Mean Solar Time in Bazi
Understand local mean solar time, how longitude shifts it from zone time, and why the equation of time is still needed for true apparent solar time.
Overview
Local mean solar time uses an imaginary “mean Sun” moving uniformly across the sky. It adjusts civil standard time for the birthplace longitude relative to the time-zone meridian, but it does not yet include the date-dependent equation of time. Civil time approximates mean solar time only near the zone’s defining meridian when daylight-saving time is absent. In a true-solar-time pipeline, mean solar time is the intermediate layer before apparent solar time.
At a glance
- Reference
- A fictitious mean Sun moving uniformly
- Location variable
- Observer longitude
- Civil comparison
- Zone meridian and standard offset
- Included correction
- Longitude difference
- Not yet included
- Equation of time
- Pipeline role
- Intermediate step before apparent solar time
Mean solar time removes the Sun’s seasonal speed variation
Astronomers define a fictitious mean Sun that moves at a uniform rate. Its local hour angle provides a smooth twenty-four-hour time scale for each longitude.
The point is mathematical rather than directly observable. A sundial follows the apparent Sun and can differ from this mean scale.
- Use “mean” and “apparent” precisely.
- Do not describe the mean Sun as a physical body.
Each longitude has its own local mean solar time
Earth rotates about fifteen degrees per hour, so one degree of longitude corresponds to roughly four minutes of mean-solar-time difference.
Civil zones place many longitudes on one clock. Only the defining meridian closely matches zone standard time before daylight-saving adjustments.
- Use signed longitude relative to the zone meridian.
- Remove any daylight-saving offset before comparing standard time.
The equation of time separates mean and apparent solar time
The equation of time is defined as apparent solar time minus mean solar time. Its value changes with the date because of Earth’s orbit and axial tilt.
Adding only longitude correction yields local mean solar time, not the final apparent time. A product should label which stage it displays.
- Do not call longitude-only output true solar time.
- Document the equation-of-time sign convention.
This time scale supplies no independent Bazi meaning
Mean solar time can support a transparent calculation pipeline, but it is not a pillar, Ten God, star or favorable element.
If all corrected times remain in the same hour and day intervals, the chart symbols remain unchanged. More calculation detail should not be presented as more destiny certainty.
- Report actual boundary effects.
- No symbolic claim from the intermediate value.
Sources and editorial basis
- U.S. Naval Observatory: The Equation of Time美國海軍天文台區分 local mean solar time 與 apparent solar time：平太陽時先反映經度，真太陽時再加入日期相關均時差An official astronomical explanation of apparent solar time, mean solar time, longitude offsets, and the equation of time.
- IANA Time Zone DatabaseIANA 歷史時區規則提供民用時刻到確定瞬間的前置轉換，不能以平太陽時取代民用時間來源The maintained reference for historical local-time rules, UTC offsets, and daylight-saving transitions by location.
- OpenFate Editorial Methodology產品時間稽核資料分列 civil time、longitude correction、mean solar time、equation of time 與 apparent solar time，避免同義混用OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.