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A nocturlabium, also known as a noctural or nocturnal, is an astronomical instrument that allows you to determine local time by the position of stars in the night sky.
The principle of the device is based on the fact that, as a result of the rotation of the Earth, the starry sky appears to rotate around a fixed point - the celestial pole.
In the Northern Hemisphere, it almost coincides with the position of the North Star (α Ursae Minoris). A complete rotation of the stars around this point takes one stellar day - approximately 23 hours 56 minutes, which is slightly less than a solar day.
Since some stars near the pole do not set below the horizon (they are called circumpolar), their position can be used to determine the time. This is done similarly to a sundial, but at night. The stars most often chosen for such measurements were Dubhe and Merak (α and β Ursae Majoris), as well as Benetnash, Kochab (β Ursae Minoris), and Shedar (α Cassiopeiae).
How does a nocturlabium work?
A typical nocturlabium consists of two rings - one with the names of the months (fixed), the other with hour divisions and a pointer (rotating), a rotating lever (or "hand") pointing to a reference star.
Before measuring, the pointer on the rotating ring is set to the current month to account for seasonal changes in the position of the stars. The device is then held vertically, with the observer looking at the North Star through the central hole and turning the lever in the direction of the chosen star. At this point, the lever points to the current stellar time on the scale.
The accuracy of the nocturlabium could reach ±15 minutes, which was considered quite acceptable for navigation and astronomical tasks. Moreover, if the exact time of midday was known according to the sundial, the nocturlabium could also be used to determine the observer's geographic latitude.
It is believed that one of the first nocturlabiums was created by Pacificus (c. 777–844), archdeacon of the Cathedral of Verona. However, the device evolved over the centuries, and at various times its invention was attributed to other authors. The device was most widely used in the 15th–17th centuries, especially among sailors. With the invention of accurate marine chronometers in the 18th century, the need for nocturlabiums disappeared, and they gave way to more modern means of measuring time.
Although nocturlabiums were used only in the Northern Hemisphere, attempts were made to create similar devices in the Southern Hemisphere. However, due to the lack of a bright star near the south celestial pole (similar to Polaris), the accuracy of such devices remained much lower.
Instead, they were guided by the constellation of the Southern Cross. The Southern Cross is like an hour hand, and the south pole is like the center of the dial. By the angle of the Cross, one could estimate how many "star hours" there were. Astronomical tables, sextants, and later marine chronometers were also used to determine the time.
The principle of the device is based on the fact that, as a result of the rotation of the Earth, the starry sky appears to rotate around a fixed point - the celestial pole.
In the Northern Hemisphere, it almost coincides with the position of the North Star (α Ursae Minoris). A complete rotation of the stars around this point takes one stellar day - approximately 23 hours 56 minutes, which is slightly less than a solar day.
Since some stars near the pole do not set below the horizon (they are called circumpolar), their position can be used to determine the time. This is done similarly to a sundial, but at night. The stars most often chosen for such measurements were Dubhe and Merak (α and β Ursae Majoris), as well as Benetnash, Kochab (β Ursae Minoris), and Shedar (α Cassiopeiae).
How does a nocturlabium work?
A typical nocturlabium consists of two rings - one with the names of the months (fixed), the other with hour divisions and a pointer (rotating), a rotating lever (or "hand") pointing to a reference star.
Before measuring, the pointer on the rotating ring is set to the current month to account for seasonal changes in the position of the stars. The device is then held vertically, with the observer looking at the North Star through the central hole and turning the lever in the direction of the chosen star. At this point, the lever points to the current stellar time on the scale.
The accuracy of the nocturlabium could reach ±15 minutes, which was considered quite acceptable for navigation and astronomical tasks. Moreover, if the exact time of midday was known according to the sundial, the nocturlabium could also be used to determine the observer's geographic latitude.
It is believed that one of the first nocturlabiums was created by Pacificus (c. 777–844), archdeacon of the Cathedral of Verona. However, the device evolved over the centuries, and at various times its invention was attributed to other authors. The device was most widely used in the 15th–17th centuries, especially among sailors. With the invention of accurate marine chronometers in the 18th century, the need for nocturlabiums disappeared, and they gave way to more modern means of measuring time.
Although nocturlabiums were used only in the Northern Hemisphere, attempts were made to create similar devices in the Southern Hemisphere. However, due to the lack of a bright star near the south celestial pole (similar to Polaris), the accuracy of such devices remained much lower.
Instead, they were guided by the constellation of the Southern Cross. The Southern Cross is like an hour hand, and the south pole is like the center of the dial. By the angle of the Cross, one could estimate how many "star hours" there were. Astronomical tables, sextants, and later marine chronometers were also used to determine the time.
