Moon Guide
Everything you need to know about the moon
Lunar Cycle
Lunar Cycle — Synodic Month
Period: 29.53 days
Move the Moon and watch how its lit side appears.
The Moon seen from Earth
Waxing Crescent
About 15% illuminated
A thin edge of the sunlit half begins to come into view from Earth.
After the waning crescent, a new Moon begins the cycle again. ↻
Jump to a phase
The moon takes about 29.5 days to cycle from new moon through full moon and back again. This is called a synodic month.
The moon doesn't produce its own light — it reflects sunlight. As the moon orbits Earth, the relative positions of the sun, Earth, and moon change, altering how much of the moon's illuminated side we can see. This is what causes the moon's phases.
Sizes, distances and orbital speed are simplified; orbital tilt is omitted. Phases are not caused by Earth’s shadow. This diagram does not predict solar or lunar eclipses.
NASA · How Moon phases work8 Moon Phases
New Moon
A moon hidden in light, holding its place in the night
The moon is positioned between Earth and the sun, so its illuminated side faces away from us. This makes it virtually invisible in the sky.
ObservationCannot be observed with the naked eye. This is also when solar eclipses can occur.
CultureMarks the 1st day of the lunar calendar and symbolizes new beginnings. In East Asian culture, this day was considered the start of a new month.
Waxing Crescent
A slender silver arc after sunset
The moon has moved about 45 degrees east of the sun, creating a thin crescent shape. Sunlight illuminates only the right edge, forming an eyebrow-like sliver.
ObservationVisible for 1-2 hours in the low western sky just after sunset.
CultureAppears around the 2nd-3rd day of the lunar month. Traditionally, people made wishes upon seeing the crescent moon.
First Quarter
A clear half-light dividing the night
The moon is 90 degrees from the sun, illuminating exactly half its face. The right half is bright and the left half is dark, forming a D-shape.
ObservationObservable in the southern sky from afternoon until midnight. Highest at sunset.
CultureFalls around the 7th-8th day of the lunar month. The terminator line reveals craters most clearly, making it ideal for telescope observation.
Waxing Gibbous
Moonlight widening slowly toward fullness
More than three-quarters of the moon is illuminated, approaching full moon. Only a slight shadow remains on the left edge.
ObservationRises in late afternoon and illuminates the sky until dawn.
CultureAround the 11th-12th lunar day, anticipation builds before the full moon. The moonlight is bright enough for outdoor activities.
Full Moon
A round light filling the long hours of night
The moon is directly opposite the sun, with its entire illuminated face turned toward Earth. This is the brightest lunar phase.
ObservationRises in the east at sunset and crosses the sky throughout the night, setting in the west at dawn.
CultureFalls on the 15th day of the lunar month — the day of Chuseok and the Lunar New Year's first full moon festival. Symbolizes abundance and completion.
Waning Gibbous
A slightly diminished light rising late
After the full moon, the left side gradually darkens. Shadows begin to appear on the right edge, though more than three-quarters remains bright.
ObservationRises late at night and is visible until morning. Best for dawn observation.
CultureAround the 18th-19th lunar day, a time to savor the afterglow of the full moon. Ancient poets wrote of impermanence while gazing at the waning moon.
Last Quarter
A half-moon left in the morning sky
The moon is 270 degrees from the sun, illuminating exactly half — the opposite of the first quarter. The left half is bright and the right half is dark.
ObservationRises after midnight and is visible in the morning sky. Observable in the early morning hours.
CultureFalls around the 22nd-23rd lunar day. Only visible at dawn, it's sometimes called the 'dawn moon.'
Waning Crescent
A final silver arc before daybreak
As the moon approaches the sun again, a thin crescent appears on the left side. It's preparing to disappear into the new moon phase.
ObservationVisible briefly in the eastern sky just before sunrise at dawn.
CultureAround the 27th-28th lunar day. The word 'Geumuem' in Korean comes from 'to hide,' as the moon hides itself during this phase.
Special Phenomena
Supermoon
When the full moon coincides with the moon's closest approach to Earth, appearing up to 14% larger and 30% brighter than usual.
Cause
The moon's orbit is elliptical, not perfectly circular. When a full moon occurs at perigee (closest point), it becomes a supermoon.
Frequency
Occurs 3-4 times per year, with the closest one being called 'the supermoon of the year.'
Origin of the Name
The term was coined in 1979 by astrologer Richard Nolle. The formal astronomical name is 'perigee-syzygy,' but the catchy 'Supermoon' became the popular term.
Lunar Eclipse
Earth's shadow falls across the moon. During a total lunar eclipse, the moon takes on a reddish hue, earning it the nickname 'Blood Moon.'
Cause
When the sun, Earth, and moon align, Earth's shadow is cast on the moon. Red light filtered through Earth's atmosphere reaches the moon, creating the red appearance.
Frequency
Occurs 0-3 times per year. A total lunar eclipse is visible from any given location about once every 2.5 years.
Origin of the Name
The term 'Blood Moon' was popularized through Pastor John Hagee's 2013 book. Once considered an ominous omen, it's now celebrated as a beautiful astronomical phenomenon.
Blue Moon
When two full moons occur in a single calendar month, the second is called a Blue Moon. It doesn't actually appear blue.
Cause
The lunar cycle (29.5 days) is shorter than a calendar month (30-31 days), so occasionally two full moons fit within one month.
Frequency
Occurs roughly once every 2.7 years. The expression 'once in a blue moon' means 'very rarely.'
Origin of the Name
Originally referred to the 3rd full moon in a season with 4. In 1946, a Sky & Telescope magazine writer mistakenly defined it as the 2nd full moon in a calendar month — and this error became the accepted modern definition.
Harvest Moon
The full moon closest to the autumn equinox (around September 23). It rises earlier than usual, providing bright moonlight throughout the evening.
Cause
In autumn, the difference in moonrise times between successive nights is smaller, allowing several consecutive nights of bright moonlight shortly after sunset.
Frequency
Occurs once a year, around the autumn equinox. Farmers historically used this moonlight to harvest crops late into the evening.
Origin of the Name
Named because, before electric farm equipment, farmers relied on this moonlight to harvest late into the night. The moon normally rises ~50 minutes later each day, but around the Harvest Moon it rises nearly at the same time for several nights.
Hunter's Moon
The full moon following the Harvest Moon (usually October), marking the ideal time for hunting before winter.
Cause
After the harvest, bare fields make animals easy to spot, and bright moonlight aids nighttime hunting.
Frequency
Occurs once a year, about one month after the Harvest Moon.
Origin of the Name
Named by Native Americans and early settlers for the tradition of hunting fattened deer and other game in harvested fields to stockpile meat for winter.
Moon's Effects on Earth
The moon is far more than a decorative object in the night sky. Earth's ecosystems, climate, and even human biological rhythms are all influenced by the moon.
Tides
The moon's gravity drives the ocean's tides — the rhythmic rise and fall of sea levels. Tides sustain coastal ecosystems and create vital habitats for countless intertidal species. During full and new moons, the combined gravitational pull of the sun and moon produces spring tides.Axial Stability
The moon's gravity keeps Earth's axial tilt steady at 23.5 degrees, ensuring consistent seasons. Without the moon, Earth's axis could wobble dramatically, causing extreme and unpredictable climate shifts.Marine Reproduction Cycles
Many marine organisms synchronize their reproduction with lunar cycles. Corals spawn en masse after the full moon, and horseshoe crabs come ashore to lay eggs during spring tides. The moon's light and tidal patterns serve as a biological clock for ocean life.Human Sleep & Circalunar Rhythm
Studies show that around the full moon, deep sleep decreases by about 30% and it takes longer to fall asleep. This is called the circalunar rhythm — possibly a trace from our ancestors who took advantage of bright moonlit nights before the age of electricity.Observation Tips
1The full moon rises in the east at sunset. Near the horizon, it appears especially large due to the 'moon illusion.'
2The crescent and waning crescent moons are only briefly visible around sunrise or sunset, so timing is key.
3Along the first quarter moon's terminator line, craters and mountain ranges are most clearly visible.
4Clear weather is essential for moon observation. Choose nights with low humidity and no clouds.
5Escape the city's light pollution for the suburbs, where you can even see shadows cast by moonlight.
6The moon repeats the same phase roughly every 29.5 days. This is called a synodic month.
A guide to the Moon
Learn how Moon phases work, explore the Sun–Earth–Moon model and find practical guidance for observing the Moon.
The orbital model simplifies size, distance and speed for illustration. Some in-depth reference articles currently use the original Korean URLs.