There are 12 windows in the frontispiece, through each of which appears
an indication relating to time. Beginning at the top of the frontispiece
of the dial, the first opening occurs on the breast of the imperial
eagle. This indicates the dominating planet, represented by its symbol,
and its house.
The opening in the eagle's left claw, labeled "Lit. Dom." is the
dominical letter. The first seven days in the month of January are each
assigned one of the letters _a_ through _g_ in order of appearance. The
letter which coincides with the first Sunday within this period is
called the dominical letter, and it serves for the following year. In
leap year, two letters are required, one to February 29th and the letter
next proceeding for the remainder of the year. This letter is used in
connection with establishing the date of Easter Sunday. The date of
Easter regulates the dates of the other movable feasts.
The eagle's right claw is labeled "Cyc. EpEC" and represents the epact,
or the age of the moon on January 1st. It serves to find the moon's age
by indicating the number of days to be added to each lunar year in order
to complete a solar year. Twelve lunar months are nearly 11 days short
of the solar year, so that the new moons in one year fall 11 days
earlier than they did the preceding year. However, 30 days are deducted
as an intercalary month since the moon has made a revolution in that
time, and the remainder, 3, would be the epact.
Below the imperial eagle two winged cherubs support a riband with three
indictions of the Julian period. This period of 7980 years is the
product derived from multiplying together the sums of 28, which
represents the cycle of the sun; 19, representing the cycle of the moon;
and 15, which represents the Roman indiction. The Julian period is
reckoned to have begun from 4713 B.C. so that the period will be
completed in A.D. 3267. The first of the three openings is marked "Ind.
Rom." or "Roman indiction," which was an edict by the Emperor
Constantine in A.D. 312, providing for the assessment of a property tax
at the beginning of each 15-year cycle. It continues to be used in
ecclesiastical contracts. The second opening, which occurs immediately
below the eagle, is marked "Cyc. Sol." (cycle of the sun). This cycle
takes a period of 28 years, after which the days of the week once again
fall upon the same days of the month as they did during the first year
of the former cycle. There is no relationship with the course of the sun
itself, but was invented for the purpose of determining the dominical
letter which designates the days of the month on which the Sundays occur
during each year of the cycle. Since cycles of the sun date from 9 years
before the Christian era, it is necessary to add the digit 9 to the
digits of the current year and then divide the result by 28. The
quotient is the number of cycles which has passed, and the remainder
will be the year of the cycle answering to the current year. The third
opening on the riband is labeled "Num. Aur." (golden number). Meton, an
astronomer of Athens, discovered in 432 B.C. that after a period of 19
years the new and the full moons returned on the same days of the month
as they had before, and this is called the cycle of the moon. The Greeks
were so impressed with this calculation that they had it inscribed in
letters of gold upon stone, hence the golden number. The First Council
of Nicaea in A.D. 325 determined that Meton's cycle was to be used to
regulate the movable feasts of the Church.
Immediately above the chapter rings is an opening through which the orb
of the sun is visible.