What angle convention does the simulator use?

Zero degrees places the Moon between the Earth and Sun and represents new Moon. Angles increase counterclockwise in the overhead diagram. Ninety degrees is first quarter, 180 degrees is full Moon and 270 degrees is last quarter. The illuminated fraction is one minus the cosine of orbital angle, divided by two.

Why does observer rotation leave illumination unchanged?

Illumination depends on the Sun, Earth and Moon geometry. Observer rotation turns the local horizon and changes where the Sun and Moon appear relative to it, but does not change the illuminated fraction. The visible disk uses a waxing-right drawing convention. The horizon is a two-dimensional direction model without refraction, parallax or geographic latitude.

Can I use the diagram without dragging?

Yes. Both coordinates have labelled number fields and sliders, and all eight named phases have buttons. The overhead orbit also accepts arrow keys for one-degree changes, Shift plus arrows for ten degrees, Page Up or Page Down for 45 degrees, Home for new Moon and End for 359 degrees. Angles from minus 36,000 to 36,000 degrees wrap through whole turns.

How do animation and time steps work?

Choose orbital motion or observer rotation, then select a day, hour or minute increment. Each step changes only that coordinate and its model clock. One orbital phase cycle is defined as 30 model days and one observer rotation as 1 model day. Animation advances the selected coordinate at 0.001 to 10 model days per real second, using elapsed time rather than a fixed amount per frame. These are teaching clocks, not calendar predictions.

What happens when I pause or leave the tab?

Pausing cancels the animation frame and preserves the current model. Hiding the tab also pauses it; returning does not add hidden time or automatically resume. With reduced motion enabled, use sliders and time steps. Export and copy pause the model so the delivered diagram or description matches a fixed current state.

What is saved or exported?

Validated settings that you change may be saved locally in your browser. Restored settings open paused. No data are uploaded. The SVG export serializes the actual current overhead, illuminated disk and horizon diagrams, including active overlays and descriptive text. The clipboard description includes angles, illumination, observer directions and the separate model clocks.

How do Moon phases work?

The Sun lights one half of the Moon. As the Moon moves around Earth, the fraction of that lit half facing Earth changes. In this model, new Moon is toward the Sun, full Moon is opposite it and the two quarter phases show a half-lit disk. Ordinary phases come from this changing viewpoint, rather than Earth casting a shadow on the Moon.

Is there a guided lunar phase lab or student exercise?

Choose New Moon, First quarter, Full Moon and Last quarter in order. Record the orbital angle and illuminated fraction for each: 0 degrees and 0%, 90 degrees and 50%, 180 degrees and 100%, then 270 degrees and 50%. Compare the Sun direction and visible disk at each stop. Next leave First quarter selected and change Observer rotation: the horizon directions change while illumination stays at 50%. Save an SVG or copy the description to record a state. This is an exercise for this model; it does not include another course's worksheet or answer key.

Lunar Phase Explorer

Move the Moon around an idealized circular orbit and watch the Sun-Earth-Moon diagram, illuminated disk and observer horizon update together. Choose a named phase, rotate the observer, or animate the model. Use the guided exercise in the FAQ to compare phases, then save the current diagrams as SVG.

One orbit. Three linked views.Idealized geometry · paused by default

Explore the geometry

Orbital angle, degrees0° new Moon toward the Sun; 180° full. Angles increase counterclockwise. Signed angles wrap through whole turns.Orbital angle slider

Observer rotation, degrees0° faces the Sun. Rotation changes the horizon view while the illuminated fraction remains fixed.Observer rotation slider

Animate or stepTime increment

Animation rate, model days / real second0.001 to 10. One orbit is 30 model days; one observer rotation is 1 model day. These are teaching clocks, with no calendar date.

Angle guidesLandmarksSunlight rays

Overhead, visible disk and horizon

Original model preview

Preparing the linked model diagrams.

Change an angle, drag the orbit or step the paused model.

Current phase: Preparing

Illuminated fraction: Preparing

Moon above horizon: Preparing

Model motion: Paused

Selectable geometry, clocks and diagram description

Preparing the current model.

Try the four landmarks

0° / new Moon0% illuminated
The Moon is toward the Sun. Its illuminated hemisphere faces away from the Earth.
90° / first quarter50% illuminated
The Moon is a quarter turn around the orbit. Half of its visible disk is bright.
180° / full Moon100% illuminated
The Moon is opposite the Sun. Its illuminated hemisphere faces the Earth.
270° / last quarter50% illuminated
The bright half switches sides in the schematic disk. Observer rotation still affects only the horizon view.

Accuracy. An idealized educational model with circular coplanar motion; not a date-specific ephemeris, tide prediction or eclipse forecast.

Common questions

What angle convention does the simulator use?: Zero degrees places the Moon between the Earth and Sun and represents new Moon. Angles increase counterclockwise in the overhead diagram. Ninety degrees is first quarter, 180 degrees is full Moon and 270 degrees is last quarter. The illuminated fraction is one minus the cosine of orbital angle, divided by two.
Why does observer rotation leave illumination unchanged?: Illumination depends on the Sun, Earth and Moon geometry. Observer rotation turns the local horizon and changes where the Sun and Moon appear relative to it, but does not change the illuminated fraction. The visible disk uses a waxing-right drawing convention. The horizon is a two-dimensional direction model without refraction, parallax or geographic latitude.
Can I use the diagram without dragging?: Yes. Both coordinates have labelled number fields and sliders, and all eight named phases have buttons. The overhead orbit also accepts arrow keys for one-degree changes, Shift plus arrows for ten degrees, Page Up or Page Down for 45 degrees, Home for new Moon and End for 359 degrees. Angles from minus 36,000 to 36,000 degrees wrap through whole turns.
How do animation and time steps work?: Choose orbital motion or observer rotation, then select a day, hour or minute increment. Each step changes only that coordinate and its model clock. One orbital phase cycle is defined as 30 model days and one observer rotation as 1 model day. Animation advances the selected coordinate at 0.001 to 10 model days per real second, using elapsed time rather than a fixed amount per frame. These are teaching clocks, not calendar predictions.
What happens when I pause or leave the tab?: Pausing cancels the animation frame and preserves the current model. Hiding the tab also pauses it; returning does not add hidden time or automatically resume. With reduced motion enabled, use sliders and time steps. Export and copy pause the model so the delivered diagram or description matches a fixed current state.
What is saved or exported?: Validated settings that you change may be saved locally in your browser. Restored settings open paused. No data are uploaded. The SVG export serializes the actual current overhead, illuminated disk and horizon diagrams, including active overlays and descriptive text. The clipboard description includes angles, illumination, observer directions and the separate model clocks.
How do Moon phases work?: The Sun lights one half of the Moon. As the Moon moves around Earth, the fraction of that lit half facing Earth changes. In this model, new Moon is toward the Sun, full Moon is opposite it and the two quarter phases show a half-lit disk. Ordinary phases come from this changing viewpoint, rather than Earth casting a shadow on the Moon.
Is there a guided lunar phase lab or student exercise?: Choose New Moon, First quarter, Full Moon and Last quarter in order. Record the orbital angle and illuminated fraction for each: 0 degrees and 0%, 90 degrees and 50%, 180 degrees and 100%, then 270 degrees and 50%. Compare the Sun direction and visible disk at each stop. Next leave First quarter selected and change Observer rotation: the horizon directions change while illumination stays at 50%. Save an SVG or copy the description to record a state. This is an exercise for this model; it does not include another course's worksheet or answer key.

An idealized educational model with circular coplanar motion; not a date-specific ephemeris, tide prediction or eclipse forecast.