Libration and the position angle of the axis #8

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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

The Moon's libration at an instant, from ch. 53: optical libration and physical libration in longitude and latitude, and their total, and the position angle of the axis.

Libration in longitude and latitude is reported in (−180°, +180°]; the position angle in [0°, 360°). Signs follow ch. 53 exactly: selenographic longitude is positive towards Mare Crisium, and the documentation says so.

Acceptance criteria

Ch. 53's worked example passes to the printed precision, for each of the optical, physical and total libration and the position angle of the axis
Libration is added to the Horizons reference tables and agrees within the stated tolerance, with the sign convention checked against Horizons' own
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 The Moon's **libration** at an **instant**, from ch. 53: **optical libration** and **physical libration** in longitude and latitude, and their total, and the **position angle of the axis**. Libration in longitude and latitude is reported in (−180°, +180°]; the position angle in [0°, 360°). Signs follow ch. 53 exactly: **selenographic** longitude is positive towards Mare Crisium, and the documentation says so. ## Acceptance criteria - [ ] Ch. 53's worked example passes to the printed precision, for each of the optical, physical and total libration and the position angle of the axis - [ ] Libration is added to the Horizons reference tables and agrees within the stated tolerance, with the sign convention checked against Horizons' own - [ ] 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

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

#5 The Moon's apparent position

jeff added a new dependency

2026-10-22 22:33:53 -05:00

#9 The subsolar point and colongitude

jeff self-assigned this

2026-10-23 00:07:15 -05:00