Sundial Types and Terms

Types and Terms

This is not a finite list but should provide a reasonable basis as a starting point on the topic of sundials.
In time examples of each will be referred to either within this blog or via links to invaluable sources of information e.g. organisations such as NASS (the North American Sundial Society) or the BCS (British Sundial Society).

[Please email any corrections to stronsy@gmail.com]

NOTE 

Use the built in Google Translate at the top right of this blog to see the content of in a language of your choice.  This blog is best viewed with Google Chrome.

Adjustable Sundial

Simply any sundial type that is adjustable.

Adjustable Sundial Alarms

See under Alarm Sundials.

Alarm Sundials

Sundials that are designed to provide some form of alarm as a result of the sun reaching a particular point.

The mechanisms for this are many and as imaginative as can be and today may include electronic devices that emit light, sound or vibration etc. at the appointed time.
The NASS conference of 2003 reported on these ingenious machines ranging from an elegantly built noon cannon to other much more fanciful devices that announce the sun’s arrival at the meridian or other appointed time with a bang from a small firecracker).

Paul Nibley, who builds them, hopes to make several more of his “industrial model” and his “butterfly model,” complete with solar cells that powers a chime when the wings of the creature opens.

Original source [September 2008]: http://www.sundials.org/conference/2003/2003conf.pdf

Altitude

The altitude of the Sun is the vertical angle it makes above the horizon (from -90° to +90°). The Sun's altitude and azimuth define its position at any given moment. At sunrise the altitude of the Sun is 0°.

Altitude Sundial

This uses the altitude of the sun at a given latitude and time of year as a means of telling the time.

Apparent Solar Time

See: Equation of time.

Azimuth

A horizontal angle measured from the meridian. It is positive towards the East and negative towards the West. The Sun's azimuth at a particular time is its bearing at that moment. The azimuth is 0° when it crosses the meridian of a place.

Analemma

If you look at the sun at exactly the same time every day you will notice the sun is not always in the same place. If you measure the sun’s location every day during a year, you will see the sun draws an analemma in the sky (a figure of 8).

This apparent motion of the sun is the result of two motions of the earth.

Let us assume that the orbit of the earth around the sun is an exact circle and let us assume that the angle of the axle of the earth is exactly perpendicular to the plane, in which the earth’s orbit is located.

In that case the sun will be exact at the same point in the sky at a fixed point in time.

However, the orbit of the earth is an ellipse and the axle of the earth with its plane of motion is approximately 23.5 degrees. Because of this, the sun makes its figure of 8-shape in the sky – the analemma.

Original source [September 2008]: http://www.analemma.biz/

Analemmatic Sundials

A type of horizontal sundial with a vertical gnomon and hour markers positioned in an elliptical pattern.

There are no hour lines on the dial and the time of day is read on the ellipse. The gnomon is not fixed and must change position daily to accurately indicate time of day.

This may include people acting as the gnomon in which case they are known as Human Gnomon.

In Biarritz, France, there are two analemmatic sundials in marvelous locations overlooking the great sweep of the bay with its Atlantic waves. These sundials are especially popular with children; you stand on a spot marked for the month of the year, and your shadow indicates the approximate time.

Modern Sunclocks of Kilwinning, Ayrshire advise these are essentially both "theft-proof" and "damage-proof", they are perfect for public spaces (children's play areas, etc) and are popular as Millennium-markers.

Made from almost any material (stone, wood, concrete, mosaic) or just painted onto the ground, which is the method normally used by Junior Schools. Set into a lawn, it will not interfere with grass-cutting.

They automatically cater for both standard and daylight- saving time - so you never need to adjust your layout. You can have a maintenance-free conversation piece that is unique - no two 'Sunclocks' are identical, as each layout is specific to its own latitude and longitude. All you need is any level area around 6 metres (20 feet) across.

Armillary Dials

Armillary dials are equatorial dials with extra circles emulating those found on armillary spheres. A true armillary sphere is not a sundial but a model of the heavens.  The extra circles on an armillary dial actually hinder time keeping by blocking the sun at various times.
Original source [September 2008]: http://www.sundials.org/faq/

Co-latitude

The complementary angle of the latitude of a place, equal to 90° latitude.

Colored/Coloured Light Sundial

Originating from the Light Sundial idea but utilising coloured filters to provide coloration to the area around the shadow or to the area around the dial itself.  By design this could be variable either mechanically or naturally based on the time.  For example the filter could be a different colour in the morning to that in the afternoon.

Cylinder Dials

Also known as the Pillar Dial or Pyrenean Shepherd’s Dial or Cylinder Dial.

Not particularly accurate but easy to make and inexpensive indicating the time from the Sun's altitude, which depends not only on the time of day, but also on the latitude and time of year.

It is designed for the specific latitude and adjustable for the date and made from a cylinder, usually hollow, capped by a movable top to which a gnomon is attached (stored inside when not in use).

Hour lines are either inscribed on the surface or drawn on a paper glued to the cylinder.

Hour lines appear as curves on the rounded face of the cylinder and the months of the year are traced around the base of the cylinder.

Declination

of a wall: the angle which a vertical wall makes relative to the meridian. When one designs a vertical sundial, one must take account of the declination of the surface on which it will be placed. In Europe, the declination of a south-facing wall is 0° of an east-facing wall is -90° of a wall facing west +90° and one facing north is 180°.

of the Sun: the position of the Sun north or south of the celestial equator.

Declining Dial

A vertical dial that does not face one of the cardinal points of the compass (see also: Direct Dial).

Dial Plate

The supporting surface of a horizontal dial. The lines and numbers of a dial are laid out on the dial plate.

Dialist

An artisan who designs, constructs and installs sundials.

Diptych dial

A diptych is any object with two flat plates attached at a hinge and devices of this type were popular in the ancient world.  The diptych dial was a portable sundial. A face was on the inside of each leaf. One leaf formed a vertical sundial, the other a horizontal sundial. The shadow caster, or gnomon was a string between them, and calibrated as to how far they should open, as the angle is critical. Such a sundial can be adjusted to any latitude by tilting it so its gnomon is parallel to the Earth's axis of rotation.

Direct Dial

A vertical dial that faces one of the cardinal points of the compass (see also: Declining Dial).

EarthDial

Sundials around the world each with a webcam whose images will then be linked to a central website so, almost in real time, these sundials in all the different time zones can be seen.

Check the first web site below for all the details on how to participate. The basic idea is for schools and individuals around the world each to build a sundial and set up a webcam that looks at it. 

These webcam images are all linked to a central website in almost real time, you can see sundials from around the world in all the different time zones!

As your eye sweeps across the images on this web page, the sun's shadow will be seen to sweep across the faces of the dials. Website viewers can gain a palpable sense of how the sun illuminates the globe of the Earth. 

This project is an adjunct to the first extraterrestrial sundial, the MarsDial on NASA's Mars Exploration Rovers that landed in January 2004 (see under: MarsDial).

Of course at any given time, half the dials will be in darkness. Some will be in the Southern Hemisphere where the sun goes through the northern sky daily, and the hour lines on sundials go in the opposite sense (anticlockwise). The shapes of the lines on dials in the tropics will contrast greatly with those in Scandinavia. There will be a multitude of languages and cultural motifs on the various dials.

http://www.planetary.org (full EarthDial and other information).

http://athena.cornell.edu/kids/sundial.html (information on the MarsDial).

Ecliptic

The plane containing the orbit of the Earth around the Sun, which by extension projects a circle on the celestial sphere.

Equation of Time

The value of the difference between Apparent Solar Time and Mean Solar Time. The difference arises from the movement of the Earth in its elliptical orbit as well as the fact that its axis of rotation is inclined to the ecliptic. The difference can vary by as much as +/- 16 minutes. The Equation of Time is sometimes represented in the form of a figure of eight placed on the noon line of a sundial.

Equatorial Dials

See: Equinoctial Sundial.

Equinoctial line

The line of declination on a sundial corresponding to the equinox. This line makes a right angle with the gnomon in a plane sundial.

Equinoctial Sundial

A type of sundial in which the plane of the shadow-receiving surface is parallel to the equator.
The equinoctial sundial is also known as equatorial dial because the dial plate rests on a plane parallel with the Earth’s equator.
When set-up properly the dial plate sits on a ring parallel to the equator and the gnomon, sometimes called a polestyle on an equatorial dial, points along a line parallel to the Earth’s axis. Weather permitting a shadow is cast by the gnomon onto the chapters that are engraved in five-minute increments.

 Equinoctial dial

The KALA Pocket Sundial for example is based on the universal equinoctial ring sundial, a historic sundial used by ancient sailors and mathematicians. This particular type of sundial also indicates the true North and is aligned with the axis of the Earth.  In this case it is designed as a contemporary response to a historic timepiece and can also be used as sun compass, a model of the world or even a piece of jewelry.

Equinox:

The day of the year when the Sun crosses the celestial equator in its apparent movement. At this moment, the Sun rises exactly in the East and sets exactly in the West. It is the period of the year when night and day are of equal length. The equinox occurs on 20 March and 23 September.

Glass Sundials

Not actually sundials themselves but pictures of all types of sundial that have been categorised on a dedicated website into four different groups: stained glass, etched glass and plastic, projection, and mosaic.

Gnomon

Also known as polestyle on an equatorial dial.
An object that by the position or length of its shadow serves as an indicator especially of the hour of the day.  The name given to a style placed vertically to the plane of a sundial. The shadow cast by the tip of the gnomon can indicate both the time and the date.

Gnomonics

The science of sundials.

Gnomonist

An amateur sundial enthusiast.

Hand Carved and Monumental Sundials

As far as John Carmichael knows, he is the only professional stone sundial maker in the United States. Unfortunately, with the perfection of mechanical clocks, sundial design and construction have nearly become a lost art and science.

The gallery for his Sundial Sculptures of Tucson Arizona shows just some of the exquisite hand-carved stone sundials with inlayed brass, beautiful artwork, and multiple scientific functions. 

Unique copyrighted brass cable gnomon is held taught by heavy counterweight. These precise timekeeping instruments are available in 25" and 40" diameter sizes and larger.

Also design large monumental public sundials. All sundials are custom-made to order, using the latitude and longitude of the sundial's location and each customer's particular tastes and requirements (sundial type, shape, size, artwork, etc.) and includes very informative Sundial Owner's Manual included.

Sundial Sculptures have a broad range of examples of such works.

Heliochronometers

These give a direct and accurate reading of Standard Time throughout the sunlit year. It is a precision timepiece determining time exactly - literally to the minute.
Heliochronometers, long considered the acme of sundials, a heliochronometer is an instrument truly worthy of the Millennium. It is only the heliochronometer that gives a direct and accurate reading of Standard Time throughout the sunlit year. It is a precision timepiece determining time exactly - literally to the minute. 
Used earlier this century to set the time of civic clocks and to standardise time on railway networks, it essentially became redundant, in 1913, with the introduction of Radio Time Signals.
Original source: Gunning Sundials, England [September 2008].

Horizontal Dials

A type of sundial in which the plane of the shadow-receiving surface is horizontal.

Hour

Babylonian Hours: an ancient system which divides the day into 24 hours, commencing at sunrise. They thus measure the number of hours elapsed since sunrise.

Italian Hours: an ancient system which divides the day into 24 hours, commencing at sunset. They thus measure the time left until sunset.

Standard Time: the time shown by clocks, which will be the same for all inhabitants of a particular time zone. It is Mean Solar Time corrected for the difference in longitude between a place and the reference longitude of the time zone in which it is located.

Mean Solar Time: Obtained by adding the Equation of Time to the Apparent Solar Time. The Mean Solar Time gives a constant hour duration over the year. It varies with the location.

Apparent Solar Time: the time naturally shown by a sundial. It is defined as 1/24th of the interval between two successive passages of the Sun across the meridian. This period of time varies throughout the year.

Temporary Hour: an ancient system dividing the interval between sunrise and sunset into 12 hours. The length of an hour therefore varied throughout the year! (between 40-80 minutes). These hours are sometimes referred to as Biblical Hours.

Human Gnomon

See under Analemmatic Sundials.

Latitude

The angle formed between the vertical of a place and the plane of the equator. Northern latitudes are positive; southern latitudes are negative.

Light Sundial

See under: Renaissance sundial.

Lines

Hour Lines: lines drawn on the dial plate of a sundial which permit us to tell the time from the shadow cast by the style. Hour lines are drawn for hours and sometimes for half-hours and quarter-hours but much more rarely for 5 minute intervals.
Declination Lines: lines drawn on the dial plate of a sundial which permit us to determine the date from the shadow cast by the style. It is conventional to show the dates on which the Sun enters certain signs of the Zodiac, for example, for the Sun declination 0° (Aries and Libra), +/- 11° 29' (Pices and Virgo), +/- 20° 20' (Gemini and Sagittarius). These lines are also called diurnal arcs.

Longitude

Longitude of a place: the angle formed by the meridian of a place with the Prime Meridian in Greenwich. Longitude is positive for locations to the west of Greenwich and negative for those to the east.

Longitude correction

The time difference between the location's meridian and the time zone’s meridian (Greenwich or local time zone). There is a difference of 4 minutes for each degree of longitude.

For example, if you buy a sundial made for Roswell, New Mexico, at longitude 105° W. and place it in Tucson, Arizona at 111° W., it can be off by about 24 minutes.

Mass Dials

Also known as ‘scratch dials’; these were inscribed on a buttress or wall close to the porch of a parish church in the Middle Ages used to indicate the time for religious service or gathering.

Mars Sundial (MarsDial)

The sundial on board NASA’s Mars Exploration Rover Spirit, which landed on Mars in January 2004.

Mean Solar Time

See: Equation of time.

Meridian:

Off a place: the geographical meridian which passes through a place.geographic: the great circle passing though the poles and the zenith of a place.

Meridian Dials

Also known as ‘Noonmark Dials’.

Nodus

Nodus: An indicator or marker on the gnomon that helps to define an exact point on the shadow.

Noonmark Dials

Also known as ‘Meridian Dials’

The simplest sundials do not give the hours but note the exact moment of 12:00 noon used historically to correct mechanical clocks sometimes so inaccurate as to lose or gain significant time daily.

In U.S. colonial-era houses a noon-mark can often be found carved into a floor or windowsill indicating local noon providing a simple, accurate time reference for households that do not possess accurate clocks.

Even today in some Asian countries post offices have set their clocks from a precision noon-mark.

The typical noon-mark sundial was a lens set above an analemmatic plate, an engraved figure-eight corresponding to plotting the equation of time versus the solar declination.

When the edge of the sun's image touches the part of the shape for the current month, it indicates noon.

Pillar Dials

Also known as the Pyrenean Shepherd’s Dial or Cylinder Dial (see under Cylinder Dials).

Polar Dials

A type of sundial in which the plane of the shadow-receiving surface is parallel to the polar axis.

Polestyle

See: Gnomon.

Polyhedral Dials

Combining various orientations on multiple faces showing, for example, faces oriented to tell time in various world cities.

Pyrenean Shepherd’s Dials

Also known as ‘Pillar Dial’ or ‘Cylinder Dial’ (see under Cylinder Dials)

Reclining Dials

See under Vertical Dials.

Renaissance Focusing Sundial

See under: Renaissance Sundial.

Renaissance Sundial

A patented sundial which illuminates the time with a crisply focused beam of light, rather than casting a fuzzy shadow.

This is a new design in sundials developed by Precision Sundials LLC in Burlington, VT.

Cylindrical focusing mirrors on the triangular support project a sharply focused beam on the inside of the helix. A timescale on the inside of the helix directly reads watch time generally to within 30 seconds of the atomic standard, when properly set up.

Ring Dial

A ring dial is for use in a fixed latitude and is a small portable dial in the shape of a cylindrical ring for determining time.  See also Universal Ring Dial.

Sawyer Equant Sundial

This dial reads solar time on its bronze plate, and civil time on its granite plate. Both sundials adjust for latitude using a precise patented wedge mechanism.

Scaphe Dial

The scaphe (or skaphe, also scaphium or scaphion) from the 3^rd century BC and was a sundial consisting of a hemispherical bowl with a vertical gnomon inside it, with the top of the gnomon level with the edge of the bowl. Twelve gradations inscribed perpendicular to the hemisphere indicated the hour of the day.

Scaphium Sundial

See under: Scaphe Sundial.

Scaphion Sundial

See under: Scaphe Sundial.

Skaphe Sundial

See under: Scaphe Sundial.

Scratch Sundial

See under Mass Dials.

Solstice

The moment in the year when the Sun reaches in maximum declination in the north or south of the celestial equator. The Summer Solstice (21 June) in Europe is the longest day of the year and the Sun is in its highest position in the sky. The Winter Solstice (21 December) is the opposite — the shortest day and the lowest point the Sun reaches in the sky.

Spot-On Sundial

Its unique feature is the split gnomon, which casts the shadow and every day at noon a line of light shines between the 2 plates for around 5 minute’s; this feature allows them to be set up precisely, so that it will tell the right "time by the sun" for years to come.

Style

The name given to the shaft or triangle which casts its shadow on a dial.  A polar style is oriented parallel to the Earth's axis. A vertical style or gnomon is placed vertical to the dial plate.

Sub-style

The line corresponding to the projection of the style on the dial plate of a sundial. The angle which the sub-style makes with the noon hour line indicates the declination of the dial.

Suncatcher

A suncatcher or light catcher is a small, reflective glass or for example mother of pearl piece that is hung indoors, so that it "catches the light" from a nearby source, usually hung at windows.

The South-western American Indians first began making them and they are now still popular in that region

Suncatchers may be mass-manufactured, or they may be created by hand, and can be of varying simplicity of design from an arts and crafts project to a professionally handmade glass sculpture.

Some designs are simple and abstract with perhaps some mobile-like chained elements, while more complex designs often evoke plants or animals.

A suncatcher is something like the optical equivalent of a wind chime, and indeed many designs combine the two.

Sundial

A device for telling solar time from the position of the Sun's shadow thrown by a gnomon.  If the dial is a horizontal or vertical plate, the hour lines are at unequal angles to each other.  A vertical dial, which faces one of the cardinal points of the compass, is called a direct dial; one which does not is a declining dial.

The Concise Oxford dictionary defines it as an instrument showing the time by the shadow of a pointer cast by the sun on to a plate marked in hours.

Thew Patent Dial

A sundial which uses numbers (for example ) cut-out of a strip or band to let light in the shape of that number aligned with a fixed marker on a surface below it to indicate the time.

Time-Mark Sundial

Sundials with an engraved message on the side of the gnomon to give a unique gift for an anniversary, birthday, wedding, or special event and the simple variant of the Time-Mark sundial idea in which the gnomon is actually set to indicate the specific time/date for the occasion concerned rather than the solar time.

Time Differentials and the need for (some) Adjustable Sundials

There is a major difficulty for sundials in some countries.

Spain for example keeps Central European Time, even though most of Spain is west of the Greenwich meridian. This means that there is always a difference of at least one-hour between sundials and clocks in Spain, and in summer time, a difference of more than 2 hours. A sundial in Pamplona should have a difference of around 2 hours and 15 minutes, since Pamplona is nearly 4 degrees West of Greenwich.

Universal Equinoctial Ring Sundial

See: Equinoctial Sundial.

Universal Ring Dial

A Ring Dial is for use in fixed latitude, but the universal ring dial can be used over a wide range of latitudes. It consists of three rings and a strip with slider. A flat vertical brass ring, with grooved outer edge, fits within a ring of circular cross-section, and slides within it. The outer ring is suspended from a shackle or mounted on a foot. One side of the inner ring carries a scale of degrees, 0-90, for adjustment to latitude using an index on the outer ring. Pivoted within the inner ring is a further ring which can be swung out at right angles, carrying the hour scale.

University of Hawaii Sundial

This sundial is one of many sundials that are more accurate--to within four seconds--than the famous Egyptian prototypes.

Vertical Dials

Vertical Dials and Reclining Dials are basic types of sundial.

With a vertical dial the plane of the shadow receiving surface is vertical and for a reclining dial it is horizontal.