Function getMoonIllumination

index.js:418–441

(date = new Date())

// moon illumination parameters, Meeus ch. 48 (and idlastro's mphase.pro)
export function getMoonIllumination(date = new Date()) {
    const d = toDaysTT(toDays(date));
    const s = sunCoords(d);
    const m = moonCoords(d);
    const sdist = 149598000; // distance from Earth to Sun in km
    const phi = acos(sin(s.dec) * sin(m.dec) + cos(s.dec) * cos(m.dec) * cos(s.ra - m.ra));
    const inc = atan(sdist * sin(phi), m.dist - sdist * cos(phi));
    const angle = atan(cos(s.dec) * sin(s.ra - m.ra), sin(s.dec) * cos(m.dec) -
        cos(s.dec) * sin(m.dec) * cos(s.ra - m.ra));

    const waxing = angle < 0; // bright limb leads → illuminated fraction is growing (new → full)

    return {
        // illuminated fraction, 0 (new) → 1 (full); reaches the exact extrema only at perfect
        // syzygy (eclipses), so a "full" moon typically peaks a hair under 1 — this is correct
        fraction: (1 + cos(inc)) / 2,
        // phase, 0 → 1: 0 new, 0.25 first quarter, 0.5 full, 0.75 last quarter (waxing for phase < 0.5)
        phase: 0.5 + 0.5 * inc * (waxing ? -1 : 1) / PI,
        // position angle of the bright limb, degrees (v2: all angles emitted in degrees) — subtract
        // getMoonPosition().parallacticAngle, also degrees, to get the zenith-relative tilt
        angle: angle / rad,
        waxing // true while the Moon is waxing (new → full), false while waning (full → new)
    };
}

function hoursLater(date, h) {
    return new Date(date.valueOf() + h * dayMs / 24);