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date1 = saint_andrew (year);
date2 = DOW_closest_to_date (DOW_SUNDAY, &date1); |
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Note that the DOW "closest" function is inclusive, so that date2 will be date1 if date1 falls on a Sunday. If the rule defining the day of observance is not inclusive, then it specifically must define an additional rule for the case that the given date already falls on the given DOW. |
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2.6.) The date N days before or after some other base date. |
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2.6.1.) The base date is an immovable feast. If no intercalary day can intervene, use the fixed-date rule. Otherwise, use the days-before or days-after rule. The example of Epiphany in this second case would be: |
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date1 = christmas (year);
date2 = date_N_days_after_date (12, &date1); |
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2.6.2.) The base date is a movable feast. |
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2.6.2.1.) The movable feast is defined in terms of its DOW. See the discussion in section 2.4.2.1. |
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2.6.2.2.) The movable feast is not defined in terms of its DOW. See the discussion in section 2.4.2.2. |
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2.7.) Dates specifically tied to astronomical events. |
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2.7.1.) Dates tied to actual astronomical events. The SCDTL system does not currently provide functions for performing the celestial mechanical, or astrophysical, calculations required to predict actual astronomical events. |
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2.7.1.1.) Lunar events. Exact predictions of the date and time of the new moon, full moon, or any given lunar phase fall into this category. Historically, many early calendars began by defining the start of a month as the day of the new moon, or the first visibility of the lunar crescent. |
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2.7.1.2.) Solar events. The actual dates and times of the summer and winter solstices and the vernal and autumnal equinoces vary with time. Many solar calendars, or calendars that attempt to correlate with solar phenomena, assume fixed dates for these phenomena. |
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2.7.1.3.) Other astronomical. The heliacal rising of Sirius in the Sothic cycle is of significance in the consideration of the ancient Egyptian calendar and is probably the example best known at the current time. Maya studies have shed light on the significance of the heliacal rising of Venus and other planets as well. |
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2.7.2.) Dates tied to calculated or averaged astronomical events. |
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2.7.2.1.) Lunar events. Easter is the best example. |
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