A thin sunlit lunar crescent with crater shadows along the terminator against black space.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Earth and Moon enlarged. correct size ratio. separation shortened.

At day 11.8 of the model month, the Moon is 90.3% illuminated as seen from Earth (waxing gibbous). The Moon's orbit is tilted by 5.145°.

How to read these results

The orbit model illustrates phase and eclipse geometry without predicting a calendar date. Lunar terrain data come from NASA SVS, LROC and LOLA, with colours adapted for this view.

Moon phases and Sun–Earth–Moon geometry

Use the positions of the Sun, Earth and Moon to explain the changing illuminated shape we see during a lunar month.

Drag to rotate. Hold Ctrl and scroll to zoom, or pinch on a touch screen.

Why it happens

Sunlight illuminates half the Moon. As the Moon orbits Earth, we see different proportions of that illuminated half, producing the phases; Earth’s shadow is involved only in a lunar eclipse.

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The Artemis II crew photographed this lunar crescent on 7 April 2026. Phase is the visible fraction of the sunlit hemisphere, not Earth's shadow.

Try a comparison

Pause at first quarter and full Moon. Turn on Eclipse alignment and explain why the tilted orbit usually prevents an eclipse.

Reading the result

Sizes and separations are enlarged for inspection; compare physical scale to see how widely separated Earth and Moon really are.

A common misunderstanding

A crescent Moon forms when Earth’s shadow covers part of the Moon.

Sunlight normally illuminates half the Moon. A crescent occurs when we see only a narrow part of that illuminated half from Earth; the rest of the visible disc is mostly on the Moon’s night side. Earth’s shadow darkens the Moon during a lunar eclipse.

Cases

Exam language

Phase

The fraction and shape of the Moon’s sunlit hemisphere visible from Earth as viewing geometry changes through its orbit. Ordinary lunar phases are not caused by Earth’s shadow; that shadow is involved in a lunar eclipse.

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The Artemis II crew photographed this lunar crescent on 7 April 2026. Phase is the visible fraction of the sunlit hemisphere, not Earth's shadow.

Synodic month

The time from one lunar phase to the next occurrence of the same phase, averaging about 29.5 days. It is longer than the Moon’s orbital period relative to the stars because Earth also moves around the Sun.

Umbra

The central part of a shadow where the light source is completely obscured. During a total lunar eclipse the Moon lies within Earth’s umbra. The surrounding penumbra receives light from part of the source and produces a less complete darkening.

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NASA's airborne view of the 21 August 2017 total solar eclipse. The Moon blocks the bright photosphere, revealing the corona.

Orbital inclination

The angle between an orbital plane and a chosen reference plane. The Moon’s orbit is inclined by about five degrees to Earth’s orbital plane. This usually carries the Moon above or below the alignment required for an eclipse.

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SDO's time sequence of the 2012 Venus transit. Such alignments require the planet to be near an orbital node. Historical transit timing helped determine the Earth–Sun distance, now represented by the precisely defined astronomical unit.

Node

One of the two places where the Moon’s orbital plane crosses Earth’s orbital plane. Eclipses require a new or full Moon close enough to a node for the Sun, Earth and Moon to align within the relevant shadow.

Ecliptic

The plane of Earth’s orbit around the Sun, projected onto the sky as the Sun’s apparent annual path. The Moon’s orbital plane is inclined by about five degrees to it, so most new and full Moons do not produce eclipses.

Near side

The hemisphere of the Moon that usually faces Earth because the Moon rotates once per orbit. Libration lets us see somewhat more than half its surface over time.

Penumbra

The part of a shadow where the light source is only partly obscured. On the Moon it produces subtler dimming than the umbra.

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NASA Kennedy Space Center photograph, 20 January 2000. Earth's shadow falls on the Moon; sunlight filtered through Earth's atmosphere gives the eclipsed surface a reddish colour.

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