Reference · Your sky: altitude and azimuth for a place and a moment
precessfunction
A J2000 position carried to the mean equator and equinox of a date: the precession of the Earth's axis (Meeus chapter 21, the rigorous method with the IAU 1976 constants, good for a few centuries either side of 2000 to far under an arc-second). It leaves out the star's own proper motion (which starPosition adds), nutation and aberration, together under a minute of arc.
Import it
import { precess } from "@johnmorrisdotca/hoshi-stars/sky";Definition
function precess(position: SkyPoint, when: When): SkyPointParameters
| Name | Type | Description |
|---|---|---|
position | SkyPoint | A right ascension and declination in degrees at J2000. |
when | When | The date to carry it to. |
Returns
SkyPoint
The right ascension (0 up to but not including 360) and declination in degrees.
Examples
import { precess } from "@johnmorrisdotca/hoshi-stars/sky";
// Meeus example 21.b: theta Persei, carried to 2028 November 13.19 (about 15 minutes of arc east in right ascension)
const there = precess({ rightAscension: 41.054063, declination: 49.227750 }, Date.UTC(2028, 10, 13, 4, 33, 36));
console.log(there.rightAscension.toFixed(3), there.declination.toFixed(3)); // 41.547 49.34841.547 49.348What it prints (run when this page was built)