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The calendar reform movement which peaked in the 1930s attempted to replace the Gregorian calendar with a calendar more oriented to strictly commercial concerns. The historical background of the various calendars is discussed later in this chapter in the sections concerning the International Fixed Calendar and The World Calendar. |
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The reader may encounter statements that the Gregorian calendar has been subjected to another reform, which has the object of further improving its accuracy. |
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The Gregorian leap year rule produces an estimate of the year length of 365.2425 days per year, which is an error of (365.242199 365.2425 =) 0.000301 days per year, or an error of one day in 3,322.26 years. This amount of error is entirely acceptable for any practical calendar. |
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The alleged reform would add a fourth part to the Gregorian leap year rule, making years evenly divisible by 4,000 common years, which would be a periodic reversal of the third part of the rule. This additional clause to the rule would shave a bit more from the error, creating an error of about 1 day in 20,000 years. |
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The sources that make this claim state the change as a fait accompli, citing no source document, quoting no authority, and giving no date (see, e.g., Abell p. 150). While this reform has been proposed occasionally, it has never been implemented. |
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9.1.4.7
Cycles in the Gregorian Calendar |
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Calendars often contain interesting cycles, in which certain conditions recur after a period of years. You may have heard it said, for example, that dates repeat every 50 years. |
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In this discussion, I will use the notion of year type within the perpetual calendar. There are 14 types of year in the Julian and Gregorian calendars. A Type 1 year is a year that begins on a Sunday. A Type 2 year begins on a Monday, and so on. Type 8 years are similar to Type 1 years, but are leap years, so they differ from Type 1 years after February 28. |
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Because a given year type has a fixed starting day and a fixed duration, it follows that any two years of the same year type have the same structure with regard to the position of the dates and their days of the week (DOW). If a given date falls on a Friday in one year, it will fall on a Friday in any year with the same year type. |
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In the Julian calendar, year types regularly cycle with a period of 28 years. The year 701 was a Type 7 year and so were 729, 757, and 785, etc. |
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The Gregorian calendar, however, interrupted this simple cycle of years by the introduction of its more complex leap year rule. Cycles are found in the Gregorian calendar, but they are more complex. |
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Beginning with the very first year of the Gregorian calendar, 1583, an interesting cycle was started, which is not found in the earlier Julian calendar. The year 1583 is a Type 7 year. So is the year 1633, which occurs exactly 50 years later. This repetition of year type does not apply to all years in the Gregorian calendar, however. The following year, 1584, is a Type 8 year, but the year 50 years later, 1634, is a Type 1 year. |
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