Sunlight creates every phase
Half of the Moon is always illuminated by the Sun. As the Moon orbits Earth, our viewing angle changes and we see a different fraction of that sunlit half.
Earth's shadow does not cause the ordinary phases. Earth's shadow reaches the Moon only during a lunar eclipse, when the Sun, Earth and Moon align near full moon.
The eight named phases
| Phase | What you see | Typical viewing |
|---|---|---|
| New moon | Moon near the Sun | Usually not visible |
| Waxing crescent | Growing thin crescent | Western sky after sunset |
| First quarter | Half-lit disc | Afternoon and evening |
| Waxing gibbous | More than half lit | Afternoon into night |
| Full moon | Fully lit near side | Rises near sunset |
| Waning gibbous | Shrinking lit area | Late evening and morning |
| Last quarter | Half-lit disc | After midnight and morning |
| Waning crescent | Shrinking thin crescent | Eastern sky before sunrise |
Waxing, waning and the terminator
Waxing means the illuminated portion is growing from night to night. Waning means it is shrinking. The dividing line between lunar day and night is called the terminator.
Low-angle sunlight near the terminator casts long shadows across craters and mountains, making quarter and crescent phases especially rewarding through binoculars or a telescope.
Why there is not an eclipse every month
The Moon's orbit is tilted by about five degrees relative to Earth's orbit around the Sun. At most new and full moons, the three bodies are not aligned closely enough for an eclipse.
A solar eclipse requires a new moon near an orbital node. A lunar eclipse requires a full moon near a node. These alignments occur during eclipse seasons spaced roughly six months apart.
Sources and methodology
Dates, paths and safety guidance use the references below. Local timings are rounded calculations for the stated reference points; recalculate your exact observing location.
- NASA: Moon Phases ↗Phase geometry, the eight named phases and the synodic cycle.
- NASA: Eclipses and the Moon ↗The distinction between normal Moon phases and eclipse alignment.