calendar-calculation-method

# Calendar Calculation Method (历法推算) This method describes how to calculate and maintain an accurate calendar for agricultural and ceremonial purposes. ## Core Principles - 履端於始: Begin calculations from the correct starting point - 举正於中: Establish the middle correctly - 归邪於终: Return errors to the end ## Key Calculations ### Determining the Year Start (正朔) Different dynasties used different months: - 夏正以正月 - 殷正以十二月 - 周正以十一月 ### Leap Month Calculation - Track discrepancy between lunar and solar years - Insert leap month when discrepancy reaches one month - 周襄王二十六年闰三月 - Example of incorrect leap month placement ### Seasonal Determination **Four Key Points:** 1. 冬至 - Shortest day 2. 夏至 - Longest day 3. 春分 - Equal day/night (spring) 4. 秋分 - Equal day/night (autumn) ## Calculation Steps ### Step 1: Establish Base Point - Use winter solstice as anchor - Measure shadow length (晷景) - Confirm with natural signs: - 鹿解角 - 兰根出 - 泉水跃 ### Step 2: Track Celestial Movement - Observe sun's position against stars - Note moon phases - Record planetary positions ### Step 3: Calculate Intercalation - 19 years = 235 lunar months (Metonic cycle) - Insert 7 leap months in 19-year cycle ### Step 4: Verify and Adjust - Compare predictions with observations - Adjust calculations as needed - Document all anomalies ## Recording Format Document for each year: - 岁名 - 月名 - 日得 - 大馀/小馀 ## Expected Outcomes - Accurate seasonal predictions - Proper timing for agricultural activities - Correct scheduling of ceremonies ## Validation 1. Confirm the winter solstice anchor point is verified by both shadow length measurement (晷景) and natural signs (鹿解角, 兰根出, 泉水跃) 2. Verify the 19-year Metonic cycle is applied correctly — 7 leap months inserted per 19-year cycle, producing 235 lunar months 3. Check that predictions are compared with actual celestial observations and discrepancies are documented for future adjustment