Only the mater of this instrument survives. Its back clearly illustrates two astrolabe scales worth commenting on.
The outer scale is the common circle of degrees and immediately within this are the signs and degrees of the zodiac. The next scale, however, separated from the zodiacal scale by a blank space, is a scale of lunar mansions, that is of the 28 sections of the zodiac occupied by the moon in the successive days of a lunar month.
Within this, separated by another blank space, is a calendar scale. The dates on this scale correspond with the positions of the sun in the zodiac on the outermost scale, but it is noticeable that the calendar scale is eccentric, that is, is offset with respect to the zodiacal scale. This is to accommodate the variation in the sun's apparent progress along the ecliptic throughout the year.