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Note that the GLC adjustment is not another version of a Metonic rule, relating lunar and solar periods. The solar year never enters this relation. Rather, the object is to relate the day and the lunar month. |
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Having adjusted the average length of the month to the average SLP, the expectation is that the GLC will remain synchronized with the moon, on average, over a period of several millennia. |
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There are several considerations which affect this argument, however. In the case of the SLP, two factors operate in opposite directions. The angular velocity of the Moon is slowing, thus increasing the length of the lunar month. The rotational speed of the Earth is decreasing, thus increasing the length of the mean solar day. If the lunar month is measured in mean solar days, then both factors must be taken into account. Calculations show that the GLC will remain synchronized with the Moon, with decreasing relative accuracy, until circa A.D. 3000. |
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The remaining question is that of choosing an epoch for the calendar. This consideration involved some research by Mr. Meyer to determine an epoch that would maximize the number of occurrences of new moons on the first day of the GLC month or the last day of the preceding month, and to maximize the number of occurrences of full moon on the night of the 14th and 15th of the GLC month. After determining the relevant parameters and generating a number of alternative epochs, Mr. Meyer selected an epoch that produced a best fit for the stated requirements. This epoch was Julian Day number 237,819, which is equated to GLC year 0, month 1, day 1. |
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The dates of solar eclipses provide one convenient relevant database for testing the fit, since these can only occur on new moons. Peter Meyer's work found that during the period from A.D. 1997 to A.D. 2032, a total of 78 solar eclipses can be dated, and that GLC dates fall in the following way: |
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| Number of New Moons | Percentage | Displacement in Days from First of GLC Month | | | | | | | | | | | | |
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In this study, 87% of the new moons occur either on the first day of the GLC month, or on the preceding day. |
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Because of the complexity of lunar motion, a schematic (rule-based) lunar calendar cannot match first-of-month to actual new moon, unless the rules are very complex. As a lunar calendar with a very simple rule set, GLC performs remarkably well. |
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The unit of the week does not enter into the GLC at all. There is, therefore, no unit between the day and the month, and therefore no default method for arranging the days of a month in a structure. The use of a seven-day week would not be incompatible, since that period tends to break lunar phase into four quarters. |
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