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Introduction    Dial types    Hours (types of)    Time (types of)    Illustration
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halcyon days

: (pron. hal-ce-on) originally, 14 days about the winter solstice. Now taken as simply calm, peaceful.

Hallomas (Halloween in USA): All Saints’ day on 1st of November. It is one of the cross-quarter days.

hectemoros angle

[hec] (pron. hec-tem-or-os) the angle from the western horizon to the sun’s position, measured around the hectemoros circle. Part of the ptolemaic co-ordinate system, and related to the seasonal hours.

hectemoros circle

The great circle that passes through the E-W points on the horizon and through the sun’s position.

heliacal rising

The instant of the earliest visibility of a star in the East at dawn. The heliacal rising of the star Sirius was used by the ancient Egyptians to predict the coming of the annual Nile flood. Since their year had 365 days, this occurrence had a variable date.

height (of a style)

See style height.


An adjective to describe a model of the solar system which places a stationary Sun in the centre, with the planets revolving around it.


Has two distinct meanings: (i) a device for transmitting morse signals over extended distances by using an accurately aligned mirror to send flashes of sunlight to the receiving station. For long messages, the ~ has a mechanism for tracking the sun’s motion. (ii) an astronomical instrument for studying sunspots, as built by George Airy at Kew in 1873.


A telescope which produces two images of the Sun which can be manipulated to determine its angular size accurately. Invented in 1754 by John Dollond of London, it is also used to measure angular distances between stars.


A scientific instrument which holds an image of the Sun stationary, allowing extended observation (e.g. for solar spectrometry).

hemisphere (northern ~ and southern ~)

One half of the Earth’s globe, either north of or south of the equator. Note that a sundial at a particular latitude in one hemisphere must be reversed for use at the reciprocal latitude in the other hemisphere.

horarius circle

The great circle that passes through the N-S points on the horizon and through the sun’s position.

horarius angle

[hor] the angle from the southern horizon to the sun’s position, measured around the horarius circle. One of the ptolemaic co-ordinates.


The line of intersection between the sky and the Earth. For normal astronomical purposes, the observer’s horizon is taken to be the great circle on the celestial sphere on which every point is 90º from the observer’s zenith. The observed horizon (accounting for the curve of the earth and the height of the observer above its surface, but excluding refraction) is below the astronomical horizon by an angle called the dip. This can have a significant affect on the times of sunrise/sunset.


The collective Latin name for dials, water-clocks and sand-glasses, used in the Middle Ages.

horologium: a name used to describe medieval manuscripts listing shadow lengths, deriving from the Latin
name for timepieces. Modern versions have also been produced. In modern astronomy, it is also the name
of a faint southern constellation (“the Clock”).


Usually means 1/24th of a mean solar day, unless otherwise stated. Scientifically, it is defined as 3600 standard seconds. See Hour (types of) for other definitions. The word derives from the Latin “hora”, which was synonomous with prayer.

hour angle

[h, HA] the angle corresponding to the sun’s position around its daily (apparent) orbit. Measured westward from local noon, it increases at a rate of 15º per hour. Thus 3pm (Local Apparent Time) is 45º and 9 am is –45º.

hour circle

A great circle on the celestial sphere that passes through the celestial poles. It is orthogonal to the celestial equator.

hour line

The line on a dial plate indicating the shadow position at a particular time (includes fractional as well as whole hours). See Figure 1.

hour line angle

[X, HLA] the angle that an hour line on a dial plate makes with the noon line. The angle increases with time (i.e. positive for the p.m. hours). Thus, for a horizontal dial, the angle increases clockwise (hence the origin of the term) whereas for a vertical south-facing dial, it increases counter-clockwise . Beware, this convention is not used by all authors, and some define the angle with respect to the sub-style line.

hour point

A point on the dial plane indicating the crossing of the gnomon‘s shadow at a particular time. Hour points replace hour lines on dials where the shadow edge does not pass through the dial centre.

hour plane

The plane which, at any instant, contains the sun, the observer and the N celestial pole. The style and the appropriate hour line lie in the hour plane.

Hours (types of):
antique ~: same as unequal hours or seasonal hours.Babylonian ~: number of hours elapsed since sunrise, with 24 equal hours per day. The origin of the term is unclear, but may be related to the fact that the ancient Babylonians originated the base-60 counting system for angles etc. They are sometimes written as “horae ab ortu solis” or H. AB ORT.” on dials. See Equations for conversion from equal hours to Babylonian hours.biblical ~: same as unequal hours or seasonal hours.Bohemian ~ : same as Italian ~.Canonical ~

The seven times of the day (as opposed to time periods) used to define the services or divine offices in the medieval church. These offices were based on the sixth century Rule of St. Benedict. See Appendix IV for details.

An alternative definition of the term canonical hours, sometimes applied to lines on early Italian dials, is the system of putting equi-angular hour lines around the base of a horizontal gnomon on a vertical south dial.

common ~

The standard 2 x 12 equal hour system, also called German or French ~. In Latin “horae communes”, they are often labelled “kleine uhr” (small hours) on Nuremberg dials.

decimal ~

: an hour system with ten equal hours per day (as sometimes used by the Chinese and ancient Egyptians, and during the French Revolution).

equal ~

Any hour system where the length of an hour is independent of the date, and the same during daytime and night-time.

French ~

An early name for the equal hour system with 2 x 12 hours per day, beginning at midday and midnight. Sometimes written “Oltramontane”.

French revolution ~

The equal hours according to French Revolution time.

Great ~:

a term for any of the unequal hour systems. Often labelled “grosse uhr” on Nuremberg dials.

Greek ~

Same as Babylonian ~.

Italian ~

Are often used synonymously in modern works but there is some evidence in older works that Italian hours were counted from 30 minutes after sunset. See Equations for conversion from equal hours to Italian hours.

modern ~

The equal hours as used in modern time systems. They may occasionally be referred to as common, European, French, German or vulgar hours.

Nuremberg ~

A hybrid equal hour system. The daylight hours were measured using the Babylonian ~ system, starting with 1 at sunrise, while the night hours started with 1 at sunset and used the Italian ~ system. In Latin, “horae norimbergenses”.

octaval ~

A time system with the period of daylight divided into eight hours. Probably introduced by the Romans, circa 250AD. See Appendix III for the names of the daylight periods.

planetary ~

A planetary hour is the time needed for 15º
of the celestial equator to rise above the horizon, counting from sunrise. As there is always 180º of the celestial equator above the horizon, there are 12 planetary hours from sunrise to sunset but they are unequal not only from day to day but also from hour to hour. Note: this definition is based on the writings of Sacrobosco, but some authors use planetary hours simply as another form of seasonal hours, with the hours associated with the “planets”. See Appendix VIII for the symbols of the planets and this association. The Zonwvlak program uses the definition of the German scholar Joseph Drecker (1925) who defines a planetary hour as the time for 15º of the ecliptic to rise above the horizon (counting from sunrise) and are hence very irregular.

seasonal ~

A form of unequal hours, usually with 12 daytime and 12 night-time hours. Named from the fact that the length of an hour varies with the seasons.

temporal ~

or temporary ~: an unequal hour system with 12 hours from sunrise to sunset, and 12 hours (of a different duration) from sunset to sunrise.

unequal ~

An hour system where the duration of an hour depends on the date and is different from day-time to night (except at the equinoxes). The number of hours during day-time is usually 12, but may be 8 and just possibly 10 (e.g. on some mass dials). Counting of the daytime hours begins at sunrise.


~: i.e. foreign. Same as Italian ~.


End of Hours (types of)


house: (astrological) a segment of the celestial sphere. Several methods of dividing the sphere into segments exist, the most common of which produce the signs of the zodiac, and the Regiomontanus houses.hyperbola: a conic section, its most common use in dialling is the shape of the declination lines on a dial.

Pages: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Introduction    Dial types    Hours (types of)    Time (types of)    Illustration
Symbols    Equations    Biographies    Chronology    Sources    Appendices