calendar would in
that case have possessed all the simplicity and uniformity of the civil
calendar, which only requires the adjustment of the civil to the solar
year; but they were probably not sufficiently versed in astronomy to be
aware of the practical difficulties which their regulation had to
encounter.

_Dominical Letter._--The first problem which the construction of the
calendar presents is to connect the week with the year, or to find the day
of the week corresponding to a given day of any year of the era. As the
number of days in the week and the number in the year are prime to one
another, two successive years cannot begin with the same day; for if a
common year begins, for example, with Sunday, the following year will begin
with Monday, and if a leap year begins with Sunday, the year following will
begin with Tuesday. For the sake of greater generality, the days of the
week are denoted by the first seven letters of the alphabet, A, B, C, D, E,
F, G, which are placed in the calendar beside the days of the year, so that
A stands opposite the first day of January, B opposite the second, and so
on to G, which stands opposite the seventh; after which A returns to the
eighth, and so on through the 365 days of the year. Now if one of the days
of the week, Sunday for example, is represented by E, Monday will be
represented by F, Tuesday by G, Wednesday by A, and so on; and every Sunday
through the year will have the same character E, every Monday F, and so
with regard to the rest. The letter which denotes Sunday is called the
_Dominical Letter_, or the _Sunday Letter_; and when the dominical letter
of the year is known, the letters which respectively correspond to the
other days of the week become known at the same time.

_Solar Cycle._--In the Julian calendar the dominical letters are readily
found by means of a short cycle, in which they recut in the same order
without interruption. The number of years in the intercalary period being
four, and the days of the week being seven, their product is 4 x 7 = 28;
twenty-eight years is therefore a period which includes all the possible
combinations of the days of the week with the commencement of the year.
This period is called the _Solar Cycle_, or the _Cycle of the Sun_, and
restores the first day of the year to the same day of the week. At the end
of the cycle the dominical letters return again in the same order on the
same days of the month; hence a table of dominical