Glossary of technical terms

The science of sundials makes use of many technical terms related to astronomy, celestial mechanics, observation instruments and more widely about measurement of time. This page gives the definition of the most common terms. This glossary is also available at the end of the user manual of Shadows.

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- A -

Abscissa – Name given to the horizontal component of a Cartesian coordinate. The abscissa is usually written x.

Alidade – Ruler equipped with pointers, that can be rotated to measure angles. Alidades are usually found at the back of astrolabes.

Almucantarat – Arc drawn on an astrolabe, to mark the altitude of a star. Sometimes called Almucantar.

Altitude – Angle measured vertically from the horizon up (or down) to the celestial object, yielding values between -90° and 90°. The altitude is part of the horizontal coordinates system (with the azimuth). At sunrise or sunset, the altitude of the Sun is 0°.

Analemma – Name given to the figure-8 -shaped curve drawn sometimes at the position of an hour line (usually the noon line).

Analemmatic – Used to qualify a sundial of elliptical shape for which the style must be moved (on a line) to a position depending on the date. Analemmatic sundials are usually big and drawn on the ground so a person’s body is used as a style.

Anomalistic year – Time between two consecutive transits of the Earth at its perihelion; it equals 365 days 6 h 13 m 53 s.

Arcdegree – Unit used to measure angles. It corresponds to 1/360 of a circumference. A degree contains 60 arcminutes of 60 arcseconds each.

Arcminute – Unit used for angle measurement. There are sixty arcminutes in a degree. One arcminute contains sixty arcseconds.

Arcsecond – Unit used for angle measurement. There are sixty arcseconds in an arcminute.

Armillary sphere – Sphere reproducing the Earth, inclined parallel to the polar axis. Its equator is graduated to give the hours by the shadow cast by the pole axis of the sphere. It is a kind of equatorial sundial.

Astronomical twilight - Instant when Sun reaches 18° below the horizon, which corresponds to totally dark skies.

Atmospheric Refraction – Deviation of light rays due to variations in atmosphere’s ability to bend light. The effect is often most visible when the celestial body is close to the horizon. For example, the image of the Sun can be visible above the horizon while the Sun is below. The effect is usually around 36 arcseconds on the horizon.

Azimuth – Angle between a vertical plane passing through a given point in the sky, and the meridian plane of the location. The azimuth is counted positively from the south to the west.

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- B -

Babylonian hours – The Babylonian hours were counted from the sunrise (with constant duration for an hour, contrary to the temporary hours). Babylonian hours were useful to know the time since the sunrise. Those hours were used by the Chaldeans, the Egyptians, the Persians, the Syrians and the Greeks.

Brachiolus - Articulted arm used on universal astrolabes (Saphae Arzachelis) to point a particular coordinate.

Butterfield - Type of portable horizontal sundial, equipped with a foldable style that can be adjusted in latitude. It has several sets of hour lines, in concentrical bands to allow the reading of time at several latitudes. These sundials are often made in precious material (such as silver).

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- C -

Celestial equator – Projection of the Earth's equator on the celestial sphere.

Circle (great circle, small circle) – A great circle is a circle on a sphere whose diameter is the same as the diameter of the sphere. Meridians and the equator are great circles of the Earth. A small circle is a circle on the sphere whose diameter is smaller than the diameter of the sphere. A parallel at a latitude greater than 0° is a small circle.

Civil time – Mean time of the reference meridian, shifted if needed by one hour if the daylight saving time is used. Civil time is the clock time.

Colatitude – Is the complementary angle of the latitude, taken from the pole. The co-latitude is (in degrees) equal to 90° - latitude.

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- D -

Declination – Angular distance of a point on the celestial sphere to the equatorial plane, with the convention of positive (0° to 90°) toward the north, negative toward the south.

Declination arc – Trajectory of the shadow's endpoint on the sundial, during the day. This curve is usually an hyperbola, except for the equinox where it is a straight line.

Declination lines – Lines indicating the date using the end of the shadow of the style. The lines are generally drawn for every Zodiac change (corresponding to the declinations 0°, +/-11°29', +/-20°20' and +/- 23°26').

Diptych sundial - Sundial formed of two articulated parts, one horizontal, one vertical, with a common style realized with a thread. It is a portable sundial that can be kept in a pocket.

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- E -

Ecliptic – [1] Plane of the Earth's orbit around the sun. [2] Large circle on the celestial sphere, where the Sun moves in its apparent motion.

Ecliptic longitude - Angle counted on the Earth orbit between the position of Earth and the Vernal point. It is separated in sectors of 30° that define astronomical zodiac signs.

Equation of time – The difference between solar time and mean time. This equation varies during the year and its value can reach ±16 minutes.

Equinox – One of the two days in a year, when the sun is crossing the celestial equator. At the equinoxes, the day and the night have the same duration.

Equatorial coordinates – Right ascension and declination of a star. Coordinate system anchored on the ecliptic. The origin of the right ascension is the vernal point; it is counted from 0 to 24 hours anticlockwise. The declination is counted from 0 to +/- 90° around the equator.

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- G -

Geographic north – Direction included in the meridian plane, pointing towards the geographic north celestial pole. It's the direction used in gnomonics (not the magnetic north).

Gnomon – Primitive sundial made of a simple rod whose shadow is cast on the ground. Today, this term is used to name the style planted perpendicularly to the sundial table.

Gnomonics – The science of sundials.

Greenwich – English city located near London.

Greenwich meridian – Meridian crossing the Greenwich observatory, used as the reference meridian to measure longitudes on Earth.

Greenwich Mean Time (GMT) – Time equals the civil time of Greenwich shifted by 12 hours. This time should not be used to call the standard time of Great Britain, which has been replaced by the Universal Time.

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- H -

Hemisphere – Half of a sphere, separated by an equator plane. On Earth, there is the northern (or boreal) hemisphere and the southern (or austral) hemisphere.

Horizontal coordinates – Azimuth and height of a star. This coordinate system is local to the observer. The azimuth is counted from 0 to 180° starting on the local meridian. The altitude is counted from 0 to 90° above the horizon.

Hour Angle – Angle between the plane of the great circle through an object and the meridian of the location. It is calculated in a retrograde sense, sometimes in degrees, but more often in hours, from 0 to 24 hours, or from -12 to +12 hours. The hour angle (H) and the right ascension (α) are related to each other and to the hour angle of the Vernal Equinox (T) by the following formula: T = α + H.

Hour coordinates – Hour angle and declination. This system is intermediate between the horizontal system and the equatorial system (using it simplifies the coordinate conversion). The origin of the hour angle is the local meridian.

Hour line – The lines that allow you to read the hour using the shadow of the style.

Hypotenuse – Longest side of a right triangle. The side opposite the right-angle.

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- I -

Italic hours – The italic hours were counted from the sunset of the previous day. Those hours were in use until the end of the 18th century in Italy.

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- L -

Latitude – Angle formed, in a given place, by the local vertical and the equatorial plane. The latitude is counted from the Earth equator, positively towards the north (from 0° to 90°), and negatively towards the south.

Leap year – Year that contains 366 days, obtained by the insertion of an additional day on February 29th, every four years. This procedure provides a mean year duration of 365.25 days which is close to the anomalistic year duration. Years for which the millennial value is dividable by 4 are leap years, except those which are not multiples of 400! For example: 1968, 1996, 2000 are leap years but 1900 and 2003 are not.

Longitude – Angle formed by the local meridian and the Greenwich meridian, counted from 0° to 180°, starting at Greenwich, positively towards the West.

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- M -

Magnetic north – Direction given by the compass. It varies from one place to another due to variations in the local magnetic field of the Earth. This direction cannot be used in gnomonics to obtain accurate results.

Mean time – It's the solar time corrected by the equation of time. From mean time, you can add the longitude correction to obtain the civil time.

Meridian – [1] Set of all locations having the same longitude on Earth. [2] Plane defined by the local vertical line (plumb line) and the axis of rotation of the Earth. (meridian plane). [3] Half circle of the celestial sphere passing through the pole and the local zenith.

Motto - Phrase written on sundials, expressing philosophical ideas, usually about the passing of time and the condition of humans. Mottoes are historically in Latin but can be in any language or dialect.

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- N -

Nautical twilight - Instant when Sun reaches 12° below the horizon, and when it becomes difficult to distinguish the horizon line at sea.

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- O -

Obliquity of the ecliptic - Angle between the Earth's rotation axis and the Aerth's orbit rotation axis. Its value is 23° 26'.

Ordinate – Name given to the vertical component of a Cartesian coordinate. It is often written y.

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- P -

Polar style – Style that points towards the celestial pole. It is represented by the hypotenuse in a triangular style. The hour lines converge at the foot of the polar style which is labeled point B, in this program. For some sundials, this point is projected far from the foot of the perpendicular style.

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- R -

Radian – Unit used in the angle measurement. There are 2π radians in the circumference of a circle. One radian equals 180/π degrees.

Revolutionary time – Decimal hour containing 100 minutes of 100 seconds. A day contains 10 revolutionary hours. This measure of time was established in 1790 by the French revolutionary council, but has never really been used. It was soon made obsolete.

Right ascension – Angle formed by the plane passing through a given point of the celestial equator and the vernal point. It is counted from 0° to 360° or more likely in hours from 0 h to 24 h.

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- S -

Sidereal Time – Hour angle of the Vernal Equinox (T). It is used to define the relative position of the equatorial and hour coordinates.

Solar hours - Simplest indication given by a sundial, obtained from the hour angle of the Sun augmented by 12 hours. To obtain civil hours, one must add the longitude correction, the equation of time and optionally the daylight savings.

Solstice – One of the two times in the year when the sun is farthest away from the equator. In the northern hemisphere, the summer solstice is on June 21st or 22nd, the winter solstice is on December 21st or 22nd. In the southern hemisphere, it is the opposite.

Solstice arc – Declination arc drawn on the solstices. The solstice arcs limit the shadow's extension on the dial.

Stereographic projection - Projection of the surface of a sphere onto a plane, seen from a particular point. It is used on a planispheric astrolabe to project the celestial sphere onto the equatorial plane, seen from the South pole.

Style – A rod inserted in the table of the dial. Its shadow gives the hour. The style (polar style) is parallel to the polar axis of the Earth.

Sub-style line – Line joining the points A and B (in the drawing), between the attach points of the polar style and the perpendicular style.

Sundial – Surface with graduations for the hours during the day, on which the shadow of a style is projected by the sun to indicate the hour.

Sundial declination – Angle measured between the normal of a wall and the local meridian plane. It is used for example on a vertical declining sundial. The declination is counted positive towards the west, from the south.

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- T -

Temporary hours – Antique hours counting 12 hours between sunrise and sunset. The duration of one hour counted in this system is varying between 40 and 80 minutes throughout the year. These hours are sometimes called Planetary hours.

Time Zone – Strip of 15° of longitude stretching from the north pole to the south pole, which allows the globe to be split into 24 segments. Each zone is centered on a meridian at a multiple of 15°. The meridian of origin is that of Greenwich, which defines Universal Time. This zone occupies the longitudes +7.5° to -7.5°. Each country uses, in principle, the hour of the nearest zone in longitude, but this is not the case in Europe where most countries (including France) have adopted the zone UT + 1h. Each time zone is one hour apart. However, some countries have adopted an offset that is not an exact hour, like the center of Australia which is at UT + 9h 30m or Nepal, at UT + 5h 45m.

Tropic – Each of the two parallels at latitude 23°27’ N and S where the sun reaches the zenith at a solstice. They define a band where the sun can pass the zenith and be in the north direction (northern hemisphere) or in the south direction (southern hemisphere). The northern Tropic is called the Tropic of Cancer and the southern tropic is called Tropic of Capricorn.

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- U -

Universal Time – Universal Time (UT): Standard Time of Greenwich.

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- V -

Vernal point – Intersection between the ecliptic and the celestial equator. The vernal point corresponds to the direction of the Sun when its declination crosses 0° upward (it is then the beginning of spring). The vernal point is noted ν.

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- Z -

Zenith – Point of the celestial sphere situated exactly above the local vertical ascending direction.

Zenithal distance – Complementary angle of the altitude, equal to 90°-altitude. The zenithal distance is counted from the zenith point above the observer.

Zodiac – A set of celestial constellations located on the ecliptic, were the sun moves in its apparent motion. This band is divided into twelve constellations (the astronomers include a thirteenth Zodiac constellation). The sun moves from one constellation of the Zodiac to another, when its declination reaches certain values (0°, ±11°29', ±20°20' and ±23°27')

Zodiac arc – Declination arc drawn for a date corresponding to a Zodiac sign change, which is around the 21st of each month. There are seven Zodiac signs, among which one is for the equinox, two for the solstices.