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.
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