The Nocturnal

Nocturnal: a fixed or, more usually, portable instrument used to tell time by the apparent revolution of the stars on the celestial sphere. The stars most often used on these instruments are either the "guards" of the Little Bear (Ursa Minor) or the "pointers" of the Great Bear or Plough (Ursa Major). They are known as the Little Dipper and Big Dipper, respectively, in the USA. Most nocturnals have inscriptions "GB" and "LB" on their scales. 

Star Trails

The nocturnal is an instrument developed for use by sailors who noticed that the northern stars appeared to rotate around the Pole Star. They would memorize the positions of some of these stars at various times of the night at the various seasons. 

Figure 1 illustrates the three parts that make up the nocturnal. They are constructed as follows and assumes that you have some sort of computer aided drafting program. If not, this is no problem, as you would only need to round off the angles.

Part A: Start with a disk about 3 inches in diameter. The diameter of this disk is divided to indicate the twelve months. This particular disk is also divided into 5-day intervals. First lay out the lines for the monthly intervals:

Number of degrees for the month = (Number of days in the month / 365) * 360°

31-day month: January, March, May, July, August, October, December = 30.575°

30-day month: April, June, September, November = 29.589°

28-day month: February = 27.616°

For 5-day intervals start at each month and lay out lines 4.932° apart. The last interval of a month can be 3,5, or 6 days long. 

Part B: Start with a disk about 2.5 inches in diameter. This smaller disk fits inside the larger disk. It is divided to indicate the 24 hours of the day and so the interval between the hour lines is 15°. The hours are further divided into 15-minute intervals with 3.75° separating each of these lines. The disk also has three teeth that are references to three particular stars located near the North Star, Polaris. These are:

• BD - "Big Dipper", "Great Bear", "Ursa Major" and references the "pointer" stars.

• LD - "Little Dipper", "Lesser Bear", "Ursa Minor" and references the star Kochab.

• "Cass" - "Cassiopeia" and references the star Shedar.

Figure 2 shows these three constellations and the reference stars in relation to Polaris.

Figure 2: The Reference Stars

Relative to midnight on the small disk, the vertical edge of the three teeth are positioned as follows:

• BD - 65.0° counterclockwise.

• LD - 121.5° counterclockwise.

• Cass - 90.5° clockwise.

Part C: The pointer must be long enough to extend beyond the larger outer disk. The bottom indicating edge lines up with the centre of the central pivot and, when assembled, the centre of the instrument.

Figure 3 shows the instrument assembled. The three pieces are held together with something that is hollow in the centre, such as a rivet. Parts B and C must be able to rotate on the rivet.

Click here if you would like to download the nocturnal shown here, in PDF format. All you have to do is print the illustration, cut out the pieces and insert the rivet. It even shows you the stars that you are looking for. Using card stock will make the instrument sturdier. Laminating it in clear plastic will also make it more rugged.

How to use "The Nocturnal".

• First decide on the reference star. It would be the one that is most visible in the night sky.

• Align the appropriate tooth of the inner disk with the current date on the outer disk.

• Hold the instrument in a vertical position away from you with the handle down.

• Sight the North Star, Polaris through the rivet at the centre of the instrument.

• Rotate the pointer until its indicating edge points to the reference star.

• Read the indicated time.

The Nocturnal is a universal device and indicates local mean time. To obtain standard time longitude correction is applied to the reading.