Lunation Calendar — Deep Dives — Moon Phases

The Moon does not change shape; our view of its daylight does.

The Sun always illuminates half of the Moon. As the Moon orbits Earth, its sunlit half turns toward us and away again, passing continuously from darkness to fullness and back. The Lunation Chart gathers that changing geometry, the names we give its familiar stages, and the time that carries one cycle into the next.

A moving view of daylight

Moonlight is reflected sunlight. One hemisphere of the Moon is in daylight while the other is in night, just as on Earth. Phase describes how much of that lunar dayside overlaps the face turned toward us. The changing angle between Sun, Moon, and Earth changes the overlap, producing the bright shapes we see in the sky.

At New Moon, the Moon is in nearly the same direction as the Sun, so its dayside faces mostly away from Earth. At Full Moon, it is opposite the Sun, turning its dayside toward us. At First and Last Quarter, the Sun and Moon are about a quarter-turn apart in our sky, and we see half of the Moon’s visible disk illuminated. The Lunation Chart’s central Orbit Diagram provides the Earth-Moon-Sun geometry behind this cycle.

Ordinary phases are not Earth’s shadow falling on the Moon. Earth’s shadow reaches the Moon only during a lunar eclipse; the Moon’s shadow reaches Earth during a solar eclipse. The Eclipses Deep Dive explains why those alignments occur only near the Moon’s orbital nodes.

One cycle, eight names

Phase changes continuously, but eight names give us landmarks within the cycle. Four principal phases name exact geometric moments; the four phases between them name the intervals through which the Moon is changing.

New Moon — the Moon is near the Sun in our sky and is normally lost in daylight.
Waxing Crescent — a growing sliver of the dayside comes into view.
First Quarter — the Moon has completed the first quarter-turn of its phase geometry and appears half illuminated.
Waxing Gibbous — more than half of the visible disk is illuminated and still growing.
Full Moon — the Moon is opposite the Sun and nearly all of its visible disk is illuminated.
Waning Gibbous — more than half remains illuminated, but the bright portion is shrinking.
Last Quarter — the Moon has completed the third quarter-turn of its phase geometry and again appears half illuminated.
Waning Crescent — the remaining bright sliver narrows toward the next New Moon.

Waxing means that the illuminated portion is growing; waning means it is shrinking. A crescent is less than half illuminated, while gibbous is more than half. First and Last Quarter name the Moon’s progress through the cycle, not the fraction of its visible disk that is bright.

Which side looks bright depends on how the Moon is oriented in the local sky, so waxing and waning are more reliable guides than “light on the right” or “light on the left.”

Phase in the daily sky

Phase also tells us when the Moon shares the sky with the Sun. A New Moon rises and sets with the Sun. A waxing Moon lingers into the evening; a Full Moon rises around sunset and remains through the night; a waning Moon belongs increasingly to the morning sky. These are broad patterns rather than exact clock times, because season and location also shape the Moon’s daily path.

The Lunation Chart’s Moon and Sun Arcs Band makes that relationship visible by tracing when each body is above the local horizon. The Sun-days and Moon-days Deep Dive follows the changing overlap of sunlight and moonlight through local time.

Uneven quarters

You might expect: New Moon, First Quarter, Full Moon, and Last Quarter divide a lunation into four equal spans.

Instead: The spans between successive principal phases range from about 6.6 to 8.2 days across the 2027 chart set—a spread of nearly thirty-eight hours. The Moon’s elliptical orbit changes its speed while Earth’s motion changes the direction to the Sun; together, these motions keep equal phase angles from taking equal time. In the Moon Phases Band, filled diamonds mark the actual phases while hollow diamonds mark equal-time quarter points. The distance between them makes the difference visible.

The interval from one New Moon to the next is a synodic month, or lunation. Its average length is about 29.5 days. This is longer than one orbit of the Moon relative to the background stars: while the Moon circles Earth, Earth also moves around the Sun, so the Moon must travel farther to regain the same Sun-Moon-Earth geometry.

The intervals from New Moon to First Quarter, from First Quarter to Full Moon, and onward differ from one another and vary from one lunation to the next. The names Waxing Crescent, Waxing Gibbous, Waning Gibbous, and Waning Crescent span the measured intervals between the principal phases.

Age is time; phase is geometry

Moon age is simply elapsed time since the New Moon that opened the lunation, counted in 24-hour periods. The Moon Age Band begins at zero and marks each whole day that follows.

Age and phase advance together, but they are not interchangeable. Age seven is roughly First Quarter and age fourteen or fifteen roughly Full Moon, yet the exact phase depends on the Moon’s changing orbital speed and the length of that particular lunation. Moon age answers how long since New Moon?; phase answers what is the current Sun-Moon-Earth geometry?

The changing lunar face

Phase describes the broad shape of the light, but the Moon’s appearance contains more. Its changing distance alters its apparent size. Libration, a gentle apparent rocking produced by the Moon’s orbit and orientation, brings slightly different edges of the familiar lunar surface into view. During a crescent phase, sunlight reflected from Earth can also faintly illuminate the dark portion as earthshine.

The Lunation Chart’s Moon Faces Band shows the Moon at the highest point of each journey above the local horizon. Its images combine phase, libration, and apparent size with the roll that orients the Moon for the chart location.

The source phase and libration imagery is geocentric: it is calculated from the center of Earth. The chart adjusts its roll for the locale, but does not fully rerender the lunar surface from the observer’s exact position. The Location Deep Dive explains the distinction between a locale-adjusted image and a fully topocentric view.