Principal phases and lunar age #7

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opened 2026-10-22 22:33:49 -05:00 by jeff · 0 comments

jeff commented

2026-10-22 22:33:49 -05:00

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Part of #1

Parent

What to build

Principal phases and lunar age, from ch. 49. Given an instant and a kind (new Moon, first quarter, full Moon, last quarter): the instant of the next one after it and of the previous one before it, and an iterator over every principal phase between two instants, in order. Results are instants in TT. Meeus's lunation number k stays internal.

Lunar age at an instant is the time since the previous new Moon as found by that search, in days.

Ch. 49 has series of its own and does not need the Moon's or the Sun's position.

The float-to-integer step that finds k must check its input first: @intFromFloat panics on NaN and on values out of range. A NaN or infinite instant gives NaN, never a panic.

Acceptance criteria

Ch. 49's worked examples pass to the printed precision
The instants agree with a published table of phases (cited) across 1900–2100 within a stated number of minutes
Properties: the next phase is after the instant and the previous one is before it; consecutive phases go new → first quarter → full → last quarter; lunar age is never negative and is under about 29.9 days
Fuzzing: no panic for any f64 Julian Day, including NaN, ±∞ and extreme finite values
Every new public declaration has a doc comment giving units, time scale and reference frame, and citing the chapter and equation of Meeus it implements
Every source used is in the README's references cited
zig build test, zig build check, zig fmt --exclude zig-pkg --check . and reuse lint pass

Part of #1 ## Parent #1 ## What to build **Principal phases** and **lunar age**, from ch. 49. Given an **instant** and a kind (new Moon, first quarter, full Moon, last quarter): the instant of the next one after it and of the previous one before it, and an iterator over every principal phase between two instants, in order. Results are instants in TT. Meeus's lunation number *k* stays internal. Lunar age at an instant is the time since the previous new Moon as found by that search, in days. Ch. 49 has series of its own and does not need the Moon's or the Sun's position. The float-to-integer step that finds *k* must check its input first: `@intFromFloat` panics on NaN and on values out of range. A NaN or infinite instant gives NaN, never a panic. ## Acceptance criteria - [ ] Ch. 49's worked examples pass to the printed precision - [ ] The instants agree with a published table of phases (cited) across 1900–2100 within a stated number of minutes - [ ] Properties: the next phase is after the instant and the previous one is before it; consecutive phases go new → first quarter → full → last quarter; lunar age is never negative and is under about 29.9 days - [ ] Fuzzing: no panic for any `f64` Julian Day, including NaN, ±∞ and extreme finite values - [ ] Every new public declaration has a doc comment giving units, time scale and reference frame, and citing the chapter and equation of Meeus it implements - [ ] Every source used is in the README's references cited - [ ] `zig build test`, `zig build check`, `zig fmt --exclude zig-pkg --check .` and `reuse lint` pass

jeff added the

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2026-10-22 22:33:49 -05:00

jeff added a new dependency

2026-10-22 22:33:52 -05:00

#2 Instants, time scales and ΔT

jeff self-assigned this

2026-10-22 23:55:39 -05:00

jeff referenced this issue from a commit

2026-10-23 00:24:55 -05:00

Add the principal phases and lunar age