PlanetSpintype

The spin of another planet, as the IAU WGCCRE gives it: where its north pole points, and how far its prime meridian has turned.

The pole is a right ascension and declination in the ICRF (the same J2000 frame as the star positions). The prime meridian is W = w0 + rate * d, with d the days since J2000.0 (2000 January 1.5): w0 is W at J2000 in degrees and rate the degrees a day (negative for a planet that turns backwards, such as Venus). W is the angle, east, from the point where the planet's equator crosses the ICRF equator going north (the right ascension of the pole plus 90 degrees) to its prime meridian.

import type { PlanetSpin } from "@johnmorrisdotca/hoshi-stars/sky";

interface PlanetSpin {
    pole: SkyPoint;
    primeMeridian: {
        w0: number;
        rate: number;
    };
}

import type { PlanetSpin } from "@johnmorrisdotca/hoshi-stars/sky";

// Mars, IAU 2015 values at J2000 (the slow drift of the pole is left out)
const mars: PlanetSpin = { pole: { rightAscension: 317.269202, declination: 54.432516 }, primeMeridian: { w0: 176.049863, rate: 350.891982443297 } };
console.log(mars.pole.declination);

54.432516