Pliny the Younger describes the eruption and gives the date in a famous letter to the historian
Tacitus, but apparently the date is corrupt in the manuscripts, whence the problem.
Here is what
Wikipedia says in its article on Mt. Vesuvius:
The year of the eruption of AD 79 was referred to by
contemporary writers (apart from being described by Pliny) and has never been seriously questioned, being determined by the well-known events of the reign of
Titus.
Vespasian died that year. When
Titus visited Pompeii to give orders for the relief of the displaced population, he was the sole ruler. In the year after the eruption, 80 AD, he faced another disaster, a great fire at
Rome.
The time of year is stated once in one historical document, the first letter of Pliny the Younger to
Tacitus,[50] as "nonum kal. Septembres", which is not a produced meaning and has no syntax (the grammarians say, indeclinable), but would seem to be an abbreviation of a
standard date, which was an abbreviation. By 79 the Julian Calendar was in use. The inscribing of dates was abbreviational and formulaic. Whether anyone knew exactly what the abbreviation stood for is questionable (compare English Mr. and Mrs.); certainly, literary representations such as Pliny's left out or misinterpreted key elements that would be required for the understanding of a produced meaning. Pliny's date (if it was Pliny's) would have been a.d. IX kal. sept., to be interpreted as "the ninth day before the Kalends of September", which would have been 8 days before Sept. 1, or August 24 (the
Romans counted Sept. 1 as one of the nine).
August 24 is not necessarily the date given by Pliny. It represents an editorial collusion to use the text of Codex Laurentianus Mediceus (a manuscript), which also appears in the 1508 printed edition of Aldus Manutius, in all recensions since then, even though the numerous Pliny manuscripts as well as the works of other authors offer many alternatives.[58] Archaeological dissent from this view began with the
work of Carlo
Maria Rosini in 1797, to be followed by a succession of archaeologists putting forward evidence to the contrary. Archaeological evidence from Pompeii suggest that the town was buried about two or three months later. For example, people interred in the ashes appear to be wearing warmer clothing than the light summer clothes that would be expected in August. The fresh fruit, olives and vegetables in the shops are typical of October, and conversely the summer fruit that would have been typical of August was already being sold in dried, or conserved form. Wine fermenting jars
had been sealed over, and this would have happened around the end of October. The coins found in the purse of a woman buried in the ash include a commemorative coin that should have been minted at the end of September.[59]
A 2007 study by Rolandi, De Lascio and Stefani of 20 years of data concerning wind direction at meteorological stations in
Rome and Brindisi established wind patterns in the Vesuvius
area above 14 kilometres (46,000 ft) with more precision than was previously known.[60] From June through August the winds blow strongly from the
west, for the rest of the time, from the east. This fact was known, but the easterly winds of the eruption were considered anomalous in August, caused (conjecturally) by the weak and shifting winds of the transition. The authors established that the winds of 79 produced long depositional patterns and were therefore not weak, and that the transition occurs in September, not August. The authors therefore reject the August date as being inconsistent with the patterns of nature.
The rejection is not of Pliny's eyewitness account or of Pliny's date, whatever it was. They focus on manuscript variants looking for possible sources of copyist alteration of Pliny's date. In many manuscripts the month has been omitted. If some original
had no month then the copyists may have felt obliged to provide one, but chose wrongly. The authors suggest an original date of a.d. IX kal
dec (November 23) or a.d. ix kal nov (October 24) more in line with the evidence of nature. This view is currently gaining wide acceptance. In its
index of volcanic activity on Earth, the Global Volcanism Program of the
Smithsonian Institution takes the view that the eruption did start around October 24 (?), 79 AD and ended on October 28 ±1 day.[14]
Notes 58-9 refer to Rolandi, G.; Paone, A.; De Lascio, M.; Stefani, G. (2008). "The 79 AD eruption of Somma: the relationship between the date of the eruption and the southeast tephra dispersion". Journal of Volcanology and Geothermal Research 169: 87–98.
