BAZI KNOWLEDGE BASE
What is true solar time? Clock time, longitude and the equation of time
True solar time usually means local apparent solar time: time measured from the Sun’s actual position, with noon at its passage across the local meridian. It differs from a clock because a whole time zone shares one clock face while solar position varies by longitude. The Sun’s apparent motion also varies through the year. Longitude correction handles location; the equation of time handles the variable part.
First distinguish the four names
Solar time does not reset everyone’s clock to one nationally correct reading. It is a local time representation; civil and solar clocks can show different readings for the same instant.
| Time scale | What it represents |
|---|---|
| Civil | The local legal clock for that date, possibly including daylight saving |
| Standard | The baseline without a seasonal daylight-saving advance |
| Local mean solar | An imaginary uniformly moving mean Sun; varies by longitude |
| Local apparent solar | The actual solar position; often called true solar time |
A degree of longitude corresponds to about four minutes
Use east-positive and west-negative longitude here. Resolve the civil record to UTC using its actual offset. Local mean solar time is approximately UTC plus longitude multiplied by four minutes. From standard time without daylight saving, use standard time plus four minutes times the difference between local longitude and the reference meridian.
Within one standard zone, a point east of the reference meridian has a larger mean-solar clock reading; a point west has a smaller one. Keep the coordinate and arithmetic sign conventions consistent instead of negating west longitude twice.
The equation of time is a separate, date-dependent step
Here the equation of time means apparent solar time minus mean solar time. Earth’s axial tilt and orbital shape make that difference vary by date. At the same longitude, different dates can therefore have different values.
Apparent solar time equals mean solar time plus this signed difference. A negative value reduces the reading; a positive value increases it. Some references use the reverse definition, so check the sign convention before importing a value into the formula.
A worked example beginning at 11:05 standard time
Suppose a location uses UTC+8 without daylight saving and lies at longitude 116.4° east. Relative to the 120° reference meridian, the difference is −3.6°. Multiplying by four minutes gives −14 minutes 24 seconds. From 11:05, local mean solar time becomes 10:50:36.
Now suppose an illustrative date has an equation of time of −6 minutes. Apparent solar time becomes 10:44:36. That −6 is a demonstration value, not a real birth-date result; an actual date requires its own value. Stopping at 10:50:36 does not yet include the equation of time.
A correction can change an hour branch, not the birth event
The example moves from a Wu-hour clock reading into Si, showing why a boundary can matter. If both readings remain in one interval, the branch stays unchanged. Near midnight, civil and solar dates may also differ, requiring a separate explanation of the Day Pillar’s date convention.
For a valid birthplace and known time, Clarify currently uses longitude and an approximate equation of time for its Hour Pillar (the two-symbol column for the birth hour). It retains the entered civil date for the Day Pillar instead of moving the whole chart to a solar date. It is not a user-configurable panel offering every mean/apparent-solar and rollover option.
Keep the original clock input to avoid double correction
If historical time-zone conversion has already handled daylight saving, do not subtract it again. Likewise, do not enter an already corrected solar time as the original local clock. Overseas births start with the local record and local zone without an intermediate Beijing-time conversion.
Astronomy explains how time scales relate. It does not by itself establish which BaZi school must be used or validate personality predictions. Understanding the correction helps explain a result and its uncertainty rather than adding unsupported precision to a vague record.