Documentation / @interstellar-tools/equations / trueAnomalyAt

Function: trueAnomalyAt()

function trueAnomalyAt(
  orbit: Pick<OrbitInterface, 'e' | 'angle' | 'period'>,
  date: Date
): Radians;

Defined in: anomalies/anomaly-at.ts:86

The true anomaly ($\nu$) of an orbit at a given date: where the body is.

Takes the mean anomaly at date from meanAnomalyAt, solves Kepler's equation with solveKepler, and converts the result with eccentricToTrueAnomaly.

Parameters

Parameter	Type	Description
orbit	Pick<OrbitInterface, "e"	"angle"
date	Date	The date to find the body at.

Returns

Radians

The true anomaly at date, in $[0,2\pi )$.

Throws

If date is not a valid Date.

Throws

If the orbit's eccentricity is outside $0\le e<1$.

Example

import { PLANETS } from '@interstellar-tools/constants';
import { trueAnomalyAt } from '@interstellar-tools/equations';

const earth = PLANETS.find((planet) => planet.id === 'earth')!;

trueAnomalyAt(earth.orbit!, new Date('2026-10-17T12:00:00Z')); // ≈ 4.901 rad (280.8°)