The ancient Celtic people used a lunar calendar in which each month
was represented by a moon. Each month started when the moon was full and
it was further divided into fortnights, or two-week periods. A dark fortnight
followed a bright fortnight. Each month had either 29 or 30 days depending
on whether it was a bright or dark month.
The calendar took into account the differing time periods taken
by the moon and the sun to circle the earth and reconciled the
differences by inserting an extra month on a regular cycle. This
method meant that most years contained twelve months, and approximately
every third year contained thirteen months.
Another Celtic calendar, known as the Coligny Calendar, was discovered
in eastern France. This calendar was a bronze plate measuring five feet
by three and a half feet. The Coligny Calendar also recorded time by lunar
months. It showed 62 lunar months, with two additional months added so
that the Coligny Calendar would match the solar timekeeping system. Experts
believe that the Celtics made this change to the calendar after learning
about the solar timekeeping system used by the Roman people.
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