Longitude Correction in Bazi
See how birthplace longitude and the time-zone standard meridian produce a signed solar-time correction of about four minutes per degree.
Overview
Longitude correction accounts for the fact that one civil time zone gives the same clock time to places on different meridians. Earth turns about fifteen degrees per hour, so the correction is approximately four minutes per degree between the birthplace longitude and the zone’s standard meridian. It must be calculated with one documented east/west sign convention after historical civil time is resolved. Longitude correction alone gives local mean solar time; the equation of time is a separate step.
At a glance
- Earth rotation rate
- About 15 degrees per hour
- Time equivalent
- About four minutes per degree
- Compared values
- Birth longitude and zone standard meridian
- Required convention
- Explicit east-positive or west-positive signs
- Output stage
- Local mean solar time
- Separate component
- Date-dependent equation of time
A zone clock hides local meridian differences
A civil time zone synchronizes a wide geographic area to one standard clock. Solar noon therefore does not occur at the same clock minute at every longitude in the zone.
Longitude correction restores the constant location-dependent part of that difference. It does not change the historical UTC instant recorded for the birth.
- Start from the correct historical zone.
- Keep civil and solar timestamps distinct.
The signed longitude difference drives the calculation
A standard meridian is commonly related to the standard UTC offset at fifteen degrees per hour. The birthplace difference from that meridian is multiplied by about four minutes per degree.
Libraries differ in whether east or west is positive. The formula, coordinate and test vectors must use one sign convention end to end.
- Declare the longitude sign convention.
- Test locations on both sides of the meridian.
Small corrections matter only near chart boundaries
A city-level coordinate is often sufficient when the birth is far from an hour boundary. Near a boundary, even a few minutes of coordinate or clock uncertainty can change the Hour Branch.
If the signed correction moves the time across midnight or the selected 23:00 boundary, the solar date and dependent Day Stem must be recalculated.
- Run boundary sensitivity when minutes matter.
- Do not hide coordinate uncertainty.
Longitude is calculation data, not geographic fate
The correction does not mean that eastern or western birthplaces are inherently favorable, stronger or more spiritual.
Latitude, city prestige and present residence do not belong in this particular formula. Other chart or location systems must remain separately labeled.
- No ranking of places from the correction.
- Do not add unrelated geographic symbolism.
Sources and editorial basis
- U.S. Naval Observatory: The Equation of Time美國海軍天文台說明一小時相當於十五度經度，以及地方經度相對標準子午線所形成的太陽時差An official astronomical explanation of apparent solar time, mean solar time, longitude offsets, and the equation of time.
- IANA Time Zone DatabaseIANA 時區規則先確定出生地歷史標準偏移，才能選出該民用時間所對應的標準子午線與有效基準The maintained reference for historical local-time rules, UTC offsets, and daylight-saving transitions by location.
- OpenFate Editorial Methodologytests/bazi/test-true-solar-time.ts 固定東西經符號與每度約四分鐘換算，並測試標準子午線兩側及跨日界案例OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.