Moon Phases
Understand why the Moon changes shape throughout its 29.5-day cycle
The Moon does not produce its own light — it reflects sunlight. As the Moon orbits Earth (taking about 29.5 days for a complete cycle), we see different portions of its sunlit half, creating the phases. At new moon (day 0), the Moon is between Earth and the Sun, so its sunlit side faces away from us and we see darkness. Over the next ~7 days, a growing sliver appears on the right side (waxing crescent). At first quarter (day ~7.4), we see exactly half the sunlit side. The illuminated area continues to grow (waxing gibbous) until full moon (day ~14.8), when Earth is between the Sun and Moon and we see the entire sunlit face. Then the process reverses: waning gibbous, third quarter (day ~22.1), waning crescent, and back to new moon. The Moon always keeps the same face toward Earth (tidal locking). Your latitude on Earth affects the Moon's apparent orientation and path across the sky — observers in the Southern Hemisphere see the Moon 'upside down' compared to Northern Hemisphere viewers.
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Sign in →What is Moon Phases?
The Moon does not make its own light — it shines by reflecting sunlight the same way a mirror reflects a flashlight beam. As the Moon travels around Earth each month, we see different portions of its sunlit half depending on where the Moon is in its orbit relative to the Sun and us. That changing view creates what we call moon phases. At new moon, the Moon sits between Earth and the Sun, so its lit side faces away from us and we see only darkness. Over the next two weeks, the Moon moves to the side and then behind Earth, gradually revealing more and more of its sunlit face. At full moon, Earth is between the Sun and Moon, and we see the entire lit hemisphere glowing in the sky. Then the cycle reverses as the lit portion shrinks back to nothing. The complete journey from one new moon to the next takes about 29.5 days — the original basis for the concept of a month in many ancient calendars. Your latitude on Earth also affects how the Moon's shape appears oriented in the sky, because observers in Australia see the Moon from a different angle than observers in Canada.
Parameters explained
- Moon Position(°)
- Moon Position sets where the Moon is in its orbit around Earth, measured as an angle from 0 to 360 degrees. At 0 degrees the Moon is near the new moon position, between Earth and the Sun, so the side facing Earth is mostly dark. Around 90 degrees, you see a quarter phase because half of the visible side is lit. Around 180 degrees, Earth is between the Sun and Moon, producing a full moon. Moving toward 270 degrees shows the waning side of the cycle. Use this slider to connect the overhead Earth-Sun-Moon model to the Moon shape an observer would see.
- Orbital Tilt(°)
- Orbital Tilt changes how far the Moon's orbit is tilted compared with the Sun-Earth line used for eclipse geometry. The real Moon's orbit is tilted by about 5 degrees, which is why we do not get a solar eclipse every new moon or a lunar eclipse every full moon. When the tilt is small and the Moon lines up closely with the Sun and Earth, shadows can fall in the right place for an eclipse. When the tilt is larger, the Moon usually passes above or below the shadow path. Use this slider to test why phases are monthly but eclipses are special alignments.
Common misconceptions
MisconceptionMoon phases are caused by Earth's shadow falling on the Moon.
CorrectMoon phases are not caused by Earth's shadow; the dark part is the Moon's own night side. Earth's shadow on the Moon creates a lunar eclipse — a relatively rare event that only happens during certain full moons when the Sun, Earth, and Moon align almost perfectly. Phases happen every month regardless of eclipses, because they are simply the result of seeing different portions of the Moon's sunlit side as it orbits. During a crescent phase, you can often faintly see the dark part of the Moon glowing with earthshine — sunlight reflected off Earth — which confirms the dark portion is not Earth's shadow.
MisconceptionThe Moon rises and sets only at night.
CorrectThe Moon is visible in daylight roughly half the time. During the waxing crescent and first quarter phases, the Moon is above the horizon in the afternoon and sets in the evening. During the waning gibbous and third quarter phases, the Moon rises after midnight and is visible in the morning sky. Only the full moon rises near sunset and is above the horizon mostly at night. The Moon follows its own schedule independent of the Sun.
MisconceptionWe see different sides of the Moon at different times of the month.
CorrectWe always see the same side of the Moon, because the Moon rotates on its axis in exactly the same time it takes to orbit Earth — a condition called tidal locking. The phases are about how much of that same familiar face is lit up by sunlight at any given point in the cycle, not about seeing different hemispheres.
MisconceptionA lunar eclipse happens every full moon.
CorrectThe Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. Most full moons, the Moon passes slightly above or below Earth's shadow and no eclipse occurs. Lunar eclipses happen only when a full moon coincides with the Moon crossing the plane of Earth's orbit — typically a few times per year at most, and often not visible from any given location.
How teachers use this lab
- Start with the Full Lunar Cycle preset, then pause at several Moon Position values as students sketch the visible Moon shape and explain how the Sun-Earth-Moon model causes each phase. This directly supports MS-ESS1-1 by using a model to describe cyclic patterns.
- Have students move only the Moon Position slider from 0 to 360 degrees while keeping Orbital Tilt at 5 degrees, then identify where new moon, first quarter, full moon, and last quarter occur. Ask them to describe the repeating pattern without relying only on memorized phase names.
- Use the Solar Eclipse Geometry preset and ask students to explain why a solar eclipse must happen near new moon. Then change Orbital Tilt upward so the Moon shadow misses Earth, showing why new moons usually do not produce eclipses.
- Use the Lunar Eclipse Geometry preset and ask students to explain why a lunar eclipse must happen near full moon. Students can adjust Orbital Tilt to compare a direct pass through Earth's shadow with a near miss, connecting alignment to MS-ESS1-1 eclipse evidence.
- Run a compare-and-contrast discussion: students test Full Lunar Cycle, Solar Eclipse Geometry, and Lunar Eclipse Geometry presets, then write a claim explaining how Moon Position controls phases while Orbital Tilt helps determine whether an eclipse alignment actually occurs.
Frequently asked questions
Why is the lunar cycle 29.5 days even though the Moon takes only 27.3 days to orbit Earth?
The 27.3-day orbit is measured relative to the distant stars (called the sidereal period). But while the Moon completes one orbit, Earth has also moved about 27 degrees along its own path around the Sun. So the Moon needs to travel a little extra — about 2.2 more days — to catch up to the same Sun-Earth-Moon alignment and produce the next new moon. This longer 29.5-day period (the synodic period) is what we experience as the monthly phase cycle.
What causes a lunar eclipse, and how is it different from a moon phase?
A lunar eclipse occurs when Earth moves directly between the Sun and a full Moon and Earth's shadow falls on the Moon, turning it a reddish-orange color (from sunlight filtered through Earth's atmosphere bending around the edges). This only happens during specific full moons when the alignment is nearly perfect, not every month. Moon phases happen every month simply because of the Moon's orbital position — no shadow is involved. They are completely separate phenomena.
Which NGSS standards does this experiment address?
This simulation primarily supports MS-ESS1-1, which asks students to develop and use a model of the Earth-Sun-Moon system to describe the cyclic patterns of lunar phases, eclipses of the Sun and Moon, and seasons. The observer latitude feature also connects to the crosscutting concept of patterns — the same Moon appears differently depending on where you are on Earth, which is itself a pattern that can be explained with a geometric model of three-dimensional space.
Why does the Moon always show us the same face?
Over billions of years, Earth's gravitational pull has gradually slowed the Moon's rotation until it now rotates exactly once for every orbit it completes around Earth. This is called tidal locking. Because the rotation and orbital periods match perfectly, the same hemisphere always faces Earth. The far side — sometimes called the dark side — receives just as much sunlight as the near side but was never seen by humans until spacecraft flew around the Moon beginning in 1959.
Can you see phases on other moons and planets from Earth?
Through a telescope, Venus shows dramatic phases similar to our Moon because it orbits closer to the Sun than Earth does — you can see a thin crescent Venus when it is between Earth and the Sun. Jupiter's large moons also show phases when viewed through a decent telescope. Mercury shows phases for the same reason as Venus. Mars, Jupiter, and Saturn show only very slight phase effects because they orbit farther from the Sun than Earth and we mostly see their fully lit side.