The Moon's phase at an instant and its named phase #6

New issue

Closed

opened 2026-10-22 22:33:48 -05:00 by jeff · 0 comments

jeff commented

2026-10-22 22:33:48 -05:00

Owner

Part of #1

Parent

What to build

The Moon's phase at an instant, from ch. 48's exact form using the apparent positions of the Moon and the Sun: elongation, phase angle, illuminated fraction, the position angle of the bright limb, whether the Moon is waxing or waning, and its named phase.

Named phases are eight equal 45° sectors of elongation centered on 0°, 45°, …, 315°: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent. The name depends only on the sky, never on a calendar day or a timezone.

Acceptance criteria

Ch. 48's worked example passes to the printed precision
The illuminated fraction is added to the Horizons reference tables and agrees within the stated tolerance
Ranges: phase angle in [0°, 180°], illuminated fraction in [0, 1], elongation and bright limb angle in [0°, 360°)
Properties: the illuminated fraction agrees with the phase angle; the named phase agrees with the elongation sector; 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 phase at an **instant**, from ch. 48's exact form using the apparent positions of the Moon and the Sun: **elongation**, **phase angle**, **illuminated fraction**, the position angle of the **bright limb**, whether the Moon is waxing or waning, and its **named phase**. Named phases are eight equal 45° sectors of elongation centered on 0°, 45°, …, 315°: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent. The name depends only on the sky, never on a calendar day or a timezone. ## Acceptance criteria - [ ] Ch. 48's worked example passes to the printed precision - [ ] The illuminated fraction is added to the Horizons reference tables and agrees within the stated tolerance - [ ] Ranges: phase angle in [0°, 180°], illuminated fraction in [0, 1], elongation and bright limb angle in [0°, 360°) - [ ] Properties: the illuminated fraction agrees with the phase angle; the named phase agrees with the elongation sector; 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

ready-for-agent

label

2026-10-22 22:33:48 -05:00

jeff added a new dependency

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

#4 The Sun's apparent position

jeff added a new dependency

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

#5 The Moon's apparent position

jeff self-assigned this

2026-10-22 23:51:41 -05:00