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Methodology in progress

The science

Real terrain, real geometry, shown plainly.

This page describes how Shadowline is designed to work. The data integrations below are planned. Every visual on the site today is procedural or illustrative.

Why actual terrain matters

The Moon's spin axis is tilted only about one and a half degrees from the plane of its orbit around the Sun. At the poles, the Sun never rises high. It circles the horizon, skimming the skyline once every lunar day.

With light arriving almost sideways, local topography decides everything. Some crater floors have not seen direct sunlight in billions of years, while nearby ridges stay lit for much of the year. Earth hangs low in the sky too, so the same ridges that cast shadows can also block communications.

A flat-ground model gets this wrong. Shadowline is designed to use measured elevation to compute each site's true horizon.

The horizon mask

One line summarises a site: the skyline angle in every direction. Anything below it is hidden.

Illustrative horizon maskTerrain elevation angle around a site plotted against azimuth. The Sun's path is lit where it rises above the terrain and blocked where the terrain is higher. Earth sits near 182° azimuth. Values are illustrative.0°90°180°270°360°0°2°4°
Terrain mask Sun path, unobstructed Sun path, blocked by terrain EarthIllustrative values
  1. Step 1

    Sample the terrain

    Read elevations around the site from LOLA polar digital elevation models.

  2. Step 2

    Build the horizon mask

    For every compass direction, find the highest angle at which terrain blocks the sky.

  3. Step 3

    Test the Sun and Earth

    Compare their apparent positions from JPL Horizons against the mask, moment by moment.

Data sources

  • NASA LRO · PDS Geosciences Node

    Lunar Orbiter Laser Altimeter (LOLA)

    Polar digital elevation models for terrain geometry and horizon computation.

    Visit LOLA (opens in a new tab)
  • NASA JPL Solar System Dynamics

    JPL Horizons (Horizons)

    Ephemerides for the apparent position of the Sun and Earth from a lunar surface site.

    Visit Horizons (opens in a new tab)

What the model must account for

  • Terrain resolution

    Features smaller than the elevation grid can't appear in the horizon. Mask accuracy depends on the DEM chosen for each region.

  • The Sun is a disc, not a point

    At about half a degree wide, the Sun can be partly hidden by a ridge. Partial illumination needs to be handled explicitly.

  • Earth's apparent size and libration

    Earth spans roughly two degrees and wanders by several degrees over a month, so visibility can change gradually.

  • Height above the surface

    A solar panel or antenna on a mast sees a lower horizon than the ground. Observer height will be a site setting.

Glossary

Horizon mask
The elevation angle of the terrain skyline in every direction around a site.
Solar elevation
The Sun's angle above the local horizontal plane.
Permanently shadowed region
Ground, often a crater floor, that never receives direct sunlight.
Libration
The slow apparent wobble of the Moon that shifts Earth's position in the lunar sky.
Direct-to-Earth
Communication from the lunar surface straight to Earth, which needs an unobstructed line of sight.
Current status: no LOLA terrain or Horizons ephemerides are loaded yet. Terrain on this site is generated procedurally, and the horizon charts use an illustrative profile.