sundial Encyclopedia > Science and Technology > Technology > Technology: Terms and Concepts sundial sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow.

The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time. A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments. The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory.

Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory. See F. W. Cousins, Sundials (1969); R. R. J. Rohr, Sundials (tr. 1970). The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved. Shadow-clock | Article about Shadow-clock by The Free Dictionary Shadow-clock | Article about Shadow-clock by The Free Dictionary Also found in: Dictionary , Thesaurus , Wikipedia . sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt.

Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow. The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time.

A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments. The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory. Bibliography Sundial | Article about sundial by The Free Dictionary Sundial | Article about sundial by The Free Dictionary Also found in: Dictionary , Thesaurus , Medical , Financial , Acronyms , Wikipedia . sundial, instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall.

Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent. A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow. The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them.

The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time. A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments. The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory. Bibliography gnomonlogic.com - About Us About Us J. Brooks Dougherty About Us Gnomon Logic was founded in 2010 by J. Brooks Dougherty , C.F.A. and Max O.

Bloomfield , Ph.D. It serves the venture capital, institutional investor, and private equity firm communities in assessing current and future capital market value of technology-based firms. Gnomons We often get questions about our name. The word gnomon (pronounced NOH-mon, from the Ancient Greek γνώμων, meaning "that which reveals or discerns") is most often used to mean the upright piece of a sundial, which casts the shadow. The first gnomons were vertical sticks or staffs, but they have evolved as our understanding of the Earth's movement around the sun has increased. Modern sundials use triangular gnomons with a shadow-casting edge that runs parallel to the axis of the Earth, allowing them to discern equal hours on any day of the year. The largest gnomon of this type is likely the cantilever spar of the Sundial Bridge at Turtle Bay located in Redding, California, pictured to the right. The largest vertical gnomon explicitly part of a sundial is Taipei 101 in Taipei, Taiwan, casting its shadow on the neighboring Sundial Park. sundial - Credo Reference Topic Page: sundial Definition: sundial from Dictionary of Energy Solar.

a timepiece employed since ancient times to indicate daylight hours by the shadow that the gnomon (a rod or fin) casts on a calibrated dial. Summary Article: sundial from The Columbia Encyclopedia Image from: clock in The Macmillan Encyclopedia instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. Although any object whose shadow is used to determine time is called a gnomon, the term is usually applied to a style, pin, metal plate, or other shadow-casting object that is an integral part of a sundial. Forerunners of the sundial include poles or upright stones used as gnomons; pyramids and obelisks were so used in Egypt. Both stationary and portable sundials were probably developed in Egypt or in Mesopotamia. The earliest extant sundial, an Egyptian instrument of c.1500 B.C., is a flat stone on which is fixed an L-shaped bar whose short vertical limb casts a shadow measured by markings on the longer horizontal limb. The sundial was greatly improved (c.1st cent.

A.D.) by setting the gnomon parallel to the earth's axis of rotation so that the apparent east-to-west motion of the sun governs the swing of the shadow. The development of trigonometry permitted precise calculations for the marking of dials and stimulated the advance of gnomonics (dial marking). Although watches and clocks came into popular use in the 18th cent., sundials were long employed for setting and checking them. The heliochronometer, a highly accurate instrument in which the shadow is cast by a fine wire, was used until c.1900 to set the watches of French railwaymen. Solar (or apparent) time indicated by sundials and clock (or mean) time are different and must be correlated by the use of tables showing daily variations in sun time. A correction must also be made for the difference in longitude between the position of a sundial and the standard time meridian of a given locality. Although sundials are still used in many areas, including Japan and China, they are regarded today chiefly as adornments.

The largest sundial in the world, constructed c.1724 in Jaipur, India, covers almost one acre (.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. Notable collections of sundials are at the Adler Planetarium, the Metropolitan Museum of Art, and the Harvard College Observatory. See F. W. Cousins, Sundials (1969);. R. R. J. Rohr, Sundials (tr. 1970). The Columbia Encyclopedia, © Columbia University Press 2016 Sundial Links Portable Sundials, for Navigation and Time During the Middle Ages advanced yet portable astronomical instruments were developed. Diptych sundial One popular portable sundial design was called a diptych. It consisted of two small flat faces, joined by a hinge. Diptychs usually folded into little flat boxes suitable for a pocket. The gnomon was a string between the two faces. When the string was tight, the two faces formed both a vertical and horizontal sundial. The best material was white ivory, inlaid with black lacquer markings.

The best gnomons were black braided silk, linen or hemp. With a knot or bead on the string as a nodus, and the correct markings, a diptych can keep a calendar well-enough to plant crops. By making the two sundials have different angles to the string (and thus different projections), a diptych can be self-aligning. When both faces show the same time, and the hinge is level, the diptych shows the local apparent solar time. Additionally, the hinge will point north (in the northern hemisphere), and the diptych will be angled so the gnomon is parallel to the Earth's axis of rotation. At solar noon, sunrise and sunset, the latitude adjustment of the diptych can't affect the time of either sundial, but at 9am and 3pm, each degree of latitude error (from holding the sundial at the wrong angle) creates four minutes of difference between the two faces. This means that a diptych can also act as a compass and even measure latitude. Some diptychs included a small scale and a plumb-bob to read the latitude. Some others included a compass rose to measure angles to geographic features.

Large (meter-sized) diptychs may have been used for navigation in ancient times. Sundial Links What is Time? We sense the passage of time in our personal experience and observe it in the world around us. We feel, think, and act in the flow of time. Einstein said, "Space and time are modes by which we think, not conditions under which we live." Time-the time that we know through clocks and calendars-was invented. The measurement of time is an ancient science, though many of its discoveries are relatively recent. The Cro-Magnons recorded the phases of the Moon some 30,000 years ago-but the first minutes were counted accurately only 400 years ago, and the atomic clocks that allow us to track the approach of the millennium by the billionth of a second are less than 50 years old. Timekeeping has been both a lens through which humanity has observed the heavens and a mirror reflecting the progress of science and civilization. At the dawn of the new millennium, the instruments that divide and measure the days on Earth have brought us to a deeper understanding of how it all began. The most beautiful thing we can experience is the mysterious. It is the source of all true art and science.

Albert Einstein (What I Believe) From Sundials to the Atomic Second A History of Timekeeping History A sundial is a device that measures time by the position of the Sun. The most commonly seen designs, such as the 'ordinary' or standard garden sundial, cast a shadow on a flat surface marked with the hours of the day. As the position of the sun changes, the time indicated by the shadow changes. However, sundials can be designed for any surface where a fixed object casts a predictable shadow. Most sundial designs indicate apparent solar time. Minor design variations can measure standard and daylight saving time, as well. Sundials in the form of obelisks (3500 BC) and shadow clocks (1500 BC) are known from ancient Egypt, and were developed further by other cultures, including the Chinese, Greek[1], and Roman cultures. A type of sundial without a gnomon is described in the Old Testament (Isaiah 38:8).(ca.700 BC). The mathematician and astronomer Theodosius of Bithynia (ca. 160 BC-ca.

100 BC) is said to have invented a universal sundial that could be used anywhere on Earth. The French astronomer Oronce Fin� constructed a sundial of ivory in 1524. The Italian astronomer Giovanni Padovani published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. Giuseppe Biancani's Constructio instrumenti ad horologia solaria (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations. The oldest sundial in Britain is incorporated into the famous Bewcastle Cross. The dial is divided into four tides, covering the parts of the working day in medieval times. This device is commonly used in the modern world because of convience and practicality. Greek equatorial sun dial, Ai-Khanoum, Afghanistan 3rd-2nd century BC. Installation of Standard Sundials Many ornamental sundials are designed to be used at 45 degrees north. By tilting such a sundial, it may be installed so that it will keep time.

However, some mass-produced garden sundials are inaccurate because of poor design and cannot be corrected. A sundial designed for one latitude can be adjusted for use at another latitude by tilting its base so that its style, or gnomon, is parallel to the Earth's axis of rotation and pointing in the direction of the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere. A local standard time zone is nominally 15 degrees wide, but may be modified to follow geographic or political boundaries. A sundial can be rotated around its style (which must remain pointed at the celestial pole) to adjust to the local time zone. In most cases, a rotation in the range of 7.5 degrees east to 23 degrees west suffices. This will introduce error in sundials that do not have equal hour angles. To correct for daylight saving time, a face needs two sets of numerals or a correction table. An informal standard is to have numerals in hot colors for summer, and in cool colors for winter. Ordinary sundials do not correct apparent solar time to clock time.

There is a 15-minute variation through the year, known as the equation of time, because the Earth's orbit is slightly elliptical and its axis is tilted relative to the plane of its orbit. A quality sundial will include a permanently-mounted table or graph giving this correction for at least each month of the year. Some more complex sundials have curved hour-lines, curved gnomons or other arrangements to directly display the clock time. Horizontal sundial in Taganrog (1833) Design and principles of operation Terminology The 'shadow-maker' of the sundial is called a gnomon[2]. In common speech, the gnomon is often called a style. [2] The sun casts a shadow from the gnomon to a surface called the dial face or dial plate (often shortened to face). Most sundials indicate time on the dial face by the shadow of a line in space called the style. On a standard garden sundial, this line is the top edge of the gnomon. The style should be parallel to the Earth's axis of rotation, and point to the celestial pole. The line on the dial plate perpendicularly beneath the style is called the substyle[3], that is below the style.

The angle the style makes perpendicularly with the dial plate is called the substyle height, an unusual use of the word height to mean an angle. On many wall dials, the substyle is not the same as the noon line. The angle, on the dial plate, the noon line makes to the substyle is called the substyle distance, an unusual use of the word distance to mean an angle. Some sundials indicate both the time and the date by the shadow of a particular point on the gnomon. That point is called the nodus. The nodus may be the tip of a gnomon with an arbitrary (usually horizontal or vertical) orientation[4]. A few sundials have both a style and a nodus, with the nodus in the form of a small sphere or a notch on a polar-pointing style, or simply the tip of the gnomon. In general, the best material for a face is a very light color to give a high contrast with the shadow. The surface should have a matt finish to prevent misleading reflections. The numerals should be dark, visible on the unshaded portion of the face.

The gnomon should be sturdy, preferably metal, because gnomons are usually thin, and can break easily. The traditional luxury materials are a white marble face, with markings inlaid in black marble. Traditional gnomons are thick bronze to prevent corrosion. In Eastern Europe, there is a tradition of painted dial faces often using fresco techniques. It is traditional for a sundial to have a motto. Equatorial or Equinoctial sundial The simplest sundial is a disk mounted on a bar. The bar must be parallel to the Earth's axis of rotation. The disk forms a plane parallel to the plane of the Earth's equator. The disk is marked so that one edge of the shadow of the bar shows the time as the Earth rotates. Usually noon will be at the bottom of the disk, 6AM on the western edge, and 6PM on the eastern edge. In the winter, the north side of the disk will be shaded, and hard to read. In the summer, the south side will be shaded. In the above design, the bar is the style. The disk in the above design is called the face.

In the summer, the north end of the bar is the nodus, but in the winter, the south end of the bar is the nodus. A series of concentric circles can be drawn on the face which plot the path of the shadow of the nodus on specific days, thus the dial can be used as a calendar as well as clock. The style shows the time and the nodus the date. One disadvantage of this design is that with a solid face, near the equinox, when sun is just on the celestial equator, the dial is hard to read. Garden sundial The classic garden sundial uses the same principle, except the lines of the disk are projected, using trigonometry, onto a face that is parallel to the ground. The advantage of the garden sundial is that it keeps time all year, and its face is never completely shaded in the daytime (as vertical sundials are). For use in a public area, this sundial can be made visible by placing it in a square, or making the face of frosted glass, elevated high in the air, and visible from underneath. The top edge of the gnomon is parallel with the axis of the Earth's rotation.

The shadow will cross time markings on the face. The markings of each edge are aligned with the edge of the gnomon that produces the shadow. The angle of the face markings from the root of the gnomon (the substyle) are calculated from the formula: face-angle = arctan(sin(latitude)*tan(hour-angle)) The angle of the style: style height = latitude An Equinoctial sundial in the Forbidden City, Beijing. Vertical Sundials They are easy to see from large distances and inexpensive to arrange. One sturdy method is to paint the sundial on the wall, and construct the gnomon as a tripod of metal bars. Fancy sundials used to have faces of inlaid stone. Vertical Direct These are placed on surfaces that align directly with the cardinal points of the compass. As mediaeval churches were aligned to face due east, this is actually quite common. A vertical direct south dial may be recognised as the hour lines are symmetrical. The hours run counter clockwise. They are similar to garden sundials in construction.

The formula for a south-facing sundial face : face-angle = arctan(cos(latitude)*tan(hour-angle)) The angle of the style: style height = co-latitude = 90-latitude. A vertical direct east or west dial has a gnomon that is parallel but raised from the wall that supports it. The hourlines run parallel to the gnomon, at an angle to the horizon. This angle is the location's latitude. The east dial drops to the right, and the west to the left. A vertical direct north dial is a mirror image of a south direct dial, it is of limited use as it only displays the hours before 6 am, and after 6 pm. It used to be traditional to place four sundials on the roof or sides of a tower to provide the time. In this way, the time was available to all for the entire day. Further a pillar could be erected to hold a a cube of stone, and the four direct dials could the inscribed.

Sundial Detail Wall sundial on East-facing wall, Prague, about 1740 South west declining face of a polyhedral pillar dial, now found in the Restoration House Gardens in Rochester, UK South reclining sundial in Recife, Brazil Designed by Fernando Almeida Vertical Declining Dials These are placed on surfaces that align directly with the cardinal points of the compass. As mediaeval churches were aligned to face due east, this is actually quite common. A vertical direct south dial may be recognised as the hour lines are symmetrical. The hours run counter clockwise. They are similar to garden sundials in construction. The formula for a south-facing sundial face : face-angle = arctan(cos(latitude)*tan(hour-angle)) The angle of the style: style height = co-latitude = 90-latitude. A vertical direct east or west dial has a gnomon that is parallel but raised from the wall that supports it. The hourlines run parallel to the gnomon, at an angle to the horizon. This angle is the location's latitude. The east dial drops to the right, and the west to the left.

A vertical direct north dial is a mirror image of a south direct dial, it is of limited use as it only displays the hours before 6 am, and after 6 pm. It used to be traditional to place four sundials on the roof or sides of a tower to provide the time. In this way, the time was available to all for the entire day. Further a pillar could be erected to hold a a cube of stone, and the four direct dials could the inscribed. A declining dial is any dial that does not directly face N, S, E or W. It can be recognised by hourlines that are not symmetrical. In every case the gnomon will point to the celestial north pole. In every case, the midday line or meridian line is vertical, but the gnomon will not be vertical. The line marked XII or 12 will be close to the meridian, but may not be exactly there as the dial has been corrected for longitude. Inclining Dials, Inclining Declining Dials Inclining dials, sometimes called reclining dials, are dials on a slope.

In principle, sundials can be placed on any surface, at any angle, given the correct trigonometric projection of the face. Sundials on sloping roofs can be calculated. This is rarely done; usually the gnomon is aligned and the rest is done by observation rather than calculation. Portable diptych sundial Early 18th Century Portable Sundials This form of sundial was about 8 cm diameter and made of brass. It had a brass lid (not shown) to protect it when travelling. Several features enabled precision to be achieved. It had an iron compass needle so that North could be accurately set. The scale division is to 5 minutes. This dial was made in Dublin in 1742 by Gabriel Stokes, a mathematical instrument maker. Elevation sundial Astrolabes were used as sundials, as well as for calendrical observations, navigation and astronomy. An even smaller design was the ring. It had a small handle, or was a fob or the decoration of a necklace. When held by its handle, a hole would cast a shadow on the inside of the ring, telling the time by markings on the inside. The user had to know if it was morning or evening.

Usually the hole was mounted in a sliding lockable piece of metal, which was adjusted to correct date. In recent times, U.S. Special Forces have taken to engraving a simple sundial on their knife-blade. It works even when a watch fails. Precision sundials (heliochronometers) A precision sundial, called a heliochronometer, corrects apparent solar time to mean solar time or another standard time. Heliochronometers usually indicate the minutes to within 1 minute of Universal Time. See this discussion of the limits of Sundial Accuracy. Equatorial bow sundial The classic shape for a heliochronometer is an equatorial bow sundial. A bar, slot or stretched wire parallel to the earth's axis forms the style. The face is a semicircle with markings on the inner surface. This pattern, built a couple of meters wide out of temperature-invariant steel invar, was used to keep the trains running on time in France before World War I. One of the simplest sundials that reads clock time is an equatorial bow with a gnomon shaped like two vases[5]. The vase-shape directly shades the hour line in the correct place as the year passes, and the sun changes elevation.

The most precise sundials ever made are monumental equatorial bows constructed of masonry, part of the Yantra mandir (Jaipur), in India, built as part of a set of astronomical instruments. Sundial in the form of a mandolin, circa 1612 Precision Noonmarks In some older houses, particularly mansions, a noon-mark can be found carved into a floor or windowsill. Such marks act as sundials to indicate local noon, and they provided a simple and accurate time reference for households that did not possess accurate clocks. In modern times, some Asian countries' post offices have set their clocks from a precision noon-mark. These in turn provided the times for the rest of the society. The typical noon-mark sundial was a lens set above an analemmatic plate. The plate has an engraved figure-eight shape. When the edge of the sun's image touches the part of the shape for the current month, it is noon!

Traveler's sundial, constructed at Paris, by Butterfield, probably in the last quarter of the 18th century Ancient Greek Sundials The ancient Greeks used a type of sundial sometimes referred to as pelekinon (axe-like, apparently because shape of the hour and day lines suggest the ancient double-headed ax pelekus). The gnomon was a rod or pole upright in a horizontal face or half-spherical face. The shadow of the tip of the rod sweeps out hyperbolic curves on a flat face, or circles on a spherical face. The advantage of these dials is that they can be marked to tell the exact time for all times of year. Analemmatic Sundials Analemmatic sundials correct solar time to mean solar time or another standard time. These usually have hour lines shaped like "figure eights" (analemmas) according to the equation of time. This compensates for the slight eccentricity in the Earth's orbit that causes up to a 15 minute variation from mean solar time.

Very accurate dials of this type fit nicely in a public square, using a ball at the tip of a flagpole as the nodus , with the face painted on or inlaid in the pavement. The geometrical construction of an analemmatic sundial is simple. First imagine an equatorial sundial floating in the air: a vertical bar directed towards the pole and a ring in the plane perpendicular to the bar. Label the lowest point of the ring "12", and the other hour marks as usual. At a certain time and date, the shadow of a certain point A on the bar (which falls here or there depending on the time of year) falls on a certain point B of the ring (which depends on the hour, and the position in the Earth's orbit). Now draw the point B' in the ground just below B and the point A' just below A. Now if you stand at A' your shadow will point at B', because the sun is somewhere in the plane A B A' B'. In middle latitudes, the ellipse with the hour-marks should be about six meters wide, so the shadow of the head of the beholder will fall near it most of the time.

[3] A less accurate version of the sundial is to lay out the hour marks on concrete, and then let the user stand in a square marked with the month. The month squares are arranged to correct the sundial for the time of year. The user's head then forms the gnomon of the dial. If the sundial is molded into the concrete, it resists vandalism and is engaging and reasonably accurate.[4] A typical Analemmatic layout, often known as a 'Human Sundial' Reflection Sundials Isaac Newton used a sundial for a south-facing window.[5] He placed a tiny mirror on the windowsill, and painted the sundial's face in a mirror-image pelekinon on the ceiling and walls. The mirror formed the gnomon by reflecting a spot of light. This provides a large, accurate, perfectly correctable sundial with minimal material, and no wasted space at all. This design could easily be made analemmatic. Analog Calculating Sundials A last, interesting variation accurately keeps clock time, while still resembling a conventional garden sundial. It is a horizontal sundial with a face cut on a cardioid (a sort of heart-shape).

A cardioid is the shape that connects the intersections between the solar-time marks of a conventional sundial, and the equal-angles of a true clock-time face. The place where the shadow crosses the cardioid's edge is the place where clock time can be read on the underlying clock-time dial. The sundial is adjusted for daylight saving time by rotating the underlying equal-angle clock-time face. The sun-time face does not move. Digital Sundials A digital sundial uses light and shadow to 'write' the time in numerals rather than marking time with position. One such design uses two parallel masks to screen sunlight into patterns appropriate for the time of day. Digital Sundial Sundials: Their Theory and Construction, Albert E. Waugh, Dover Publications, Inc., 1973. "Sundials Old and New", A.P.Herbert, Methuen & Co. Ltd, 1967. "Illustrating Shadows", Simon Wheaton-Smith, LCN: 2005900674. Footnotes Sundials: Their Theory and Construction, Albert E. Waugh, Dover Publications, Inc., 1973.

Websites of Interest Analemmatic Sundials How to build one and why they work. Plus is an internet magazine which aims to introduce readers to the beauty and the practical applications of mathematics. The British Sundial Society Its objects are to advance the education of the public in the art and science of gnomonics and the knowledge of all types of sundial; to catalogue and advise on the restoration of sundials that still exist in the British Isles, and to research their history. Brooke Clarke's Web-site Brooke has links to organizations, calculations, for sale, construction, photographs, descriptions, homepages, books, patents, as well as links to related subjects such as time & frequency, and astronomy. Heavenly Mathematics: Cultural Astronomy The goal of this course is to study astronomy in a cultural context. This is a very good site. Check out the section on ancient astronomical instruments. #Instruments Manor House Museum The Manor is a museum of art and horology, which includes the development of timekeeping in Britain, Europe and America. There is even a portable sundial.

MarsDials The scientific purpose of the MarsDial is as a calibration target for the camera, providing a known set of colors and gray scales, both in sunlight and in shadow, much like a television test pattern. Also available in Czech (web page) Polish (3 parts) Russian (PDF) and Brazilian Portuguese (4 parts) Click on the part number above, or on 'Next Part' to start reading a part at the top of the screen. Part 1. How Sundials Work. From the earliest times people have regulated their lives by the apparent motions of the Sun and the shadows cast by its rays. We say ‘apparent’ because of course it is the rotation of the Earth on its axis which causes the moving shadows we observe every day. When the Earth rotates 15° it is just as though the Sun had moved 15° around its daily track. Diallists use both conventions but it is usually easier to regard the Sun as being in motion. Perhaps the best place to begin an understanding of how sundials work would be to imagine a pictorial view of the Earth at the North Pole In this diagram it is the Sun which seems to move at 15° every hour.

The shadow-casting element of a sundial is usually called the ‘gnomon’ of which the edges are its ‘styles’. Part 12. Solar Time, Correction for Longitude and Greenwich Mean Time (England). The Sun takes four minutes to traverse one degree of longitude from East to West. Sundials on the same line of longitude all tell the same time (shown on clocks for clarity). Sundials on the 4° meridian are 16 minutes behind Greenwich time and those at 8° West are 32 minutes ‘slow’. Example: Plymouth is 4° 08’ West of Greenwich. Sundials in Plymouth are always 16 minutes 32 seconds slow. In 1880, to avoid chaos when the first railways gave rapid east/west travel, Parliament established Greenwich Mean Time (GMT) so all UK clocks told the same time as Big Ben. Part 13. Introduction to the analemma. The first accurate mechanical clocks were made in 1656 by Dutch Scientist Christian Huygens. His later models were accurate to within one second per day.

If he set the clocks daily by the sundial he may have initially thought that the clock was varying in accuracy at different times of the year when in fact it was the clock which was accurate and the sundial which was varying. We now know that all clocks are continually ‘out of step’ with the sundial because day lengths, measured by the Sun, increase by a few seconds over a period of about three months then decrease by similar amounts over the next three months only to repeat the process for the next half year. We can see what is actually happening on the next page. Part 17. Earth orbit diagram. If the Earth stayed in one place and just rotated on an upright axis* then all days would have the same length. When we look at the Sun we are observing it from a moving platform. It is the varying speed around its elliptical orbit and the tilted axis which are responsible for the daily variations accounted for by the Equation of Time. · *Impossible of course as it is the orbital motion which prevents the Earth being dragged inwards by the Sun’s gravity. · Part 18. Introduction to decliners.

A vertical sundial on a truly south-facing East/West wall has a central vertical noon line and a symmetrical arrangement of hour lines. The hours will begin at 6am and end at 6pm Sundials on walls lying at an angle to truly East/West are said to ‘decline’. In most cases the noon line will still be vertical but the gnomon will be rotated to so that it continues to align with the Earth’s axis.

Question: What is the word for the upright shadow-casting part of a sundial?
Answer: Gnomon