Part of the Astronomy Bundle.

Lunar Limb Profile

The lunar-limb-profile package provides the LunarLimbProfile object for computing the height of the Moon's visible limb silhouette from LOLA/LDEM terrain data. Given a libration state and the observer's distance, it evaluates the apparent terrain radius along the line of sight and returns the limb height above the LDEM reference sphere at a celestial position angle. This is useful for modelling the irregular lunar limb in occultations, grazing occultations, and the Baily's beads phase of a solar eclipse.

Contents

• Install
• Terrain data
• API Reference
  • Create the lunar limb profile
  • Reference radius
  • Limb height at a position angle

Install

With npm: npm install @astronomy-bundle/lunar-limb-profile
With yarn: yarn add @astronomy-bundle/lunar-limb-profile
With pnpm: pnpm add @astronomy-bundle/lunar-limb-profile

Terrain data

The profile is derived from the LOLA Lunar Digital Elevation Model (LDEM), distributed as a raw 16-bit little-endian raster (.img) alongside a PDS label (.lbl) that describes the grid geometry. Both are passed to create as plain data — reading them from disk (or fetching them over the network) is the caller's responsibility. The ldem_16 product (16 pixels/degree) is a good default; higher-resolution products such as ldem_64 trade a much larger file for finer detail.

The datasets can be downloaded from the LRO/LOLA GDR DEM archive: https://ode.rsl.wustl.edu/moon/pagehelp/Content/Missions_Instruments/LRO/LOLA/GDR/GDRDEM.htm

API Reference

Create the lunar limb profile

Description: The LunarLimbProfile object is created from the LDEM raster bytes and the text of its PDS label. The label is parsed immediately to derive the grid geometry; the raster is validated against that geometry on first use.

Example: Create a lunar limb profile from the ldem_16 product

import {readFileSync} from 'node:fs';
import {LunarLimbProfile} from '@astronomy-bundle/lunar-limb-profile';

const imgData = readFileSync('ldem_16.img');
const label = readFileSync('ldem_16.lbl', 'utf8');

const profile = LunarLimbProfile.create(imgData, label);

Reference radius

Description: getReferenceRadiusKm() returns the radius of the LDEM reference sphere in kilometres. Limb heights are measured relative to this sphere.

Example: Get the reference radius

import {readFileSync} from 'node:fs';
import {LunarLimbProfile} from '@astronomy-bundle/lunar-limb-profile';

const imgData = readFileSync('ldem_16.img');
const label = readFileSync('ldem_16.lbl', 'utf8');

const profile = LunarLimbProfile.create(imgData, label);

const referenceRadiusKm = profile.getReferenceRadiusKm();

The result of the calculation should be:
Reference radius: 1737.4 km

Limb height at a position angle

Description: getLimbHeightKm(positionAngleDeg, libration, axisPositionAngleDeg, observerDistanceKm) returns the height of the visible limb silhouette in kilometres — the maximum apparent terrain radius along the line of sight, in perspective from the given observer distance — above the apparent radius of the LDEM reference sphere, at the given celestial position angle.

• positionAngleDeg — celestial position angle of the limb point, measured eastward from the north point of the disk.
• libration — the Moon's optical libration as {longitude, latitude} in degrees.
• axisPositionAngleDeg — position angle of the Moon's axis of rotation, used to rotate the celestial position angle into the selenographic frame.
• observerDistanceKm — distance from the observer to the Moon in kilometres.

Example: Get the limb height at several position angles

import {readFileSync} from 'node:fs';
import {LunarLimbProfile} from '@astronomy-bundle/lunar-limb-profile';

const imgData = readFileSync('ldem_16.img');
const label = readFileSync('ldem_16.lbl', 'utf8');

const profile = LunarLimbProfile.create(imgData, label);

const libration = {longitude: 2.0, latitude: -0.1};
const axisPositionAngleDeg = 340;
const observerDistanceKm = 380000;

const north = profile.getLimbHeightKm(0, libration, axisPositionAngleDeg, observerDistanceKm);
const east  = profile.getLimbHeightKm(90, libration, axisPositionAngleDeg, observerDistanceKm);
const south = profile.getLimbHeightKm(180, libration, axisPositionAngleDeg, observerDistanceKm);
const west  = profile.getLimbHeightKm(270, libration, axisPositionAngleDeg, observerDistanceKm);

The result of the calculation should be:
Height at PA 0°: -0.185596 km
Height at PA 90°: 2.115692 km
Height at PA 180°: 1.296247 km
Height at PA 270°: -0.927427 km