Terminology commonly used in Gnomonics
ALTITUDE SUNDIALS indicate the time based on the date and altitude of the sun on the horizon.
ANALEMMA OF THE MEAN TIME curved hour line that has this name for its resemblance to the lemniscate of Bernoulli; automatically corrects the reading of true time by providing the mean time.
ANALEMMATIC SUNDIALS, particular types of azimuth clocks that require placement of the style by date.
ASTRONOMICAL HOURS, an hour system that includes apparent local time, apparent time of the STZ, or apparent time referring to a particular location. The astronomical hour system includes all kinds of hours that use of the hour angle with start of count from a predetermined meridian.
AZIMUTH, it is the spherical angle described by the sun, measured from the south, having the zenith as vertex.
AZIMUTAL SUNDIALS, indicate the time by exploiting the azimuth of the sun.
BABYLONIC OR BABYLONIAN HOURS. A system similar to the Italic one but with the beginning of the hourly clock calculation from dawn (now zero).
CANONICAL HOURS, a system used in monasteries that sometimes used temporary time division, sometimes more simply a system of convergent lines (monastic hours) but indicated by different names such as: the Terce, Sext, the Compline etc.
CENTRE, meeting point of the astronomical hours where the style is applied.
EQUATION OF TIME, the difference between true (apparent) time and mean time. It mainly depends on the inclination of the Earth's axis on the plane of the ecliptic and the eccentricity of the earth's orbit. On the more sophisticated sundials are represented by an eight-figure curve, known as "analemma of the mean time", which automatically corrects the reading of true time by providing mean time.
FALSE style, ambiguous term: gnomon emerging from a point other than the centre or foot and ending on the tip of the style but also the method of detecting the declination of a wall.
FOOT, orthogonal projection of the tip of the style on the dial plate.
FRENCH HOURS, astronomical hour system, introduced in the 18th century instead of Italic system. The 24th hour is midnight, 12 noon. The time given by a sundial to French hours is the true local time.
GNOMON, a generic term that indicates an object whose shadow indicates time and / or date.
GNOMONIC DECLINATION, orientation of a vertical plane or reclining dial with respect to the east - west line. Azimuth of the normal on the dial surface. A dial facing south has 0 ° declination; facing east has declination - 90 °; facing west 90 °.
GNOMONIC POINT is that point of the style or the gnomon whose shadow indicates the time and date. It can be represented by a nodus or a gnomonic hole or the style tip.
GNOMONICS, science of sundials and solar clocks.
HEIGHT of the style or elevation of the style, angle between the style and its projection on the surface of the dial.
HORIZONTAL LINE is a horizontal line that indicates the line of the horizon on the dial by separating the daytime part of the dial from the night.
HOUR ANGLE is the spherical angle described by the sun around the pole, measured from the south.
HOUR INVERSION, illusory phenomenon, occurring in vertical quadrants with a declination greater than ± 90 ° or reclining dials whose inclination exceeds the value of a certain latitude and / or declination limit value. Looking at two vertical quadrants at the same time, the former with a declination of less than 90 ° and the latter declining beyond 90 °, it is noticed that the movements of the shadow of the styles take place in opposite directions and that the style of the second quadrant has the tip pointing upwards.
INCLINATION (zenith), reclining, angle between the dial and the lead wire. It is positive if the dial is farther from the face than from the feet of an observer post in front.
INCLINATION, see zenith distance, used to determine the inclination of a plane especially in France. When it is null the plan is horizontal.
ITALIC or ITALIAN HOURS, a time division system measuring from sunset or, in some cases, half an hour after sunset (bell tower clocks). The hours are 24: the hour 24 coincides with the sunset. The hour 23 indicates one hour to sunset and so on.
MEAN TIME is the time you get by correcting the true apparent local time with the time equation.
MERIDIAN Line, a sundial line that exploits the shadow of a gnomon to indicate the apparent noon. It is usually present in any type of solar clock.
NODUS is, in general, the seat of the gnomonic point. Its shadow indicates the daily path of the sun and then the time, date, sunset, etc.
NOON, instant when the sun goes to the meridian dividing the clear day into two equal parts. It is present in the sundials with a line that is vertical in the flat and cylindrical vertical sundials. It does not, however, coincide with the highest slope in declining, reclining dial planes. The noon line is an hour line that goes to the centre.
PERPENDICULAR STYLE (PILLAR) gnomon emerging from the foot and ending on the tip of the style.
PLANETARY HOURS, an ancient hour system of profane and astrological tradition that divides time in a similar way to the temporal one but also indicates the influence of the planets.
SIDEREAL TIME, time system that takes as reference the passage of the vernal point to the meridian.
SUBSTYLE segment between the foot and centre of the dial.
SUBSTYLE DISTANCE, tabular angle, is the angle between the (max) slope and the sub-style line.
SUBSTYLE HOUR. Hour line that passes through the foot of the dial.
SUBSTILE LINE, hour line of orthogonal projection of the style on the dial plate. It passes through the foot and the centre.
STANDARD TIME, time used after the introduction of the time zone convention. Civil time and even legal time. It is strictly constant and is determined by a fictitious sun moving smoothly on the celestial equator, in contrast to the true time that is measured on the real sun that moves on the ecliptic.
STYLE or POLAR STYLE, dial element aligned with the Earth's axis projecting its shadow on the dial, determining hours, daytime lines, and so on.
TEMPORARY OR TEMPORARY HOURS. Time-division system dating back to Assyrians and Babylonians. Hours are the twelfth part of the light day so in summer they are longer than in the winter. At night is the opposite.
TIME READERS that take advantage of the direct clock angle are usually the normal wall dials, the flat ones, the equatorial ones that use, in fact, the sun hour angle.
TRUE (APPARENT) TIME, astronomical time system that indicates the true local time. When the sun meets the local meridian is noon (12 o'clock).
ZENITH DISTANCE, the angle between the normal to the dial plane and the zenith.