It took two millennia to get the one we now use; we owe a lot to the sun and moon, to Caesar, Pope Gregory and, oh yes, the Earl of Chesterfield

Equinox seen from the astronomic calendar of Pizzo Vento at Fondachelli Fantina, Sicily
Equinox seen from the astronomic calendar of Pizzo Vento at Fondachelli Fantina, Sicily Wikimedia Commons

When New Year's Day rolls around again 11 months from now, it will be 2000 A.D. — the A.D., in case anyone has forgotten, short for anno Domini nostri Jesu Christi (the year of our Lord Jesus Christ). That designation was set in place in 6th-century Rome by an obscure abbot and mathematician called Dennis the Little. It happened while he was trying to solve one of the persistent calendrical problems of Christianity, figuring out exactly when Easter ought to be celebrated. In the process Dennis the Little came up with a big idea: Why not peg Rome's calendar to the year of Christ's birth rather than to (as was then the case) the first year of the reign of Emperor Diocletian, a notorious persecutor of Christians? So what if Dennis got Jesus' birthday wrong...

David Duncan's article on the calendar, adapted from his best-selling book of the same title, traces the long and often tortuous history of human attempts to measure (and number) the passing of months and years. The moon was always alluring, Duncan points out, what with its predictable rhythm of waxing and waning. The ancients recognized that 12 lunar cycles came very close to a year of seasons — but, alas, they ultimately found, it wasn’t close enough. Only the position of the sun could predict a year from summer solstice to summer solstice or spring equinox to spring equinox. Figuring out ways to bring the two time-keeping systems (not to mention the influences of science and religion) into alignment — as Duncan recounts in unexpected and amusing detail — has kept priests, kings and mathematicians busy for millennia.

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