixed earlier than several days after the 29th of August of the Julian calendar – the date of the letter in which he informed Cicero that he was on the point of bringing back the army. Bearing in mind that it occurred when “the equinox was at hand” [‘The Gallic War’ Book V Chapter 23], we may place it about the middle of September.”
‘The Gallic War’ Book V Chapter 2 and Chapter 5.
Caeasar, using the usual Roman method of reckoning, would have included both the day of his arrival and the day the eighteen transports set off in his count. By this inclusive method of counting, therefore, four days before the 30th August is the 27th. However, though the ships became visible off the coast of Britain on the 30th, they quite possibly had (like Caesar) travelled through the night – writing at the beginning of the 1st century AD, Strabo (c.63BC–c.AD24) suggests (‘Geography’ Book IV Chapter 3) that an overnight passage of the Channel was normal practice. They might have actually set sail on the night of the 29th August, which would make Caesar's arrival in Britain the 26th.
The Nones itself is the count-of-one. The count-of-two, i.e. the day before the Nones (this also applies to the the Kalends and the Ides), is ‘Pridie’. Then it's the third day before (a.d. iii), and so on. So, for Quintilis, the sequence from Kalends to Nones is:
Kal. Quint.
a.d. vi Non. Quint.
a.d. v Non. Quint.
a.d. iv (or iiii) Non. Quint.
a.d. iii Non. Quint.
Prid. Non. Quint.
Non. Quint.
1st July
2nd July
3rd July
4th July
5th July
6th July
7th July
T. Rice Holmes, in ‘Ancient Britain and the Invasions of Julius Caesar’, explains:
“We have ascertained that the Kalends of January, 709, fell on the 1st of January, 45 B.C.; that 90 days were intercalated in 708, which accordingly consisted of 445 days; that a month of 23 days was intercalated in 702, which accordingly comprised 378 days; and that 701, 703, 704, 705, 706, and 707 were ordinary years, each comprising 355 days. It follows that the last day of 700, the year in which Caesar made his second expedition to Britain, corresponded with the 30th of November, 54 B.C., and that the sixth day before the Kalends of October, the day on which he wrote to tell Cicero that he was on the point of bringing back his army from Britain to Gaul, corresponded with the 29th of August. From these data it will be easy to ascertain the correspondence of any date in the year 700 which we find in our authorities with the Julian calendar.”
During the reigns of Claudius (41–54AD) and Nero (54–68AD), Quintus Asconius Pedianus composed commentaries on Cicero's speeches. Only five survive.
In a footnote, T. Rice Holmes cites some comments made by Cicero to substantiate this claim. He suggests that one particular letter took an “extraordinarily long time – 33 days” to reach Cicero because he “was not at Rome when he received it, but at Laterium, near Arpinum, about 70 Roman miles E. by S. of Rome”. Rice Holmes also refutes the claim of Napoleon III (‘Histoire de Jules César’, 1865) “that, in favourable circumstances, letters only required 20 days for transmission from Britain to Rome”.