- Museum number
This brass nocturnal consists of a range of scales, discs and indices. Attached to the main disc is a small handle in the shape of a pierced fleur-de-lis, inscribed with a pair of initials.
The FRONT of the MAIN DISC has a circular calendrical scale with the usual Latin names of the months, numbered 1 to 12 starting with January. Each month has the corresponding number of days (December has 311/4 to indicate the leap year). The centre of the handle corresponds to 24th October.
In the centre of this scale is a VOLVELLE with an hour scale, numbered anticlockwise 1 to 12 twice. Teeth of alternating size indicate the full and half hours. An index with a sun effigy at 12 o'clock allows this volvelle to be set to the correct date. The centre of the disc is engraved with scrollwork.
Over this volvelle rotates an INDEX ARM, marked around the central hole 'STELLA POLARIS' and along the arm 'INDEX HORA NOCTIS'. The signature of the maker is inscribed on the same arm.
The REVERSE is inscribed with several circular scales as follows (from the outside):
1) a degree scale 0° to 360°.
2) a 24 hour scale, numbered clockwise 1 to 12 twice.
3) a compass rose of 32 points, with the eight cardinal points shown by their first letter(s), North indicated by a fleur-de-lis.
In the centre of this scale is a fixed DISC inscribed with the age of the moon from 1 to 30.
Over this disc rotates a VOLVELLE with an index. A circular aperture shows the phases of the moon. The plate is marked with planetary aspects but has not been completed.
- Production date
- 1570-1580 (ca.1575)
Diameter: 95 millimetres
- Curator's comments
[In] this instrument by the famed Elizabeth maker Humfrey Cole … the combination of scales 2 and 3 with the central disc and volvelle allow the prediction of high tide at a particular port (provided the direction of this place is known) as described by Digges in his Prognostication, printed by Thomas Gemini in 1555.
[S.Ackermann, EPACT 1998, http://www.mhs.ox.ac.uk/epact/catalogue.php?ENumber=24134]
According to Waters (1958, pl. xliv caption), the earliest known English tide computer is that on Cole's compendium of 1569 (held at the National Martime Museum, Greenwich, London (AST0172; old no. D.318)) and he considers that this example can be dated to the period shortly after 1570 because the Moon's age is given as 30 days and not 29½ days. However, 30 is more practical for seamen when working out the date of the New Moon.
Turner (Turner 200, p. 129) dates this instrument to ca. 1570.
How to use a tide calculator on a nocturnal
1. Hold the nocturnal upright with the handle at the bottom
2. Look at the disc numbered 1 to 30 (age of the Moon in days) and turn it until the small index above the number 30 is aligned with the fleur-de-lys at north.
3. Next, align the straight edge of the indicator on the inner disc to the corresponding age of the Moon. If you don’t know the age of the Moon you can approximate by matching the phase demonstrated in the cut out with the actual phase seen in the sky.
4. Read the number of hours on the outer scale marked 1 to 12 twice.
5. Look up your location in the relevant tables and find the value of the Establishment of Port. Add this value to the number of hours read off before. The result is the time of high tide at this particular location.
The Establishment of Port is a correction value to account for the variation of tide times between different locations. Every location has a specific value.
- On display (G46/dc6/s4/sh2)
- Exhibition history
2003 1 May-15 Sept, London, National Maritime Museum, Elizabeth I
- Latest: Greenish-white deposit under volvelle on age-of-moon dial.
- Acquisition date
- Britain, Europe and Prehistory
- Registration number
- Additional IDs
Miscellaneous number: OA.265 (?)