This device is used to tell time using the celestial clock. One ring is set to the the date while Polaris will be seen through the hole in the centre. The horizontal arm attached to the centre of the ring is then rotated around and aligned with pointer stars, such as the Big Dipper, Little Dipper or Cassiopeia. The point where the arm coincide with the marked ring will be taken as the time.
It is only used to measure Polaris's distance in minutes of arc from true North, thus, there are some corrections that can be applied.
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