Lunar phase

Alunar phaseorMoon phaseis the apparent shape of theMoon's directly sunlit portion as viewed from theEarth.Because the Moon istidally lockedwith the Earth, the samehemisphereis always facing the Earth. In common usage, the four major phases are thenew moon,the first quarter, thefull moonand the last quarter; the four minor phases are wa xing crescent, wa xing gibbous, waning gibbous, and waning crescent. Alunar monthis the time between successive recurrences of the same phase: due to theeccentricityof the Moon's orbit, this duration is not perfectly constant but averages about 29.5 days.

The lunar phases andlibrationsin 2024 as viewed from theNorthern Hemisphereat hourly intervals, with titles and supplemental graphics

The lunar phases andlibrationsin 2024 as viewed from theSouthern Hemisphereat hourly intervals, with titles and supplemental graphics

Afull moonsets behindSan Gorgonio Mountainin California on a midsummer's morning.

The appearance of the Moon (its phase) gradually changes over a lunar month as the relative orbital positions of the Moon around Earth, and Earth around the Sun, shift. The visible side of the Moon is sunlit to varying extents, depending on the position of the Moon in its orbit, with the sunlit portion varying from 0% (at new moon) to nearly 100% (at full moon).

Phases of the Moon

"Wa xing gibbous" redirects here. For the album, seeWa xing Gibbous.

The phases of the Moon as viewed looking southward from theNorthern Hemisphere.Each phase would be rotated 180° if seen looking northward from theSouthern Hemisphere.The upper part of the diagram is not to scale, as the Moon, the Earth, and the Moon's orbit are all much smaller relative to the Earth's orbit than shown here.

There are fourprincipal(primary, or major) lunar phases: thenew moon,first quarter,full moon,and last quarter (also known as third or final quarter), when the Moon'secliptic longitudeis at an angle to the Sun (as viewed from the center of the Earth) of 0°, 90°, 180°, and 270° respectively.Each of these phases appears at slightly different times at different locations on Earth, and tabulated times are therefore alwaysgeocentric(calculated for the Earth's center).

Between the principal phases areintermediatephases, during which the apparent shape of the illuminated Moon is eithercrescentorgibbous.On average, the intermediate phases last one-quarter of asynodic month,or 7.38 days.

The termwa xingis used for an intermediate phase when the Moon's apparent shape is thickening, from new to a full moon; andwaningwhen the shape is thinning. The duration from full moon to new moon (or new moon to full moon) varies from approximately13 days22+1⁄2hoursto about15 days14+1⁄2hours.

Due to lunar motion relative to the meridian and theecliptic,in Earth'snorthern hemisphere:

• A new moon appears highest at the summersolsticeand lowest at the winter solstice.
• A first-quarter moon appears highest at the springequinoxand lowest at the autumn equinox.
• A full moon appears highest at the winter solstice and lowest at the summer solstice.
• A last-quarter moon appears highest at the autumn equinox and lowest at the spring equinox.

Non-Western cultures may use a different number of lunar phases; for example, traditionalHawaiian culturehas a total of 30 phases (one per day).

Lunar libration

Animation showing progression of the Moon's phases.

As seen from Earth, the Moon's eccentric orbit makes it both slightly change its apparent size, and to be seen from slightly different angles. The effect is subtle to the naked eye, from night to night, yet somewhat obvious in time-lapse photography.

Lunar libration causes part of the back side of the Moon to be visible to a terrestrial observer some of the time. Because of this, around 59% of the Moon's surface has been imaged from the ground.

Principal and intermediate phases of the Moon

Moon phase	Illuminated portion		Visibility	Average moonrise time	Culminationtime (highest point)	Average moonset time	Illustration		Photograph (view from Northern Hemisphere)
	Northern Hemisphere	Southern Hemisphere					Northern Hemisphere	Southern Hemisphere
New Moon	Disc completely in Sun's shadow (lit byearthshineonly)		Invisible (too close to Sun) except during a solar eclipse	06:00	12:00	18:00
Wa xingcrescent	Right side, (0%–50%) lit disc	Left side, (0%–50%) lit disc	Late morning to post-dusk	09:00	15:00	21:00
First quarter	Right side, 50.1% lit disc	Left side, 50.1% lit disc	Afternoon and early night	12:00	18:00	00:00
Wa xing gibbous	Right side, (50%–100%) lit disc	Left side, (50%–100%) lit disc	Late afternoon and most of night	15:00	21:00	03:00
Full Moon	100% illuminated disc		Sunset to sunrise (all night)	18:00	00:00	06:00
Waning gibbous	Left side, (100%–50%) lit disc	Right side, (100%–50%) lit disc	Most of night and early morning	21:00	03:00	09:00
Last quarter	Left side, 50.1% lit disc	Right side, 50.1% lit disc	Late night and morning	00:00	06:00	12:00
Waning crescent	Left side, (50%–0%) lit disc	Right side, (50%–0%) lit disc	Pre-dawn to early afternoon	03:00	09:00	15:00

This video provides an illustration of how the Moon passes through its phases – a product of its orbit, which allows different parts of its surface to be illuminated by the Sun over the course of a month. The camera is locked to the Moon as Earth rapidly rotates in the foreground.

Wa xing and waning

Diagram of the Moon's phases: The Earth is at the center of the diagram and the Moon is shown orbiting.

When the Sun and Moon arealigned on the same sideof the Earth (conjunct), the Moon is "new", and the side of the Moon facing Earth is not illuminated by the Sun. As the Moonwaxes(the amount of illuminated surface as seen from Earth increases), the lunar phases progress through the new moon, crescent moon, first-quarter moon,gibbousmoon, and full moon phases. The Moon thenwanesas it passes through the gibbous moon, third-quarter moon, and crescent moon phases, before returning back to new moon.

The termsold moonandnew moonare not interchangeable. The "old moon" is a waning sliver (which eventually becomes undetectable to the naked eye) until the moment it aligns with the Sun and begins to wax, at which point it becomes new again.Half moonis often used to mean the first- and third-quarter moons, while the termquarterrefers to the extent of the Moon's cycle around the Earth, not its shape.

When an illuminated hemisphere is viewed from a certain angle, the portion of the illuminated area that is visible will have a two-dimensional shape as defined by the intersection of anellipseand circle (in which the ellipse'smajor axiscoincides with the circle's diameter). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards),whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be acrescent.When a crescent moon occurs, the phenomenon ofearthshinemay be apparent, where the night side of the Moon dimly reflects indirect sunlight reflected from Earth.

Orientation by latitude

The observed orientation of the Moon at different phases from different latitudes on Earth (the different orientation displayed between the phases at each latitude show merely the extremes of orientation due tolibration)

In theNorthern Hemisphere,if the left side of the Moon is dark, then the bright part is thickening, and the Moon is described aswa xing(shifting toward full moon). If the right side of the Moon is dark, then the bright part is thinning, and the Moon is described as waning (past full and shifting toward new moon). Assuming that the viewer is in the Northern Hemisphere, the right side of the Moon is the part that is always wa xing. (That is, if the right side is dark, the Moon is becoming darker; if the right side is lit, the Moon is getting brighter.)

In theSouthern Hemisphere,the Moon is observed from a perspective inverted, or rotated 180°, to that of the Northern and to all of the images in this article, so that the opposite sides appear to wax or wane.

Closer to theEquator,thelunar terminatorwill appear horizontal during the morning and evening. Since the above descriptions of the lunar phases only apply atmiddleorhigh latitudes,observers moving towards thetropicsfrom northern or southern latitudes will see the Moon rotated anti-clockwise or clockwise with respect to the images in this article.

The lunar crescent can open upward or downward, with the "horns" of the crescent pointing up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens downward; when the Moon is above the Sun,the crescent opens upward.The crescent Moon is most clearly and brightly visible when the Sun is below the horizon, which implies that the Moon must be above the Sun, and the crescent must open upward. This is therefore the orientation in which the crescent Moon is most often seen from the tropics. The wa xing and waning crescents look very similar. The wa xing crescent appears in the western sky in the evening, and the waning crescent in the eastern sky in the morning.

Earthshine

Anoverexposedphotograph of a crescent Moon reveals earthshine and stars.

Main article:Earthlight (astronomy)

When the Moon (seen from Earth) is a thincrescent,Earth (as viewed from the Moon) is almost fully lit by the Sun. Often, the dark side of the Moon is dimly illuminated by indirect sunlight reflected from Earth, but is bright enough to be easily visible from Earth. This phenomenon is calledearthshine,sometimes picturesquely described as "the old moon in the new moon's arms" or "the new moon in the old moon's arms".

Timekeeping

Main articles:Lunar calendar,Lunisolar calendar,Metonic cycle,Intercalation,andHistory of calendars

Archaeologists have reconstructed methods oftimekeepingthat go back to prehistoric times, at least as old as theNeolithic.The natural units for timekeeping used by most historical societies are theday,thesolar yearand thelunation.The first crescent of the new moon provides a clear and regular marker in time and pure lunar calendars (such as the IslamicHijri calendar) rely completely on this metric. The fact, however, that a year of twelve lunar months is ten or eleven days shorter than the solar year means that a lunar calendar drifts out of step with the seasons. Lunisolar calendars resolve this issue with a year of thirteen lunar months every few years, or by restarting the count at the first new (or full) moon after thewinter solstice.TheSumerian calendaris the first recorded to have used the former method;Chinese calendaruses the latter, despite delaying its startuntil the second or even third new moonafter the solstice. TheHindu calendar,also a lunisolar calendar, further divides the month intotwo fourteen day periodsthat mark the wa xing moon and the waning moon.

The ancientRoman calendarwas broadly a lunisolar one; on the decree ofJulius Caesarin the first century BCE, Rome changed to asolar calendarof twelve months, each of a fixed number of days except in aleap year.This, theJulian calendar(slightly revised in 1582 to correct theleap yearrule), is the basis for theGregorian calendarthat is almost exclusively thecivil calendarin use worldwide today.

Calculating phase

A crescent Moon overKingman, Arizona

Each of the four intermediate phases lasts approximately seven days (7.38 days on average), but varies ±11.25% due to lunarapogee and perigee.

The number of days counted from the time of thenew moonis the Moon's "age". Each complete cycle of phases is called a "lunation".

The approximate age of the Moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as 1 January 1900 or 11 August 1999) and reducing thismodulo29.53059 days (the mean length of asynodic month).The difference between two dates can be calculated by subtracting theJulian day numberof one from that of the other, or there are simpler formulae giving (for instance) the number of days since 31 December 1899. However, this calculation assumes a perfectlycircular orbitand makes no allowance for the time of day at which the new moon occurred and therefore may be incorrect by several hours. (It also becomes less accurate the larger the difference between the required date and the reference date.) It is accurate enough to use in a novelty clock application showing lunar phase, but specialist usage taking account of lunar apogee and perigee requires a more elaborate calculation. Also, due tolunar librationit is not uncommon to see up to 101% of the full moon or even up to 5% of the lunar backside.

Effect of parallax

Main article:Lunar parallax

TheEarthsubtends an angle of about two degrees when seen from the Moon. This means that an observer on Earth who sees the Moon when it is close to the eastern horizon sees it from an angle that is about 2 degrees different from the line of sight of an observer who sees the Moon on thewestern horizon.The Moon moves about 12 degrees around its orbit per day, so, if these observers were stationary, they would see the phases of the Moon at times that differ by about one-sixth of a day, or 4 hours. But in reality, the observers are on the surface of the rotating Earth, so someone who sees the Moon on theeastern horizonat one moment sees it on the western horizon about 12 hours later. This adds an oscillation to the apparent progression of the lunar phases. They appear to occur more slowly when the Moon is high in the sky than when it is below the horizon. The Moon appears to move jerkily, and the phases do the same. The amplitude of this oscillation is never more than about four hours, which is a small fraction of amonth.It does not have any obvious effect on the appearance of the Moon. It does however affect accurate calculations of the times of lunar phases.

Misconceptions

Orbital period

It can be confusing that the Moon's orbital sidereal period is 27.3 days while the phases complete a cycle once every 29.5 days (synodic period). This is due to the Earth's orbit around the Sun. The Moon orbits the Earth 13.4 times a year, but only passes between the Earth and Sun 12.4 times.

Eclipses

As the Earth revolves around the Sun, approximateaxial parallelismof the Moon's orbital plane (tiltedfive degrees to theEarth's orbital plane) results in the revolution of thelunar nodesrelative to the Earth. This causes aneclipse seasonapproximately every six months, in which asolar eclipsecan occur at thenew moonphase and alunar eclipsecan occur at thefull moonphase.

Thelunar phasedepends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the Sun. This animation (not to scale) looks down on Earth from the north pole of the ecliptic.

It might be expected that once every month, when the Moon passes between Earth and the Sun during a new moon, its shadow would fall on Earth causing asolar eclipse,but this does not happen every month. Nor is it true that during every full moon, theEarth's shadowfalls on the Moon, causing alunar eclipse.Solar and lunar eclipses are not observedeverymonth because the plane of theMoon's orbitaround the Earth is tilted by about 5° with respect to the plane ofEarth's orbitaround the Sun (the plane of theecliptic). Thus, when new and full moons occur, the Moon usually lies to the north or south of a direct line through the Earth and Sun. Although aneclipsecan only occur when the Moon is either new (solar) or full (lunar), it must also be positioned very near the intersection of Earth's orbital plane about the Sun and the Moon's orbital plane about the Earth (that is, at one ofits nodes). This happens about twice per year, and so there are between four and seven eclipses in a calendar year. Most of these eclipses are partial; total eclipses of the Moon or Sun are less frequent.

Mechanism

The phases are not caused by the Earth's shadow falling on the moon, as some people believe.

See also

• Blue moon– Metaphor for a rare event, or lunar calendrical term
• Earth phase– Phases of the Earth as seen from the Moon
• Lunar effect– Unproven proposal of influence of lunar cycle on terrestrial creatures
• Lunar month– Time between successive new moons. (Also known as a "lunation".)
• Lunar observation– Methods and instruments used to observe the Moon
• Planetary phase– Part of planet seen to reflect sunlight
• Planetshine– Illumination by reflected sunlight from a planet
• Tide– Rise and fall of the sea level under astronomical gravitational influences
• Week– Time unit equal to seven days
• Month– Unit of time about as long the orbital period of the Moon
• Parmenides– 5th-century BC Greek philosopher, who tried to explain lunar phases

References

Sources

• Asmelash, Leah; Allan, David (30 July 2019)."A black moon is coming on July 31. Here's what that means".CNN.
• Buick, Tony; Pugh, Philip (2011).How to Photograph the Moon and Planets with Your Digital Camera.Springer.ISBN978-1-4419-5828-0.
• Kelley, David H.; Milone, Eugene F. (2011).Exploring Ancient Skies: A Survey of Ancient and Cultural Astronomy(2nd ed.). Springer.ISBN978-1-4419-7624-6.
• Kutner, Marc L. (2003).Astronomy: A Physical Perspective.Cambridge University Press. p.435.ISBN978-0-521-52927-3.
• Lynch, Mike (2007).Texas Starwatch.Voyageur Press.ISBN978-1-61060-511-3.
• Naylor, John (2002).Out of the Blue: A 24-Hour Skywatcher's Guide.Cambridge University Press.ISBN978-0-521-80925-2.
• Ruggles, Clive L. N. (2005).Ancient Astronomy: An Encyclopedia of Cosmologies and Myth.ABC-CLIO.ISBN978-1-85109-477-6.
• Seidelmann, P. Kenneth, ed. (1992).Explanatory Supplement to the Astronomical Almanac.Mill Valley: University Science Books.

External links

• Six Millennium Catalog of Phases of the Moon: Moon Phases from -1999 to +4000 (2000 BCE to 4000 CE).