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Nocturnal or nocturlabes, as they were sometimes called, are devices for telling the time of night. Their operation is based on the fact that the stars while remaining fixed relative to one another, appear to rotate around the North star (Polaris). It is of course the earth which is rotating and Polaris which remains fixed because it lies along the earth´s axis of rotation. As the others stars appear to rotate, their position at any moment indicates the time. Sometimes called horologium nocturnum, it is related to the astrolabe and sundials.
With Martin Cortés de Albacar´s book “Arte de Navegar” published in 1551 the name and the instrument gained a larger popularity.Timekeeping was very important to navigators at sea. Precise time was needed to use tide tables to safely enter harbours and also to regulate work shifts aboard. Thus, the navigators were able to tell the time at night if their weather was clear. The problem was that in foul weather, when the sky was obscured, they had no way of knowing the exact time except by using an hourglass. By the middle of the 18th century, more accurate clocks started to become available and the nocturnals fell out of use.

Nocturnals are actually simple analogue computers. Compared to others instruments which requiere mathematical tables and trigonometry, the nocturnal is simple to use. Nocturnals are accurate to within 15'.

Using a nocturnal in conjuntion with a quadrant will yield a more accurate latitude reading and also the use of a planisphere is most helpful to find the correct stars to use.

It´s compossed by several pieces which are attached at the center so they can rotate relative to one another. At the axis of rotation there is a hole to observe the Polaris.

The nocturlabe we feature is one of the most complete known, because as you can see it requires several rings with which to do the math:
-The large ring or mater contains list of the days and months and also zodiac signs.
-The hour circle contains the hours and constellations with which we will find the time. This noctolabe is provided with three:  Big Dipper (Ursa Major), Little Dipper (Ursa Minor), and Cassiopeia
-The third circle corresponds to the time and moon age.
-The fourth part is the arm that is going to tell the time on the corresponding circle.
-The back of the nocturnals was usually utilized to register any other quadrant, usually a sundial as Regiomontanus type, other, as in our case, is provided with a dial deflection directions, although for use need help of a height quadrant , that allows us to determine the latitude in which we find ourselves.
It is made of a durable metal alloy coated in bronze.
It comes in a beautiful box with a display stand and with a comprehensive instruction booklet.
Historical Examples

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