A history of astronomical timekeeping

The gnomon – a vertical stick casting a shadow – may be the simplest astronomical instrument ever devised, yet it could reveal an astonishing amount. Its noon shadow pointed north (in the northern hemisphere). Its shortest noon shadow of the year marked the summer solstice; the longest, the winter solstice. The length of the noon shadow also indicated latitude. Eratosthenes famously used gnomon shadows in two Egyptian cities to calculate Earth's circumference around 240 BCE – and got within 2% of the true value.^[1]

Egyptian obelisks cast shadows that swept across marked ground, dividing the day into segments. The Egyptians and Romans divided daylight into 12 "temporal hours" – but these were not equal hours like ours. Each hour was 1/12 of the daylight period, so summer hours were longer than winter hours. A Roman counting hora prima, hora secunda from sunrise needed no AM/PM distinctions – the hours simply stretched and compressed with the seasons.

The reason traditional clocks go "clockwise" comes from the sundial: in the northern hemisphere, the shadow sweeps in that direction as the Sun crosses the sky from east to west. The Egyptians gave us the 12-hour division (though they counted hours separately for day and night). The 24-hour cycle split into two 12-hour periods came later, once mechanical clocks needed to distinguish morning from afternoon.

Temporal hours served well enough for millennia. But they made astronomy difficult – how do you compare observations made in different seasons when your unit of time keeps changing?

The key innovation came from Islamic astronomers in the 9th–10th century. They discovered that if you tilt the gnomon to point at the celestial pole (parallel to Earth's axis), the shadow rotates at a constant 15° per hour year-round. This polar alignment gives equal hours in all seasons – no more stretching and compressing.

The innovation spread to Europe through Spain and the Crusades. By the 14th century, European sundials combined the Islamic polar gnomon with Roman numerals. Equal hours eventually became universal, driven partly by the needs of precise prayer times in Islam and partly by the mechanical clocks that would soon appear. Our modern 60-minute hours descend directly from this choice.

The Greeks conceived of the cosmos as a series of nested crystalline spheres, with Earth at the center and the fixed stars on the outermost sphere. The Sun, Moon, and planets each occupied their own sphere, rotating at different rates. This model was wrong about Earth's position, but it was brilliantly useful – it explained the daily rotation of stars, the monthly motion of the Moon, and the yearly journey of the Sun along the ecliptic.

To represent this nested-sphere cosmos, astronomers built armillary spheres – skeletal globes made of interlocking rings representing the celestial equator, the ecliptic, the tropics, and the polar circles. A small Earth sat at the center. The device was invented independently in China (possibly as early as the 4th century BCE) and Greece (3rd century BCE).

In 125 CE, the Chinese polymath Zhāng Héng (张衡) created the first water-powered armillary sphere that rotated automatically to track the heavens.^[2] Chinese astronomical innovation continued for centuries. Sū Sòng's (苏颂) astronomical clock tower (1088 CE) was an extraordinary mechanism combining an armillary sphere, celestial globe, and time-announcing system – all driven by an escapement mechanism that regulated time through controlled, intermittent motion. This innovation predated comparable European clockwork by centuries.

The armillary sphere was more than astronomy to medieval minds. In both Jewish and Christian cosmology, the celestial spheres (Hebrew: galgalim) were thought to be animated by angelic intelligences. Maimonides identified the sphere-movers with angels; Dante placed angelic orders in each planetary sphere. The same Hebrew word galgalim appears in Ezekiel's vision of the divine throne – wheels within wheels, covered in eyes. To medieval thinkers, modeling the celestial spheres was modeling angelic reality itself.

More than decorative, armillary spheres were working instruments. Astronomers used them to measure star positions, track the Sun's path, and teach the geometry of the heavens. They remained essential astronomical tools for nearly two thousand years.

In 1901, sponge divers discovered a Roman-era shipwreck off the Greek island of Antikythera. Among the artifacts was a corroded bronze mass that would take decades to understand. Modern imaging revealed an intricate clockwork mechanism dating to around 100 BCE – over a thousand years before anything of comparable complexity would appear anywhere in the world.^[3]

The Antikythera mechanism was a mechanical computer. Its interlocking gears tracked the positions of the Sun and Moon, predicted eclipses based on the Saros cycle, indicated the phase of the Moon, and may have shown planetary positions. It even tracked the four-year cycle of the Olympic Games.

We don't know who built it or how widespread such devices were. It stands as humbling evidence that ancient astronomical sophistication far exceeded what we once assumed. The tradition of building mechanical models of the cosmos would later resurface in orreries – clockwork solar system models that showed planets orbiting the Sun at their correct relative speeds. But the Antikythera mechanism predates them by nearly two millennia.

Armillary spheres are beautiful but cumbersome. You cannot slip one into your pocket or carry it easily on a ship. Around 200 BCE, Greek mathematicians solved this problem through stereographic projection – a technique for mapping the celestial sphere onto a flat surface while preserving the angular relationships between stars.

The result was the astrolabe: a brass disc perhaps 15 centimeters across, yet containing the entire visible sky. The key insight was that circles on a sphere project as circles on the plane. The celestial equator, the ecliptic, the horizon for any latitude – all become circles or arcs that can be engraved in metal.

But it was during the Islamic Golden Age, from the 8th century onward, that the astrolabe reached its full sophistication. Islamic craftsmen elevated astrolabe-making to high art, producing instruments of extraordinary beauty and precision.

A brass disc called the "mater" holds a removable plate (the "tympan") engraved with a coordinate grid for a specific latitude. Over this sits the "rete" – an intricate openwork pattern representing the positions of major stars and the ecliptic circle.

The astrolabe was remarkably versatile. Rotate the rete to match the current sky, and you could tell the time from the stars. Aim the alidade (a sighting bar on the back) at the Sun or a star, read its altitude, and you could determine your latitude or the time of day. Set it for a future date, and you could predict when any star would rise or set.

For Muslim astronomers and the faithful, the astrolabe solved critical problems: determining the direction of Mecca from any location and calculating the times for the five daily prayers. It was both scientific instrument and religious tool.

Through Islamic Spain and the Crusades, the astrolabe passed to medieval Europe, where it became essential for astronomers, navigators, and astrologers alike. Geoffrey Chaucer wrote a treatise on the astrolabe for his son in 1391 – one of the oldest technical manuals in English. Owning an astrolabe marked one as learned and sophisticated.

What makes the astrolabe remarkable is that it puts the entire visible sky in your hands. The rete is literally a map of the stars – Vega, Altair, Aldebaran, Rigel – their positions fixed in brass. To use it is to engage with the real sky, matching the instrument to what you observe above. This is what the Spacetime Watch attempts to revive: a portable window into the cosmos that you carry with you.

Medieval craftsmen found a way to make the astrolabe tell time automatically: add clockwork. The Prague Orloj, installed in 1410 and still running today, is essentially a giant mechanical astrolabe. Its main dial shows the Sun and Moon moving against the zodiac ring, with horizon lines marking sunrise and sunset for Prague's latitude – exactly like a traditional astrolabe, but driven by gears instead of human hands.

But simpler mechanical clocks, without astronomical displays, were spreading faster. They worked anywhere, required no astronomical knowledge, and could be read at a glance. Christiaan Huygens introduced the pendulum clock in 1656, dramatically improving accuracy. His second great invention, the balance spring (1675), made accurate portable timekeeping possible – the direct ancestor of every mechanical watch since.

The clock face abstracted time completely. Twelve hours, sixty minutes – these divisions have no cosmic significance. They're arbitrary conventions that could be anything. Time became a number rather than a position in the sky.

The astrolabe had served sailors for centuries, but measuring star altitudes on a rolling ship was difficult. The sextant, perfected in the 1750s, solved this by using mirrors to bring the horizon and a celestial body into the same view – you could measure altitude precisely even on a heaving deck.

But knowing your latitude (from star altitudes) was only half the navigation problem. Longitude required knowing the exact time back home, and pendulum clocks couldn't survive at sea. John Harrison spent decades building increasingly precise marine chronometers, culminating in his H4 of 1761 – a watch accurate enough to determine longitude within a few miles after a months-long voyage.

The combination of sextant and chronometer finally made precise ocean navigation possible. But it also made the astrolabe obsolete. Sailors no longer needed a device that modeled the whole sky – they needed only to measure specific angles and consult tables. The cosmic context was stripped away; navigation became a technical procedure.

Until the 19th century, each town kept its own local solar time. Noon was when the Sun reached its highest point. Bristol ran ten minutes behind London; Paris ran nine minutes ahead.

Railways broke this system. Trains needed synchronized schedules across distances, and the chaos of local times made timetables impossible. By the 1880s, most of the world had adopted standardized time zones, each one hour apart. Clock time became completely divorced from the Sun's actual position. Today, "noon" might mean the Sun is due south – or it might be an hour off, depending on where you are in your timezone and whether daylight saving time is in effect.

Then came electric light. Thomas Edison demonstrated his practical incandescent light bulb in 1879. Within decades, cities glowed through the night. The stars didn't disappear – we just stopped being able to see them.

Today, over 80% of the world's population lives under light-polluted skies. For more than a third of humanity – including 60% of Europeans and nearly 80% of North Americans – the light pollution is severe enough that the Milky Way is completely invisible.^[4] The very phenomenon that inspired millennia of astronomy, mythology, and wonder has been erased from most people's experience.

The final severance came in 1967, when the second was officially redefined. No longer was a second 1/86,400 of a solar day. Instead, it became exactly 9,192,631,770 oscillations of a cesium-133 atom. Time was now measured by quantum mechanics, not the turning of the Earth. For the first time in human history, time had no definitional connection to the sky.

Even as ordinary clocks abstracted time away from the heavens, some watchmakers refused to let go. Abraham-Louis Breguet (1747–1823), perhaps the greatest watchmaker who ever lived, spent decades perfecting astronomical complications – mechanisms that kept celestial awareness alive in pocket-sized form. His watches could show the equation of time, moon phases, and perpetual calendars. His masterpiece, the "Marie Antoinette" watch, took 44 years to complete^[5] and incorporated every known complication of its era.

This tradition reached its apex in 1933 with the Patek Philippe "Graves Supercomplication," commissioned by banker Henry Graves Jr. With 920 hand-crafted components and eight years of work, it remained the most complicated watch ever made without computer assistance for over 50 years. Among its 24 complications was a celestial chart showing the night sky exactly as it appeared from Graves's Fifth Avenue apartment in New York. The chart rotated in real time, tracking the stars as they wheeled overhead – an astrolabe for the 20th century.

But at $24 million (the Supercomplication's 2014 auction price)^[6], such marvels remained the province of the ultra-wealthy. The cosmos stayed out of reach for ordinary people.

There is an irony here. The same forces that disconnected us from the sky – mechanical clocks abstracting time, railways demanding standardization, electric light drowning the stars – eventually created the conditions for reconnection.

A modern smartwatch contains more computational power than existed in the entire world when Graves commissioned his masterpiece. What once required 920 hand-crafted components and eight years of labor can now be calculated in milliseconds. Real-time planetary positions, moon phases, the equation of time, a rotating star chart – all the astronomical complications that were once reserved for kings and bankers can now run on anyone's wrist.

The Spacetime Watch is a digital astrolabe. Like its brass ancestor, it puts the sky in your hands – showing where the Sun, Moon, and planets are right now, where the stars wheel overhead, where you stand in the turning cosmos. Unlike the mechanical marvels of Breguet and Patek Philippe, it costs nothing beyond the watch you already own.

Technology first pulled us away from the sky. Now it can bring us back.

1. Carman, C.C. & Evans, J. (2015). The Two Earths of Eratosthenes. Isis, 106(1), 1-16.
2. Needham, J. (1959). Science and Civilisation in China, Vol. 3: Mathematics and the Sciences of the Heavens and the Earth. Cambridge University Press.
3. Freeth, T., Bitsakis, Y., Moussas, X., Seiradakis, J.H., Tselikas, A., Mangou, H., Zafeiropoulou, M., Hadland, R., Bate, D., Ramsey, A., Allen, M., Crawley, A., Hockley, P., Malzbender, T., Gelb, D., Ambrisco, W. & Edmunds, M.G. (2006). Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism. Nature, 444, 587-591.
4. Falchi, F., Cinzano, P., Duriscoe, D., Kyba, C.C.M., Elvidge, C.D., Baugh, K., Portnov, B.A., Rybnikova, N. & Furgoni, R. (2016). The new world atlas of artificial night sky brightness. Science Advances, 2(6), e1600377.
5. Breguet, E., Aked, C.K. et al. (2015). Breguet: Art and Innovation in Watchmaking. Fine Arts Museums of San Francisco / Legion of Honor exhibition catalog.
6. Sotheby's (2014). Henry Graves Supercomplication by Patek Philippe. Important Watches auction, Geneva.