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  • Writer's pictureMarc Jenni

The 10 days which never existed...

Location: Zurich, Switzerland

Date: March 15, 1996


I just love the story about the 10 days which never existed! It illustrates how difficult and challenging the measurement of exact timing was in the past.



Today’s internationally accepted civil calendar was introduced by Pope Gregory XIII (1502 - 1585) who signed on February 24, 1582, in Rome the decree «Inter gravissimas» the foundation for a new calendar - the Gregorian calendar.


The older calendar system introduced by Julius Caesar which became effective on January 1, 45 BC, was not precise enough (365 1/4 days). The Julian calendar year gained about three days every four centuries compared to observed equinox times and the seasons.


Imagine, the Greek astronomer Hipparchos from Nicaea already determined around 150 BC the length of the tropical year (or solar year) and verified the original approach of the Babylonians. The established value was 365 days, 5 hours and 55 minutes.


In comparison, as of January 1, 2000, the mean tropical year current value is of 365 days, 5 hours, 48 minutes and  45.19 seconds.


The tropical year was a few minutes shorter than 365.25 days - the Julian calendar did not compensate for this difference.


Pope Gregory XIII therefore reformed the Julian calendar and corrected 10 days of accumulated timing difference. After October 4, 1582, followed immediately October 15.


The 10 days between October 4 and October 15, 1582, have never existed!

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