Why was the date moved by 10 days instead of 9 during Gregorian calendar reform?
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The date was moved by ten days because that's how many days the Julian calendar was off by.
You may have thought the time skip was meant to compensate for the drift of nine days between 325 and 1582. Actually, it appears they were correcting the whole Julian calendar around having the equinox fall on the calendar date decided on by the Council of Nicaea.
That is to say, when Wikipedia says the reform was to the vernal equinox to the date set in 325, it meant the literal date, i.e. 21 March. Not the date of 325's equinox corrected for 1582. From inter gravissimas, the papal bull ordering the changes:
Therefore, in order to restore the vernal equinox, which was placed by the fathers of the Council of Nicaea at the twelfth day before the Kalends of April, and to return it to that same place, we direct and ordain: that ten days shall be removed from the month of October of the year 1582
Since the Council used the Julian calendar, in 325 their 21 March was actually 22 March on the proleptic Gregorian calendar. And in fact, the true astronomical equinox that year was on the Julian 20 March. By 1582, the total accumulated discrepancy had reached 10 days.
The 1582 equinox fell on 11 March, also 10 days from ideal. Hence, the number of days skipped.
Upvote:3
The reform corrected the timepoint of Easter, the highest religious ceremony in Christendom. Easter is the first Sunday after the first full moon after spring (fixed on March 21st). Equinox correction (so that the seasons are correct again) needs the true year's length.
The Julian year is 0.0078 days (1 day in 128.2 years) longer than the correct year. As the Julian year was too long, Easter was more and more delayed in time. 1582-325 = 1257 years divided by 128.2 yields 9.8 days. Now we do not have 0.8 days, so we are using 10 days, adding effectively an offset of 0.2 days.
If you now use the shifted value and calculate backwards as the proleptic Gregorian calendar does, you have this 0.2 offset, so the original 10 days offset will only come a bit before 325, in this case February 28th, 300.
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