Moon Phase Explorer
Follow New Moon, crescents, quarters, and Full Moon in a living 3D system.
First quarter
Phases reveal different amounts of the Moon’s sunlit half. They are not caused by Earth’s shadow.
Dark core: umbra · soft edge: penumbra.
Close-ups are separate inspection cameras.
Drag to orbit. Right-drag or Shift-drag to pan. Scroll to zoom. Touch: one finger rotates; two fingers pan and pinch. Arrow keys rotate; Shift + arrows pan.
- Solar shadow on Earth Computed from physical geometry
- Earth shadow at the Moon
- Moon phase from Earth
- Your surface observer
Surface station
The observer’s location rotates with Earth. Change the station here, then use Surface observer to look around its sky.
Time & visibility
Physical scale reveals the geometric umbra and penumbra cones. Classroom mode uses surface close-ups instead of distorted cones.
Camera field of view: 45°.
Starting 3D renderer…
Classroom layout compresses distances so the bodies are easy to see; it does not preserve apparent sizes or sightlines. Surface lighting and eclipses always use physical geometry. Surface observer automatically uses physical sightlines. Use Physical distances for undistorted space geometry.
Start paused at First quarter. Drag the labeled time slider or click a phase to explore the lunar month.
Description
Explore the Moon’s changing appearance from New Moon through crescents, quarters, and Full Moon, and back again. The large phase disk and Earth-facing Moon close-up show the part of the sunlit hemisphere visible from Earth. Use the four 3D cameras to connect that view to the positions of Earth, Moon, and Sun. Solar and lunar eclipses remain available in “Also explore eclipses.”
Instructions
- Slide through a lunar month. Drag the labeled time bar or click any of its nine markers. Phase markers use this model’s actual orbital geometry, so they are not spaced at exactly equal day intervals. Use the cycle arrows to explore another month. Playback and reverse time work with every camera.
- Look from a spinning Earth. Surface observer begins with the Moon overhead at a useful telescope magnification. Track Moon keeps it centered with a stable telescope orientation as Earth turns. Drag to release tracking and look freely. Wide sky gives a 65° view of the horizon; Moon telescope centers and magnifies the Moon. Aim at Moon or Home camera also restores tracking. Earth can carry the Moon below your horizon—this is normal. Moon overhead relocates your station once so you can see it again, without changing time.
- Use a predictable camera. Drag to rotate or look. Choose Pan / move station to pan with an ordinary drag, or use right-drag, middle-drag, or Shift-drag. Scroll to zoom; sensitivity scales with magnification. Touch supports one-finger dragging and two-finger pan/pinch. Arrow keys rotate/look, Shift + arrows pan, +/− zoom, Home resets, and Space plays or pauses while the canvas is focused.
- Compare viewpoints. Earth-centered follows Earth around the Sun. Sun viewpoint is a lookout near the light source. Free space is independent. Space cameras are prevented from entering a planet, and Home camera recovers a useful view. Classroom layout compresses distances; surface view always uses physical sightlines.
- Explore eclipses separately. Solar eclipse and Lunar eclipse jump to model alignments and switch the time bar to a 6-hour window. Their close-ups both face the Sun to reveal moving shadows. Return to Moon phases restores a month-long timeline and the Earth-facing Moon close-up. A New Moon is usually dark even without a solar eclipse; most Full Moons do not enter Earth’s shadow.
Why eclipses are occasional
The Moon’s orbit is inclined by about 5.145° to Earth’s orbital plane. New and full moons therefore often pass above or below the required alignment. The model includes the Moon’s orbital eccentricity, slow nodal regression, and apsidal precession, so shadow placement changes through the year. Earth’s 23.439° axial tilt and approximately 23 h 56 min sidereal rotation move the surface station through the sky.
How shadows are calculated
At each point on Earth or the Moon, the renderer compares the angular disks of the Sun and the intervening body. Their overlap gives the fraction of sunlight blocked. This produces a soft penumbra, a fully blocked umbra, or an annular alignment when the Moon looks smaller than the Sun. The lunar umbra receives a small illustrative red tint to suggest atmospheric refracted light.
This is a teaching model, not an eclipse prediction service. Earth follows a circular orbit; the Moon follows a simplified precessing Kepler ellipse. The reference epoch is an invented central solar alignment. Solar and lunar event classifications use a shadow cross-section near the target center; surface overlap and local observer obscuration are calculated separately. Terrain, atmospheric refraction, detailed scattering, tides, barycentric motion, and real ephemerides are not simulated. Continents are stylized rather than a navigation map. Time stops while the page is hidden. A software 3D renderer is used when WebGL is unavailable. No external graphics libraries, textures, or API requests are needed.
Reference: NASA: Moon phases; NASA: eclipses and the Moon; NASA: orbital inclination and eclipse periodicity. Size and orbital constants use standard approximate Earth, Moon, and Sun values.