The shadow as a clock
On a sunny day, the shadow of any upright object moves in a predictable way. It is long and points west in the morning, shortest at midday, and long again, pointing east, in the evening. Early people did not need mathematics to notice this. What they needed was a way to mark and compare positions, and that is what made a sundial out of a stick.
Sundials are among the oldest scientific instruments, and the earliest ones are very hard to date. Shadow-reading probably existed long before the first surviving examples.
Egyptian shadow clocks and obelisks
Ancient Egypt provides some of the oldest surviving evidence. Obelisks, tall stone pillars, cast shadows that could be used as large sundials, though their main purpose was ceremonial and the timekeeping use is partly an inference. More clearly built for the job are small portable shadow clocks, with a long bar marked with divisions and a raised crosspiece at one end. In the morning, the device was pointed east so that the crosspiece’s shadow fell along the bar. At noon it was turned around to face west. Examples are dated to roughly the second half of the second millennium BCE.
Babylon, Greece and Rome
Mesopotamian astronomers, whose records survive on cuneiform tablets, tracked shadow lengths and the Sun’s movements across the year. The Greeks inherited some of this knowledge and developed the geometry of sundials. A later writer credited the Babylonian priest Berosus with a hollow bowl-shaped dial, the scaphe, in which the tip of the gnomon’s shadow traced a curve across a hollowed surface. The attribution is traditional rather than certain.
Roman sundials were common in public spaces and houses. The architect Vitruvius, who also wrote about aqueduct engineering, described many types of dials in the first century BCE and listed designs said to be invented by earlier Greek scholars. Rome also had large public installations, for example the Horologium that Augustus had built using an Egyptian obelisk as the gnomon, though how well it worked as a clock is debated.
Hours that changed length
One feature of ancient dials surprises modern readers. In most of the ancient Mediterranean, daylight was divided into twelve equal parts, whatever the season. A summer hour was therefore longer than a winter hour. These are called seasonal or temporal hours. A dial for seasonal hours uses a set of curves that change over the year; the dial lines are not straight.
A different idea, equal hours of constant length, came to dominate later. Dials built on this idea require the gnomon to point toward the celestial pole, which means it is angled to match the latitude of the place. With that tilt, the shadow turns at nearly constant speed, and the hour lines on the plate become simple to draw.
Chinese and Islamic traditions
Chinese astronomers measured the length of a gnomon’s noon shadow to find the dates of solstices and the length of the year. They also used equatorial dials, which have a flat plate parallel to the Earth’s equator, with the shadow reading hours from marks on that plate. Records on this stretch back more than two thousand years, though dating individual devices is difficult.
In the Islamic world, scholars wrote detailed mathematical treatments of dials, and the office of the muwaqqit, a timekeeper attached to a mosque, relied on tables and instruments to fix prayer times. Dials with gnomons aligned to the pole were developed in this period, and many surviving dials show an advanced understanding of spherical geometry.
India and large observatory dials
In India, astronomers had used shadow-based instruments for centuries. The best-known large examples are the early-eighteenth-century observatories built under Maharaja Jai Singh II, including the one in Jaipur. The giant Samrat Yantra there has a triangular gnomon and curved quadrants on each side, and its scale lets observers read time to within a few seconds, according to the usual descriptions. Whether practical readings reached that level is unclear, but the scale shows the ambition of these instruments.
How a sundial is designed
Most designs combine the same elements:
- A gnomon, the part that casts the shadow.
- A dial plate marked with hour lines.
- An orientation that suits the location, with the gnomon angle usually set to the local latitude.
Different dial types suit different surfaces. Horizontal dials sit on a pedestal in a garden. Vertical dials hang on walls and are easier to see from a distance. Equatorial dials are easy to understand but need to be read from both sides in different seasons. Portable designs, including ring dials and pocket dials, used a compass or a hole through which sunlight passed.
Why a sundial is not the same as a clock
A sundial shows local apparent solar time, the time by the Sun’s actual position. This differs from the time on a clock for several reasons. The Earth’s orbit and tilt make the Sun run ahead or behind clock time by up to roughly a quarter of an hour at different times of the year, a difference called the equation of time. Many modern dials include a correction table or a figure-eight curve. Also, a sundial gives local time, so places east or west within the same time zone read differently. And in many regions, daylight saving time adds another hour.
Before railroads linked towns, these differences did not matter much; each town kept its own solar time. Later, standard time zones replaced local time for public use.
What happened to the sundial
When mechanical clocks appeared in the late Middle Ages, dials kept an important role, often as the reference used to set the clock. For a long time, the public clock in a town was checked against a noon mark. Sundials never entirely disappeared and are still built as garden features and teaching tools. Looking at one is a good way to see for yourself how the Earth’s rotation turns into the hours.