Sundial Vocabulary

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.

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.

Co-latitude:

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

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.

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.

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.

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.

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.

Gnomon:

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.

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.

Latitude:

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

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' (Pisces and Virgo), +/- 20 20' (Gemini and Sagittarius). These lines are also called diurnal arcs.

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.

Meridian:

of a place: the geographical meridian which passes through a place.

geographic: the great circle passing though the poles and the zenith of a place.

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.

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






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  Glossary of Sundial and Armillary related terms