Sundials: Telling Time by Shadow
The oldest way to read the time is still one of the cleverest, and it explains why the sun itself is a slightly unreliable timekeeper.
The simplest clock there is
A sundial tells the time by where a shadow falls. As the sun crosses the sky, the shadow of a fixed object sweeps across a marked surface, and the markings turn that moving shadow into hours.
The object that casts the shadow is called the gnomon. On a garden sundial it is usually the triangular blade standing up from the dial plate. Older descriptions, including Britannica's account of ancient dials, also call the pointer the style.
The sun appears to move 15 degrees across the sky every hour, and a properly set gnomon throws a shadow that moves at that same steady rate. That regular motion is what lets a slab of stone or brass act as a clock.
Why the gnomon points at the pole
The trick to an accurate sundial is tilting the shadow-casting edge so it runs parallel to the Earth's axis.
The simplest design, the equatorial dial, puts the hour lines on a ring or plate set square to that slanted gnomon, and the lines sit an even 15 degrees apart. According to the British Sundial Society, horizontal garden dials, vertical wall dials and polar dials are all derived from that basic type.
On a flat horizontal dial the hour lines are not evenly spaced, and the angles between them change with latitude. That is why a sundial made for one city will not read correctly if you move it far north or south.
A very old idea
People were using shadows to track the day thousands of years before mechanical clocks. NIST notes that Egyptian obelisks, built as early as 3500 BCE, cast moving shadows that split the day into morning and afternoon. Britannica suggests that a simple upright stick or pillar was probably the first time-telling device of all.
By around 1500 BCE, Egyptians had a portable shadow clock. NIST describes it as dividing the sunlit day into 10 parts plus two twilight hours, and it had to be turned around at noon to measure the afternoon. Sources disagree on the oldest surviving sundial: Royal Museums Greenwich dates the earliest known Egyptian example to at least 3,500 years ago, while Britannica names a green schist shadow clock from the 8th century BCE or earlier as the oldest preserved one.
The Greeks and Romans refined the design. Britannica credits the hemicycle, a bowl-shaped dial cut into a block of stone, to Aristarchus of Samos around 280 BCE, while NIST says only that it was said to have been invented about 300 BCE. By 30 BCE, according to NIST, the Roman architect Vitruvius could describe 13 sundial styles in use in Greece, Asia Minor and Italy.
Rome came to sundials partly by conquest. Britannica records that the city's first sundial, set up in 290 BCE, had been captured from the Samnites, and that the first dial actually designed for Rome followed around 164 BCE. In Athens, the Tower of the Winds, built around 100 BCE, still carries eight flat sundials facing different points of the compass.
Hours that changed with the seasons
Most early sundials did not measure equal hours. They divided daylight into 12 parts whatever the season, so a summer hour was longer than a winter hour. Royal Museums Greenwich points out that in the British Isles a summer hour could be twice as long as a winter one.
Medieval Muslim astronomers, who used sundials to find prayer times, developed the gnomon set parallel to the Earth's axis, and most of their dials carried lines for the hours of prayer. Britannica credits Abu al-Hasan al-Marrakushi, writing at the beginning of the 13th century, with introducing equal hours, at least for astronomy. Royal Museums Greenwich says equal hours were in general use by the 15th century.
The Renaissance brought a burst of new designs, and Royal Museums Greenwich notes that portable dials were made in large numbers from the 16th to the 19th centuries.
Even after mechanical clocks spread in the 14th century, sundials stayed essential, because they were how people reset those clocks. Royal Museums Greenwich notes that French railways were still regulating their clocks by sundials until the end of the 19th century.
See also: sunrise and sunset times where you live on OnlineClock.app.
Why a sundial and your watch disagree
A sundial shows apparent solar time, the time according to the real sun. Your clock shows civil time, which follows an imaginary average sun that moves at a perfectly even pace. The gap between the two is called the equation of time.
According to the U.S. Naval Observatory, there are two causes. The Earth's orbit is slightly elliptical, so the planet moves fastest in early January and slowest in early July. The Earth's axis is also tilted about 23.4 degrees, which changes how quickly the sun appears to move east to west through the seasons. Together these effects can open a gap of as much as 16 minutes.
Taken on its own, the orbit effect peaks at about 7.5 minutes either way, while the tilt effect reaches about 10 minutes. The Naval Observatory stresses that the two are similar in size, so neither one deserves to be called the main cause.
The British Sundial Society gives worked examples: on October 13 you subtract about 14 minutes from the sundial reading, while on July 15 you add about 6 minutes. Many sundials carry a small table or graph so you can make the adjustment yourself.
Longitude and summer time
There are two more corrections. Clocks across a time zone all show the same time, but the sun reaches its highest point earlier in the east of the zone and later in the west. The difference is 4 minutes for every degree of longitude away from the zone's reference meridian: add it if you are west of that line, subtract it if you are east.
Royal Museums Greenwich points out that Bristol is about 10 minutes west of Greenwich, so a Bristol sundial needs 10 minutes added to match Greenwich Mean Time unless the maker built that correction into the dial. The U.S. Naval Observatory gives Omaha, at about 96 degrees west, as an example: at about 96 degrees west, it is 6 degrees from the zone's 90-degree meridian, so its mean solar time is offset from Central Standard Time by 24 minutes.
The British Sundial Society suggests a quick check. If the noon line runs down the middle of the dial, the maker did not correct for longitude. If it sits off to one side, the correction is already built in.
Finally, if summer time or daylight saving time is in force, add an hour. The society's own example puts it all together: a dial in Plymouth, about 4 degrees west, reading 10:15 on July 15 needs 6 minutes for the equation of time, 16 minutes for longitude and an hour for British Summer Time, giving a clock time of 11:37.
See also: how daylight saving time shifts your clocks on OnlineClock.app.
Questions people ask
What is the pointer on a sundial called?
It is called the gnomon, and older sources also call it the style. On an accurate dial, its shadow-casting edge is set parallel to the Earth's axis.
Why is my sundial wrong?
Usually it is not wrong, it is showing sun time. To get clock time you correct for the equation of time, which can reach about 16 minutes, then for your longitude at 4 minutes per degree, and then add an hour during daylight saving time.
Does a sundial work anywhere?
Only at the latitude it was designed for. The spacing of the hour lines on a horizontal dial depends on latitude, so a dial made for one place reads incorrectly far to the north or south.
How old are sundials?
Shadow timekeeping goes back to Egyptian obelisks as early as 3500 BCE. Sources differ on the oldest surviving sundial: Royal Museums Greenwich says at least 3,500 years old, while Britannica points to an Egyptian shadow clock from the 8th century BCE or earlier.