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Introduction    Dial types    Hours (types of)    Time (types of)    Illustration
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: the point on the celestial sphere that is diametrically opposite the observer’s zenith.

nautical mile

A distance (6080 feet or 1853 metres) determined as 1 arc-minute of longitude at the equator.


(or night-time): the period of darkness between sunset and sunrise.


. These are known as the Little Dipper and Big Dipper, respectively, in the USA. Most nocturnals have inscriptions “GB” and “LB” on their scales. The term ~ can also be used as an adjective, meaning “of the night”.


A point which casts a shadow to indicate the time and/or 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. See Figure 1.

nodus height

[N, NH] the height (distance) of a nodus perpendicular to the dial plane. It is also the same as a vertical style height.

nomogram (

sometimes nomograph): a system of graphs showing relationships between three or more variables. From the Greek “nomos” (law).


A device similar to a vernier for interpolating readings on an angular scale, but using a large number of concentric scales rather than a single movable one. Named after the 16th century Portuguese mathematician Pedro Nõnes.


The time of the sun’s transit each day. Equivalently, the time that the Sun reaches its largest altitude for that day. Note that noon is specific to the observer’s location, unlike 12:00 o’clock with which it is often confused.

The word ~ originates from the Latin ‘nonus’ or ninth, indicating the ninth hour of the day counting from sunrise. By 1420 it meant the hour or ecclesiastical office of Nones, so noon gradually became associated with the beginning of this office.

noon cross

A cross shape often seen instead of XII on the noon line of dials. It can have many forms, many of which look like an Iron or Maltese cross. The nearest heraldic term is the cross patty.

noon gap

The gap in the hour scale of a dial to account for the finite thickness of the gnomon. It is positioned on the dial plate where the Sun is in the same plane as the gnomon, i.e. at noon for horizontal or direct S dials. A gnomon gap is occasionally seen on the sub-style of a declining dial. See Figure 1.

noon line

(on a dial): simply the hour line corresponding to noon, it is the most important line from which the others are usually calculated. It is the line which most often carries an analemma.

noon marker

A single mark or stone in the ground (or on a wall) set to show noon when crossed by the shadow of a convenient vertical; for example, a stick or edge of a wall. Sometimes also called a shepherd’s dial.


The intersection of the local meridian with the horizon, in the direction of the north celestial pole.

North Pole

The point on the Earth’s surface and its axis with a latitude of +90º
. It lies in the direction of the North celestial pole, from which the Earth is seen to rotate anti-clockwise.


The numerals on dials are usually either Arabic (the usual 0-9 used in English) or, especially on older dials, Roman numerals (I, II,..XII etc.). Note that it is common to find IIII in place of the later IV on some dials. A convention sometimes used on dials with more than one hour ring is to use Roman numerals for Local Apparent Time, and Arabic ones for civil time (often BST etc.). Many other forms of numerals (e.g. Chinese, Turkish) are used world-wide.


A small periodic (principal time constant of 18 years 220 days) oscillation of the rotational axis of the Earth about its mean position. Discovered by James Bradley (1693-1762), the third Astronomer Royal, in 1748. The disturbance of the idealised orbit of the Earth (as a two-body system) is due to the gravitational attraction of the Moon and, to a lesser extent, the other planets. Nutation introduces small changes, typically 7 arcseconds annually, to the precession of the equinoxes.

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