Moon Phases Explained
Moon phases have nothing to do with Earth's shadow. As the Moon orbits Earth once every 29.5 days, we see different fractions of its sunlit half. This diagram shows both the space-view geometry and the corresponding phase from Earth.
Key takeaways
- The Moon's synodic period is about 29.5 days, from one new Moon to the next.
- Phases show which fraction of the sunlit half faces Earth.
- Earth's shadow only touches the Moon during a lunar eclipse, not every full Moon.
- Phase orientations look mirrored to observers in the Southern Hemisphere.
About this visual
The Moon takes about 29.5 days to complete a phase cycle, from one new Moon to the next. That period is called the synodic month, and it is what our calendars broadly follow.
What actually changes
The Moon always has one half lit by the Sun. As it orbits Earth, we see the sunlit half from different angles. When the Moon is between Earth and the Sun we see the dark side (new Moon). When Earth is between the Moon and the Sun we see the fully lit side (full Moon). In between we see growing (waxing) or shrinking (waning) crescents and gibbous phases.
Phases are not eclipses
Moon phases are geometry, not shadow. Earth's shadow only touches the Moon during a lunar eclipse, which requires the Sun, Earth and full Moon to line up almost exactly along a straight line — an alignment that does not happen every full Moon because the Moon's orbit is tilted about five degrees from Earth's.
Hemisphere matters
A waxing crescent in the Northern Hemisphere looks like the letter D: the right side of the Moon glows. In the Southern Hemisphere the same phase looks like the letter C because the sky is effectively flipped for the observer. This is why guides written for one hemisphere can confuse readers in the other.
Naming order
The cycle in order: New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Third (Last) Quarter → Waning Crescent → New Moon.
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Sources and methodology
Values verified against official mission and archive pages on the updated date shown above. Numeric quantities are cited to the precision typical of introductory astronomy references.