Epact Definition & Meaning

Origin of Epact

French épacte from Late Latin epacta from Greek epaktē (hēmera) intercalary (day) feminine of epaktos brought in, inserted from epagein to bring in, introduce ep-, epi- epi- agein to lead ag- in Indo-European roots

From American Heritage Dictionary of the English Language, 5th Edition

Epact Is A Word Of Greek Origin, Employed In The Calendar To Signify The Moon'S Age At The Beginning Of The Year.
Another Addition Of Eleven Gives Thirty Three For The Epact Of The Fourth Year; But In Consequence Of The Insertion Of The Intercalary Month In Each Third Year Of The Lunar Cycle, This Epact Is Reduced To Three.
In Like Manner The Epacts Of All The Following Years Of The Cycle Are Obtained By Successively Adding Eleven To The Epact Of The Former Year, And Rejecting Thirty As Often As The Sum Exceeds That Number.
But The Order Is Interrupted At The End Of The Cycle; For The Epact Of The Following Year, Found In The Same Manner, Would Be 29 11=40 Or 10, Whereas It Ought Again To Be 1S To Correspond With The Moon'S Age And The Golden Number 1.
The Reason Of This Is, That The Intercalary Month, Inserted At The End Of The Cycle, Contains Only Twenty Nine Days Instead Of Thirty; Whence, After 11 Has Been Added To The Epact Of The Year Corresponding To The Golden Number 19, We Must Reject Twenty Nine Instead Of Thirty, In Order To Have The Epact Of The Succeeding Year; Or, Which Comes To The Same Thing, We Must Add Twelve To The Epact Of The Last Year Of The Cycle, And Then Reject Thirty As Before.

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