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C

Candlemas: the festival of the purification of the Virgin Mary, on 2nd February. It also corresponds to the
Celtic festival of Imbolic, and is a cross-quarter day.

calendar

A system for counting days and defining the date.

Campbell-Stokes

See sunshine recorder.

Cancer

See tropics.

canting out

See wedging out.

Capricorn

See tropics.

cardinal point

With the horizon. Note that the Latin terms are Septentrio, Meridies, Oriens and Occidens, so that a compass rose on a mediaeval dial simply identifying “S” is ambi
guous.

cartesian co-ordinates

See co-ordinates

celestial equator

The intersection of the extended plane of the Earth’s equator with the celestial sphere.

celestial latitude

See ecliptic latitude.

celestial longitude

See ecliptic longitude.

celestial pole

The points on the celestial sphere where it meets the Earth’s axis. The stars appear to rotate around these poles.

celestial sphere

An imaginary sphere, arbitrarily large and co-centred with the Earth, on which all the stars appear to be fixed.

centre (of a dial)

The point where all the hour lines, and a polar-pointing style, meet. This point does not always exist (e.g. on polar dial and direct E or W dials, the lines meet at infinity). In simple horizontal or vertical dials, this point coincides with the root of a (thin) gnomon. In the case of a thick gnomon having two styles, there are two centres to the dial. The centre is often, but not necessarily, the origin of the co-ordinate system used to describe the dial. See Figure 1.

chapter ring

The ring on a dial face carrying the hour numerals. The term is more widely used for clocks, but it also finds use, for example, on dials with several separate rings for different locations.

chilindrum

See Dial (types of); cylinder ~.

civil time

See Time (types of) civil~.

clinometer

An instrument for measuring the inclination or slope of a surface. Also called an inclinometer.

cloisonné

A term sometimes used to describe the technique of making metal dials by deeply etching the lines and numerals and then filling them with coloured material. It derives from the jewellery method of separating enamels into shallow compartments with metal edges.

co-latitude

Equals 90° – latitude.

compass bowl

A bowl sunk into the dial plate of a (portable) horizontal dial to house a magnetic compass.

compass rose

A drawing of the compass directions, showing as a bare minimum the cardinal points, but more usually eight, sixteen or thirty-two points.

compendium

Normally used to describe a collection of scientific instruments in one case. Also, Compendium: the journal of the NASS.

conic section

Any of the range of geometric curves produced by the intersection of plane with a cone (i.e. circles, ellipses, parabolas and hyperbolas).

coordinates: a system of measurements used to describe any point in two or three dimensions.


 

Co-ordinate Systems:

Cartesian ~ [x,y,z]

in which the axes are mutually perpendicular, are normally used for positions of points within a dial. For simple horizontal dials the preferred axes have x increasing to the E of the dial plane, y increasing to the N of the dial plane and z (in 3-D only) increasing. perpendicularly to the dial plane (upwards). For vertical and other plane dials, x increases to the left, y increases downwards, and z perpendicular to the plane in the direction towards the observer. The origin of the system must be defined explicitly. Note that these definitions produce a conventional right-handed co-ordinate system, and are also those used by the Zonwvlak programs.

ecliptic ~

[ß , ?, e ] or [ELAT, ELON] the system of ecliptic (or celestial) latitude and longitude, defined with respect to the ecliptic and the celestial poles. Ecliptic co-ordinates predominated in Western astronomy until the Renaissance but, with the advent of national nautical almanacs, the equatorial system, more suited to observation and navigation, gained ascendancy.

Celestial co-ordinates

Figure 3.

    Celestial co-ordinates seen by an observer in mid-northern latitudes.

equatorial ~

: [a, d] or [RA, DEC] is the most common astronomical co-ordinate system and is defined by the celestial equator and poles. The right ascension and declination are directly analogous to terrestrial latitude and longitude.

Equatorial co-ordinates

Figure 4.

    The equatorial system of celestial co-ordinates, showing the right ascension (RA) and declination (dec) of a star.

galactic ~

: is used for studying the structure of the galaxy. It is unlikely to be encountered in dialling.

geographic ~

[f, ? ] or [Lat, LON] the standard method of determining any location on the Earth’s globe, using latitude and longitude.

horizon ~ system

[a,A] or [ALT,AZ] the simplest celestial co-ordinate system, it is based on altitude and azimuth. It is fundamental in navigation as well as in terrestrial surveying. However, for specifying the position of the Sun or other celestial bodies, other co-ordinate systems fixed with respect to the celestial sphere are far more suitable.

Ordnance Survey co-ordinates

Of the datum point at the bottom left of the map. Note that the OS maps on which the co-ordinates are based use the transverse Mercator projection, with a projection origin at 49º
N; 2° W.

polar ~

:[r, ?] an angle-based co-ordinate systemal sometimes used for defining points on a dial plane, where r is the distance from the origin and
? is measured anti-clockwise from the S. Note: navigators also make use of polar co-ordinates and usually define them as (?, r).

Ptolemaic co-ordinates

[hec, hor]: an angular co-ordinate system loosely based on the geometry of Ptolemy.

terrestrial :

See geographic ~.

 

End of Co-ordinate systems

cross: for a discussion of the cross sometimes seen on the noon line of a dial, see noon cross and cross patty.

cross-quarter days: days which are (approximately) midway between the Quarter days, hence dividing the year into eight parts. They are occasionally used instead of the zodiac signs for declination lines on dials, and have become adopted as modern celebrations or holidays. See Appendix XII for their names and dates.

cross patty{c. pattée or c. formée}: an heraldic term for a form of square cross shape (cross patty ) sometimes seen instead of XII on the noon line of dials. It is perhaps the most common of the noon crosses.

cross-staff: a simple instrument for determining the altitude of a celestial body. A cross piece or transom is moved along a staff, calibrated with a cotangent scale, and sighted by eye against the body and the horizon. Old illustrations often show a ~ with three transoms fitted but, in use, only one would be used at a time. Also called a fore-staff or Jacob’s staff.

culmination (of the sun): to lie on the meridian or, in more general language, to reach its highest point. Equivalent to the superior transit. From the Latin “culmen”, meaning summit.

cursor: a part of a mathematical instrument which slides backwards and forwards over a scale.

cusp: (astrological) the initial point of an astrological house or sign.

cycloid: (pron. si-cloy-d) a geometric curve which is traced out by a point on the circumference of a circular disk rolling (without slipping) along a straight line.

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