Astronomical Synchronisms

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 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:

Lunar data

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:

Solar data

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:

Sidereal data

A small amount of sidereal data is useful to bound Roman dates.

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