CALENDAR. Circular luni-solar volvelle calendar of vellum.

A luni-solar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year, then a luni-solar calendar will give an indication of the season; if it is taken as a sidereal year, then the calendar will predict the constellation near which the full moon may occur. Usually there is an additional requirement that the year have a whole number of months, in which case most years have 12 months but every second or third year has 13.

The main disc is entirely filled with circular and semi-circular scales.
Reading from the outer edge inwards, these are:

I. A scale of 90 numbered divisions, alternate numbers being distinguished by alternating red and black divisions.

II. Ten repeating groups lettered A to I each letter corresponding to one division of scale I.

III. Nine divisions, each containing letters from A to I and numbers from 10 to 90, commencing at the points 1 and A on scales I and II and with each division equal to two and half divisions of the first two scales.
This first quadrant is succeeded by a second with smaller divisions embracing only two divisions of scales I and II, each division containing a sign of the zodiac in sequence from Aries each subdivided to 4, but numbered only 10, 20, 30.
The third quadrant of the same scale divides (like quadrant one) to two and a half divisions of scales I and II, the first two divisions containing emblems for the Sun and the Moon followed by the planetary symbols for the days of the week. These divisions are sequentially numbered 10 to 90.
The fourth quadrant contains twelve sequentially numbered divisions.

IV. Corresponding with the divisions of scale III, each cell of this scale contains a set of sixteen numbers which, since none of them exceed 31, are presumably dates.

V. A scale of thirty divisions, coincident with every three groups of scale II, each division containing the names of four months and the numbers 3, 5, 7, 9 in different combinations coinciding with four divisions for each cell on the outer edge.
VI. A series of nine semi-circles, six containing the name of one month, and three (the winter quarter, November to April), two months. In addition each contains a sequence of ten numbers which read in an alternating sequence: viz May reads 80, 11, 79, 12, 78, 13, 77, 14, 76, 15.
VII. A scale of thirty sequentially numbered divisions. Within this, at the center, are seven leaf-shaped diagrams radiating out from the center, half of each being cross-hatched in black. These diagrams however are concealed by two volvelles mounted above them, the first of which supplies a zodiacal calendar (0° Aries = 20 March) followed by a scale of ten divisions each divided to ten and carrying a letter from A to 1.

Mounted at the center of this is a second volvelle with an index which reaches out to scale II carrying a lunar calendar of 30 with diagrams of the four phases of the Moon at the center.

The fact that lunar calendars do not keep pace with the passage of the seasons has been regarded as a disadvantage. To overcome this many societies have a adopted a hybrid luni-solar calendar. These are lunar calendars which use various devices to bridge the missing 11.25 days so that the calendar year ends up close in length to the tropical year; the year of the seasons.