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真太阳时

钟表上的时间是按时区统一的“民用时”。真太阳时以太阳在当地实际的位置为准,正午即太阳最高的时刻。一些排盘流派主张按真太阳时定时辰。

换算的两部分

  1. 经度修正:经度每差 1°,太阳时差 4 分钟。修正值 = 出生地经度 × 4 分钟 − 当时的 UTC 偏移(含夏令时)。例如北京时间(UTC+8,基准 120°E)在乌鲁木齐(约 87.6°E),约慢 130 分钟。
  2. 均时差:地球轨道是椭圆且地轴倾斜,真太阳时与平太阳时一年中相差约 −14 到 +16 分钟。应用使用美国 NOAA 公布的近似公式。

真太阳时 = 民用时 + 经度修正 + 均时差。修正后可能跨到前一天或后一天,应用会显示换算明细与实际用于排盘的日期和时辰。

应用中的做法

是否采用真太阳时属于流派选择,没有对错之分。

True solar time

Clock time is civil time, shared across a timezone. True (apparent) solar time follows the sun's actual position at the birthplace, with noon when the sun is highest. Some schools assign the birth hour by true solar time.

  1. Longitude correction: 4 minutes per degree. Correction = longitude × 4 min − the UTC offset in effect (including daylight saving).
  2. Equation of time: because of the elliptical orbit and axial tilt, apparent and mean solar time differ by about −14 to +16 minutes over the year. The app uses NOAA's published approximation.

True solar time = civil time + longitude correction + equation of time. The result can cross midnight; the app shows the breakdown and the date and hour actually used.

It is off by default and enabled per profile under Advanced calculation. The longitude must be entered by hand. Nonexistent and repeated clock times at daylight-saving changes are detected. Results within 2 minutes of an hour boundary get a warning, since the equation of time is approximate.

Whether to use true solar time is a choice of school, not a matter of right or wrong.