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Questions and Answers
Which calendar did the Gregorian calendar replace?
What was the reason for advancing the date by 10 days in the Gregorian calendar reform?
What was the last European country to adopt the Gregorian calendar for civil use?
What is the difference between the Julian and Gregorian calendars?
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How often is a leap year in the Gregorian calendar?
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What is the reason for the Catholic Church delaying February feasts after the 23rd by one day in leap years?
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Do all Orthodox churches use the Julian calendar for religious rites and the dating of major feasts?
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Which council authorised Pope Paul III to reform the calendar?
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Who proposed the modification of the leap year in the Gregorian calendar?
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What was the obstacle to calendar reform that Lilius solved?
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Which expert believed that Easter should be computed from the true motions of the Sun and Moon?
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What was the Gregorian leap year rule?
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What did Petrus Pitatus of Verona note about the Gregorian leap year rule?
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What is the mean length of the Gregorian year?
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What was the length of the mean tropical year according to the Prutenic tables?
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Who noticed discrepancies in the tables that provided the Sun's mean longitude?
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Which pope instituted the Gregorian calendar reform?
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What was the motivation for the Gregorian calendar reform?
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What was the error in the Julian calendar?
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When did the Catholic Church consider the increasing divergence between the canonical date of the equinox and observed reality unacceptable?
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Who estimated the error in the Julian calendar to be seven or eight days?
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When did the project for calendar reform begin to be taken up again after it was interrupted by the death of Regiomontanus?
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What is the length of a mean calendar year in the Gregorian calendar?
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Which calendar did the Gregorian calendar replace?
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What was the principal change made in the Gregorian calendar?
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What is the rule for leap years in the Gregorian calendar?
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What is the reason for shortening the average (calendar) year in the Gregorian calendar?
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Which years are not leap years in the Gregorian calendar?
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What was the issue with the Julian calendar that caused the need for the Gregorian reform?
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What was the issue with the Julian algorithm that caused the calendar to drift?
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Study Notes
Calendar Reforms
- The Gregorian calendar replaced the Julian calendar.
Reasons for Reform
- The Julian calendar had an error of 11 minutes per year, resulting in a discrepancy of 10 days from the astronomical seasons.
- The Catholic Church considered the increasing divergence between the canonical date of the equinox and observed reality unacceptable by the 16th century.
Gregorian Calendar Characteristics
- The Gregorian calendar advances the date by 10 days to align with the astronomical seasons.
- The mean length of the Gregorian year is 365.2425 days.
- The length of a mean calendar year in the Gregorian calendar is 365.2425 days.
- The rule for leap years in the Gregorian calendar is that years divisible by 4 are leap years, except for years divisible by 100, but not by 400.
History of Reform
- The Council of Trent authorized Pope Paul III to reform the calendar.
- The project for calendar reform began to be taken up again after it was interrupted by the death of Regiomontanus in 1476.
- Pope Gregory XIII instituted the Gregorian calendar reform in 1582.
People Involved
- Luigi Lilius proposed the modification of the leap year in the Gregorian calendar.
- Christophorus Clavius believed that Easter should be computed from the true motions of the Sun and Moon.
- Petrus Pitatus of Verona noted that the Gregorian leap year rule omitted 3 leap years every 400 years.
Adoption and Obstacles
- The last European country to adopt the Gregorian calendar for civil use was Greece in 1924.
- The obstacle to calendar reform that Lilius solved was the mistake in the Julian calendar's calculation of the solar year.
- Aloysius Lilius noticed discrepancies in the tables that provided the Sun's mean longitude.
Julian Calendar Issues
- The Julian calendar had an error of 7 or 8 days, as estimated by Roger Bacon.
- The issue with the Julian calendar that caused the need for the Gregorian reform was the drift of the calendar away from the astronomical seasons.
- The issue with the Julian algorithm was that it was too long, resulting in a drift of about 11 minutes per year.
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Description
Test your knowledge of the Gregorian calendar and leap years with this quiz. Learn about the 400-year cycle and how February 29 falls on different days of the week.
