The Moon’s appearance changes in a regular cycle as it orbits Earth, because the portion of the Sun’s light that reaches and illuminates the Moon’s surface that we can see from Earth changes over time. Direct answer
- The cycle is called the lunar phase cycle and repeats about every 29.5 days (the synodic month). During this cycle the Moon goes through eight primary shapes: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third (last) quarter, and waning crescent. Each phase corresponds to how much of the Moon’s near side is sunlit and visible from Earth.
- Why this happens: the Moon orbits Earth, while both bodies orbit the Sun. We always see the same side of the Moon because its rotation period is locked to its orbital period around Earth, but the angle of sunlight on the near side changes as the Moon’s position in its orbit changes. This combination creates the familiar progression of phases from dark (new moon) to fully illuminated (full moon) and back again.
- Visual progression in simple terms:
- New Moon: Moon lies between Earth and Sun; the side facing us is dark.
- Waxing crescent to first quarter: a sliver of the sunlit side becomes visible, growing larger each night.
- Full Moon: the entire near side is illuminated.
- Waning phases: the illuminated portion decreases, fading through gibbous, last quarter, and waning crescent, until the cycle restarts at new moon.
Additional context and nuances
- The Moon’s apparent size or brightness can vary due to its orbital distance (perigee vs apogee) and atmospheric effects, but the basic shape sequence is governed by the geometry of Sun–Moon–Earth alignment described above.
- The Moon’s day length and synchronous rotation mean we always see the same face, but this does not affect the cycle of illumination from the Sun that creates the observed phases; it only explains why we never see the far side from Earth.
If you’d like, I can tailor this into a quick, printable reference card or create a simple diagram description to help visualize the phase progression.
