Sidereal time #11

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opened 2026-10-23 20:43:19 -05:00 by jeff · 1 comment

jeff commented

2026-10-23 20:43:19 -05:00

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Part of #10

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#10

What to build

Sidereal time in astro.earth: Greenwich mean sidereal time (eq. 12.4) and Greenwich apparent sidereal time, which adds the equation of the equinoxes, Δψ cos ε with the true obliquity. Both accept an instant in either time scale; a TT instant is converted to UT through the ΔT model, as positions already are. Results are in [0°, 360°).

Acceptance criteria

Meeus's examples 12.a and 12.b pass to the printed precision
Apparent sidereal time agrees with JPL Horizons' over 1900–2100
Properties: a TT instant and the same moment in UT give the same sidereal time; non-finite instants give NaN
A fuzz target: any finite instant gives a result in [0°, 360°)
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 Horizons tolerance is set only after measuring, with the worst error recorded beside it
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 #10 ## Parent #10 ## What to build **Sidereal time** in `astro.earth`: Greenwich mean sidereal time (eq. 12.4) and Greenwich apparent sidereal time, which adds the equation of the equinoxes, Δψ cos ε with the true obliquity. Both accept an **instant** in either **time scale**; a TT instant is converted to UT through the ΔT model, as positions already are. Results are in [0°, 360°). ## Acceptance criteria - [ ] Meeus's examples 12.a and 12.b pass to the printed precision - [ ] Apparent sidereal time agrees with JPL Horizons' over 1900–2100 - [ ] Properties: a TT instant and the same moment in UT give the same sidereal time; non-finite instants give NaN - [ ] A fuzz target: any finite instant gives a result in [0°, 360°) - [ ] 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 Horizons tolerance is set only after measuring, with the worst error recorded beside it - [ ] 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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2026-10-23 20:43:19 -05:00

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2026-10-23 20:43:29 -05:00

Observer, and the topocentric Sun and Moon #12

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2026-10-23 20:43:48 -05:00

#12 Observer, and the topocentric Sun and Moon

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2026-10-23 20:44:31 -05:00

jeff referenced this issue from a commit

2026-10-23 20:50:41 -05:00

Add mean and apparent sidereal time at Greenwich

jeff closed this issue

2026-10-23 20:50:41 -05:00

jeff commented

2026-10-23 20:50:44 -05:00

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Done in 2491b18. Examples 12.a and 12.b pass (the book's 128.7378734° is a unit out in its last digit). Against Horizons: 0.0042 s at worst before 1962, where its UT is UT1; after that its UT is UTC, and the 0.73 s worst is UT1 − UTC, held to 0.9 s.

Done in 2491b18. Examples 12.a and 12.b pass (the book's 128.7378734° is a unit out in its last digit). Against Horizons: 0.0042 s at worst before 1962, where its UT is UT1; after that its UT is UTC, and the 0.73 s worst is UT1 − UTC, held to 0.9 s.