In the article Calendar (q.v.), that part of chronology is treated which relates to the measurement of time, and the principal methods are explained that have been employed, or are still in use, for adjusting the lunar months of the solar year, as well as the intercalations necessary for regulating the civil year according to the celestial motions.
This Mistake Having Been Discovered, Augustus Ordered That All The Years From The Thirtyseventh Of The Era To The Forty Eighth Inclusive Should Be Common Years, By Which Means The Intercalations Were Reduced To The Proper Number Of Twelve In Forty Eight Years.
Directed Ten Days To Be Suppressed In The Calendar; And As The Error Of The Julian Intercalation Was Now Found To Amount To Three Days In 400 Years, He Ordered The Intercalations To Be Omitted On All The Centenary Years Excepting Those Which Are Multiples Of 400.
46 Sec. Now The Gregorian Rule Gives 97 Intercalations In 400 Years; 400 Years Therefore Contain 365 X400 97, That Is, 146,097 Days; And Consequently One Year Contains 365.2425 Days, Or 365 Days 5 Hours 49 Min.
The Third 3 3, Gives Eight Intercalations In Thirty Three Years Or, Seven Successive Intercalations At The End Of Four Years Respectively, And The Eighth At The End Of Five Years.
If The Commencement Of The Year, Instead Of Being Retained At The Same Place In The Seasons By A Uniform Method Of Intercalation, Were Made To Depend On Astronomical Phenomena, The Intercalations Would Succeed Each Other In An Irregular Manner, Sometimes After Four Years And Sometimes After Five; And It Would Occasionally, Though Rarely Indeed, Happen, That It Would Be Impossible To Determine The Day On Which The Year Ought To Begin.
But A System Of 31 Intercalations In 128 Years Would Be By Far The Most Perfect As Regards Mathematical Accuracy.
This Long Period, However, May Be Reduced To Four Hundred Years; For Since The Dominical Letter Goes Back Five Places Every Four Years, Its Variation In Four Hundred Years, In The Julian Calendar, Was Five Hundred Places, Which Is Equivalent To Only Three Places (For Five Hundred Divided By Seven Leaves Three); But The Gregorian Calendar Suppresses Exactly Three Intercalations In Four Hundred Years, So That After Four Hundred Years The Dominical Letters Must Again Return In The Same Order.
It Has Already Been Mentioned That The Error Of The Julian Year Was Corrected In The Gregorian Calendar By The Suppression Of Three Intercalations In 400 Years.