How Moon Phases Work: The Science Behind the Lunar Cycle

The Moon does not produce its own light. What people on Earth call moonlight is sunlight reflected off the lunar surface, according to NASA's Science Mission Directorate. At any given moment, the Sun illuminates exactly half of the Moon, just as it illuminates half of Earth. The portion of that lit half visible from Earth changes constantly as the Moon travels along its orbit, and that changing view is what creates the sequence known as moon phases.

Why the Moon appears to change shape

According to NASA, the cause of lunar phases is simple geometry: the relative positions of the Sun, Earth and Moon shift every day as the Moon moves around Earth. When the Moon sits between Earth and the Sun, its lit side faces away from us, producing a new moon. Roughly two weeks later, when Earth sits between the Sun and the Moon, the entire lit side faces Earth, producing a full moon. Every phase in between reflects a different viewing angle on that same half-lit sphere.

NASA also notes that the Moon is tidally locked to Earth: it spins on its own axis in almost exactly the same time it takes to orbit Earth, so the same lunar hemisphere always faces our planet. Tidal locking explains why people on Earth never see the Moon's far side, but it is a separate phenomenon from the Sun-Earth-Moon geometry that produces phases.

The eight recognised phases

NASA describes the lunar month as containing eight named phases, split into four primary phases and four intermediate ones. The primary phases — new moon, first quarter, full moon and third (or last) quarter — occur at a specific, calculable moment. The intermediate phases describe the gradual transitions between them.

• New Moon — the Moon is between Earth and the Sun; the near side is unlit.
• Waxing Crescent — a thin sliver of light grows on the right side (Northern Hemisphere view).
• First Quarter — half the visible disc is lit; the Moon is 90° from the Sun in the sky.
• Waxing Gibbous — more than half lit, the lit area still growing.
• Full Moon — Earth is roughly between the Sun and Moon; the whole near side is lit.
• Waning Gibbous — more than half lit, the lit area shrinking.
• Third (Last) Quarter — the other half of the disc is lit.
• Waning Crescent — a thin sliver remains before the next new moon.

The terms waxing and waning simply describe direction: waxing means the illuminated portion is growing, from new moon toward full moon, while waning means it is shrinking, from full moon back toward new moon, per NASA's description.

Why the cycle is 29.5 days, not 27.3

Two different numbers describe the Moon's motion, and mixing them up is a common source of confusion. The sidereal month — 27.32166 days, or 27 days, 7 hours, 43 minutes and 12 seconds — is how long the Moon takes to complete one orbit relative to the fixed stars, according to NASA's eclipse website at the Goddard Space Flight Center. The synodic month — 29.53059 days, or 29 days, 12 hours, 44 minutes and 3 seconds — is how long it takes for the same phase (say, new moon to new moon) to recur.

The synodic month is longer because Earth itself is moving around the Sun while the Moon orbits Earth. After completing one full 360° orbit relative to the stars, the Moon has to travel an additional stretch — roughly 2.2 extra days' worth — to catch up to the same Sun-Earth-Moon alignment it started from, NASA's eclipse site explains. This 29.5-day synodic month is the basis for calendar "months" in many cultures and for the common shorthand that a lunar cycle lasts about 29.5 days.

The Moon's orbit is not a perfect circle

The Moon's path around Earth is an ellipse with a mean eccentricity of 0.0549, NASA's eclipse site states, meaning the Earth-Moon distance varies throughout each orbit. Perigee, the closest point, has ranged between about 356,355 and 370,399 km over the past five millennia, while apogee, the farthest point, has ranged between roughly 404,042 and 406,725 km in the same period. A separate orbital period — the anomalistic month, averaging 27.55455 days — measures the time from one perigee to the next.

The Moon's orbital plane is also tilted about 5.145° relative to Earth's orbital plane around the Sun (the ecliptic), according to the same NASA source. That tilt is the reason solar and lunar eclipses do not happen every month even though new and full moons do: the Moon usually passes slightly above or below the Sun-Earth line. Eclipses are possible only during two "eclipse seasons" each year, when a new or full moon coincides with the Moon crossing the ecliptic plane, NASA's eclipse-geometry page explains.

Phases and ocean tides

Moon phases also correspond to a predictable tidal pattern. The U.S. National Oceanic and Atmospheric Administration (NOAA) explains that during new and full moons — when the Sun, Earth and Moon are nearly aligned — the gravitational pulls of the Sun and Moon reinforce each other, producing "spring tides" with higher high tides and lower low tides. During the first and third quarter phases, when the Sun's pull on the ocean partly cancels out the Moon's pull, the result is a smaller-range "neap tide." Both spring and neap tides occur twice in every lunar month, NOAA notes.

What counts as a "blue moon"

Because the synodic month (about 29.5 days) is shorter than most calendar months, two full moons occasionally fall within the same calendar month. The science outreach site EarthSky, run by astronomer Deborah Byrd, describes this as the modern, popular definition of a "blue moon." A second, older definition — the third full moon in a season that contains four full moons instead of the usual three — comes from the 1937 Maine Farmers' Almanac. EarthSky traces the now-common "second full moon in a month" version to a 1946 Sky & Telescope magazine article by amateur astronomer James Hugh Pruett, who misread the almanac's seasonal rule. Monthly blue moons recur about seven times every 19 years, or roughly every two to three years, per EarthSky.