From antiquity it has been observed that the night sky containing the stars rotates incessantly about an immovable point, close to the Pole Star. This could therefore be used as a point of reference. It was also known that the moon underwent periodic transformations: invisible for a certain period, it gradually displayed a greater and greater luminous surface, until it became full and then began to diminish before vanishing once again. Even the moon could constitute a visible point of reference to calculate the passing of time.

While scholars studied the phases of the moon and astronomers observed the stars and calculated their number, the instrument makers occupied themselves with making devices which would allow the passage of time to be measured even in the dark. Nocturnals evolved between the 15th and 16th century, but declined in popularity with the rapid improvements subsequently made in mechanical clocks.

The nocturnal is made up of a series of superimposed disks of different diameters. The largest disk bears the days of the month, the median one bears the months and the signs of the zodiac, and the smallest one bears a series of divisions. The polar star can be sighted through the central hole, after the disks have been arranged according to the day and month of the observation. An index arm is then oriented in such a way that it can follow the movement of a certain star, close to the chariot of Ursa Major (The Great Bear). Other nocturnals were used for calculations of the phases of the moon and allow the duration of the phases themselves to be checked.

Mara Miniati
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