altitude (of the sun): the angular distance of the (centre of) the sun’s disk above the observer’s horizon.

altitude dial: any dial which uses the sun’s altitude, rather than its azimuth, for indicating the time. Usually does not need to be aligned N-S. Examples are analemmatic dials, ring dials, and shepherds’ dials.

axis (of the Earth’s rotation) or polar axis: the line running through the true North and South poles about which the Earth rotates.

azimuth (of the sun): the angle of the sun, measured in the horizontal plane and from true south. Angles to the west are positive, those to the east, negative. Thus due west is 90°, north is ±180°, east –90°.

centre (of a dial): the point where all the hour lines, and a polar-pointing style, meet. 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.

declination (of the sun): the angular distance of the Sun above or below the celestial equator. Its value follows an annual sine wave like curve, varying between 0º at the equinoxes and ±23.4º (approx.) at the solstices. It has positive values when the Sun is above the celestial equator (summer in the Northern hemisphere) and negative when below.

dial face (or dial plate): the physical surface on which the hour lines and furniture lie. It (usually) supports the gnomon.

ecliptic (plane): (pron. e-clip-tic) the plane that the Earth’s orbit traces during a year. The orbits of the Moon and the planets are also close to this plane. It is a great circle on the celestial sphere.

Equation of Time: the time difference between Local Apparent Time (apparent solar time) and mean solar time at the same location. Its value varies between extremes of about +14 minutes in February and –16 minutes in October. It arises because of the elliptical orbit of the Earth, and the tilt of the Earth’s axis to the ecliptic. The preferred usage by diallists is:

mean solar time = apparent solar time + EoT

but this sign convention is by no means universal and the opposite sign is used in modern almanacs. Irrespective of the sign convention adopted, sundials will always appear slow compared to mean time in February, and fast in October/November.

equator: the great circle of the Earth (or other celestial body) which is equidistant from the poles. It has, by definition, a latitude of 0º.

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

equatorial plane: the plane through the Earth defined by the equator.

equinoxes: (vernal or spring, autumnal or fall) literally "equal nights" i.e. equal amounts of daylight and night-time. The Sun’s declination at the equinoxes is 0º. The vernal equinox is around 20-21 March, the autumnal equinox is around 22-23 September.

furniture: all features on a dial face other than the hour lines and their numerals are referred to as dial furniture. This may include declination lines and curves, a compass rose, Equation of Time  graphs or tables, mottoes etc. Other common furniture includes: date, maker’s and/or benefactor’s name, coats of arms, and latitude and (rarer) longitude.

gnomon: (pron. no-mon) the physical structure of a sundial which casts the shadow (from the Greek for "indicator"). The gnomon today is most-often polar pointing (sometimes described as an "axial gnomon"), although it may also be horizontal or vertical. The distinction between gnomon and style made (and encouraged in this Glossary) in modern dialling literature is not the one used in early works, and the two words are still sometimes used interchangably.

horizontal dial: the common or garden sundial with a horizontal dial face and polar-pointing gnomon.

hour angle: Local Apparent Time expressed as the angular position of the Sun in its daily track. Measured from noon, it increases by 15º per hour with increasing time (i.e., morning hours are negative). Beware, this convention is not universal.

hour line: the line on a dial face indicating the shadow position at a particular time (includes fractional as well as whole hours).

latitude: The angular position of a place north or south of the equator. Positive values in the Northern hemisphere, negative in the South (i.e., the South Pole = -90º).

Local Apparent Time: solar time, as derived from the real Sun at any particular location. It is the hour angle of the Sun + 12 hours. Some authors (non-UK) may refer to it a Local True Time.

longitude: the angular location of a place on the Earth’s surface measured east or west of the Prime meridian though Greenwich, England. Longitudes W are positive, E are negative.

Mean Solar Time: the authoritative (by the National Physical Laboratory) definition is: a measure of time based conceptually on the diurnal motion of the fictitious mean Sun, under the assumption that the Earth's rate of rotation is constant.

nodus: a point which casts a shadow to indicate the time and/or (more often) the date on a dial face. It may take the form of a small sphere or a notch on a polar-pointing gnomon, or it may be the tip of a gnomon with an arbitrary (usually horizontal or vertical) orientation.

noon, solar : the time when the sun is due south of the observer's location. At solar noon the sun is at it's highest altitude, or angular distance above the observer's horizon. Not to be confused with 12:00 standard time.

noon gap (or gnomon gap or split noon ): the gap in the hour scale of a dial to account for the finite thickness of the gnomon. It is positioned on the dial face where the Sun is in the same plane as the gnomon, i.e. at noon for horizontal or direct S dials.

orbit (of the Earth): the path of the Earth around the sun. For dialling purposes, this is taken as elliptical, with a very small eccentricity, i.e., it ignores the small perturbations due to the effects of the Moon and other planets.

poles (N and S of the Earth): the locations on the Earth’s sphere with latitudes of +90º (N) and –90º (S).

pole, celestial: the points where the Earth’s axis meet the celestial sphere. The stars appear to rotate around these poles.

solar time: the same as Local Apparent Time.

solstices: (Summer, Winter) literally, "Sun stands still". In the Northern hemisphere, they represent the beginning of summer (on or around 21 June) and the beginning of winter (on or around 21 December). They are the days with the shortest and longest night-times and correspond to the extreme values of declination.

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

style {stile}: the line in space which generates the shadow edge used to indicate the time on the dial face. Note that a gnomon with finite thickness will have two styles (one along each of the upper edges) which will each be operational for parts of every day. If the gnomon is in the form of a long rod, the style will be the virtual line running along the centre of the rod and the dial is read by estimating the centre of the shadow.

style height: of a polar style is the angle that the style makes with the sub-style line. Note that this is an unusual use of the word "height", and style angle could be regarded as a better term.

sub-style (line): the line lying in the dial plane which is perpendicularly below (or behind for a vertical dial) the style.

vertical dial: any dial in which the dial face is vertical.