Angles, coordinates, nutation and obliquity #3

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opened 2026-10-22 22:33:46 -05:00 by jeff · 0 comments

jeff commented

2026-10-22 22:33:46 -05:00

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Part of #1

Parent

What to build

astro.Angle, astro.coord and astro.earth. An angle can be made from degrees, radians, or degrees–minutes–seconds, and read in degrees or radians. Ecliptic coordinates convert to equatorial and back for a given obliquity (ch. 13). astro.earth gives the nutation in longitude and in obliquity from the full 63-term IAU 1980 table, and the mean and true obliquity, at an instant in either time scale (ch. 22).

Results are normalized: longitudes and right ascensions in [0°, 360°), latitudes and declinations in [−90°, +90°].

Acceptance criteria

Ch. 13's and ch. 22's worked examples pass to the printed precision
Ecliptic → equatorial → ecliptic round-trips
Properties: every finite result is in its documented range; NaN or ±∞ in gives NaN out
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 `astro.Angle`, `astro.coord` and `astro.earth`. An angle can be made from degrees, radians, or degrees–minutes–seconds, and read in degrees or radians. Ecliptic coordinates convert to equatorial and back for a given **obliquity** (ch. 13). `astro.earth` gives the **nutation** in longitude and in obliquity from the full 63-term IAU 1980 table, and the mean and true obliquity, at an **instant** in either time scale (ch. 22). Results are normalized: longitudes and right ascensions in [0°, 360°), latitudes and declinations in [−90°, +90°]. ## Acceptance criteria - [ ] Ch. 13's and ch. 22's worked examples pass to the printed precision - [ ] Ecliptic → equatorial → ecliptic round-trips - [ ] Properties: every finite result is in its documented range; NaN or ±∞ in gives NaN out - [ ] 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

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2026-10-22 22:33:46 -05:00

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2026-10-22 22:33:51 -05:00

#2 Instants, time scales and ΔT

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2026-10-22 22:33:51 -05:00

#4 The Sun's apparent position

jeff added a new dependency

2026-10-22 22:33:51 -05:00