The Moon’s phases are a result of the varying amounts of its surface illuminated by sunlight that we can see from Earth. This changing visibility gives us the lunar phases.
The Moon Reflects Sunlight
The Moon does not emit its own light but reflects the light of the Sun. As it orbits Earth, the portion of the Moon that is illuminated changes, creating the phases we observe.
Primary and Intermediate Phases
In Western culture, the lunar month is divided into eight phases: four primary phases and four intermediate phases.
- Primary Phases: These occur at specific moments in time.
- Intermediate Phases: These are the transitions between the primary phases.
New Moon
New Moon is the first primary phase. It occurs when the Sun and the Moon are aligned, with the Sun and Earth on opposite sides of the Moon. During a New Moon, the side of the Moon facing Earth is not illuminated, making it invisible to us. If the New Moon is near the lunar nodes, it can cause a solar eclipse. Spring tides, the largest difference between high and low tides, occur around the New Moon and Full Moon.
Waxing Crescent
Waxing Crescent is the first intermediate phase, starting when a thin sliver of the Moon becomes visible after the New Moon. Historically, this was called the New Moon, with the dark phase referred to as the Dark Moon. The traditional definition is still used in some cultures, such as in the Islamic calendar. During this phase, Earthshine, or Da Vinci glow, may also be visible, where sunlight reflected from Earth faintly illuminates the dark part of the Moon.
First Quarter
First Quarter is the second primary phase, occurring when the Moon has completed one-quarter of its orbit around Earth. Also called the Half Moon, we see exactly 50% of the Moon’s surface illuminated. The appearance of the illuminated half (left or right) depends on the observer’s location. Neap tides, the smallest difference between high and low tides, occur around the First and Third Quarters.
Waxing Gibbous
Waxing Gibbous is the second intermediate phase. “Waxing” means increasing, and “gibbous” refers to the shape that is larger than a semicircle but not yet a full circle.
Full Moon
Full Moon is the brightest phase, occurring when the Sun and the Moon are on opposite sides of Earth. The alignment lasts only a moment, but the Moon can appear full for about a day before and after. Occasionally, a lunar eclipse happens when the Full Moon passes through Earth’s shadow. If the Full Moon occurs near the closest or farthest points in its orbit, it is called a Supermoon or Micromoon, respectively.
Waning Gibbous
Waning Gibbous follows the Full Moon and is the period when the visible portion of the Moon starts to decrease.
Third Quarter
Third Quarter is the last primary phase, occurring when the opposite half of the Moon is illuminated compared to the First Quarter.
Waning Crescent
Waning Crescent is the final intermediate phase. The illuminated portion continues to shrink until only a thin crescent is visible. Earthshine may be seen again towards the end of this phase.
Understanding the Moon’s Cycle
The Moon’s cycle from New Moon to New Moon takes about 29.5 days, known as a lunar month. Each phase has its unique appearance and timing, influenced by the Moon’s position relative to Earth and the Sun. By observing the Moon’s phases, we gain insights into its orbit and the dynamic relationship between Earth, the Moon, and the Sun.