ABSTRACT
The reformed calendar which Julius Caesar instituted in 46 BC, on the advice of the Peripatetic Alexandrian philosopher and astronomer Sosigenes, enabled the traditional agricultural year, based on the circuit of the earth round the sun, to be reconciled with the duration of a complete revolution of the moon round the earth. His year of 365 days (366 every fourth year) consisted of seven months of 31 days, four of 30 days, and one of 28 days (29 every fourth year). A slight, and temporary, adjustment had to be made in the time of Augustus when it was discovered that the pontifices had misinterpreted the instructions and had decreed a leap year every three years instead of four. The year was still actually 11 minutes 14 seconds too long, which it remained until Pope Gregory XIII in the sixteenth century corrected the error and adjusted the incidence of the leap year so that it does not fall on the opening year of a new century unless that year is divisible by 400. Thus the year 2000 was a leap year; 2100 will not be.
