And some more..........
"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 and align the correct stars, 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.
Design:
The early nocturnals were made of wood and later made in brass. They were approximately 7" to 10" in diameter plus the length of the handle and arm. The large circle, that has the handle attached to it, lists the days and months. The center rotating disk has the time. The arm has a notch on one edge of it so you know to use the opposite edge for lining it up with the stars. Both the center circle and arm rotate on an arbor which is hollowed out to use it as a sight."
