Equation of Time in Bazi
Understand the equation of time, its sign and seasonal variation, and why it is separate from longitude correction in Bazi true solar time.
Overview
The equation of time is apparent solar time minus mean solar time. It changes with the date because Earth’s orbital motion and axial tilt make the apparent Sun run unevenly relative to a uniform mean Sun. The value is essentially the same for all longitudes on a given date and can approach about sixteen minutes in magnitude. It is added only after longitude has established local mean solar time. OpenFate records the signed value and astronomical method so the final apparent-solar reading can be reproduced.
At a glance
- Definition
- Apparent solar time minus mean solar time
- Cause
- Orbital geometry and axial tilt
- Varies by
- Date, with small year-to-year differences
- Not the same as
- Longitude correction
- Approximate range
- Up to about 16 minutes in magnitude
- Required record
- Signed value, model and calculation date
The equation compares the real Sun with a uniform mean Sun
The U.S. Naval Observatory defines it as apparent solar time minus mean solar time. A positive value means the apparent solar reading is ahead of mean solar time; a negative value means it is behind.
The sign convention must be stated. Reversing the subtraction silently moves the final correction in the wrong direction.
- Store the signed value.
- Name the subtraction convention.
The value changes through the year, not by time zone
Earth’s orbit and axial tilt make the apparent Sun’s motion non-uniform. The difference accumulates and changes sign during the year, reaching roughly sixteen minutes at its largest.
The value is associated with the date and astronomical model. It is not a fixed monthly table entry and not a local daylight-saving adjustment.
- Calculate for the relevant date.
- Do not substitute a seasonal rule of thumb.
Longitude and equation-of-time corrections solve different differences
Longitude correction converts the effective standard-time basis to local mean solar time. The equation of time then converts mean solar time to apparent solar time.
Combining them into one unexplained number prevents auditing and can double-count a correction. Show both components and their units separately.
- Apply longitude before the equation term.
- Keep both corrections independently visible.
Its Bazi effect depends on proximity to a boundary
A few minutes matter only when the corrected reading crosses an hour branch, the selected day boundary or another explicitly solar-time-based cutoff.
Otherwise the Four Pillars remain unchanged. The equation of time has no independent personality, fortune or event meaning.
- Report unchanged pillars when appropriate.
- Do not interpret the correction as a symbol.
Sources and editorial basis
- U.S. Naval Observatory: The Equation of Time美國海軍天文台將均時差定義為視太陽時減平太陽時，說明符號、年度變化、約十六分鐘幅度及與經度修正的區別An official astronomical explanation of apparent solar time, mean solar time, longitude offsets, and the equation of time.
- IANA: Theory and Pragmatics of the Time Zone DatabaseIANA 民用時間與歷史偏移提供前置時間軸基準；均時差不是時區、UTC 偏移或日光節約調整The IANA reference for time-zone identifiers, historical coverage, aliases, offsets, transitions, and known limits of the database.
- OpenFate Editorial Methodology產品把經度修正與均時差分欄保存，只在帶符號修正跨過已聲明時辰或日界時改柱，不賦予均時差獨立象徵OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.