A number of astronomical synchronisms exist which give exact synchronisms or, more usually, approximate synchronisms or bounds. These are summarised here.
The most useful astronomical synchronisms are precisely dated eclipses. Three are crucial to this subject:
The solar eclipse of Kal. Aug. A.U.C. 798 = 1 August A.D. 45. This establishes that the Julian dates of the early empire correspond to the modern Julian calendar.
- The lunar eclipse of a.d. III Non. Sept. A.U.C. 586 = 21 June 168, on the eve of the battle of Pydna.
- The solar eclipse of a.d. V Id. Quint. A.U.C. 564 = 14 March 190, which makes this year the earliest for which precise dates can be established.
The last two eclipses are the basic anchors for Roman chronology in the second century.
All three Roman dates are only known through literary sources. While there has never been any serious reason to doubt the first synchronism (though some have tried), the veracity of the other two have been challenged. The second is strongly supported by contemporary epigraphical evidence giving a close Athenian synchronism. The first is mostly supported by the detailed consistency of the internal chronology of the account of the remaining events of that year, leading finally to a rough seasonal synchronism for the following winter. I am not aware of any serious reason to doubt either.
The so-called eclipse of Ennius is sometimes identified with an eclipse in May 203, which would allow us to fix chronology in that year precisely. However, I cannot accept the arguments for this date. According to Cicero, this eclipse was a cornerstone of Varro's chronology, i.e. a key step in establishing his equation of A.U.C. 1 = 753. As far as we can determine, the major chronological issue for Varro was to establish the date of the sack of Rome by the Gauls. Most likely, therefore, this eclipse occurred in the late fifth or early fourth century, where several possible candidates are located. On the analysis given here, it is suggested that this eclipse is that of Non. Iun. A.U.C. 349 = 20 March 405, a solution which may imply that all intercalations before A.U.C. 563 = 191 were 23 days long, a result that, if correct, allows precise conversions for many years in the third century.
Two other eclipses are helpful:
The solar eclipse of 19 October 202 may be identifiable as the eclipse which Dio, as preserved by Zonaras, says took place shortly before the battle of Zama, fought a few days before a.d. XIV Kal. Ian. A.U.C. 552 = 202. This eclipse appears to be a later calculation rather than a contemporary reference since it was only visible in central Africa.
- The solar eclipse of 11 February 217 was one of the portents for the consulship that started on Id. Mart. A.U.C. 537 = 217, according to Livy.
There are a few lunar synchronisms which can be used to fix or bound Roman dates in addition to the eclipse of 168. The major ones are:
A.U.C. 813 = A.D. 60: Pompeian graffito CIL IV 4182 gives us that a.d. XI Kal. Feb. A.U.C 813 was the start of a lunar month. Since 22 January A.D. 60 was a new moon this gives us that a.d. XI Kal. Feb. A.U.C 813 = 22 January A.D. 60.
- A.U.C. 730 = 24: The lunar ephemeris data for Iulius, Sextilis and September given in pOxy 61.4175 establish that the Roman calendar was exactly synchronised to the Julian calendar in that year. This data is new, first published only in 1999, and is central to this study.
- A.U.C. 701 = 53: The full moon rose in Scorpio, i.e. on 11 or 12 May 53, a few days after the battle of Carrhae, which was fought on a.d. V Id. Iun A.U.C. 701.
- A.U.C. 687 = 67: The inscription CIL I2 2511 says that a.d. XV Kal. Oct. A.U.C. 687 was day 3 of a lunar month. This synchronism is critical to establishing exact Roman chronology in the 70s, 60s and early 50s of the first century B.C.
There are a few solar synchronisms which can be used to bound Roman dates in addition to the eclipses of 190 and A.D. 45. The major ones are:
A.U.C. 717 = 37: Varro states that the vernal and autumnal equinoctes were on a.d. IX Kal. Apr. and a.d. VI Kal. Oct.
- A.U.C. 705 = 49: Cicero states that the vernal equinox was close to a.d. XVII Kal. Iun. A.U.C. 705.
- A.U.C. 700 = 54: Cicero states that Caesar left Britain shortly after a.d. VI Kal. Oct. A.U.C. 700; Caesar states that he left shortly before the autumnal equinox
- A.U.C. 653 = 101: Marius' battle against the Cimbri was fought on a.d. III Kal. Sex. A.U.C. 653, which Plutarch says was after the summer solstice
A small amount of sidereal data is useful to bound Roman dates.
A.U.C. 708 = 46: Caesar struck camp and was hit by a massive hailstorm on a.d. VI Kal. Feb. A.U.C. 708, very shortly before the setting of the Pleiades, which was on 8 November 47.
- A.U.C. 691 = 63: Augustus was born under Libra on a.d. IX Kal. Oct. A.U.C. 691, which therefore lies between 17 September and 17 October 63.
- A.U.C. 605 = 149: The consul Censorinus left the camp of the army beseiging Carthage some time after the rising of Sirius, 27 July 149, in order to conduct elections in Rome, which would date his departure to around mid-late (civil) September.
- A.U.C. 536 = 218: Hannibal arrived in Italy across the Alps around the setting of the Pleiades, c. 7 November 218, which was approximately 5 months after the founding of Placentia on prid. Kal. Iun. A.U.C. 536.
- A.U.C. 499 = 255: A fleet assembled shortly after the start of the consulate, Kal. Mai. 499 A.U.C., was wrecked in a storm off the coast of Sicily before the rising of Sirius, c. 26 July 255, after a series of actions which must have taken at least two months.
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