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From Wikipedia:

... the Council of Trent authorized Pope Paul III to reform the calendar, requiring that the date of the vernal equinox be restored to that which it held at the time of the First Council of Nicaea in 325 ...

At the time of Gregory's reform there had already been a drift of 10 days since the Council of Nicaea ...

Lilius's formula was a 10-day correction to revert the drift since the Council of Nicaea ...

When the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days.

However, this article shows that this 10 day shift made Gregorian calendar in sync with Julian between 200 and 300 AD, and the drift between Council of Nicaea (325 AD) and the reform (1582 AD) was in fact 9 days.

It’s simple to calculate that between 325 and 1582, 9 years were leap years in Julian but not in (proleptic) Gregorian: 500, 600, 700, 900, 1000, 1100, 1300, 1400, 1500.

So, why did they choose to shift the date by 10 days? Was it a glaring mistake?

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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

(English Translation from Wikisource)

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.

  • Short: "The date was 10 days off. The papal bull said Remove 10 days. Now the retrocalculated date is only 9 day off in comparison, so the original day must have been 9 days off." Youre only stating the original question in different form. – Thorsten S. Jan 16 '18 at 10:55
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    @ThorstenS. I don't know where you're getting 9 days from. Actually this comment makes no sense to me; it reads like series of disjointed partial sentences. Can you restate? – Semaphore Jan 16 '18 at 11:32
  • I keep wondering why they chose to anchor it on Nicaea, not correcting tu Julian, not re-anchoring it to Pessach, not on presumed actual date of crucifixion (in ~33). Know of any debates before the decree that argued in such a way? – LаngLаngС Jan 16 '18 at 15:54
  • @LangLangC Well Nicaea was Passover standardised for Christians, and I suppose no one had anything better to go on for the Crucifixion than the biblical link to Passover. – Semaphore Jan 16 '18 at 17:11
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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.

  • According to en.wikipedia.org/wiki/Gregorian_calendar#Preparation , astronomers of the time did not know the true length of the year with the same precision as we do, and Gregorian mean year was their best approximation. The difference between Gregorian and Julian is 3 days in 400 years, and 1257/400*3=9,4275 days, which rounds to 9, not 10. – Neith Jan 16 '18 at 0:41
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    Anyway, I’d prefer an answer based on historical sources telling how the reform authors’ reasoning went. – Neith Jan 16 '18 at 0:51
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    Using two succesive divisions is a funny way to calculate 1257 * 0.0078 = 9.8 ... – Evargalo Jan 16 '18 at 10:08

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