Tomorrow at dusk, to mark the 1600th anniversary of the departure of the Romans from Britain, flaming torches will be lit every 250 metres along the length of Hadrian's Wall, starting at the North Tyneside end, to create a 'line of light from coast to coast'. It will take about an hour for the light to reach Carlisle,where it will be greeted with a variety of neopagan festivities:

Led by the stirring sounds of street band Tongues of Fire and impressive fiery engines (from Pandeamonium) and lit by thousands of flickering flames, a parade of costumed characters and musicians will follow the elusive and beautiful airborne Heliosphere through the streets.

It should all be very pretty, as long as it doesn't rain.

You may be wondering, though, how the date of the end of the Roman occupation of Britain can appear to be known so precisely. It's not as if there are any surviving fragments of any ancient Briton's diary: 'Saturday, 13 March 410. That's the last of the buggers loaded onto their galleys and hightailing it for Gaul. And good riddance.' Or indeed of any Roman's: 'As I was the last legionary to leave Britain the centurion asked me to turn the lights off. Very funny sir I said.'

The Romans are thought to have left by 410 because, according to the historian Zosimus, following Olympiodorus, the Emperor Honorius wrote to the Britons in that year telling them they'd have to fend for themselves; he needed all the troops he could muster to defend Rome from Alaric, who sacked the city anyway.

As for why the wall's being lit up tomorrow, rather than on any other day of the year: to coincide with a conference at the British Museum this weekend, called 'Debating the End'. As so often, the answer to the question 'how do we know?' turns out to be 'we don't.'