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Astronomical basis

This star chart is of the region around Ursa Major (the Great Bear) and Ursa Minor (Little Dipper). The stars used for setting the nocturnal were the "pointers" of Ursa Major (Merek and Dubhe) or the two bright outer stars in the Little Bear (Kochab and Pherkad).

What makes the nocturnal a useful instrument for navigation? It relies on the motion of the stars caused by the Earth's daily rotation. If the stars can be seen, their apparent position act as the hands of a clock and thereby provide a navigator with the time of night. The specific stars that are used are the pointers in the Big Dipper of Ursa Major or, alternately, the bright stars, Kochab and Pherkad, of Ursa Minor, the Little Dipper.

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Method of use

The navigator holds the nocturnal at arms length sighting Polaris, the North Star, through the hole in the central pivot. The straight side of the index arm of the nocturnal is then rotated to coincide with the "pointers" in the Big Dipper or the 2 bright stars of the Little Dipper, Kochab and Pherkad.

    This image by Apianus (1539) illustrates the use of an early nocturnal.  

Early nocturnals were only designed for use with the Pointers of the Big Dipper. Later, an additional "ear" or pointer was added for the stars of the Little Dipper.

With Polaris sighted, the index arm is rotated until the chosen stars in one of the Dippers is aligned with it. The time can then be read off the hour scale.

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A complicating factor

The Earth, of course, orbits the Sun in its annual path. This motion causes the position of the stars to drift about 1° westward each night which is equivalent to almost 4 minutes of time. Time reckoned by the stars is called "sidereal time" and 1 "sidereal day" equals 23 h 56 minutes of solar time.

For a couple of days this difference is not important but the cumulative error soon becomes significant. In just two weeks, the error would be 1 hour. That is, if this night to night drift of the stars is not accounted for, the position of the stars at 10 o'clock tonight would be read off the nocturnal as 9 o'clock in two weeks - an error of 1 hour. The solution for the navigator is to rotate the plate with the hour scale and with the Dipper pointers to compensate.

The pointer for either the Great or Little Bear is set to the date on the scale on the outer edge. This scale is divided into weekly intervals, subdivided into five units, throughout the year. The sighting arm was then aligned with the stars of the Great or Little Bear as chosen, and the hour read off the inner scales.

Another function of importance to mariners was the nocturnal's use to indicate the time of high tides of ports--day or night. Each port has a characteristic "establishment of port" which is the number of hours that a high tide lags behind the Moon's transit across the meridian (north/south line through the sky's zenith). Knowing the Moon's age (number of days since new Moon), the time of high tide can be calculated with the nocturnal. Bennett provides details of this procedure.

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Three main sources of error affect the accuracy of a nocturnal.

The first source of error, usually the largest, is in aligning the index arm on the chosen stars. Some practice with a home made model will show the difficulty of holding the device steady and aligning the index arm to the stars. Add to that, the rolling motion of a ship and you will easily imagine how this error could be significant. A skilled navigator would be able to achieve an accuracy of about ± 15 minutes of time.

The second source of error is inaccurate division of the scales of the nocturnal. Scales were laid off using a set of dividers (similar to a drawing compass but with two sharp metal points). Because of the small size of the nocturnal's disc, these may be inaccurate by ± 1/2° or more. (close up of scale) The maximum size of the nocturnal's disc is in fact limited by the diameter of the arcs made by the stars of the Dippers as the Earth rotates (apx. 20°). With the nocturnal at arm's length, the disc must not obscure the sighting stars of either Dipper.

The third error results from the fact that Polaris, during the times when the nocturnal was popular, was about 2° or 2.5° from the North Celestial Pole. This is like having the pivot of a clock's hands offset from the centre of the dial. This results in times that would be systematically too fast and then systematically too slow. However, this was never taken into account in making or using the nocturnal.

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