Sundial

A sundial is an instrument for telling the time that reads the hour from the position of a shadow cast by the sun on a marked surface. It is one of the oldest forms of timekeeper known and developed over a long history across many cultures.

A sundial works through the cooperation of two principal parts: a dial surface bearing hour lines, and a gnomon whose shadow falls across them. The gnomon was originally a staff or pillar standing perpendicular in a place exposed to the sun, so that the length of the shadow it threw could easily be ascertained. The name later came to be applied to any sun-dial and especially to its finger — the shadow-throwing part that points to the hour. The shadow-casting edge of the gnomon is called the style.

Horologium was the name the ancients gave to the various instruments by which they measured the time of day and night, the earliest and simplest being the polos and the gnomon. The first crude sundial was probably no more than a vertical stake in the ground, and the changing length of its shadow came to be used, with stone markers, much as the hands of a clock are used today.

Sundials are commonly grouped into four types: hemispherical, cylindrical, conical and planar, the last being either horizontal or vertical. They were usually made in stone, with a concave surface marked out on which a gnomon cast its shadow.

In an equatorial dial the dial surface is aligned parallel to the Earth's equator, and the gnomon stands at right angles to it and parallel to the rotation axis; because the Sun completes 360 degrees in 24 hours, the hour lines are set 15 degrees apart. In a horizontal dial the style is set at an angle equal to the local latitude, and its hour lines are not equidistant. Vertical dials, common on old houses and churches, likewise carry hour lines that are not equally spaced.

The underlying principle is that a rod placed parallel to the Earth's axis casts a shadow that moves uniformly, the Sun's passage through 15 degrees in one hour moving the shadow at the same rate. Arab dialists replaced vertical and horizontal gnomons with gnomons aligned parallel to the Earth's axis, which made equal-length hours possible throughout the year.

Because the apparent sun describes a figure 8 through its annual motion in the sky, the analemma is visible only when the sun's position is compared with a mean time kept by a precise clock. The concept of the analemma was invented by Grandjean de Fouchy in 1740 to describe the apparent irregular motion of the sun.

A treatise the Analemma written by Diodoros was translated into Arabic, and the Persian polymath al-Biruni of Ghazna quotes it in his treatise On Shadows. The scientists of the Islamic world included al-Biruni, who wrote al-Qanun al-Mas'udi on astronomy and solar, lunar and planetary motions, and Ibn al-Haytham, who measured the thickness of the atmosphere.

Because the Sun's disc covers half a degree of the sky, which takes about two minutes of time to traverse, any measurement based on the solar motion carries a minimum error of about one minute.

Vitruvius devotes the ninth book of De Architectura to the rules of dialling, which he derives from the shadow that a gnomon casts in the rays of the sun. Because the equinoctial shadow differs from place to place, the construction of a dial must be varied according to where it is to be erected.

At the equinoxes the shadow of a gnomon at Rome is eight ninths of its length, at Athens three fourths, at Rhodes five sevenths, at Tarentum nine elevenths and at Alexandria three fifths.

To regulate a water-dial for winter Vitruvius places the hours drawn from the analemma on a turning column together with the lines of the months, so that the indicator makes the hours long or short.

Egyptian obelisks, built as early as 3500 BCE, served as a kind of sundial, their moving shadows partitioning the day into morning and afternoon and marking the year's longest and shortest days. The earliest known sundial was found in Egypt and dates from at least 3500 years ago. The Egyptians were apparently the first civilization after the Sumerians to formally divide the day into parts something like our hours.

Another Egyptian shadow clock, possibly the first portable timepiece, came into use around 1500 BCE and divided a sunlit day into 10 parts plus two twilight hours. The merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 BCE; a pair of merkhets allowed night hours to be marked off as stars crossed the meridian.

In ancient Egypt the daily cycle was divided into twenty-four hours, twelve of day and twelve of night, the latter reckoned by the movement of groups of stars called decans. Beginning in the New Kingdom, about 1500 B.C., there is evidence that sundials, shadow clocks and water clocks were used to measure the passing of the hours, though no evidence shows the Egyptians tracked minutes or seconds.

The Egyptian altitude sundial in the Science Museum Group collection is a wooden board with markings for units of time and a raised horizontal wooden gnomon, made in Egypt between 1000 and 800 BCE.

In the quest for year-round accuracy sundials evolved from flat plates into the hemispherical dial, a bowl cut into stone with a central vertical gnomon, and then into the hemicycle, said to have been invented about 300 BCE.

On the plain wall below the entablature of each face of the Tower of the Winds, lines are still visible which, with the gnomons that stood above them, formed a series of sundials, and a clepsydra stood at the centre of the building.

A 2,000-year-old intact and inscribed sundial, one of only a handful known to have survived, was recovered during the excavation of a roofed theatre in the Roman town of Interamna Lirenas near Monte Cassino in Italy.

Sundials were ubiquitous in medieval Islam because their shadows showed the time of the midday and afternoon prayers, two of the five prayers prescribed for faithful Muslims; by the eighth century the daytime prayers had come to be defined by the increase of the shadow over its minimum at midday, and the afternoon prayer began when the shadow of a vertical gnomon exceeded its noon length by the length of the gnomon; once the daytime prayers were defined by shadows, the sundial became the most appropriate device for regulating them.

Islamic prayer sets five daily times calculated from the position of the sun, and finding both those times and the direction of Mecca presented problems that scholars solved with mathematics, geography and astronomy. Astrolabes used by a mosque's timekeeper, the muwaqqit, carried lines for calculating prayer times by true astronomical time, among them lines for the dawn, midday and sunset prayers. Knowledge of timekeeping, ilm al-miqat, covered the finding of instantaneous time, the times of the five daily prayers, the direction of Mecca and the making of instruments, and those who worked on it were called muwaqqit or miqati.

Timekeeping in the medieval Muslim world started in Baghdad in the 9th century, but it was the Mamluk-era astronomers of Egypt, Syria and Palestine who specialised in it and made some of the best astrolabes, astrolabe quadrants and sundials in the history of astronomy.

Habash al-Hasib, who worked in Baghdad in the ninth and tenth centuries, composed perhaps the first tables for making sundials, recording for ten latitudes and each seasonal hour the sun's height and the length and direction of the gnomon's shadow. Ibn al-Haytham showed, in his Treatise on the Hour Lines, that hour lines other than the noon line were not straight, but that each lay between two straight lines forming a very small angle, so that they were nearly straight.

The earliest surviving Islamic dial comes from eleventh century Cordoba and is the work of the astronomer Ibn al-Saffar, displaying lines for the seasonal hours, the beginnings of the seasons and the midday and afternoon prayers. In the second half of the fourteenth century the astronomer Ibn al-Shatir built for the great Umayyad Mosque in Damascus a horizontal sundial on a rectangular marble slate about two metres long, placed on one of the minarets; its gnomon was parallel to the polar axis and inclined to the horizon at about 33 and a half degrees, a common medieval value for the latitude of Damascus, and this was the first known proof that Muslims introduced the polar-axis sundial. Al-Marrakushi, working in Cairo in the late thirteenth century, composed a standard compendium on astronomical instruments with an extensive treatment of sundials, and the cylindrical dial he described was a Muslim invention of the ninth century.

Through most of the Middle Ages, sundial styles continued to evolve, yet they did not depart far from ancient Egyptian principles. By the 10th century, several types of pocket sundials were in use, and one English model even compensated for the seasonal changes in the Sun's altitude.

The Renaissance brought rapid development in the making of sundials. Dials of this period were often given extra features, indicating the seasons, the calendar date, the times of sunrise and sunset, and the points of the compass.

In 1656, Christiaan Huygens made the first pendulum clock, regulated by a mechanism with a natural period of oscillation. Its error was less than a minute a day, and his later refinements reduced that error to less than 10 seconds a day.

The Jantar Mantar in Jaipur is an astronomical observation site built in the early 18th century, comprising some 20 main fixed instruments — monumental masonry versions of known instruments designed for naked-eye observation of astronomical positions. It constitutes the most significant and best preserved set of fixed monumental instruments built in India in the first half of the 18th century, and some of them are the largest ever built in their categories. The site belongs to the tradition of Ptolemaic positional astronomy shared by many civilizations, and through this type of observation it contributed to the completion of the astronomical tables of Zij. Under its creator, the prince Jai Singh II, the observatory served as a meeting point for different scientific cultures and as a symbol of royal authority, expressed through its urban dimensions, its control of time and its rational and astrological forecasting capacities.

UNESCO judged the Jantar Mantar in Jaipur to be an outstanding example of the coming together of observation of the universe, society and beliefs, and a testimony to the ultimate culmination of the scientific and technical conceptions of the observatory tradition devised in the medieval world. Its inscription carries criteria (iii) and (iv) and a property of 1.8652 ha.

Most people relied on public sundials to tell the time, so local times differed across the country by up to about 30 minutes, and the difficulties this created eventually led to the creation of Standard Time based on the Prime Meridian at Greenwich.

A conference held at Washington in 1884 adopted the meridian passing through Greenwich as the initial or prime meridian for longitude and timekeeping, and the world's 24 time zones are on average 15 degrees wide. railroad industry implemented its Standard Railway Time System on 18 November 1883 in advance of the international accord, and in March 1918 an Act of Congress made standard timekeeping official in the United States.

• 3500 BCE
  Egyptian obelisks as sundials
  Obelisks are used as sundials in Egypt.^{9A Walk Through Time - Early Clocks · National Institute of Standards and Technology (NIST) ↗}
• around 1500 BCE
  Portable shadow clock
  An Egyptian shadow clock, possibly the first portable timepiece, divides a sunlit day into 10 parts plus two twilight hours.^{9A Walk Through Time - Early Clocks · National Institute of Standards and Technology (NIST) ↗}
• about 1500 B.C.
  New Kingdom timekeeping
  Evidence from the New Kingdom shows sundials, shadow clocks and water clocks used to measure the passing of the hours, though nothing shows the Egyptians tracked minutes or seconds.^{10Telling Time in Ancient Egypt · The Metropolitan Museum of Art, Heilbrunn Timeline of Art History ↗}
• 1000–800 BCE
  Egyptian altitude sundial
  An Egyptian altitude sundial, object 1926-992.^{11Ancient Egyptian Altitude Sundial, or Shadow Clock, in pine (wood) · Science Museum Group Collection ↗}
• around 600 BCE
  The merkhet
  The merkhet comes into use.^{9A Walk Through Time - Early Clocks · National Institute of Standards and Technology (NIST) ↗}
• 3rd century BCE
  Earliest surviving sundial
  The earliest surviving sundial, from Delos.^{4Ancient Timekeeping · World History Encyclopedia ↗}
• about 300 BCE
  The hemicycle
  The hemicycle is invented.^{9A Walk Through Time - Early Clocks · National Institute of Standards and Technology (NIST) ↗}
• 159 B.C.
  Public clepsydra
  Scipio Nasica establishes a public clepsydra.^{1Horologium (Smith's Dictionary of Greek and Roman Antiquities, 1875) · LacusCurtius / penelope.uchicago.edu (Bill Thayer) ↗}
• 30 BCE
  Vitruvius on sundial styles
  Vitruvius describes 13 sundial styles.^{9A Walk Through Time - Early Clocks · National Institute of Standards and Technology (NIST) ↗}
• 9th century
  Baghdad timekeeping
  Timekeeping begins in Baghdad.^{14Building time machines · History of Science Museum, University of Oxford (Inside HSM Oxford) ↗}
• 11th century
  Cordoba dial
  A dial by Ibn al-Saffar in Cordoba.^{6Sundials in Medieval Islamic Science and Civilization · J. L. Berggren, Simon Fraser University (The Compendium, NASS) ↗}
• 13th century
  Equal-length hours
  Abul-Hassan introduces equal-length hours.^{3Sundials · Royal Museums Greenwich ↗}
• late 13th century
  al-Marrakushi in Cairo
  al-Marrakushi works in Cairo.^{6Sundials in Medieval Islamic Science and Civilization · J. L. Berggren, Simon Fraser University (The Compendium, NASS) ↗}
• second half of the 14th century
  Ibn al-Shatir's Damascus dial
  Ibn al-Shatir's dial in Damascus.^{6Sundials in Medieval Islamic Science and Civilization · J. L. Berggren, Simon Fraser University (The Compendium, NASS) ↗}
• 15th century
  Equal hours in general use
  Equal hours come into general use.^{3Sundials · Royal Museums Greenwich ↗}
• 1607–1653
  Elias Allen sun-and-moon dial
  Elias Allen's sun-and-moon dial.^{19Sun-and-moon dial by Elias Allen · Science Museum Group Collection ↗}
• 1656
  Huygens's pendulum clock
  Huygens's first pendulum clock.^{15A Walk Through Time - A Revolution in Timekeeping · National Institute of Standards and Technology (NIST) ↗}
• early 18th century
  Jantar Mantar built
  The Jantar Mantar is built.^{16The Jantar Mantar, Jaipur · UNESCO World Heritage Centre ↗}
• 1740
  The analemma
  Grandjean de Fouchy invents the concept of the analemma.^{5Analemma and the Equation of Time (Sundials for Starters) · North American Sundial Society ↗}
• 1833
  Time Ball first used
  The Time Ball is first used.^{17The Greenwich Time Ball and one time for all · Royal Museums Greenwich ↗}
• 1840s
  Railway standard time in Britain
  Railway standard time is adopted in England, Scotland and Wales.^{18A Walk Through Time - World Time Scales · National Institute of Standards and Technology (NIST) ↗}
• 18 November 1883
  U.S. Standard Railway Time System
  The U.S. railroad industry implements its Standard Railway Time System.^{18A Walk Through Time - World Time Scales · National Institute of Standards and Technology (NIST) ↗}
• 1884
  Greenwich prime meridian
  The Washington conference adopts the Greenwich prime meridian.^{18A Walk Through Time - World Time Scales · National Institute of Standards and Technology (NIST) ↗}
• March 1918
  Standard timekeeping made official
  An Act of Congress makes standard timekeeping official in the United States.^{18A Walk Through Time - World Time Scales · National Institute of Standards and Technology (NIST) ↗}
• 1976
  Horologium Augusti pavement
  Edmund Buchner predicts the location of the Horologium Augusti pavement.^{20Rome, Horologium Augusti · Livius.org ↗}
• 2010
  Jantar Mantar inscription
  The Jantar Mantar is inscribed as a UNESCO World Heritage Site.^{16The Jantar Mantar, Jaipur · UNESCO World Heritage Centre ↗}