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

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 in the night sky. Sometimes called a "horologium nocturnum" (time instrument for night) or nocturlabe (in French and occasionally used by English writers), it is closely related to the sun dial

Sun Dial

Sun Dial is a British space rock band formed in 1990 by Gary Ramon.-History:The precursor to Sun Dial was Ramon's the Modern Art, formed in the mid-'80s with a loose lineup that never played gigs but did see the release of two studio albums...

. A nocturnal is typically a navigational instrument. Knowing the time is important in piloting

Pilotage

Pilotage is the use of fixed visual references on the ground or sea by means of sight or radar to guide oneself to a destination, sometimes with the help of a map or nautical chart. People use pilotage for activities such as guiding vessels and aircraft, hiking and Scuba diving...

 for calculating tides and some nocturnals incorporate tide charts for important ports.

The nocturnal was first mentioned by Martín Cortés de Albacar

Martín Cortés de Albacar

Martín Cortés de Albacar was a Spanish cosmographer. In 1551 he published the standard navigitional textbook Arte de navigar Cortés was born in Bujaraloz, province of Zaragoza, Aragon...

 in his Arte de Navegar, published in 1551.

The first known description of the instrument was done by Ramon Llull

Ramon Llull

Ramon Llull was a Majorcan writer and philosopher, logician and tertiary Franciscan. He wrote the first major work of Catalan literature. Recently-surfaced manuscripts show him to have anticipated by several centuries prominent work on elections theory...

 by the end of the thirteenth century in his book upon the names of and .

Construction

Nocturnals have been most commonly constructed of wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 or brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

.

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) as well as locations for one or more reference stars. It will also have a pointer rotating on the same axis as the discs. 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

In the northern hemisphere

Northern Hemisphere

The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

, all stars will appear to rotate about the North Star (known as Polaris) during the night, and their positions, like the progress of the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, can be used to determine the time. The positions of the stars will change based on the time of year. A nocturnal is a simple analog computer

Analog computer

An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...

, made of several dials, that will provide the time of day based on a time of year and a sighting of Polaris

Polaris

Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

 and some other common star.

The most commonly used reference stars are the pointer stars from the Big Dipper

Big Dipper

The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...

 (Ursa Major

Ursa Major

Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...

) or Kochab from the Little Dipper (Ursa Minor

Ursa Minor

Ursa Minor , also known as the Little Bear, is a constellation in the northern sky. Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, whence the name Little Dipper...

). The star Shedar in Cassiopeia

Cassiopeia (constellation)

Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...

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.

External links

• British Museum – Nocturnal from an astrological compendium
• Simulation – Video and description, also, lots of devices
• A working nocturnal in coin form