Julius Caesar's Timeline in Egypt
Caesar and Gnaeus Pompeius Magnus (Pompey) were fighting a civil war for control of Rome. Caesar had "crossed the Rubicon", cities in his path quickly surrenedered, and the anti-Caesar senators and Pompey fled from Rome to the East. What followed were skirmishes in Italy, a brief interlude of conquering Pompey's troops in Spain, then head-to-head battles in Greece and Asia. Pompey lost to Caesar at the Battle of Pharsalus, and tried to regroup and add to his forces. He was the "protector" in Rome of Egypt at the time, and ultimately wound up seeking military aid from Ptolemy XIII.
Ptolemy XIII was 14 years old, and had ascended to throne 3 yrs previously at the death of his father. He "officially" ruled with older sister Cleopatra VII (it is not clear if they were technically married). Cleopatra side-lined him early in his reign, but his trio of advisors (the eunuch Pothinus, the scholar Theodotus of Chios, the general Achillas) ultimately forced Cleopatra into exile and took over rule of Egypt. This was the state of Egypt when Pompey came looking for assistance.
The trio of advisors thought they would "please" Caesar, who they deemed the most likely to win the Roman civil war, and beheaded Pompey … presenting the head to Caesar when he arrived shortly after. Caesar was not pleased.
Caesar attempted to resolve the dynastic dispute, as Rome had done with previous Egyptian rebellions., and he put Ptolemy and Cleopatra back on their joint thrones. Ptolemy's advisors did not appreciate the loss of power and assembled Ptolemy's troops out side the city. Caesar put Ptolemy into "protective custody". Ptolemy's general Achillas executed the ambassadors who had brokered the truce, and the Egyptian army attacked the palace. Full-out civil war started between Ptolemy's forces and Cleopatra's (bolstered by Caesar's legions).
It took reinforcements of Roman allies to significantly turn the tide in Caesar's favor. Attempting to escape, Ptolemy XIII drowned in the Nile, in armor. Caesar stayed in Alexandria for a while, famously going on a cruise down the Nile with Cleopatra. He then went to fight Pharnaces at the Battle of Zela.
There are a number of historical accounts that all agree on the big picture. What's not clear is the timeline, since some authors conflict with days between some events. Fortunately, this has all been finally straightened out with the scholarship of John T. Ramsey (University of Illinois at Chicago) and Kurt A. Raaflaub (Brown University). They have worked out the details between the "hard" dates (Caesar's arrival in Egypt, Ptolemy XIII's death, Battle of Zela, etc.) using details like average marching speed, sailing speed of transports, and seasonal weather.
Caesar was very, very busy. He didn't "relax" at the death of Pompey: the Civil War wasn't over, he still had to battle troops loyal to Pompey, as well as deal with an uprising in client kingdom of Pontus.
HOW LONG WAS THE NILE CRUISE OF CAESAR AND CLEOPATRA?
He had love affairs with queens too, including Eunoe the Moor, wife of Bogudes, on whom, as well as on her husband, he bestowed many splendid presents, as Naso writes; but above all with Cleopatra, with whom he often feasted until daybreak, and he would have gone through Egypt with her in her state-barge almost to Aethiopia, had not his soldiers refused to follow him.
-- Suetonius, Life of Caesar, LII.1
He consumed nine months in this strife, at the end of which he established Cleopatra on the throne of Egypt in place of her brother. He ascended the Nile with 400 ships, exploring the country in company with Cleopatra and generally enjoying himself with her. ...
After Caesar had performed these exploits in Alexandria he hastened by way of Syria against Pharnaces
-- Appian, The Civil Wars, II:90-91
Taking the sixth veteran legion with him into Syria, he left the rest in Egypt to support the authority of the king and queen, neither of whom stood well in the affections of their subjects, on account of their attachment to Caesar, nor could be supposed to have given any fixed foundation to their power, in an administration of only a few days' continuance.
-- Aulus Hirtius, Alexandrian Wars, 8
In this way Caesar overcame Egypt. He did not, however, make it subject to the Romans, but bestowed it upon Cleopatra, for whose sake he had waged the conflict. Yet, being afraid that the Egyptians might rebel again, because they were delivered over to a woman to rule, and that the Romans might be angry, both on this account and because he was living with the woman, he commanded her to "marry" her other brother, and gave the kingdom to both of them, at least nominally. For in reality Cleopatra was to hold all the power alone, since her husband was still a boy, and in view of Caesar's favour there was nothing that she could not do. Hence her living with her brother and sharing the rule with him was a mere pretence which she accepted, whereas in truth she ruled alone and spent her time in Caesar's company.
For now that Ptolemy was dead and Domitius vanquished, Caesar had decided that his delay in Egypt was neither creditable nor profitable to him, and had set out from there and had come with great speed into Armenia,
-- Cassius Dio, Roman History, XLII.44,47
What's a Thalamegos?
Suetonius: He is the only author who actually mentions a "state barge". His actual words:
"et eadem nave thalamego paene Aethiopia tenus Aegyptum penetravit".
He doesn't actually say they went anywhere, but why would he mention "her state-barge" if they didn't? Technically, "thalamegos" is a type of boat used to ferry administrators around the delta, but it's a safe bet that if they were going to spend a good deal of time on it, it would be the only fancy Thalamegos we know about.
Appian: 400 ships? Seriously? Ocatavian/Augustus had 400 ships at Actium. Where did Caesar get them on short notice? Maybe it was a mixture of Roman and Egyptian river boats. Maybe Appian means "a lot". I like how he says "he enjoyed himself" THEN "he hastened".
Hirtius: The author is a guess. Historians are pretty sure that Caesar didn't write it (in the way he wrote his previous books), and they're pretty sure Hirtius wrote Book VIII of the Gallic Wars. In any case, he completely rolls over any possible Nile cruise with "only a few days." Perhaps this is because he was an officer in Caesar's military and chose to ignore a non-serious expenditure of time when there were pressing military needs elsewhere.
Dio: In a pleasant change from his usual, Dio says less, rather than inventing stuff from thin air to say more. He does mention a "neither creditable nor profitable" delay. Perhaps, this is the cruise. Knowing how he bends things to match his opinions, IMHO he doesn't want to talk about it. As far as I can tell, it's rare for him to let a rumor go.
Did they actually take a lover's trip up the Nile?
Most historians agree, that despite the lack of direct evidence, it is entirely possible that Cleopatra and Caesar had a luxury cruise up the Nile. Consider that Cleopatra, like the Ptolemies before her, needed to cement a strong relationship with Rome. There probably wasn't a lot deep emotional conversation while Ptolemy XIII's troops were attacking the palace. If Cleopatra could manage a boy child to the currently childless Caesar, she could bind him more strongly that just a political allegiance.
How far did they go?
I'm afraid Suetonius suffers from a lack of geography, in the same manner that first-time European tourists think they can drive from New York to Chicago in a day. The best guess is that they had 27 days of travel: ~14 days in each direction. A "barge" of any kind is not really a sailing vessel. Ptolemy IV's Thalamegos had sails, but with it's enormous girth, they were probably only an assist. Barges generally need to be towed, and are consquently quite a bit slower than sailed or rowed ships. Cleopatra and Caesar would have been lucky to reach Philae at the first cataract. If Caesar actually wanted to walk around the Sphinx, and the Pyramids ... and Dendera, and Luxor, and Karnak ... they probably didn't get past Thebes. More than likely, if they spent ANY time sight-seeing off the boat, they only made it to Memphis, then turned around. Besides, there is NO way they could have dragged the Thalamegos up the slip-way at the first cataract to head towards "Aethiopia".
Did they use Ptolemy IV's Thalamegos?
Put yourself in Cleopatra's sandals. At 21 yrs old, she's got to impress Caesar that she can control an empire. He's just bailed her out of a serious war. Her palace and a good portion of Alexandria is in ruins. She's still dead broke due to her father's policies, but she's managed to save up just enough to pay the Roman debt when Caesar demands it: there's no money for building a luxury boat overnight. Caesar doesn't want to "rough it" and travel around in those uncomfortable minimal papyrus riverboats ... he wants to be rewarded for his efforts, in grand style, by the queen he just placed on the throne. The Thalamegos was about 150 yrs old at this time, but I'm reminded that the USS Constitution was launched in 1797 and the last time it sailed under its own power was 2012. Presuming the Thalamegos was maintained over that period, all they had to do was load up food and go.
At the time all this is going on, the Roman Empire is using a broken lunar calendar. It's several months off. When Caesar gets in power, he converts it to a solar based calendar ... like Egypt. This new calendar is the "Julian" one that we use today. The authors of the paper give you the choice of lunar or solar calendar dates for all these events: I used the Julian ones because it gives a better idea of the seasons, IMHO.
Just This This & More Caesar
... And a Bit of Fun
This is a small section of a very large mosaic that was found on the wall of a 1st century BCE shrine grotto in Palestrina, Italy (just outside of ancient Rome) -- click on it. The mural-sized mosaic shows the Hellenistic/Ptolemaic Nile in a compressed back & forth depiction (similar to Philip Jose Farmer's Riverworld). Clothing is Greek and Egyptian. This particular style of artwork is credited to have been invented by "Demetrius the Topographer" in 165 BCE. There are many animals represented: those known to Greeks are not named, those from the interior of the continent have names. This is said to be an Alexandrian style. It permanently resides in the Antikensammlung, Staatliche Museen in Berlin, although a quick search just told me it's on loan to the Getty in Los Angeles right now (some of the best Egyptian art is elsewhere other than Egypt).
There are many references to the people on the barge as Cleopatra and Julius Caesar (more like a raft, actually). It's supposed to lend credence to the idea that at the time of the mosaic, before Suetonius, et al., there was at least a legend of a cruise in place.
The problem is that the best dating puts this mosaic at 100 BCE ... 41 years before Cleopatra's birth. Everyone wants to say they have a picture of Cleopatra.
Here's a scholarly article on just this topic: Caesar, the Thalamegos, how long on the cruise. Did he use the Thalamegos? Maybe? Maybe not.