What is a lunar phase?

What is a lunar phase?

A lunar phase is the portion of the Moon's sunlit surface that is visible from Earth on a given date. The Moon produces no light of its own; it shines by reflecting sunlight, and sunlight — arriving from effectively one direction — illuminates exactly one half of the sphere at all times. NASA states the geometry plainly: "The Sun's light comes from one direction, and it always illuminates, or lights up, one half of the Moon."³ The half that is lit does not change in size. What changes, over the course of a month, is the angle from which we view it, and therefore how much of the lit half is turned toward us.

That viewing angle is the angle between the Sun, the Earth, and the Moon — formally the Moon's elongation, the apparent separation between the Moon and the Sun in the sky. When the Moon sits between the Earth and the Sun, its lit half faces away from us and we see a dark disc: new moon. When the Earth sits between the Sun and the Moon, the lit half faces us fully: full moon. At the in-between positions we see a slice of the lit half, growing or shrinking as the Moon moves around its orbit.³

For precise work the phase is defined not by appearance but by coordinates. The U.S. Naval Observatory — the body that publishes the official astronomical data of the United States — defines the principal phases as the moments when "the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively."¹ In other words, the phase is fixed by how far ahead of the Sun the Moon has moved along the sky, measured in degrees. This makes each principal phase a single, well-defined instant in time rather than a day-long state, which is why almanacs can list the new moon or full moon to the minute.⁶

What are the eight phases of the Moon?

The lunar cycle is conventionally divided into eight phases. NASA gives the order: "new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third quarter, and waning crescent."⁴ Four of these — new moon, first quarter, full moon, and last quarter (also called the third quarter) — are the principal phases, the precise instants defined by the Moon running 0°, 90°, 180°, and 270° ahead of the Sun.¹ The other four are the gradual stages between them.

The intermediate stages are named by how much of the disc is lit. The U.S. Naval Observatory defines a crescent as the stage when the illuminated fraction is between 0 and 50 percent, and a gibbous Moon as the stage when it is between 50 and 100 percent.¹ Each lasts roughly a week, bridging two principal phases. The terms waxing and waning record the direction of change: a waxing Moon is growing more lit each night, on its way from new to full; a waning Moon is shrinking, on its way from full back to new.

The names "first quarter" and "last quarter" are a common source of confusion, because both show a half-lit disc rather than a quarter-lit one. The label refers to position in the cycle, not to the illuminated area: at first quarter the Moon has completed one quarter of the synodic month and stands 90° — a quarter-turn — ahead of the Sun, and at last quarter it has completed three quarters and stands 270° ahead.¹ The half-lit appearance is simply what 90° of elongation looks like from Earth.

How long is the lunar phase cycle?

One full cycle of the phases — from one new moon to the next — is the synodic month, and its mean length is 29.53059 days. NASA's figure, carried to the second, is "29.53059 days (29d 12h 44m 03s)."² This is the "month" that the word originally meant, and the interval that lunar and lunisolar calendars are built to track. The U.S. Naval Observatory tabulates the dates and times of the four principal phases for every year from 1700 to 2100, all computed from this cycle.⁶

The 29.53-day figure is an average, not a fixed period. The length of any single lunation drifts from the mean because the Moon's orbit is an ellipse and its speed varies along it, and because the Sun's gravity tugs unevenly on the orbit through the year. NASA notes that "the duration of the lunation actually varies from its mean value by up to seven hours."² Over many cycles those departures average out, which is why a single mean value serves for calendar design even though no individual month matches it exactly.

Why is the synodic month longer than the sidereal month?

The Moon completes one orbit of the Earth, measured against the fixed background stars, in 27.32166 days — the sidereal month, or "27d 07h 43m 12s" in NASA's reckoning.² Yet the cycle of phases takes 29.53 days. The phases run about 2.21 days behind the orbit because the phase depends on the Moon's position relative to the Sun, and the Sun's direction is not fixed: the Earth carries the Moon roughly one degree per day around its own yearly orbit.

NASA describes the catch-up directly: "As the Moon revolves around Earth, both objects also progress in orbit around the Sun. After completing one revolution with respect to the stars, the Moon must continue a little farther along its orbit to catch up to the same position it started from relative to the Sun and Earth."² By the time the Moon has finished one true orbit, the Earth–Sun line has swung forward by about 27°, so the Moon needs roughly two more days of travel to return to the same phase. The same relationship — an orbital period measured against the stars running shorter than the cycle measured against the Sun — is the lunar counterpart of the gap between the sidereal year and the tropical year that governs the seasons.

Are the Moon's phases caused by the Earth's shadow?

No. This is the most persistent misconception about the Moon, and it confuses two unrelated phenomena. The phases are a matter of viewing angle: the Sun lights one half of the Moon, and we see a changing slice of that lit half as the Moon orbits.³ The Earth's shadow plays no part. When the Earth's shadow does fall across the Moon, the event is a lunar eclipse, and NASA is specific that "lunar eclipses occur at the full Moon phase," when the Earth passes between the Sun and the Moon and "Earth's shadow falls upon the surface of the Moon."⁵

If the shadow caused the phases, there would be an eclipse every month at full moon — but there is not, typically only a few a year. The reason is that the Moon's orbit is tilted. NASA puts the tilt at "about 5 degrees compared to the plane of Earth's orbit around the Sun," so at most full moons the Moon passes above or below the Earth's shadow rather than through it.⁵ The phases, by contrast, repeat every synodic month without exception, because they depend only on the Sun–Earth–Moon angle and not on a precise alignment. Eclipses, supermoons, and the apparent size of the Moon are separate topics from the phase cycle described here.

How do calendars use the synodic month?

The synodic month is the basis of every calendar that counts time by the Moon, and those calendars fall into two families. A purely lunar calendar follows the synodic month and ignores the solar year. The Islamic calendar is the leading example: the U.S. Naval Observatory describes it as "based on lunar months" of 12 months that are "on average, 11 days shorter than the (Gregorian) civil year," with each month beginning when "the thin crescent Moon is actually sighted in the western sky after sunset a day or so after New Moon."⁷ Because twelve synodic months fall short of a solar year by about eleven days, observances such as Ramadan drift earlier through the seasons from one year to the next.

A lunisolar calendar keeps the months tied to the Moon but periodically inserts an extra leap month to stay aligned with the solar year, so its festivals do not drift through the seasons. The Hebrew and traditional Chinese calendars work this way, which is why the Chinese New Year and the Jewish holidays fall within a bounded window of the Gregorian year rather than circling it. The full mechanics of intercalation belong to a separate page on the lunisolar calendar.

Even the Gregorian calendar, which is solar and whose months no longer track the Moon, carries one lunar inheritance: the date of Easter. Since the Council of Nicaea in 325, Easter has been set as the first Sunday after the first full moon on or after the March equinox, computed from an ecclesiastical approximation of the lunar cycle rather than the observed Moon. It is the clearest surviving trace of the synodic month inside an otherwise solar civil calendar.

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