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About this tool
Interactive celestial optics simulator for Solar and Lunar eclipses with Umbra and Penumbra shadow cone geometry.
Solar & Lunar Eclipse Simulator draws the Sun–Moon–Earth shadow geometry on a live canvas so you can see why an eclipse is total, annular, partial or penumbral. You switch between the solar arrangement (Sun → Moon → Earth) and the lunar one (Sun → Earth → Moon), slide the alignment off centre, and move the Moon between perigee and apogee, while the umbra and penumbra cones and the resulting eclipse classification update in real time. It is a schematic teaching model rather than a to-scale ephemeris, aimed at students and teachers who need to see the shadow cones rather than compute a date.
Open Solar & Lunar Eclipse Simulator on AltFTool — it loads instantly in your browser.
Press Solar Eclipse for the Sun-Moon-Earth arrangement or Lunar Eclipse for Sun-Earth-Moon.
Drag Orbital Alignment Shift across its -45 to 45 range and move Earth-Moon Distance between Perigee (Near) and Apogee (Far).
Read Umbra Coverage and the Shadow Zone label of Umbra Cone, Penumbra Cone or Outside Shadow, or press Animate Transit to sweep the whole alignment range.
The shadow is rendered as tangent-line geometry from the Sun's limbs rather than a flat disc, which is what makes the total-versus-partial distinction visible.
Perigee-to-apogee distance and orbital alignment offset are independent sliders, so you can isolate which one turns a total eclipse into an annular one.
One button sweeps the alignment through the full range so the eclipse progresses from outside the shadow, through partial, to maximum and out again — switch to Lunar mode first to also see the penumbral stage.
The umbra is the inner cone where the light source is completely blocked; the penumbra is the surrounding region where only part of the Sun's disc is hidden. Standing in the umbra during a solar eclipse gives you totality, while the penumbra gives a partial eclipse — the simulator shades the two regions differently so you can see which one a location falls in.
Because the Moon is too far from Earth for its umbra to reach the surface, so a ring of the Sun's disc stays visible. In the simulator, pushing the Earth–Moon distance slider past the perigee/apogee midpoint switches a perfectly aligned solar eclipse from total to annular for exactly that reason.
The Moon's orbit is tilted about 5 degrees to the plane of Earth's orbit, so at most new and full moons the shadow cone passes above or below its target. Moving the alignment slider away from zero reproduces this: past a small offset the umbra misses entirely and only a partial or penumbral event remains.
No — the Sun, Earth and Moon are drawn at exaggerated relative sizes and distances so the shadow cones fit on one screen. The geometry of how the cones form and which eclipse type results is faithful; the numbers are not, so use it to understand mechanism rather than to predict a real eclipse.