In ancient Rome, an hour was not a consistent unit of time. In the summer, it could be as long as 75 minutes – and in the winter it sometimes lasted just 45 minutes.

At first it just appeared to be a plain block of carved stone.

The limestone lump was lying face-down in the mud at the site of an ancient Roman town in central Italy, and it was thoroughly stuck. It’s thought that the block was stolen in the medieval era – plucked from the metropolis' antique remains and dragged away, possibly with the intention of using it as building material. But the mud had thwarted this attempt, and here it was, still in position, hundreds of years later.

It took a team of three people to prise the stone out, and as they did so, Alessandro Launaro yelped with delight. "The first thing I remember saying is 'Woah! Inscription!'", he says. The block had left behind an imprint of Latin letters and mysterious lines, pressed into its muddy grave like a stamp. "And that I found really puzzling," says Launaro, an associate professor of classics at the University of Cambridge.

Once the rock had been safely extracted from the ground, its purpose became clear: this was an ancient Roman sundial, one of many hundreds which have been discovered across the globe. After its peaceful sleep entombed in mud, it was exceptionally well-preserved – with lines that demarcated each passing hour, and an inscription crediting the official who paid for it. But perhaps the most exciting part was the way it demonstrated the ancient Roman solution to a perennial dilemma: how to make the most of the daylight available at different times of the year.

Twice a year, around a third of the world's countries perform a hotly-debated ritual: meddling with time to create longer summer evenings and brighter winter mornings. The US, the UK, and most of Europe implement Daylight Savings Time (DST), which involves pushing the clocks forward by an hour for the spring, then pulling them back by an hour to regular Standard Time in the autumn. But the ancient Romans had no such system – instead, they practised the long-forgotten art of seasonal hour-stretching.

Just like we do today, the ancient Romans divided up each day into 24 units – but for the vast majority of the year, they were not of equal length. All the daylight hours were divided by 12, all year round. This meant that at the peak of summer, when the Sun is up for longest, an hour took 75 minutes during the day, and just 45 minutes at night. In the middle of winter, meanwhile, when daylight was in shortest supply, the pattern was reversed – and during the day an hour occupied just 45 minutes.

"And then gradually between the summer solstice and the winter solstice, the length of those hours would change day by day, just a little bit each day," says James Ker, professor of classical studies at the University of Pennsylvania. By the equinox – a moment that occurs twice a year, when the Sun is directly above the equator, and days and nights are roughly the same length – an hour was a familiar 60 minutes.

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