The Moon appears to change shape in the sky across nights and months. These changes are the phases of the Moon, caused by the shifting angles of the Sun, Earth, and Moon. In brief, there are eight principal phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. Each phase reflects how much of the sunlit half of the Moon faces Earth and how that illuminated portion grows or shrinks over the lunar month.
What Causes the Moon’s Phases
The phases result from the geometry of the Sun, Earth, and Moon. The Moon orbits Earth about once every 27.3 days relative to the stars. As it moves, the portion of its day side that we can see changes. When the Moon sits between Earth and the Sun, its sunlit side faces away from us, producing the New Moon. When Earth sits between the Sun and the Moon, the full day side faces us, giving the Full Moon. The intermediate positions create the crescent, quarter, and gibbous shapes we observe.
Phase Characteristics and Timing
Each phase has distinct appearances and practical timing. Waxing phases grow from New toward Full, with the right side illuminated in the Northern Hemisphere. Waning phases shrink from Full back to New, with the left side lit. The first and last quarter phases show exactly half of the Moon illuminated, but at different times of night. Knowing these traits helps you identify the current phase and predict the next change.
New Moon
During New Moon, the Moon is close to the Sun in the sky. Its illuminated side faces the Sun, and the night side mostly faces Earth. The Moon rises and sets near the same times as the Sun, so it is usually not visible. This phase marks the start of the lunar cycle and is a key moment for moonless night sky observations.
Waxing Crescent
After New Moon, a thin crescent becomes visible just after sunset. The rest of the Moon may appear as earthshine, sunlight reflected from Earth onto the Moon’s surface. This phase occurs in the early evening sky and is a good period to spot the Moon low above the horizon before it sets several hours after sunset.
First Quarter
At First Quarter, half of the Moon’s visible disk is illuminated, with the right half lit in the Northern Hemisphere. It rises around noon, sits high in the sky at sunset, and sets around midnight. The terminator, the line between day and night, runs top to bottom, making surface features easy to observe with binoculars or a telescope.
Waxing Gibbous
Between First Quarter and Full Moon, more than half but not all of the Moon is illuminated. The shape continues to grow nightly, and the Moon is visible for most of the afternoon and evening. Features along the terminator become more detailed as the angle of sunlight provides strong shadows.
Full Moon
At Full Moon, Earth is between the Sun and the Moon, so the entire day side faces us. It rises at sunset, stays up all night, and sets at sunrise. The Full Moon can appear larger and brighter than other phases, though these changes are subtle. This phase is traditionally associated with high tides and cultural events around the world.
Waning Gibbous
After Full Moon, the illuminated portion begins to shrink. The Moon rises after sunset and remains visible through the early morning hours. The left side of the Moon grows darker each night as the angle of sunlight shifts and less of the day side is turned toward Earth.
Last Quarter
Last Quarter, also called Third Quarter, again shows half of the Moon illuminated, but the left half is lit in the Northern Hemisphere. It rises around midnight, is high in the sky at sunrise, and sets around noon. Like First Quarter, the terminator provides clear views of lunar features.
Waning Crescent
Before New Moon, only a thin crescent remains visible in the early morning sky. This phase appears low before dawn and sets a few hours after sunrise. Earthshine can again be visible on the darker portion, creating the old Moon in the new Moon’s arms effect.
Quarter-Day and Intermediate Phases
In addition to the eight principal phases, intermediate names are sometimes used to describe the Moon’s position. These include the exact quarter points and broader transitions such as the beginning and end of waxing or waning. The table below summarizes key phase attributes, approximate timing from New Moon, and the visible window from Earth.
Moon Phase Reference Table
| Phase | Day of Cycle (Approximate) | Lunar Visibility Window | Notable Feature |
|---|---|---|---|
| New Moon | Day 0 | Not generally visible | Start of lunar cycle |
| Waxing Crescent | Day 1–7 | Early evening, after sunset | Thin crescent, earthshine |
| First Quarter | Day ~7 | Afternoon to midnight | 50% illuminated, rising at noon |
| Waxing Gibbous | Day 7–14 | Afternoon to after midnight | More than 50% illuminated |
| Full Moon | Day ~14 | Sunset to sunrise | 100% illuminated, opposite the Sun |
| Waning Gibbous | Day 14–21 | After sunset to morning | Decreasing illumination |
| Last Quarter | Day ~21 | Midnight to noon | 50% illuminated, rising at midnight |
| Waning Crescent | Day 21–29 | Late night to sunrise | Thin crescent, old Moon in new Moon’s arms |
Synodic Month and Cycle Length
The cycle from one New Moon to the next is called a synodic month, averaging about 29.5 days. This differs from the sidereal month (~27.3 days), the time to complete one orbit relative to the stars, because Earth moves along its orbit around the Sun while the Moon orbits Earth. The extra days allow the Moon to catch up to the same lunar phase as seen from Earth. Tracking this cycle is essential for calendars, eclipse prediction, and planning observations.
How to Identify the Current Phase
To determine the Moon’s phase on any given night, note the time of sunset and when the Moon rises. A thin evening crescent indicates a Waxing Crescent; a Moon visible nearly all night and fully illuminated at midnight is likely Full. If you see half the Moon illuminated and it sets around midnight, it is likely Last Quarter. With simple observing habits, you can confidently identify each phase without tools.
Common Misconceptions About the Moon’s Phases
Some believe the phases are caused by Earth’s shadow, which describes a lunar eclipse, not the monthly cycle. Others think the Moon is completely dark during New Moon, but earthshine often reveals a dimly lit disk. Understanding the true cause—changing angles of sunlight on the Moon—clarifies why the pattern is regular, predictable, and global, visible everywhere on Earth at appropriate times.
Why the Phases Matter Beyond Observation
Lunar phases influence ocean tides, with the strongest tides occurring around Full Moon and New Moon when the Sun and Moon are aligned. Many cultural, religious, and agricultural traditions reference the phases. In modern astronomy and space missions, phase knowledge aids in planning observations, communication, and lighting conditions for lunar exploration. Recognizing the cycle builds a deeper connection to the rhythms of the night sky.