北京古天文台的銅制月晷 moondial

又名太陰晷

可用作量度月出月落，或任何時間的角度（真太陰時）。

中間刻有農曆初一至三十的日期。

Moondial (TaiYin Gui) is used to determine time through observation the directional changes of the moon. It consists of two concentric disks and a vernier in the center. The vernier is used for indicating the time according to the moon

外圈：

24 大格（每時辰分為初正二格）

1大格分 4中格

1格分3小格

24x4x3=288小格

每小格=5分鐘

內圈有360小格

有兩指針。

右方短的為“時針”，中間較長為“分針”。

我的理解：

本身並沒有置閏的概念，並非顯示一般人所認識的農歷。現今很多時月圓（望）並非出現在農曆十五，通常是下一天。

很可能是應用在沒有置閏的年代。所以當中初一至三十的刻度需作調整置閏（correction table)作能對應現今的農曆。大月時有30天，小月有29天。

當月圓時, 月球會在午夜12時經過天頂。

但月球每日會遲~49分鐘才到天頂，需要人手補償/調整這差異。

實際應用在time reckoning意義不大, 祗能“報時”，不能“報日”。祗能大概知道小時或 1/3小時。“銅壺滴漏”或水鐘會更準確ź

用法

1 見視將長針兩端對正成一線

2. 讀出 “時辰”

3. 用補償表，找出相應加或減多少時數。eg 初八就要減 5小時 36分

Reference

Queen's college moondial

Planting by lunar phases

Astroseek on gardening

Blessings of lunar calendar

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