Lesson 1 of 5 · Sunlight, phases & observing
Why the Moon changes shape
8 minute lesson · 3 checks along the way
Your goal for this lessonPredict the illuminated part of the Moon visible from Earth, and distinguish the phase cycle from its orbital period.
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One world, different views of its daylight
The Moon does not physically grow from a sliver into a sphere each month. Its visible light is mainly reflected sunlight. Except during an eclipse, the Sun illuminates roughly half of the lunar globe. As the Moon travels around Earth, we see different amounts of that sunlit hemisphere. Those changing views are the , not pieces of the Moon appearing and disappearing.
A crescent therefore shows a small visible portion of the Moon’s day side; a full Moon shows nearly the whole Earth-facing disk illuminated. Earth’s shadow does not normally cause either shape. Shadows become central during a lunar eclipse, which is a particular alignment we will investigate later. Keep the physical globe, its illuminated half, and the part visible to an observer as three separate ideas.
The Moon photographed from Apollo 8 on December 22, 1968. This view comes from a traveling spacecraft, not an observer standing on Earth. Sunlight illuminates the bright terrain; the visible day-night boundary depends on the observing geometry.
Look closely
Find the curved boundary between bright and dark terrain. Before naming a phase seen from Earth, what should you check about this photograph?
See what to notice
Check the observer’s location. Apollo 8 saw the Moon from a different direction than an observer on Earth. Ordinary lunar phases describe how much of the sunlit hemisphere we see from our viewpoint; this dark region is lunar night, not evidence that Earth’s shadow caused the phase.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Move the Moon, keep track of the observer
At new Moon, the Moon is in approximately the Sun’s direction from Earth, so its sunlit half faces mostly away from us. Around full Moon it is in the opposite direction, so the lit face points toward us. First and last quarter occur between these configurations; half the visible disk is illuminated. Quarter names refer to progress around the phase cycle, not a quarter of the visible disk.
In the activity, keep the sunlight direction fixed and compare the overview with the view from Earth. Predict the illuminated fraction before changing the Moon’s position. The bodies and gaps are enlarged or compressed for clarity. This model isolates phase geometry; it does not show whether the Moon passes exactly through a shadow. It therefore cannot predict an eclipse at every new or full Moon.
Compare two viewpoints
Change your view of the sunlit Moon
Move the Moon through a phase cycle. Compare the view from above with the face we see from Earth. Which part changes: the half lit by the Sun, or how much of that half faces us?
Use the slider’s arrow keys or choose a named phase. A complete cycle takes about 29.5 days; these angles describe geometry, not today’s Moon.
From above Earth’s north side
As seen from Earth
First quarter · 50% illuminated
The visible sunlit area is growing as the Moon waxes. The boundary across the disk divides lunar day from lunar night.
Ordinary phases are different views of the sunlit half, not Earth’s shadow moving across the Moon.
How is this view simplified?
The two pictures use different viewpoints and different scales. The orbit is circular, sunlight is parallel, and the Moon’s disk is a smooth sphere. The diagram omits earthshine, surface markings, libration, and eclipses. The next geometry activity adds the Moon’s tilted orbit to explain why new and full Moons usually avoid an eclipse.
The 29.5-day phase cycle follows the Moon’s position relative to the Sun. Its roughly 27.3-day orbit relative to distant stars is shorter because Earth is also moving around the Sun.
Sources: NASA: Moon phases ↗ and NASA: the Moon’s motion and viewpoints ↗.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Two different ways to count a month
means the illuminated portion we see is increasing; waning means it is decreasing. The sequence runs from new through waxing crescent, first quarter, and waxing gibbous to full, then through waning gibbous, last quarter, and waning crescent. Which side looks bright in a picture depends on the observer’s orientation, so a universal left-side or right-side shortcut can mislead.
The measures the Moon’s return to approximately the same direction relative to distant stars: about 27.3 days. The , a new-Moon-to-new-Moon phase cycle, averages about 29.5 days. During the orbit, Earth also moves around the Sun, so the Moon must travel farther to repeat the Sun–Earth–Moon geometry. The two numbers measure different reference cycles, rather than competing estimates of one interval.
Turn a sighting into a useful record
A Moon journal becomes more useful when it records date, time, location, phase, and the direction of the horizon. A first-quarter Moon is often visible in the afternoon and evening; a full Moon generally rises near sunset. Exact rise times depend on date and location. The Moon’s daytime appearance is ordinary: reflected sunlight can remain visible against a blue sky.
Compare several observations instead of forcing one drawing to answer every question. Has the illuminated fraction increased? Has the Moon moved relative to the background stars? Was a missing observation caused by cloud rather than a missing Moon? Use the on-screen activity or published imagery for Sun–Moon comparisons; never search close to the Sun with your eyes, binoculars, or a telescope for this lesson.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
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