Do it like the sundial…

On February 19, 2013, an excavation team from the University of Basel discovered a limestone slab in Egypt’s Valley of the Kings with a sundial painted on the front. At the top there is a hole intended for the shadow caster, a stick made of wood or metal. The clock was made by workers who were busy painting the nearby graves of famous people. It dates back to the time of Pharaoh Seti II and his wife Tausret around 1200 BC. BC and is therefore one of the oldest in human history.

The semi-oval dial on the flat stone of 15.5 by 17.5 centimeters, in technical terms “Ostrakon” called, is divided into twelve sections depending on the number of hours in the day. The hour lines, applied in a flowing brushstroke and without a square or ruler, are surprisingly even. The two horizontal boundary lines above represent six o’clock in the morning and six o’clock in the evening; the noon line points vertically downwards. The sundial was portable and, in order to tell the time correctly, had to be hung or placed on a south-facing surface – in fact, its location was found near a south-facing rock face.

At first glance, a vertical sundial is simple. As the sun travels from east to west, it casts a shadow that moves counterclockwise across the dial. Hour lines indicate the time. But here’s where things get more complicated. In ancient Egypt, the time between sunrise and sunset was divided into twelve equal periods. Because the days are shorter in winter than in summer, a pharaonic hour can last between 51 and 69 minutes, depending on the season.

That’s why people talk about it “Temporary hours” – unlike our current hours, which only came about through the construction of mechanical clocks and which divide the day into 24 units of exactly the same length. Since the temporal and modern hours are exactly the same length only twice a year (on the equinoxes, i.e. between March 19 and 21 in spring and between September 22 and 24 in autumn), the latter are also called “Equinoctial hours”.

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The shadow caster of the Valley of the Kings sundial did not survive. The opening intended for him was drilled vertically into the stone, so that the rod extended perpendicular to the dial. Sundials of this type do “canonical” named because they were used in medieval monasteries to indicate prayer times more or less accurately.

But there’s a problem with these canonical sundials: the angle of the sun’s shadow at any given time depends on the time of year. In the Valley of the Kings, the difference between the solstices (June 21 and December 21) at nine in the morning is more than five degrees. Canonical sundials that want to provide reliable values ​​all year round therefore require multiple scales for all months or constellations that are not easy to read.

The Moroccan astronomer and mathematician Abu Ali Hassan al Marrakushi described a practical solution in his work in the 13th century “Collection of the Principles and Objectives of Scientific Timekeeping”: If you place the shadow caster not perpendicular to the screen surface, but rather diagonally down below a geographic latitude so that it is parallel to the Earth’s axis, then the shadow will point in the same direction at the same point. time, year in, year out.

This is called a shadow staff «pole rod», because it points toward the North Celestial Pole, which the North Star is close to. Today, most vertical sundials have polar bars that point downward according to latitude in Switzerland at angles between 45.83 (Chiasso) and 47.56 degrees (Basel).

Astronomical accuracy was not the pharaonic painter’s concern. And yet the thousands-year-old sundial has two important insights to offer. First, the sundial had to be placed on a rock wall and was therefore visible to everyone, both colleagues and managers. Workers in ancient Egypt must have been granted certain rights because an exploitative regime would have been careful not to relinquish control over working hours.

And second, at 11am, 12pm and 2pm the lines coincide surprisingly well with our current times all year round. This could indicate a stoppage in work at a time when temperatures in the Valley of the Kings become too high and when workers have sought the shade of nearby huts. The limestone shard from the Valley of the Kings is said to be the first evidence of a modern form of working time regulation.

3200 years later, in the case of the sundial at Muottas Muragl in the municipality of Samedan, built in 2011, precision is the measure of all things. According to designer Fred Bangerter “most accurate sundial in the world” with the Latin name “Sine sole sileo” (“Without the sun I remain silent”) With its finely adjustable shadow thrower, the time can be read with an accuracy of 10 seconds – but only from March 21 to September 23. In the winter months the sun is too low and the extremely accurate sundial stops working.