Purple–White Stars 紫白 Glossary

The annual/monthly (and notoriously disputed daily/hourly) flying-star cycles used in date selection — where our engine refuses to output a day/hour star at all rather than guess one.

Zi Bai · 紫白飛星 · 紫白 · zǐ bái · purple-white · 年白 月白 日白 時白

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

What it is

A star cycle layered on the almanac: a given year, month (solar-term month), day or hour carries one of the nine stars in its own progression — year purple-white （年白）, month （月白）, day （日白）, hour （時白）. The year and month cycles key to the solar-term boundaries; day and hour cycles have multiple transmitted starting rules that do not agree.

School differences — and the fail-closed line

Day and hour purple-white starting rules diverge materially across the literature: this is why the engine does not compute them. Rather than pick one transmission and present its numbers with false confidence, the day/hour fields return an explicit "not closed" error — the fail-closed policy. Fewer numbers, no invented ones. (Year and month cycles are closed and shipped.)

How this engine computes it

• Year/month stars cycle from the exact solar-term boundaries; results appear on almanac pages and in /v1/calendar as year_zi_bai / month_zi_bai.
• day_zi_bai and hour_zi_bai return an explicit error object — surfaced verbatim, never silently omitted or replaced.
• The fail-closed status is registered in the Rule Provenance Register alongside every closed rule.

Related entries

Nine Stars · Xuan Kong Flying Star · Solar Terms · Rule Provenance Register

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