The Council of Nice, which was held in the year 325 of the Christian era, adopted a fixed rule to determine the time at which Easter falls. This rule was based on the supposed fact that the spring equinox happened every year on the 21st of March, as it did at the time of the meeting of the Council. This would indeed be the case if the mean value of the civil year of the Julian calendar was exactly equal to the tropical year. But while the first is 365.25 days, the second is 365.242264 days; so that the tropical year is too small by 11 minutes and 8 seconds. It follows hence that after the lapse of four Julian years the vernal equinox, instead of happening exactly at the same time as it did four years before, will happen 44 minutes 32 seconds too soon; and will gain as much in each succeeding four years. So that at the end of a certain number of years, after the year 325, the equinox will happen on the 20th of March, afterwards on the 19th, and so on. This continual advance notified by the astronomers, determined Pope Gregory XIII. to introduce a new reform into the calendar. It was in the year 1582 that the _Gregorian reform_ was put into operation. At that epoch the vernal equinox happened on the 11th instead of the 21st of March. To get rid of this advance of ten days that the equinox had made and to bring it back to the original date, Pope Gregory decided that the day after the 4th of October, 1582, should be called the 15th instead of the 5th. This change only did away with the inconvenience at the time attaching to the Julian calendar; it was necessary to make also some modification in the rule which served to determine the lengths of the civil years, in order to avoid the same error for the future. So the Pope determined that in each 400 years there should be only 97 bissextile years, instead of 100, as there used to be in the Julian calendar. This made three days taken off the 400 years, and in consequence the mean value of the civil year is reduced to 365.2425 days, which is not far from the true tropical year. The Gregorian year thus obtained is still too great by .000226 of a day; the date of the vernal equinox will still then advance in virtue of this excess, but it is easy to see that the Gregorian reform will suffice for a great number of centuries. The method in which it is carried out is as follows:--In the Julian calendar each year that divided by four when expressed in its usual way, by A.D., was a leap year, and therefore each year that completed a century was such, as 1300, 1400 and so on--but in the Gregorian reform, all these century numbers are to be reckoned common years, unless the number without the two cyphers divides by four; thus 1,900 will be a common year and 2,000 a leap year. It is easy to see that this will leave out three leap years in every 400 years.