Nocturnal (instrument)

A Nocturnal (also Nokturnal or Nocturlabium ) is a medieval device for determining the hours of the night from the position of the stars.

A Nocturnal used in the northern hemisphere usually uses a star of the Great Bear, Kochab of the Little Bear or the star Schedir Cassiopeia as a reference point.

The earliest known description of the Nocturnal comes from Ramon Llull in 1295th A detailed description can be found in the instrument book with Peter Apian. Another mention of the Nocturlabiums was made by Martín Cortés de Albacar 1551 in his work Arte de Navegar.

Operation

The instrument has two superposed rotatable discs with a pointer and a hole in the center. On one disc with the months of the year, the date is set. Through the hole the pole star is sighted and meanwhile put ( α and β the Stars) of the Big Dipper, the pointer to Polweiser. On the second disc, the time can be read off.

In Horologien, or sundials by Sebastian Münster from 1553 finally finds a contemporary operating instruction that fits to the one shown here historical representation of Nocturnal by Peter Apian:

So " If you now want to know the night time, the hour of the night, so you should do so. Rotation during the day the moving wheel around until the tooth in which the Twelve stands, falls on your day in the calendar or on the sun degrees therein which is in that day. So is the instrument for the same day and is addressed as it should be. Now if you erwachest to night and desirest from heaven to know the hour before or after midnight, so take the Nocturnal it in your hand with its handle and lift before your eyes against the North Star and I care that it does not hang to one side namely to the left or to the right hand and look through the hole to the North Star, see also the same for the two rear star that you see the three stars together. A through the hole and the other two outside next to the perimeter of this instrument and if you 've seen them all three, so halt the Nocturnal so quiet until you rule the pointer umhertreibst on the two rear star. Then 'm starting to count from the ravages of the twelfth hour, behind or in front of what you'll have to put the next rule, so you can find the hour of the night. You count two teeth from the large tooth of the twelfth hour, and to the left hand in, so it will be two hours after midnight. You count one tooth at the right hand of the large tooth, so it will be about the eleventh hour before midnight. "