constellationAtfunction

The constellation a point on the sky is in. The position is precessed to the equator and equinox of B1875.0, where the IAU's boundaries (Delporte 1930) are defined, and looked up in N. G. Roman's table of them (1987, PASP 99, 695). Coordinates are J2000, the frame of this package's stars, unless when says the position is of date. The constellations tile the sky, so every point is in exactly one; Serpens is one constellation (Ser) although it is in two pieces.

A point within a few arc-seconds of a boundary can fall either side of it: Roman's table has the boundaries to a hundredth of a second of time and the precession here is IAU 1976, not the Newcomb-based one of 1930.

import { constellationAt } from "@johnmorrisdotca/hoshi-stars/boundaries";

function constellationAt(position: { rightAscension: number; declination: number; }, when?: When): Constellation

import { starByName } from "@johnmorrisdotca/hoshi-stars";
import { constellationAt } from "@johnmorrisdotca/hoshi-stars/boundaries";

console.log(constellationAt(starByName("Betelgeuse")!).name);                       // Orion
console.log(constellationAt({ rightAscension: 0, declination: 90 }).abbreviation);   // UMi
console.log(constellationAt({ rightAscension: 83.8, declination: -5.4 }, Date.UTC(2026, 10, 8)).english);

Orion
UMi
Hunter