The Nocturnal is an old device, used before there were reliable clocks, to tell the time at night. It uses the positions of three constellations – the Little Bear, Great Bear, and Cassiopeia – to determine the hour. The positions of the constellations will change throughout the year.
First, see which constellations are visible – most often, you can see either the Great Bear or Cassiopeia but not always both. Say that today is August 16th, and you can see the Great Bear.
Turn the time ring disc so that the GB tab lines up with today’s date. The months are approximated to 30 days each so just get as close as you can if it’s the 31st. (Smaller versions of this device only ever had the months listed without the days, so don’t fret about this)
Hold the device up in front of you, being careful not to move the time ring. Make sure it hangs vertically. Look through the hole in the middle and find Polaris, the North Star.
Then turn the arm so that it lines up with the two stars in the Great Bear, as shown on the rear of the device.
The arm should now be pointing to the correct time of night (or early morning). In this example it would be about 8am.
Now, there will most likely be some variation between your reading and the actual time.
1) You’re probably not going to be situated in the very middle of your time zone. A time zone is 15° wide, and the time will change by 4 minutes for each degree. So if you’re at the very edge of your time zone, you might be (7.5° x 4) = 30 minutes different from the clock time. See below for a time zone map of the USA.
2) Don’t forget daylight savings time! You are measuring local time with the Nocturnal; your reading will need an hour added on in the summer to match your clock time.
The back of the device shows the main three constellations and the sight lines, through the specific stars that you need to line up on. The outer scale is Right Ascension, for reference.
References:
https://sundials.org/index.php/teachers-corner/sundial-construction/61-longitude-correction.html