True solar time correction
One double-hour spans two hours — a few minutes' difference in birth time can cross a boundary and produce a completely different chart. So the clock time must first be converted to local true solar time: this is the first gate of chart correctness.
1. Why clock time is not charting time
Traditional calendrics sets the hour by the sun's actual position at the birthplace (sundial time), while clocks run on the “mean solar time” of the time zone's central meridian. Two layers of deviation sit between them:
- ① Longitude deviation: for every degree the birthplace lies off the zone's central meridian, solar time differs by 4 minutes. Take China: the whole country uses UTC+8 (central meridian 120°E), so Ürümqi (about 87.6°E) runs its clocks more than two hours ahead of local solar time — uncorrected, the double-hour can be off by a full step.
- ② Equation of time: Earth's orbit is elliptical and its axis is tilted, so true solar days vary in length through the year; the gap between true and mean solar time swings between about −14 and +16 minutes over the year.
2. Our correction chain (fixed order)
Step 1 Time-zone normalization: 17 common zones available; clock time is first brought to a uniform base.
Step 2 Longitude correction: ±4 minutes per degree, adjusting for the offset from the zone's central meridian.
Step 3 Equation of time: looked up by birth date, yielding true solar time.
Step 4 Early/late Zi convention: whether the late Zi hour after 23:00 belongs to the same or the next day follows the chosen convention (see the “Early/late Zi hour” article).
Every correction is labeled on the chart — you can always see the hour “before → after”, with no invisible black-box conversion.
3. What if the hour is uncertain
Remembering only “morning” or “afternoon” but not the exact time is common. We suggest casting a reference chart at the Wu hour (11:00–13:00) first, then using theHour Calibrationto back-solve the best-fitting hour from known life events.