So we have come to the last and most lasting of Caesar’s scientific achievements - his calendar. In a sense it was not his at all. He recognized the astronomical superiority of the Egyptian calendar, originally of twelve 30-day months plus five end-of-year days, to the confusing and by now retarded Roman calendar. This had followed a pattern of years 355 days long based on more or less accurate lunar months, brought into line with the sun by the priestly insertion every three years of an additional ‘‘intercalary’’ month of 22 days: this was inserted at the end of the religious calendar after the last feast day in February. But for some years now the college of pontiffs (of whom Caesar was the senior man, but absent in Gaul) had refused to insert the extra days from political motives, and the Roman calendar was now three months ahead of the real solar year. Caesar must have already experienced the frustrations of this situation and he brought the Egyptian astronomer Sosigenes to Rome to help him implement a new calendar - the calendar we still observe. This involved two steps: the calculation of the number of supplementary days needed to bring the calendar date into coincidence with the winter solstice, and the adjustment of the number of days in the year. First then he inserted an intercalary month after February 24, 46 BC, plus two intercalary months after the end of November, to make this a year of 445 days (Macr. Sat. 1.14.3). Starting in January 45, Caesar extended the year from 355 to 365 days by distributing ten extra days over seven of the old Roman months. Apart from February these seven months had been of either 29 or 30 days (see Feeney 2007: 152-3).
What Caesar did makes sound sense if we take into account respect for the old calendar, and, I would suggest, the desire to equalize the four seasons. The days he added to the old calendar were inserted after the last festival of the month, and so did not affect the old religious calendar. But by adding two days to the 29 days of January, August, and December, and one each to the 30 days of April, June, September, and November (and inserting a leap day every four years), Caesar achieved as nearly as possible ayear of 365(+%) days containing four equal seasons: January, February, and March make a winter of31+ 28 + 31 = 90 days (91 days in leap years), April to June a spring of 30 + 31 + 30 = 91 days, July to September a summer of 31 + 31 + 30 = 92 days, and October through December an autumn of 31 + 30 + 31 = 92 days. This would also keep as close as possible to the orbit of the moon, which takes between 29 and 30 days to complete each cycle. And we must not forget leap year, with the extra day which Caesar intended to be added every four years to cover the six extra hours of the sun’s orbit.
After Caesar’s murder the leap day was mistakenly inserted every three years and had to be adjusted by his heir Augustus in AD 8. Cicero might joke that on January 5, 45 the constellation Lyre was rising ‘‘by Caesar’s decree,’’ but it did take a decree to impose the new calendar, and a good deal of scholarly explanation to relate it to the old Roman calendar and the expected risings and settings of constellations. Sosigenes, according to Pliny the Elder (Nat. Hist. 18.211), wrote three books of Commentationes to explain his calendar. But what was Caesar’s De astris (‘‘On the constellations’’)?
As Feeney shows, this work of Caesar’s affected the way both Varro and Pliny the Elder indicated the significant days in their agricultural calendars (Varro On Agriculture 1; Pliny Nat. Hist. 18.211-end): Varro can now give stable calendar dates for his stars rising and setting, and Pliny quotes Caesar for a number of synchronizations between his astronomical data and the civil calendar. Starting at 18.234 he lists Caesarian synchronisms for December 30, for February 16 and 22, for the ‘‘deadly’’ Ides of March, for March 18, and for other days from the winter solstice to the coming of the warm south wind (a bruma in favonium), the first of Varro’s eight divisions of the farmer’s year. Caesar must have depended on the collaboration of Sosigenes, but Macrobius, more oddly, claims (Sat. 1.14.2) that Caesar had the support of a scribe, M. Flavius, ‘‘who brought the individual days written out to Caesar in such a way that their order could easily be found and once it was found the fixed system (certus status) would persist.’’ I think we can make sense of this if we assume Sosigenes and Caesar harmonized the new months with the celestial calendar (which would have to be adjusted from the sky at Alexandria, quoted by Pliny at e. g. Nat. Hist. 18, 246, 270 to the sky at Rome). Then Flavius applied this to the civil calendar as modified by Caesar, providing celestial data for each civil day.
Where does Caesar come into this? Suetonius describes the calendar (Iul. 40) but does not include any volume on this topic in his list of Caesar’s published works. Was it simply ascribed to Caesar because he had decreed the calendar? Scholars like Le Boeuffle (1972, in Le Bonniec’s edition of N. H. 18) and Domenicucci (1990) rightly consider a middle possibility: that Caesar did not make all the calculations in the detailed calendar listing of the volume, but authorized its issue, and, I would suggest, composed the introduction in the form of his actual public decree; that is, De astris was in two parts: the preamble Caesar had composed for his decree, and its implementation in the form of a parapegma, or representation of the traditional calendar board with its movable wooden pegs, now in fact no longer needed, because the new calendar would ensure that constellations rose and set on the same day each succeeding year. It would be this latter part which Sosigenes and Flavius assembled for Caesar. If this is right, Caesar had actually carried his enthusiasm to the point of promulgating a work reconciling the civil and astronomical calendars.
One more general idea may emerge from the range of Caesar’s investigations. According to Varro (De Lingua Latina 9.24, written for Cicero but in the time of Caesar’s dictatorship), analogists justified their imposition of regularized forms on Latin nouns by analogy with the regularity of the tides and of the seasons. We know that Posidonius wrote about the tides of the Ocean, so that this topic was at the center of scientific theorizing when Caesar wrote his Gallic Wars. But rather than see Caesar’s comments as a reaction for or against Posidonius, I would suggest that the three topics are linked in Varro’s comment because they all reflect rudimentary systems, and that Caesar commented on these topics because he was naturally attracted to systematization. Most recently, Feeney (2007: 197) rightly diagnoses ‘‘the same Caesarian regularizing and ordering urge at work in the reform of the calendar as in his grammatical work on Analogy.’’
World History
