Encyclopedia
A
sundial 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.
History
Sundials in the form of
obelisks and shadow clocks are known from ancient
Egypt, and were developed further by other cultures, including the
Chinese,
Greek, and
Roman cultures. A type of sundial without
gnomon is described in the old Old Testament .
The mathematician and astronomer Theodosius of Bithynia is said to have invented a universal sundial that could be used anywhere on Earth. The French astronomer
Oronce Finé 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 and horizontal sundials.
Giuseppe Biancani's Constructio instrumenti ad horologia solaria discusses how to make a perfect sundial, with accompanying illustrations.
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, so that it points at 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 and political boundaries. A sundial can be rotated around its style or gnomon 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.
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. Rotating the sundial will not work well because most sundials do not have equal hour angles.
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.
Design & principles of operation
Terminology
The 'shadow-maker' of the sundial is called a
gnomon.The sun casts a shadow from the gnomon to a surface called the
dial face or
dial plate .
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.
In common speech, sometimes
style refers to the entire gnomon.
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 orientation.
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 gnomon, 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 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 styles are thick bronze to prevent corrosion.
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 . 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 are calculated from the formula face-angle = arctan.
The angle of the style = latitude.
Vertical sundials
Although they are rare in modern life, sundials on vertical south-facing walls are a traditional ancient convenience. 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.
A problem is that vertical sundials only keep time for the part of the year in which the sun illuminates the wall. They are very similar to garden sundials. The formula for a south-facing sundial face is face-angle = arctan. The angle of the style = co-latitude = 90-latitude.
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 year. In principle, sundials can be placed on any surface, at any angle, given the correct trigonometric projection of the face. For example, sundials on roofs are harder to calculate but quite practical.
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 , 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 , 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 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 diptychs may have been used for
navigation in ancient times.
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
A precision sundial, called a
heliochronometer,
corrects
apparent solar timeto
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 .
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. 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 , in India, built as part of a set of astronomical instruments.
Precision noonmarks
In some older houses, particularly farmhouses, 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 Oriental 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!
Ancient Greek sundials
The ancient
Greeks used a type of sundial sometimes referred to as
pelekinon . 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" according to the
equation of time. This compensates for the slight eccentricity in the Earth's orbit that causes 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.
A fun, 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 is almost perfectly immune to vandalism, as well as truly fun and reasonably accurate.
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 falls on a certain point B of the ring . 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.
Article of interest: Reflection sundials
Isaac Newton invented a sundial for a south-facing window. 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
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.
Reference
Sundials: Their Theory and Construction, Albert E. Waugh, Dover Publications, Inc., 1973, ISBN 0-486-22947-5.
"Sundials Old and New", A.P.Herbert, Methuen & Co. Ltd, 1967.
"Illustrating Shadows", Simon Wheaton-Smith, ISBN 0-9765286-8-1, LCN: 2005900674.
Footnotes
See also
- Sundial Bridge at Turtle Bay — a single spar cable stayed bridge that casts a shadow into a garden where the time may be read.
- Scottish sundial — the ancient renaissance sundials of Scotland.
- Francesco Bianchini
- Tempometer: a clock displaying the time of a sundial.
Sundial Societies, Groups and Organizations
External links
- Guide to the sun, the seasons and sundials
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- Sundials on the Internet, the leading information site on sundials: http://www.sundials.co.uk
- A virtual sundial: http://www.quns.cam.ac.uk/Queens/virtualdial/VirtDial.html
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- . The patent was filed June 1995 .
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- Sundials carried on Mars rovers Spirit is the first of the two Mars Exploration Rover [i] missions. ...
and Opportunity
[i]
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- - collection of sundial links
- The has a and a
- Learn about many different kinds of sundials and how to make them. At "Sunny Day U" you will find paper sundial kits that are fun to make, dialling related devices and lots of sundial templates. Learn how to build a "human" sundial.
- Designing small and large sundials using many methods.
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- - Generate the dial of a horizontal/vertical/cylindrical sundial online.
- - Sundials and shadow clocks
- - Egyptian shadow clock
- - Earliest clocks
