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Telling the time with the Sun

Published: 26 September 2018

Today we live in a world of artificial time—digital signals beamed to smartphones and computers. 

But the way we define time ultimately comes from the Sun. Here we’ll explore the deep connections between the Earth’s motion around the Sun, and our concept of time.

The oldest mechanical pocket calendar?

Byzantine portable sundial with geared calendrical device, 401-600.
Science Museum Group Collection More information about Byzantine portable sundial with geared calendrical device, 401-600.

This curious instrument is known as the Byzantine Sundial-Calendar. Dating from around 500 CE, it is the second oldest geared mechanical device ever discovered. 

So how did it work? The Sun was central to its operation—as our curator Harry Cliff explains:

In other words, its Byzantine owner would have been able to use it to read a date.

So how did our ancestors work out how to use the Sun to determine a year?

The Earth’s orbit, a year, and the seasons

Our planet is hurtling around the Sun at about 108,000 kilometres per hour, held in an almost circular orbit by its enormous gravitational pull.

To complete one orbit, the Earth has to travel almost a billion kilometres, and the time this takes is what we call a year.

However, we only notice that there is such a thing as a year because the axis of the Earth’s rotation is tilted compared to the plane of its orbit around the Sun.

Gif showing the tilt of the Earth

By Tfr000 (talk) [CC BY-SA 3.0], via Wikimedia Commons 

This tilt causes the changing lengths of the day and night, and hence, the seasons. For half the year, the northern hemisphere is tilted closer to the Sun, leading to longer days, more sunlight and warmer weather, while the opposite is true in the southern hemisphere. For the other half of the year the situation is reversed. 

But if you didn’t already know there are 365 sunrises in a year, how could you determine when a full year had passed? 

Using the solstice to mark a year

The summer solstice is the point in the year when the places where the sun rises and sets are at their furthest points apart on the horizon. It then reverses until—roughly six lunar cycles later in mid-winter—the sunrise and sunset occur closest together. And then they grow further apart again.

By observing the direction of the sunrise and sunset from a fixed point—for example, by placing a series of stakes in the ground to mark their positions over the course of the year—you’d gradually build up a partial circle of markers tracking the Sun’s progress across the horizon.

We can see this at many of the ancient stone and earth circles built by our ancestors across the globe.

Using the solstice to mark a year

Day and night: solar phenomena

So we see how the Sun has defined our concept of a year.

It also defines day and night. In fact, these are the only truly natural units of time, and they are solar phenomena.

The ground beneath your feet is currently rushing eastwards, following a circle around an imaginary line passing through north and south poles. In other words, the Earth is spinning on its axis. At the equator, the ground is moving at around 1600 kilometres per hour, and the further north or south you, go the slower the pace of the spin.

The rotation of the Earth gives us day and night as the planet continually changes the face that it shows to the Sun.

For most of human history, the day was all you needed to organise your activity, with the Sun itself giving an imprecise sense of how much time was left before nightfall.

However, as societies become more complex, the need for a smaller unit of time arose. The solution was something that we could now hardly live without: the hour.

The sundial: A time-finder

Even though the concept of an hour (and minutes and seconds) is a human invention, the Sun would have been instrumental in defining it. 

As the Sun moves across the sky, the shadows cast by objects change in length and direction.

By placing an object in the ground, you can use these shadows to chart the passage of the day. By scratching lines into the earth, our ancestors could define an ‘hours’ system and begin to count the passing of daylight hours for the very first time. 

And so we have a sundial—a time-finder.

The 12-hour day that we are familiar with can be traced back to ancient Mesopotamia and Egypt.

And the oldest known sundial dates from the reign of Thutmosis III, who ruled Egypt around 1500 BCE. 

PIcture of the earliest sundial from the Staatliche Museen zu Berlin © Photo: Egyptian Museum and Papyrus Collection of the Staatliche Museen zu Berlin - Preußischer Kulturbesitz

It is a simple L-shaped piece of stone with hour lines scored out along its upper face. At dawn, it would have been aligned towards the rising Sun, and a bar mounted atop the short part of the L would have cast a shadow on the first hour line of the day.

As the Sun moved, the position of the shadow progressed along the hour lines, until noon, when the direction of the sundial was reversed and the same six hour lines where used to count out the afternoon hours.

We don’t know who the owner of the sundial was, but it is likely they were part of the priesthood, who needed to know the time in order to schedule religious rituals.

The development of timekeeping

So the Earth’s relationship to the Sun has defined our concept of years, days and hours—all represented in the Byzantine sundial-calendar.

Our ancestors made increasingly complex instruments to use the Sun to help schedule their days. But even the first mechanical clocks did not stop the use of sundials.

The development of timekeeping

Keeping time not finding time

But when did we stop relying on the Sun as our ultimate timekeeper? 

There were two major influences here.

The first came during the 1600s. The development of accurate pendulum clocks meant that clocks could keep their time for weeks, months or even years before without needing to be reset using a sundial.

One of the earliest pendulum clocks made for sale
Science Museum Group Collection More information about One of the earliest pendulum clocks made for sale

The second influence was the advent of the railways in the 19th century. 

For centuries, British towns and cities used the Sun to tell the time, with differences of up to 20 minutes between the west and east of the country. 

But now there needed to be a standard time set across the country to avoid collisions and ensure that passengers departed on time.

This pocket watch has two minute hands, one displaying the time used by the railways, while the other could be adjusted to local time
Science Museum Group Collection More information about This pocket watch has two minute hands, one displaying the time used by the railways, while the other could be adjusted to local time

Our ultimate timekeeper

Time is still integral to our lives, much like it was for the Byzantine owner of the mechanical calendar. 

Even though we no longer directly rely on the Sun to know the date or tell the time, it has been instrumental in defining our temporal systems and—in the end—it is our ultimate timekeeper.

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