A nocturnal is an instrument used to determine the local time based on the position of a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

relative to the

pole star A pole star is a visible star that is approximately aligned with the axis of rotation of an astronomical body; that is, a star whose apparent position is close to one of the celestial poles. On Earth, a pole star would lie directly overhead when ...

. As a result of the

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in progra ...

, any fixed star makes a full revolution around the pole star in 23 hours and 56 minutes and therefore can be used as an hour hand. The 4-minute difference between the

solar day A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal day, which is ...

and

sidereal day Sidereal time ("sidereal" pronounced ) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal t ...

requires a correction of this giant clock based on the date of observation, and nocturnal helps to apply this correction. Sometimes called a ''horologium nocturnum'' (time instrument for night) or ''nocturlabe'' (in French and occasionally used by English writers), it is related to the

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

and

sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

. Knowing the time is important in piloting for calculating

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

s and some nocturnals incorporate tide charts for important ports.

History

The actual ''horologium nocturnum'', a precursor for the later nocturnal instruments, was invented in the 9th century by Pacificus of Verona. Even if the nightly course of the stars has been known since antiquity, mentions of a dedicated instrument for its measurement are not found before the Middle Ages. The earliest image presenting the use of a nocturnal is in a manuscript dated from the 12th century.Farre i Olive E., (1996): ''La Sphaera Horarum Noctis de Ramon Llull'', "La Busca de Paper" n. 22, Primavera, p. 3-12 (edicion bilingue) Raymond Lull repeatedly described the use of a ''sphaera horarum noctis'' or ''astrolabium nocturnum''.Raimon Lull, ''Liber Principorum medicinae'' in ''Opera omnia'', vol.1, Chap. 36, par.30, Mainz, 1721; Earlier Lull mentions the nocturnal in his ''Nova geometria'' (1299). With Martín Cortés de Albacar's book ''Arte de Navegar'', published in 1551 the name and the instrument gained a larger popularity.Harriet Wynter and Anthony Turner, ''Scientific Instruments'', Studio Vista, 1975, It was described also c. 1530 by Petrus Apianus in his ''Cosmographicus Liber'', republished later by

Gemma Frisius Gemma Frisius (; born Jemme Reinerszoon; December 9, 1508 – May 25, 1555) was a Dutch physician, mathematician, cartographer, philosopher, and instrument maker. He created important globes, improved the mathematical instruments of his day ...

with a widely circulated illustration of the instrument while being used by an observer.

Construction

Nocturnals have been most commonly constructed of

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

brass Brass is an alloy of copper and zinc, in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic and chemical properties, but copper typically has the larger proportion, generally copper and zinc. I ...

. A nocturnal will have an outer disc marked with the months of the year, and an inner disc marked with hours (and perhaps half hours, or quarter hours on the largest instruments) as well as locations for one or more reference stars. It will also have a pointer rotating on the same axis as the discs, sometimes extended beyond the rim. The axis, or pivot point, must be such that a star can be sighted through it; usually a hollow rivet is used. Since the instrument is used at night, markings may be exaggerated or raised. Often the inner disc has a diagram of the necessary constellations and stars, to aid in locating them.

Usage

A nocturnal is a simple

analog computer An analog computer or analogue computer is a type of computation machine (computer) that uses physical phenomena such as Electrical network, electrical, Mechanics, mechanical, or Hydraulics, hydraulic quantities behaving according to the math ...

, made of two or more dials, that will provide the local time based on the time of year and a sighting of

Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an ...

, the North Star, and one or more other stars. In the

northern hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

, all stars will appear to rotate about the North Star during the night, and their positions, like the progress of the

sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, can be used to determine the time. The positions of the stars will change based on the time of year. The most commonly used reference stars are the pointer stars from the

Big Dipper The Big Dipper (American English, US, Canadian English, Canada) or the Plough (British English, UK, Hiberno-English, Ireland) is an asterism (astronomy), asterism consisting of seven bright stars of the constellation Ursa Major; six of them ar ...

(

Ursa Major Ursa Major, also known as the Great Bear, is a constellation in the Northern Sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear", referring to and contrasting it with nearby Ursa M ...

) or Kochab from the Little Dipper (Ursa Minor). The star Schedar in

Cassiopeia Cassiopeia or Cassiopea may refer to: Greek mythology * Cassiopeia (mother of Andromeda), queen of Aethiopia and mother of Andromeda * Cassiopeia (wife of Phoenix), wife of Phoenix, king of Phoenicia * Cassiopeia, wife of Epaphus, king of Egy ...

may also be used, since it is on the opposite side of the sky from Ursa Major. The inner disc is rotated so that the mark for the chosen reference star points to the current date on the outer disc. The north star is sighted through the center of the device, and the pointer arm is rotated to point at the chosen reference star. The intersection of the pointer arm with the hour markings on the inner disc indicates the time. The instrument must be held upright, and should have a handle or similar hint as to which direction is down. It is not possible to convert the local time to a standard time such as UTC without accurate knowledge of the observer's

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

. Similarly, it is not possible to determine longitude unless the observer also knows the standard time from a chronometer.

References

Sources

External links