Walk through a shadow
Play an eclipse from beginning to end. Watch the penumbra touch the Moon, then the umbra. Compare the shadow view with the close-up.
FIELD NOTE 05 / ECLIPSE LAB
When worlds align.
Follow the Moon into Earth’s shadow. Then find out why it usually misses.
EARTH’S SHADOW AT THE MOON
Before the shadow arrives.The Moon begins outside the penumbra. Press Play to follow the whole crossing.
Cross-section sizes to scale- Lunar eclipse type
- No eclipse
- Moon inside the umbra
- 0 %
- From the shadow’s centre
- — km
Explore lunar and solar eclipses through large shadow diagrams, a moving tilted orbit, a close-up Moon and the red light bent through Earth’s atmosphere.
The Moon’s passage through Earth’s shadow
Fraction of the Moon’s disk · orbital offset from full moonInside any Earth shadowInside the umbra
Try a starting point
Ready to explore.
Make a prediction. Change one thing. See what happens.
Model notes & references ↗FOLLOW THE SCIENCE
A shadow with more than one story.
At full moon, the Sun and Moon lie on opposite sides of Earth. A lunar eclipse occurs when the Moon also crosses Earth’s shadow. The darker inner region is the umbra, where Earth blocks the entire direct solar disk. In the outer penumbra, part of the Sun remains visible.
The shadow view shows a cross-section at the Moon’s distance. The close-up uses the same geometry to calculate how much direct sunlight reaches different parts of its surface. The smaller cone diagram shows where that cross-section comes from.
A FEW QUESTIONS TO TRY
Let’s see what happens.
Keep the full Moon. Lose the eclipse.
Open Orbit & nodes. Leave the tilt near 5.1°, then move the node away from the Sun. Now let the Moon travel a full orbit.
Remove the borrowed red light
Open Why red? Compare the Moon with and without atmospheric light. The solid Earth still blocks the direct Sun in both cases.
The science in the infographic, one question at a time.
01 / HOW THE SHADOW FORMS
The Sun has a visible width.
Earth can block all of the Sun from one point and only part of it from another. This produces a narrowing umbra and a widening penumbra behind Earth. At the Moon’s distance, the umbra is roughly 2.6 Moon diameters wide in this model.
02 / WHY FULL MOON ISN’T ENOUGH
Two planes must meet.
The Moon’s orbit is tilted about 5.1° to the ecliptic—Earth’s orbital plane. It crosses that plane at two nodes: ascending from south to north, and descending from north to south. A full Moon must be near a node for an eclipse.
| Type | Geometry | What changes on the Moon? |
|---|---|---|
| Penumbral | The Moon enters the penumbra without reaching the umbra. | A subtle loss of light, often difficult to notice. |
| Partial | Part of the Moon enters the umbra. | A curved dark region grows across the surface. |
| Total | The whole Moon fits inside the umbra. | The direct Sun is blocked everywhere on the disk. Atmospheric light can make it look copper-red. |
04 / Earth’s atmosphere both bends and filters the light.
Refraction bends some sunlight around Earth’s limb and into the geometric shadow. Scattering removes more short-wavelength light along the atmospheric path. The light that reaches the eclipsed Moon is therefore rich in reds and oranges. Turning off atmospheric light in the experiment leaves a dark Moon in the umbra.
Dust, clouds and atmospheric structure change the brightness and colour of real eclipses. This illustration explains the route of the light; it does not predict a particular eclipse’s colour. NASA: lunar eclipses and their colours ↗
05 / How often can this happen?
Counting penumbral, partial and total events together, NASA’s catalogue gives two to five lunar eclipses worldwide per calendar year. That is a global count, not the number visible from one place, and many events are penumbral.
Eclipse seasons recur roughly every six months, when the Sun lies near the line of nodes. Whether a particular full Moon makes a penumbral, partial or total eclipse depends on its precise alignment and distance. The seasons permit eclipses; they do not guarantee totality. NASA’s lunar eclipse catalogue ↗ · NASA’s eclipse-season overview ↗
How does a solar eclipse reverse the story?
Choose Solar eclipse in the controls. At new moon, the Moon can stand between Earth and the Sun. The observer view compares the apparent disks from one point on Earth.
Some overlap produces a partial eclipse. If the Moon’s apparent disk covers the Sun, the eclipse is total. If the Moon looks smaller and lies completely within the Sun’s disk, the remaining ring makes an annular eclipse. Solar eclipse type depends on the observer’s position as well as the alignment and Moon distance.
The lunar umbra in this lab belongs to Earth. The dark disk in the solar observer view belongs to the Moon. They are two different consequences of the same three-body alignment.
The mathematics, assumptions & references
A tilted orbit
The Moon moves on a circular ring with adjustable radius, inclination and node direction. The model uses Earth radius 6,371 km, Moon radius 1,737.4 km, Sun radius 695,700 km and a fixed Earth–Sun distance of 149,597,870.7 km. The Sun lies on the positive x-axis.
Lunar offset 0° places the Moon near the anti-solar direction (full moon). Solar offset 0° places it near the solar direction (new moon). The actual position is rotated through the tilted orbital plane. The node control rotates the line where that plane intersects the ecliptic. Lunar illuminated fraction in the orbit view is the geometric approximation (1 − x/d)/2.
Earth’s umbra and penumbra
Ru ≈ RE − L(RS − RE) / D
Rp ≈ RE + L(RS + RE) / D
Rp ≈ RE + L(RS + RE) / D
L is the Moon’s axial distance behind Earth; D is Earth–Sun distance. RE and RS are the physical radii. These similar-triangle, small-angle expressions describe the direct-light shadow of spherical bodies. At L = 384,400 km, the umbral radius is about 4,600 km and the penumbral radius about 8,175 km.
The Moon centre’s perpendicular distance from the shadow axis is √(y² + z²). Circle-overlap areas determine the fraction of the projected lunar disk in the umbra and in the entire shadow. The larger shadow fraction includes the umbra. These are geometric area fractions, not photometric brightnesses.
Across the Moon’s face
For each point in the close-up, the renderer calculates the fraction of the Sun’s disk blocked by Earth. Projected into Earth’s plane, the Sun has radius RSL/(D + L), and the centre separation is qD/(D + L), where q is the point’s offset from the shadow axis. The unblocked fraction sets direct illumination; this makes the penumbral boundary gradual.
The disk uses NASA’s lunar colour texture mapped to a sphere. Depth across the Moon, limb topography and libration are omitted. Atmospheric red light is a small illustrative RGB contribution in the geometric shadow, with display exposure increased during totality. It is not a radiative-transfer solution or a prediction of lunar surface brightness. The atmospheric toggle controls this contribution; it does not change the geometric shadow radii.
Reading the drawings
The shadow cross-section preserves the Moon-to-shadow scale. The cone overview compresses longitudinal distances. The orbit view enlarges Earth and the Moon, and multiplies vertical orbital displacement by four so the small inclination is easy to inspect. The marked nodes remain the true plane intersections. The Why red? paths exaggerate refraction and atmospheric thickness; the spectrum and rays describe different directions, not numerical photon trajectories.
Solar mode
Angular radius α = asin(R / d)
Obscuration = overlap area / (π αSun²)
Obscuration = overlap area / (π αSun²)
Solar mode retains an observer on the subsolar surface of Earth. It computes Sun–Moon angular separation and disk overlap, preserving angular scale in the observer view. A missed solar eclipse at this observer need not be missed everywhere on Earth. The displayed corona is illustrative.
Neither mode includes Earth rotation, real ephemerides, orbital eccentricity, atmospheric enlargement of Earth’s effective eclipse shadow, topography or a location/date forecast. Animations compress hours or a whole orbit into seconds. Hidden pages and Pause stop the simulation clock.
NASA — lunar eclipse types, tilt and red light ↗NASA / Espenak & Meeus — eclipse statistics, nodes and prediction limits ↗NASA — solar eclipses and eclipse seasons ↗Moon texture: NASA Scientific Visualization Studio / LROC CGI Moon Kit ↗Earth texture: NASA/GSFC, Reto Stöckli and Robert Simmon / Blue Marble ↗