This is a functional nocturnal coin which you can actually use to tell time during the night and get your latitudinal deviation.

The nocturnal navigational tool stood the test of time. Its purpose is to tell time at night so you can gain a more accurate latitude and longitude reading. The first written evidence of a nocturnal was discovered in the year 1272. It was refined in the 1500s to tell more accurate time and was used well in to the late 1800s and partially used into the early 1900s. Two disadvantages of the Nocturnal is it can only be used at night, and only when the pole star Polaris and either of the dippers are visible. However, it also had a great advantage since it can be used at night when a sundial could not be used. The nocturnal can only be used in the northern hemisphere unless you know how to triangulate the southern axis point, but the deviation scales can only be used in the Northern Hemisphere. Using a nocturnal in conjunction with a quadrant will yield a more accurate latitude reading. The use of a planisphere is most helpful to find the correct stars to use.

Even though mechanical clocks have been around for several centuries, they could not keep accurate time on a ship that is pitching and rolling on the ocean waves. When an accurate chronometer good enough to be used at sea was invented and after it was able to be massed produced is when the nocturnal finally became obsolete.

More sophisticated nocturnals also listed tide charts and deviation gauges on its surfaces.

How to use it

1. Determine which of the dipper constellations you will use to take your reading.

2. Then choose the corresponding tab on the time disk and align it with the current date.

3. Keep the time disk side of the coin facing towards you and hold the coin upright so that the top point of the coin is directly up and the bottom indention is centered directly down. A more accurate reading will be achieved if you ensure to do this. There are several ways to hold this coin; however, one way we found to be very effective is to place your middle finger on the top point, and place your thumb on the bottom indention. This automatically causes your hand to naturally vertically center the top and bottom of coin. Then by placing your index finger over both the time disk and the edge of the arm will prevent the pieces from moving out of adjustment once you have the moving pieces set.

4. Hold the hole of the coin close to your eye and center Polaris in it. It is more effective to keep both eyes open.

5. Then move the coin a few inches straight out from your eye while keeping Polaris centered in the hole.

6. Once you find the correct distance from your eye, align the un-notched edge of the arm with the corresponding stars in the dipper you had previously chosen (Kochab in the Litter Dipper, the pointer stars in the Big Dipper, and Shedir in Cassiopeia).

This example shows lining up the arm with the star Kochab in the Ursa Minor constellation:

This example shows lining up the arm with the star Shedir in the Cassiopeia constellation:

7. Once set, ensure the time disk and arm do not move.

8. Pull the coin away from your face and take a reading from the time ring. You now have the approximate time of night. The two examples below show the time readings from the example settings displayed above. The first picture is the time reading from the Little Dipper example, and the second picture shows the time reading from the Cassiopeia example:

9. If you know the time of your port of departure, then you can use the time reading to ensure you are keeping within the same longitude. Once you get too far to the east or west of your departure point, then the longitude calculations become inaccurate. As opposed to the nocturnal, the chronometer is the device which allowed accurate longitude readings at any point on earth.

10. The back of the coin is used to calculate latitude deviation and is scaled in minutes. You take readings with it in the same way as the time side of the nocturnal: hold the nocturnal so that the deviation scale is facing you; rotate the arm to one of the dipper constellations; then subtract or add the corresponding number which the arm references to from the latitude reading you took with your quadrant. For example: if your quadrant reads 35 degrees and the deviation of the nocturnal reads -23, then your latitude is approximately 34 degrees and 37 minutes.