A date that is still open
Textbooks traditionally say that Vesuvius erupted on 24 August 79 CE. That date has a long history, but it is not secure. The year 79 is well supported, and the sequence of the eruption is understood. The exact day, and especially the season, is a genuine open question. Many specialists now lean towards the autumn, probably October or later, although no one claims the matter is closed.
The word “mystery” in the title should be read modestly. No one doubts that the town was destroyed. What is uncertain is when, and the reason it matters is that a date helps interpret what people were wearing, eating, harvesting and doing when the eruption began.
The case for 24 August
The traditional date goes back to the letters of Pliny the Younger to the historian Tacitus. In them he describes the death of his uncle, Pliny the Elder, and the first hours of the eruption. In the surviving manuscripts, the start of the event is given as “the ninth day before the Kalends of September”, which corresponds to 24 August.
There are good reasons to take the letters seriously. Pliny was an eyewitness to the cloud, and he wrote for Tacitus, who was preparing a historical work. But the letters were written some twenty-five years after the event, and they reach us only through medieval copies. In the process of copying, numbers and dates are especially prone to mistakes, and different manuscripts of the same letter can vary. The date therefore rests on a chain of transmission, not on a document written on the day.
Evidence that points to autumn
Much of the contrary evidence comes from the ground itself. Several things found at Pompeii seem more natural in the autumn than the late summer:
- Fruits and nuts associated with autumn, such as pomegranates, have been identified in the remains, although some of these could in principle have been stored.
- Large storage jars for new wine were found in a state suggesting the harvest had already taken place or was under way.
- Braziers for heating appear in some rooms, and some of the casts seem to be wearing heavier clothing, which is a less obvious choice for August heat.
- A coin found in the ruins appears to carry a title for the emperor Titus that, on the usual reading, was awarded after the date in the traditional account. Because coin evidence relies on how the legend is dated and read, it is treated as supportive rather than decisive.
Each item has an alternative explanation. People kept warm clothing for rain, stored fruit for months and used braziers for cooking. No single clue overturns the date in Pliny. Together, however, they have made many researchers suspicious.
The 2018 inscription
In 2018, archaeologists working on a house in a part of the town then being cleared found a charcoal inscription on a wall in a room that was being renovated. The text includes a date given in the Roman way of counting back from the Kalends, a system that belonged to the Roman festival calendar, as the sixteenth day before the Kalends of November, which corresponds to 17 October. It was apparently written in the middle of the building work, perhaps by someone doing the job, and the writing was done in charcoal, which is a fragile material.
The inscription’s value is that it provides a date written close to the time of the event. If the reading is right, the eruption must have happened on or after 17 October. A date earlier than that would be hard to square with it. The find is therefore the strongest reason for supporting an autumn date and has been widely discussed.
There are some uncertainties. Charcoal graffiti do not last long indoors, but they could survive for days or weeks, so the lag between writing and the eruption can only be estimated. The reading of the numeral depends on the shape of the letters.
Some scholars have suggested that the eruption took place around the end of October or later, but these are proposals, not settled results, and the possibility of a copying error in Pliny’s date is one explanation that has been put forward.
How historians weigh the evidence
The debate illustrates a common problem in the study of the past: a written source and the physical record seem to disagree. Historians generally prefer contemporary physical evidence when it is clear, but they also know that any single find can mislead. The strongest argument for autumn is the combination of several independent clues, not any one of them.
Dating the eruption by scientific methods does not solve the problem. Techniques such as radiocarbon dating are too coarse to separate one season from another in the first century, and the analysis of annual layers or tree-ring dating in a single year would require unusually well-preserved material.
What this means for visitors
The date question does not change the basic story of the destruction. It does affect captions. A guidebook that states 24 August as fact is simplifying. A careful statement is that the eruption took place in 79 CE, traditionally dated to 24 August, with evidence now favouring a date in the autumn. The sequence of ash and surges described in the explanation of the Vesuvius eruption remains the same either way.