True Solar Time

Rule statement

Civil clocks unify by time zone; true solar time reflects local solar position:

1. Take birth longitude L.
2. Take the zone’s central meridian L0 (e.g. UTC+8 → 120°E).
3. Longitude time offset ≈ 4 × (L − L0) minutes.
4. Add or subtract that day’s equation of time (from an astronomical almanac).
5. Clock time + correction = true solar time for charting.

When it must be considered

• Birth place far from the zone’s central meridian.
• Moment near a double-hour boundary, Zi hour, or solar-term change.

If charting with clock time only, write trueSolarTime = false on the chart header.

Relation to charting

Correction only affects “the moment used to fix day and hour,” which may change the hour pillar and, extremely near day change, the day pillar. Steps embed in Four-Pillar Charting Method.

Boundaries

• Historical time zones and daylight saving need separate handling.
• Different equation-of-time sources differ by seconds to minutes; borderline charts should publish the data source.