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The 19th century saw a good deal more participation by amateurs in such matters as calendric calculations. The periodicals of the day are filled with simple schemes for calculating the dates of Easter, reckoning the day of the week (DOW) from a date, producing a better calendar, and so on. |
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The author's grandmother, Esther Gillett Latham, encountered such an article in her own research and passed it along. The original unfortunately lacks a bibliographic citation, so no credit properly can be given to the original author. The article describes a simple rule for determining the DOW for any date. Although the rule does not work as described, I learned much by deduction from the pattern of limitations and errors and ended by writing several functions for performing the original task. |
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These functions are defined and tested in the test program DTLT_154.C. |
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The estimated_delivery_date () function is presented in Chapter 1 as an example, to illustrate the general form of an SCDTL function. |
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This function is defined and tested in the test program DTLT_177.C. |
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The bulk of the calendars treated in this work are schematic, in the sense that they calculate the position of the Sun or Moon by a rule, which is often simple. Calendars that depend upon the observation of an actual astronomical event, such as the new moon, however, are an entirely different matter. In this work, only the Chinese calendar falls into this category, and adequate historical records exist for the past for this calendar. |
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The position of the moon can be calculated, but the required code involves quite a bit of celestial mechanics and mathematical astronomy. To treat these subjects at a level of detail consistent with the rest of this work would involve at least another large chapter, and introduce a large amount of material largely for the single purpose of allowing the limited extension of the Chinese calendar to future dates. |
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After some debate, it was decided that the current edition of this work should not be expanded in such a way. Rather, a type of compromise was achieved, by the terms of which some canned restricted-purpose lunar phase routines would be introduced. These routines allow the calculation of lunar phases with good accuracy for about 100 years on either side of the epoch A.D. 2000. They may thus be used to extend the Chinese calendar into the future in a restricted way and for other lunar phase applications. |
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These routines are not, however, explained in detail, nor is any background material included for them. They are presented in hard-coded form, along with some auxiliary routines. |
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