Function getMoonPosition

index.js:393–415

(date, lat, lng)

}

export function getMoonPosition(date, lat, lng) {
    const lw = rad * -lng;
    const phi = rad * lat;
    const d = toDays(date);
    const c = moonCoords(toDaysTT(d));    // position series run on Terrestrial Time
    const H = siderealTime(d, lw) - c.ra; // sidereal time stays on UT

    // geocentric parallax (Meeus ch.40) lowers the moon along its vertical circle, leaving
    // azimuth unchanged; sin(parallax) = (Earth radius / distance) * cos(geocentric altitude).
    const hGeo = altitude(H, phi, c.dec);
    const h = hGeo - asin(earthRadius / c.dist * cos(hGeo));

    // parallactic angle, Meeus 14.1
    const pa = atan(sin(H), tan(phi) * cos(c.dec) - sin(c.dec) * cos(H));

    return {
        azimuth: azimuth(H, phi, c.dec),
        // apparent (refraction-corrected) altitude in degrees
        altitude: (h + astroRefraction(h)) / rad,
        distance: c.dist,
        parallacticAngle: pa / rad
    };
}

// moon illumination parameters, Meeus ch. 48 (and idlastro's mphase.pro)
export function getMoonIllumination(date = new Date()) {