Perpetual calendar with astronomical data

Unknownc. 1385-1400

The Royal Society

The Royal Society
London, United Kingdom

Part of a folded medieval almanac, showing a perpetual astronomical calendar for January 1st to June 15th.

Located on the exterior side of the almanac, below the cardinal fold when parchment fully opened (24 compartments).

Each complete month occupies four consecutive compartments, two for the first part of the month, two for the latter.

Each compartment is divided into nine columns. The first compartment for each half of the month records (from right to left):

1. The day of the month (in Arabic numerals)

2. The dominical letter (one of seven letters A-G indicating days of the week, ‘A’ highlighted in red. On any given year if you know the date e.g. a Sunday or ‘domenica’ falls, every date marked with the same dominical letter will also be a Sunday etc throughout the weekdays).

3-4. The day of the month according to the Julian calendar (organised by Kalends ‘KL’, Nones ‘Non’ and Ides ‘Idus’—across two columns, in the first column the day number counting down from the previous ides and in the second the abbreviations as noted).

5. The Golden Number ‘aureus num’ (indicates the date of a new moon for each year in a 19 year lunar cycle. Calculated by dividing the year by 19 and subtracting one the resulting golden number listed against a date in the calendar indicates the date a new moon will fall for years with that number).

6. The duration of a half night ‘nox’ (period between sunset and midnight also equal to period between midnight to sunrise, hence half night and doubled will equate to total period of darkness. Given in hours (red ink) and minutes (black ink) the hour is only listed when it changes. Column topped with a figurehead in black ink).

7. The duration of a half day ‘diem’ (period between sunrise and midday also equal to period midday and sunset – hence half day. Given in hours (black ink) and minutes (red ink), the hour is only listed when it changes. Column topped with a figure head in red ink. The values given for a half day and half night will always total twelve and allow for the calculation of the time of sunrise and sunset).

8-9. The solar longitude and its angle at sunrise. (In two columns under the heading ‘Sol’ with a symbol of the sun. In black ink the position of the sun on the eliptic given in a degree ‘gre’ of the thirty assigned to each sign of the zodiac. Running continuously across the months this column also marks the points where the sun moves into the next sign with the name of the new zodiac and re-iterates the current sign at the top of each new compartment. In red ink the Sun’s angle at sunrise in relation to the eliptic, given in minutes).

The nine columns on the second compartment for each half of the month record the date, hour and minute in a year on which the conjunction of the Sun and Moon will occur for three 19-year lunar cycles.

The first two folds on this side were left blank, possibly for the addition of notes and dates of personal or local significance (though this practice is more common in examples of later, annual almanacs from 1560 onwards1).


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