Sundials Shed Light on Man’s Quest to Deal With Time : Timekeeping: Many view the instruments as nostalgic pieces, but they have had an important functional place in history.

With extra daylight and warm temperatures, clock watchers everywhere become more anxious by the hour to get outside. Away from the confines of school, home or office, these people are still likely to confront their nemesis--time--on the pleasant face of a sundial.

While sundials today may seem mere quaint garden ornaments, for more than 3,000 years various forms of dials have divided the day for people. Sundials have marked the time for the faithful to pray, and have even been used to correct mechanical clocks.

“Sundials can be accurate to the minute,” said Carlene Stephens, a historian and curator of a sundial collection at the Smithsonian’s National Museum of American History in Washington, D.C. “But people today are attracted by sundials’ nostalgic appeal. They remind us that, for thousands of years, the fundamental time standard was the Earth-sun relationship.

“Sundials are sculptural and functional at the same time,” added Stephens, whose research is focused on timekeeping devices. As late as 1900, French railroaders used a precision sundial to set their pocket watches. Digital wristwatches and atomic clocks have since surpassed sundial accuracy.

The earliest known example of a sundial is a shadow clock from an Egyptian burial in the reign of Thutmose III (circa 1500 BC). Egyptians measured the daylight hours by marking the heavenly progress of their sun god, Ra. The shadow clock, reminiscent of a T-square, was placed with the T facing east in the morning, west in the afternoon. The crossbar cast a shadow across calibration marks on the horizontal piece.

The horizontal shadow clock was limited, however. It could not mark sunrise or sunset because the shadows were too long. A great advance in sundials was the hemicycle, developed about 300 BC. This dial had the sun’s path marked inside the bowl of a half-sphere, facing upward. A shadow cast by a pointer passed over 12 equal divisions of the bowl. The shadow’s path on the bowl accurately followed the sun’s motion across the sky.

The Greeks improved this design and developed other timekeeping dials. But like all sundials of the time, these showed “seasonal” hours. Although the dial divided the day into equal parts, the length of the “hours” varied with the season--being long in summer and shorter in winter.

In the 10th Century, a Muslim astronomer devised a dial that could accurately mark hours of uniform length year-round. By the 16th Century, sundials were marked with what has become the standard 60-minute hour.

Different cultures divided daylight in different ways. Several forms of sundials were known in China from at least the 1st Century BC. In 1932, archeologists found a stone sundial from the 3rd Century BC that was marked to divide the day into 100 equal parts, each equal to 14 standard minutes today. Later, Chinese dials used much larger segments of time.

The Bible provides perhaps the earliest literary reference to a sundial--the Dial of Ahaz. As described in the Old Testament books 2 Kings and Isaiah 38:8, the prophet Isaiah invoked a miracle to reassure the gravely ill King Hezekiah that he would live long enough to put his house in order: “ ‘Look, I shall make the shadow cast by the declining sun go back 10 steps on the Dial of Ahaz.’ And the sun went back the 10 steps by which it had declined.”

While no one today knows what the 8th Century BC Dial of Ahaz looked like, Renaissance German craftsman Christopher Schissler fashioned a bowl-shaped sundial to re-create the biblical miracle. The Bowl of Ahaz, completed in 1578, uses the way water bends light to create the illusion of the shadow of the sun moving backward. Filling the dial’s basin with water causes the shadow to move back to an earlier time of day.

Although Latin inscriptions on the sundial--now displayed at the American Philosophical Society in Philadelphia--refer to the biblical passages and describe what will happen if the bowl is filled with water, no one understood how it worked. That is, not until historian Owen J. Gingerich and his colleague Philip Sadler, both of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., became intrigued by and then studied the sundial.

They discovered that the bowl had been incorrectly reassembled in the 19th Century. The Philosophical Society had received the bowl as a gift in 1755, when Benjamin Franklin was society president. “The string that casts a time-telling shadow across the bowl’s interior was in the wrong place,” Gingerich said. “It didn’t work as a sundial.”

Sadler and Gingerich compared the sun’s position to the bowl’s markings at several times during a year. In this way, they could calculate where the indicator string should be to produce accurate results. When the bowl is filled with water, the shadow moves backward on the dial by as much as one hour.

“Our findings make the bowl even more impressive,” Gingerich said. “We will wonder how Schissler succeeded in calibrating the ellipsoidal surface of the bowl.”

While the Bowl of Ahaz re-created a miracle, sundials used by Muslims helped determine the times when miracles might be requested. It is the duty of every Muslim to pray each day at five astronomically defined times. One 14th-Century Tunisian dial, for example, has special markings that show precisely when to begin prayers at midafternoon, nightfall and daybreak.

During prayer, the believer is required to face Mecca. Islamic instrument makers devised portable sundials with compasses to allow the user to determine prayer times and the direction of Mecca.

Medieval churches in Europe also used sundials to establish prayer hours. A form characteristic of the Church of England as early as the 7th Century was the “Scratch Dial.” Early types consisted of a semicircle with the hour lines radiating from the center where a horizontal gnomon, or style, was inserted. At first these dials were scratched or inscribed on the south walls of churches. Later, they were separate stone slabs attached to the walls.

In the late 18th Century, the unusual cannon dial brought sound to shadow clocks. As envisioned by a French court functionary, the Comte d’Angivillers, a magnifying glass would concentrate the heat of noon sunlight to ignite the powder charge of a small cannon. About 1790, a French military engineer installed such a cannon dial on the grounds of the Royal Palace at Versailles.

As Silvio A. Bedini, another historian of science at the Smithsonian’s American History Museum, describes the scene:

“The cannon dial became instantly popular as a place for ladies and gentlemen to gather each day, ostensibly to set their watches but also to be seen and to exchange the news and gossip of the day.” This loud noontime marker was used until about 1916.

While sundials are naturally an outdoor art form, at least one sundial has been designed to work indoors. In 1974, William Shrader of Wayland, Mass., built a sundial on his office ceiling.

Shrader’s sundial worked by means of a small mirror, mounted on a south-facing window sill so it reflected a spot of sunlight onto the ceiling. Shrader wrote a computer program to determine the mirror’s position and the layout of the sundial’s hour lines. He then marked the lines by pinning colored ribbons to the acoustical ceiling tiles.

“The sundial was a center of attraction for many of my co-workers,” Shrader said. “I literally had people coming in to set their watches by observing the time on the ceiling.” Shrader was temporarily moved to an office in the Aleutian Islands. “When I returned,” he said, “the sundial was gone.” Now Shrader and his colleagues are back to watching the numbers flip on their digital watches.