Thomas Selden married his wife, Sarah, on 16 April 1717. Their son Thomas was born on 25 February 1717/18. Was he born on the wrong side of the blanket?
Probably not. It’s a confusing story. Julius Caesar created a calendar that had 365 days and a leap year with an extra day every fourth year. This calendar was used throughout the Roman Empire. But there was a problem: every 128 years, the Julian calendar gained a day, putting it out of sync with the solar year.
By the time Pope Gregory XIII’s new calendar was adopted in 1582, the Julian calendar was ten days out of sync with astronomical time. The Gregorian calendar also set 1 January as the first day of the year.
Prior to the Gregorian calendar being adopted in the British Isles in 1752, England celebrated the new year on 25 March (the Feast of the Annunciation or Lady Day). So Thomas Selden Jr. was born on 25 February 1717 OS (old style—when the new year was 25 March), also known as 25 February 1718 NS (new style—when the Gregorian calendar started the new year on 1 January).
Additional Reading/Viewing:
Rupert Shepherd, “The Problem With Dates,” 16 February 2020; Rupert Shepherd.
“Introduction to Calendars;” Astronomical Applications Department of the U.S. Naval Observatory.
Patricia Law Hatcher, CG, “Dates: Recording and Interpreting;” Dallas Genealogical Society.
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I have always found this switch to the Gregorian calendar an interesting turning point in history. Good reminder re dates.
But we still have 365 day years with every fourth one a leap year...so how are we not gaining a day every 128 years? How did Pope Gregory change the calendar so it doesn't do that?
I've never really thought about it - I just knew about the change and how to adjust my dating.