Lunar phase: Difference between revisions

Browse history interactively

← Previous edit Next edit →

Content deleted Content added

VisualWikitext

Inline

Revision as of 03:30, 11 November 2013

The phase and libration of the Moon for 2013 at hourly intervals, with music, titles and supplemental graphics.

The moon is really really big and fat.

Lunar phase or phase of the moon refers to the shape of the illuminated (sunlit) portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. The half of the lunar surface facing the Sun is always sunlit, but the portion of this illuminated hemisphere that is visible to an observer on Earth can vary from about 100% (full moon) to 0% (new moon). The lunar terminator is the boundary between the illuminated and unilluminated hemispheres. Aside from some craters near the lunar poles such as Shoemaker, all parts of the Moon see around 14.77 days of sunlight followed by 14.77 days of "night" (the "dark side" of the Moon is a reference to radio darkness, not visible light darkness).

The principal lunar phases are new moon, first quarter moon, full moon and last quarter moon. These are the instants, respectively, when the Moon's celestial longitude minus the Sun's celestial longitude is 0°, 90°, 180° and 270°.

Overview

The lunar phase depends 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.

The Lunar phases are the result of looking at the illuminated half of the Moon from different viewing geometries; they are not caused by the shadow of the Earth or umbra falling on the Moon's surface (this occurs only during a lunar eclipse).

The Moon exhibits different phases as the relative position of the Sun, Earth and Moon changes, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth and as a new moon (dark moon) when they are on the same side. The phases of full moon and new moon are examples of syzygies, which occur when the Earth, Moon, and Sun lie (approximately) in a straight line. The time between two full moons (a Lunar month) is about 29.53 days^[1] (29 days, 12 hours, 44 minutes) on average (hence, the concept of the time frame of an approximated month was derived). This synodic month is longer than the time it takes the Moon to make one orbit around the Earth with respect to the fixed stars (the sidereal month), which is about 27.32 days.^[1] This difference is caused by the fact that the Earth-Moon system is orbiting around the Sun at the same time the Moon is orbiting around the Earth.

The actual time between two syzygies or two phases is quite variable because the orbit of the Moon is elliptic and subject to various periodic perturbations, which change the velocity of the Moon. When the moon is closer to the earth, it moves faster; when it is farther, it moves slower. The orbit of the Earth around the Sun is also elliptic, so the speed of the Earth also varies, which also affects the phases of the Moon.

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 a solar eclipse but it does not happen every month. Nor is it true that during every full moon the Earth's shadow to fall on the Moon, causing a lunar eclipse. Solar and lunar eclipses are not observed every month because the plane of the Moon's orbit around the Earth is tilted by about five degrees with respect to the plane of Earth's orbit around the Sun (the plane of the ecliptic). 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 an eclipse can only occur when the Moon is either new (solar) or full (lunar), it must also be positioned very near the intersection of Earth's orbit plane about the Sun and the Moon's orbit plane about the Earth (that is, at one of its nodes). This happens about twice per year, and so there are between four and seven eclipses in a calendar year. Most of these events are quite insignificant; major eclipses of the Moon or Sun are less frequent.

Names of lunar phases

In Western culture, the phases of the Moon have been given the following names, in sequential order:

Phase	Northern Hemisphere	Southern Hemisphere	Visibility	Mid-phase standard time
New moon	Not visible (too close to the Sun) Later to be followed by the Moon's first visible crescent		(too close to Sun)	(too close to Sun)
'Waxing (young)'crescent moon	Right side, 1–49% lit disc	Left side, 1–49% lit disc	late afternoon and post-dusk	3 pm
'First quarter 'moon	Right side, 50%-lit disc	Left side, 50%-lit disc	afternoon and early evening	6 pm
'Waxing 'gibbous moon	Right side, 51–99% lit disc	Left side, 51–99% lit disc	late afternoon and most of night	9 pm
Full moon	Completely illuminated disc		Sunset to sunrise (all night)	sunset to sunrise (all night)
'Waning 'gibbous moon	Left side, 51–99% lit disc	Right side, 51–99% lit disc	most of night and early morning	3 am
Last (third) quarter moon	Left side, 50%-lit disc	Right side, 50%-lit disc	late night and morning	6 am
Waning (old) crescent moon	Left side, 49-1% lit disc Diminishing to the Moon's last visible crescent	Right 1–49% lit disc	pre-dawn and morning	9 am

When the Sun and Moon are aligned on the same side of the Earth the Moon is "new", and the side of the Moon visible from Earth is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress from new moon, crescent moon, first-quarter moon, gibbous moon and full moon phases, before returning through the gibbous moon, last-quarter (or last quarter) moon, crescent moon and new moon phases. The terms old moon and new moon are interchangeable, although new moon is more common. Half moon is often used to mean the first- and third-quarter moons, while the term 'quarter' refers to the extent of the moon's cycle around the Earth, not its shape.

When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion of the illuminated area that is visible will have a two-dimensional shape defined by the intersection of an ellipse and circle (where the major axis of the ellipse coincides with a diameter of the circle). 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 a crescent. When a crescent Moon occurs, the phenomenon of Earthshine may be apparent, where the night side of the Moon faintly reflects light from the Earth.

In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving toward a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (past full and moving toward a new moon). Assuming that the viewer is in the northern hemisphere, the right portion of the Moon is the part that is always growing (i.e., if the right side is dark, the Moon is growing darker; if the right side is lit, the Moon is growing lighter). In the southern hemisphere the Moon is observed from a perspective inverted to that of the northern hemisphere, so the opposite sides appear to grow (wax) and shrink (wane).

Non-Western cultures may use a different number of Moon phases, for example traditional Hawaiian culture has a total of 30 different Moon phases.^[2]

The Calendar

The average calendrical month, which is 1⁄12 of a year, is about 30.44 days, while the Moon's phase (synodic) cycle repeats on average every 29.53 days. Therefore the timing of the Moon's phases shifts by an average of almost one day for each successive month. Photographing the Moon's phase every day for a month, starting in the evening after sunset, and repeating approximately 25 minutes later each successive day, ending in the morning before sunrise, would create a composite image like the example calendar from May 8, 2005, to June 6, 2005. There is no picture on May 20 since a picture would be taken before midnight on May 19, and after midnight on May 21. Similarly, on a calendar listing moon rise or set times, some days will appear to be skipped. When the Moon rises just before midnight one night it will rise just after midnight the next (so too with setting). The 'skipped day' is just a calendar artifact and not the Moon behaving strangely.

Calculating phase

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 January 1, 1900 or August 11, 1999) and reducing this modulo 29.530588853 (the length of a synodic month). The difference between two dates can be calculated by subtracting the Julian Day Number of one from that of the other, or there are simpler formulae giving (for instance) the number of days since December 31, 1899. However, this calculation assumes a perfectly circular orbit 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 moon phase, but specialist usage taking account of lunar apogee and perigee requires a more elaborate calculation. ^{[citation needed]}

References

^ ^a ^b Pamela J. W. Gore (1996-01-22). "Phases of the Moon". Georgia Perimeter College. Retrieved 2010-06-14.
^ Hawaiian Moon Names. Imiloa, Hilo Attractions. http://www.imiloahawaii.org/57/

External links

General

Six Millennium Catalog of Phases of the Moon hosted by NASA
Virtual Reality Moon Phase
US Naval Service on Moon Phase / What the Moon Looks Like Today (United States Naval Observatory)
Full Moon Names
Telescopic moon photos through the phases by Michael Myers
Current Moon Phase
The Length of the Lunar Cycle (numerical integration analysis)
Moon Phases (by TimeTofish.info)

Educational aids

[gore1996-1] Pamela J. W. Gore (1996-01-22). "Phases of the Moon". Georgia Perimeter College. Retrieved 2010-06-14.

[2] Hawaiian Moon Names. Imiloa, Hilo Attractions. http://www.imiloahawaii.org/57/

[1]

[2]

@@ Line 1: / Line 1: @@
 {{Redirect|Moon phase|the manga and anime series|Tsukuyomi: Moon Phase}}
 [[File:Moon Phase and Libration, 2013.ogv|thumb|300px|The phase and libration of the Moon for 2013 at hourly intervals, with music, titles and supplemental graphics.]]
-[[Image:Lunar libration with phase Oct 2007 450px.gif|thumb|right|200px|Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere.  The apparent wobbling of the Moon is known as [[libration]]. The apparent change in size is due to the eccentricity of the lunar orbit.]]
+[[Image:Lunar libration with phase Oct 2007 450px.gif|thumb|right|200px|Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere.  The apparent wobbling of the Moon is known as [[libration]]. The apparent change in size is due to the eccentricity of the lunar orbit.]] The moon is really really big and fat.
 '''Lunar phase''' or '''phase of the moon''' refers to the shape of the illuminated (sunlit) portion of the [[Moon]] as seen by an observer, usually on Earth.  The lunar phases change cyclically as the Moon [[orbit]]s the Earth, according to the changing relative positions of the [[Earth]], [[Moon]], and [[Sun]]. The half of the lunar surface facing the Sun is always sunlit, but the portion of this illuminated hemisphere that is visible to an observer on Earth can vary from about 100% ([[full moon]]) to 0% ([[new moon]]). The [[lunar terminator]] is the boundary between the illuminated and unilluminated hemispheres. Aside from some craters near the lunar poles such as [[Shoemaker (lunar crater)|Shoemaker]], all parts of the Moon see around 14.77 days of sunlight followed by 14.77 days of "night" (the "dark side" of the Moon is a reference to radio darkness, not visible light darkness).

v t e Moon
Outline
Physical properties	Internal structure Topography Atmosphere Gravity field Hill sphere Magnetic field Sodium tail Moonlight Earthshine
Orbit	Lunar distance Orbital elements Distance Perigee and apogee Libration Nodes Nodal period Precession Syzygy New moon Full moon Eclipses Lunar eclipse Total penumbral lunar eclipse Tetrad Solar eclipse Solar eclipses on the Moon Eclipse cycle Supermoon Tide Tidal force Tidal locking Tidal acceleration Tidal range Lunar station
Surface and features	Selenography Terminator Limb Hemispheres Near side Far side Poles North pole South pole Face Maria List Mountains Peak of eternal light Valleys Volcanic features Domes Calderas Lava tubes Craters List Ray systems Permanently shadowed craters South Pole–Aitken basin Soil swirls Rilles Wrinkle ridges Rocks Lunar basalt 70017 Changesite-(Y) Water Space weathering Micrometeorite Sputtering Quakes Transient lunar phenomenon Selenographic coordinate system
Science	Observation Libration Lunar theory Origin Giant-impact hypothesis Theia Lunar magma ocean Geology Timescale Late Heavy Bombardment Lunar meteorites KREEP Volcanism Experiments Lunar laser ranging ALSEP Lunar sample displays Apollo 11 Apollo 17 Lunar seismology
Exploration	Missions Apollo program Explorers Probes Landing Colonization Moonbase Tourism Lunar resources
Time-telling and navigation	Lunar calendar Lunisolar calendar Month Lunar month Nodal period Fortnight Sennight Lunar station Lunar distance
Phases and names	New Full Names Crescent Super and micro Blood Blue Black Dark Wet Tetrad
Daily phenomena	Moonrise Meridian passage Moonset
Related	Lunar deities Lunar effect Earth phase Moon illusion Pareidolia Man in the Moon Moon rabbit Craters named after people Artificial objects on the Moon Memorials on the Moon Moon in science fiction list Apollo era futuristic exploration Hollow Moon Moon landing conspiracy theories Moon Treaty "Moon is made of green cheese" Natural satellite Double planet Lilith (hypothetical second moon) Splitting of the Moon
Category