A sundial at Tapeley Park, Instow, Devon
©TopFoto.co.uk

Using the sun

Humans evolved to suit the rhythm of night and day, caused by the earth's rotation around the sun. The daylight hours were for work, and the darkness was the time to sleep, or sit around the fire. The simplest way to mark the passing of time in daylight is by the movement of a shadow, cast by the sun as it travels on its daily journey across the sky. When the sun is at its height, at midday, the shadow is shortest. All around the world, people have used the sun's shadow to measure time passing.

The Ancient Egyptians were the first people to divide day and night into 24 hours, with 12 hours each for day and night. To measure daytime hours, they invented the sundial, in which the shadow of an upright rod fell on a circular scale. Since day was measured from sunrise to sunset, the length of hours varied from season to season. In winter, daylight hours were shorter, while the hours of night lengthened.

Using water

At some point before the 15th century BC, the Egyptians invented the water clock, in which water steadily flowed from an upper vessel through a small hole into a lower one. Marks on the inside of either container could be used to show the passing hours. Different sets of markings were used at different times of the year, to make the water clock match the changing length of hours on sundials. Unlike sundials, water clocks could be used at night as well as in daytime.

Greek and Roman clocks

Water clocks and sundials were adopted by the Greeks and Romans, who used them to determine times for public business, religious worship, a visit to a theatre or bath-house, or when to go to a dinner party. They also used water clocks to time speeches in law cases, so that each side had the same time. A water clock was called a "clepsydra" (Greek for water thief).

Since the sun is lower in the sky the further north you travel, sundials had to be specially constructed for different latitudes. It took the Romans a long time to realise this. The earliest sundial in Rome was a Greek one, brought there from Catania in Sicily in 263 BC. This showed the wrong time for more than 100 years!

Sundials are a much more natural way to measure time than mechanical clocks, and they only mark broad divisions of daytime. Yet, even in Roman times, some people were beginning to resent the power they held over their daily lives. A fragment of a play by an unknown Roman author, quoted by Aulus Gellius in about AD 150, curses the tyranny of sundials:

The gods confound the man who first found out
How to distinguish hours. Confound him too,
Who in this place set up a sundial,
To cut and hack my days so wretchedly
Into small pieces! When I was a boy,
My belly was my sundial - one surer,
Truer and more exact than any of them.
This dial told me when 'twas the proper time
To go to dinner, when I ought to eat;
But nowadays, why even when I have,
I can't fall to unless the sun gives leave.
The town's so full of these confounded dials.

Aulus Gellius, Noctes Atticae, from the 1927 Loeb translation by JC Rolfe

Whistling water clocks

Although no Roman water clocks have survived, we know from descriptions that they were ingeniously made, and fitted with automatic floats which moved pointers and dials, and struck the hour by blowing whistles and tossing pebbles or eggs into the air. Such clocks were status symbols, used by the rich to show off their wealth and taste. Yet they rarely showed time accurately. The writer Seneca said that it was easier to get the philosophers to agree among themselves than Rome's clocks.

Equal Hours

The only people who needed to know the time with great accuracy were the astronomers, who wanted to record the exact times that constellations appeared in the night sky. It was a Greek astronomer, Hipparchus, working in Egypt in around 130 BC, who first proposed a fixed length for an hour. He based its length on the time of an hour at an equinox, when the day is as long as the night. Hipparchus's ''equinoctial hours" were useful for astronomers, yet ordinary people would continue to use temporal (flexible) hours for more than 1,000 years.