A history of astronomical timekeeping

To make this nested-sphere cosmos tangible, astronomers built armillary spheres: globes of interlocking rings representing the celestial equator, the ecliptic, the tropics, and the polar circles, with a small Earth 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) built the first water-powered armillary sphere – one that rotated automatically to track the heavens.^[2] Chinese innovation continued for centuries. Sū Sòng's (苏颂) astronomical clock tower of 1088 CE combined an armillary sphere, a celestial globe, and a time-announcing system, all driven by an escapement – a device that regulates motion through controlled, intermittent release. This predated comparable European clockwork by centuries.

To medieval minds, the armillary sphere was more than astronomy. In 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 word galgalim appears in the ancient Israelite priest Ezekiel's vision of the divine throne – wheels within wheels, covered in eyes – and medieval commentators read this as a description of the celestial spheres themselves. To model those spheres in brass was, in a sense, to model angelic reality.

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 decipher. Modern imaging eventually 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 using the Saros cycle (explained on the astronomy page), indicated the lunar phase, 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 evidence that ancient astronomical sophistication far exceeded what we once assumed. There is a gulf of more than eight centuries between the Antikythera mechanism and the next comparably advanced mechanical device – Yī Xíng's water-powered astronomical clock in 8th-century China. European geared clockwork would not appear for another five hundred years after that.

Around 200 BCE, Greek mathematicians discovered that stereographic projection could flatten the celestial sphere onto a plane while preserving a crucial property: circles on the sphere project as circles (or straight lines) on the plane. This meant that the celestial equator, the ecliptic, the horizon for any latitude – all the great circles an astronomer cares about – could be engraved as precise circular arcs on a flat metal disc. Two such discs, one representing the local sky and one the stars, could then be stacked and rotated against each other to simulate the turning of the heavens. The result was the astrolabe – the entire visible sky compressed into a brass disc you can hold in one hand.

A disc with a wide, raised rim – the "mater" (mother) – holds one or more removable plates called "tympans," each made for a specific latitude and engraved with azimuth and altitude circles representing the local sky above the horizon. Over the tympan sits the "rete," an intricate openwork framework bearing the ecliptic and pointers marking the brightest stars. The rete represents the heavens: rotate it, and the stars and ecliptic move over the coordinate lines on the tympan below, one full turn for each day. On the back, a sighting bar called the alidade lets you measure the altitude of the Sun or a star. Read the altitude, set the rete accordingly, and you could determine the time, your latitude, or predict when any star would rise or set.

Greek and Hellenistic astronomers developed the principles, 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. For Muslim astronomers and the faithful, the astrolabe also solved critical practical 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.

The rete is a map of the stars, their positions fixed in brass. To use the astrolabe is to engage directly with the cosmos above. The Spacetime Watch alludes to this tradition – a portable illustration of the sky's current state, carried on your wrist.

Sometime in the late 13th century, European craftsmen built the first clocks driven by a falling weight regulated by a verge-and-foliot escapement. The early tower clocks were installed in monasteries and cathedrals, where the regulation of communal prayer had always driven the demand for timekeeping. They often had only an hour hand. But unlike the water clocks that came before, they could run continuously for days. They still needed regular calibration against sundials, but the leash to the sky was growing longer.

Christiaan Huygens introduced the pendulum clock in 1656, dramatically improving accuracy. His balance spring of 1675 made accurate portable timekeeping possible – the direct ancestor of every mechanical watch. The clock face abstracted time completely. Time became a number rather than a state of the sky.

At sea, the astrolabe had served navigators for centuries, but measuring star altitudes on a rolling deck was difficult. Simpler instruments like the cross-staff and quadrant gradually replaced it, and by the 18th century the sextant – which used mirrors to bring the horizon and a celestial body into the same view – had superseded them all. But latitude was only half the problem. Longitude required knowing the exact time at a reference port, 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 months at sea.

The sextant and chronometer completed the astrolabe's long obsolescence, and by the 19th century it was forgotten by everyone but historians.

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 ten minutes ahead. Railways shattered this. 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" can mean the Sun is half an hour or more from its highest point – and over an hour off during daylight saving time.

Then came the artificial light. Thomas Edison demonstrated his practical incandescent bulb in 1879. Within decades, cities glowed through the night. Light pollution drowned out the stars. For more than a third of humanity, the Milky Way is completely invisible.^[4]

The final severance came in 1967, when the second was redefined. No longer was it 1/86,400 of a solar day. 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 pulled 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 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 – an astrolabe for the 20th century, its stars turning in real time overhead.

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.

No freely licensed images of the Graves Supercomplication exist, but a contemporary descendant illustrates the tradition well: the Patek Philippe Celestial ref. 6102. With its own rotating star chart and moon phase display, it is perhaps the closest thing in traditional watchmaking to what the Spacetime Watch does digitally. It costs "only" about $450,000.^[7]