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.

Lunar Phase Explorer

Move the Moon around an idealized circular orbit and see the overhead geometry, visible illuminated disk and observer horizon update together. Choose all eight named phases, rotate the observer independently, or animate and step a selected model coordinate. Export the exact current diagrams as SVG with a selectable description of the geometry.

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.

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