What causes the Moon’s phases
The Moon’s phases result from its orbit around Earth and the shifting angles of sunlight that reach the near side. The Moon does not produce its own light; it reflects sunlight. As the Moon circles Earth roughly every 27.3 days (a sidereal month) while Earth orbits the Sun, the portion of the lunar hemisphere we see illuminated changes in a repeating sequence. This geometric relationship among Sun, Earth, and Moon defines the cycle from New Moon through Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent, then back to New Moon. Each phase reflects how much of the sunlit side faces Earth and how much daylight covers the hemisphere we observe.
The eight principal lunar phases
There are eight commonly recognized phases, each tied to specific angles and observable shapes. They form a continuous cycle that repeats approximately every 29.5 days (a synodic month), the period from one New Moon to the next. Below is a concise reference outlining each phase, its place in the cycle, and what you can expect to see from Earth.
| Phase | Approximate Timing in Cycle | Visible Hemisphere Illumination | Best Observed When |
|---|---|---|---|
| New Moon | 0–5% | Near side almost entirely dark | Not visible; occurs during daytime |
| Waxing Crescent | 5–50% | Thin crescent in western sky after sunset | Low in west shortly after sunset |
| First Quarter | 50% | Right half (northern hemisphere) illuminated | Evening, highest near sunset |
| Waxing Gibbous | 50–100% | Most of the near side lit | Afternoon and evening |
| Full Moon | 100%Entire near side sunlit | Rises at sunset, visible all night | |
| Waning Gibbous | 100–50% | Most of the near side lit, decreasing | Evening and early morning |
| Last Quarter | 50% | Left half (northern hemisphere) illuminated | Pre-dawn and morning |
| Waning Crescent | 50–0% | Thin crescent in eastern sky before sunrise | Low in east before dawn |
New Moon
During New Moon, the Moon lies between Earth and Sun. The side illuminated by the Sun faces entirely away from Earth, so the lunar disk is not visible from most locations. This is the only phase where the Moon rises and sets roughly with the Sun, remaining in the sky’s glare during daylight hours. Astronomers often study this phase with instruments that block direct sunlight to examine the faint dusty streamers and Earthshine reflections off the unlit portion.
Waxing Crescent
After New Moon, a thin crescent becomes visible in the western sky after sunset. The crescent grows larger and higher each evening as the Moon moves eastward in its orbit. Skyglow and clear horizons affect visibility; binoculars can help early in this phase. The rest of the near side remains dark due to the Sun’s overwhelming brightness, but faint earthshine may outline the rest of the Moon’s disk.
First Quarter
At First Quarter, the Moon has completed about one quarter of its orbit around Earth since New Moon. Exactly half of the near side is illuminated, with the terminator (the dividing line between day and night on the Moon) running north to south. In the northern hemisphere, the right half appears bright. This phase rises around noon and sets around midnight, providing high-contrast viewing of surface features in the evening.
Waxing Gibbous
Between First Quarter and Full Moon, more than half but not all of the near side is lit. The Moon rises in the afternoon, is well placed in the evening sky, and sets after sunrise. Surface details are visible with moderate magnification; the increasing shadows near the terminator enhance crater and mountain relief.
Full Moon
At Full Moon, Earth is between the Moon and Sun. The entire near side receives sunlight, making the Moon appear fully lit from sunrise to sunset and visible most of the night. Full Moons can appear larger near the horizon due to an optical illusion and may have a warm hue when low through the atmosphere. Because shadows are minimal, crater relief is less pronounced than at quarter phases.
Waning Gibbous
After Full Moon, the illuminated portion begins to shrink, yet more than half of the near side remains lit. The Moon rises after sunset and remains visible through much of the night. Features stand out well along the shrinking dusk terminator, making this a good phase for detailed observation before sky brightness increases.
Last Quarter
Last Quarter occurs when the Moon has completed about three quarters of its orbit since New Moon. Again, half of the near side is illuminated, but the left half is bright in the northern hemisphere. This phase rises around midnight and sets around noon, favoring pre-dawn and morning observation. The terminator provides strong shadows that highlight surface textures.
Waning Crescent
As the Moon approaches New Moon again, a thin crescent appears in the eastern sky before sunrise. The crescent grows thinner and rises closer to sunrise, becoming harder to see as daylight brightens the sky. Earthshine may briefly make the darker portion visible, creating the “old Moon in the new Moon’s arms” effect under clear conditions.
Key facts and timing
The table below summarizes typical durations and observational notes. Exact timing varies by a few hours depending on orbital specifics and your location on Earth.
| Attribute | Verified Detail | Source Type |
|---|---|---|
| Cycle length (synodic month) | ~29.5 days | Lunar orbital mechanics |
| Sidereal orbital period | ~27.3 days | Sidereal motion relative to stars |
| New to New interval | ~29 days 12 hours 44 minutes (average) | Lunar ephemerides |
| Visibility of thin crescents | Typically 1–4 days after New Moon and before New Moon | Observational astronomy |
| Twilight visibility of crescent | 30–60 minutes after sunset (low latitudes) or longer at higher latitudesObservational conditions