The activity pairs an original overhead orbit diagram with a separate Earth-view panel. Four position buttons update both views, and students enter a phase name, the fraction of the whole Moon illuminated, and the bright portion of the apparent disk. A fixed answer check keeps those three answers separate; a written reason remains available for teacher discussion.
Establish the model before playing
Place Earth at the center and the Sun to the right. Read the overhead diagram as a view from north of the orbit. Move counterclockwise through right, top, left, and bottom. The right-hand side of every small Moon marker faces the Sun and stays illuminated. The diagram is schematic and not to scale.
NASA explains ordinary phases as a changing view of the Moon's sunlit half. That gives the activity its constant: half of the whole Moon is sunlit at each of these ordinary positions. What changes is how much of that half appears in Earth's view. The model excludes eclipse events. NASA: Moon phases.
Match the four positions
Start at the right, toward the Sun. The key is new Moon, with none of the directly sunlit portion visible in the idealized Earth-view disk. Move to the top for first quarter: half the sphere remains sunlit, and half the apparent disk is bright. At the left, full Moon shows a full bright disk. At the bottom, last quarter again shows a half-bright disk.
Give one partner the job of naming the phase and the other the job of explaining the two fractions. At first quarter, the successful response is “first quarter / half / half.” “First quarter / quarter / half” gets two of the three fixed answers right and exposes exactly which quantity needs another look.
The quarter name describes progress through the phase cycle. Full describes the appearance of the disk facing Earth; it does not mean sunlight reaches the entire spherical surface. Ask students to point to the overhead marker while explaining the whole Moon, then to the Earth-view panel while explaining its appearance.
Keep first and last quarter distinct
Both quarter positions have a half-bright disk. Their place in the declared counterclockwise sequence distinguishes them. Use the position labels and text fraction for this comparison. A half-bright drawing here does not promise a universal left/right orientation in everyone's sky.
Students can check any position, reveal that position's key, then return to revise an answer. Each position keeps its own responses while the page is open. Switching positions clears the previous feedback, and Reset clears all four response records. There is no date input or prediction of tonight's sky.
Ask the eclipse question separately
The flat overhead drawing puts the new-Moon marker between Earth and the Sun in projection. That does not establish the precise physical alignment needed for an eclipse. The Moon's orbit is tilted; ordinary new and full phases do not imply an eclipse every cycle. Do not introduce an Earth-shadow wedge as the explanation for ordinary phases.
For the last discussion, ask, “Which part changed: the amount lit, or our view of it?” Have students use both labeled panels in their explanation. A correct phase name alone leaves that question unanswered. On paper, record the position, three answers, and reason, with the complete four-position key on a separate teacher sheet.
Copy a prompt for this example
Build an original four-position Moon reasoning activity. Overhead view is from north of the orbit: Earth center, Sun right, light rays leftward, progression counterclockwise right → top → left → bottom. Not to scale; projected positions exclude eclipse events. Each Moon marker has its right, Sun-facing hemisphere illuminated. Keep this diagram separate from a labeled View from Earth panel. Fixed key: right=new Moon/half sphere sunlit/none of disk directly bright; top=first quarter/half/half; left=full Moon/half/full; bottom=last quarter/half/half. Use half-disk illustrations only as fraction schematics, not a universal sky orientation. Explain orbital tilt, quarter as cycle progress, and full as disk appearance. Provide position buttons, three blank answer fields, a teacher-reviewed reason, fixed checks, key reveal, and reset. Preserve separate per-position responses, clear stale feedback on edits, and clear all records on reset. No live phase calendar or Sun-observation activity.
Arrange the overhead model and Earth-view disk side by side on a wide screen and stack them on a phone. Keep the Sun, Earth, position labels and illuminated fractions readable. Put the three answer fields and written reason beneath the diagrams, with a Check answers button and a separate current-key disclosure. Accept first quarter moon as well as first quarter, and last quarter moon or third quarter for the final position. Return one complete HTML file with embedded CSS and JavaScript.
For the next small-group rotation, NOS can display this prepared HTML model with both views together. Let one pair defend its first-quarter answer before revealing the key, then ask the next pair to explain why full Moon still leaves a night side.