True Solar Time 真太陽時 Glossary

The sun's actual position expressed as time: local mean solar time plus the equation of time — a correction of up to ±16 minutes against the clock, and far more against a time-zone offset.

Zhēn tàiyáng shí · 真太阳时 (simplified) · 真太陽時 (traditional)

Source credibility (S/A/B/C/X): S    Rule computability (A/B/C): A

What it is

The traditional double-hour （時辰） was defined by the sun, not by a government time zone: noon is when the sun crosses the local meridian. A birth certificate records civil clock time, so converting it to the chart's time pillar requires undoing two artificial layers. The first is longitude: every degree west of the time zone's central meridian shifts local mean solar time by 4 minutes (for UTC+8, central meridian 120°E). The second is the equation of time （均時差）: the sun runs ahead of or behind mean solar time by up to about ±16 minutes over the year. In formula form, the total displacement is s = 4 × (λ − 120°) + EoT minutes.

The combined correction is large. Beijing sits about 14.4 minutes "behind" the clock before the equation of time is added; Urumqi, nearly 130 minutes. Since a double-hour lasts 120 minutes, the probability that the correction alone moves a birth across an hour-pillar boundary is roughly |s|/120 — about 12% for Beijing and effectively 100% for Urumqi. For a birth time known only to within ±30 minutes, the hour pillar is uncertain with about 50% probability; the day pillar (one of twelve, cycling every 720 minutes) moves with probability Δ/720.

School differences

The physics is not disputed; the practice is. Many charts use civil clock time directly, and among tools that do correct, most apply the longitude term only and skip the equation of time — a difference that is directly testable and that the project treats as a public evaluation point rather than a hidden setting. The choice between clock time, mean solar time and true solar time is presented as an explicit comparison, not a default disguised as orthodoxy. Historical time-zone practice (pre-1949 regional zones, wartime UTC+9 in Hong Kong, daylight-saving windows) is handled by the same engine layer; see the School Dispute Register for how boundary-day conventions are disclosed.

How this engine computes it

• True solar time is computed as standard time + 4 min/degree × (local longitude − 120°) + equation of time, with the equation of time sourced from astronomical tables (USNO/IMCCE-grade) and validated across 200 cities × 24 solar terms.
• The engine outputs both hour pillars — computed with and without the true-solar-time correction — and quantifies the difference; it never makes the choice silently.
• Birth-time uncertainty is propagated into explicit probabilities: P(hour pillar changes) = Δ/60, P(day pillar changes) = Δ/720, so a user who "remembers roughly" sees what that actually means.
• Longitude resolution follows a fixed priority (GPS coordinates > district centre > user-entered), and the resolution used is part of the reproducible input record (see input_digest).

Related entries

Early / Late Zi Hour · Year Pillar (Lichun boundary) · input_digest (recomputability)

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