In astronomy, a phase of the Moon is any of the aspects or appearances presented by the Moon as seen from Earth, determined by the portion of the Moon that is visibly illuminated by the Sun. The lunar phases vary cyclically as the Moon orbits the Earth, according to the relative positions of the Earth, the Moon, and the Sun. Since the Moon appears bright only due to the Sun's reflected light, only the half of the Moon closest to the Sun is illuminated. In ancient Greece and other early civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third planet outward from the Sun. ... A sun is the star at the center of a planetary system. ... Cycles are series of states or conditions that repeat themselves, usually after a regular or nearly regular period. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... A sun is the star at the center of a planetary system. ... The word reflection (also spelt reflexion in British English) can refer to several different concepts: In mathematics, reflection is the transformation of a space. ...

The lunar phase depends on the Moon's position in orbit around the Earth. This diagram looks down on Earth from the north. Earth's rotation and the Moon's orbit are both counter-clockwise here. From this diagram, we can see, for example, that the full moon will always rise at sunset, and that the waning crescent moon is high overhead around 9:00 AM local time.

Lunar phases are the result of our seeing the illuminated half of the Moon at different angles. The Moon exhibits different phases as the relative positions of the Sun, Earth and Moon change, appearing as the full moon when the Sun and Moon are on opposite sides of the Earth, and becoming invisible as the new moon (also named dark moon) when they are on the same side: these two phases are called syzygies. The time between two full moons is about 29.5 days; it is longer than the time it takes the Moon to orbit the Earth since the Earth-Moon system is orbiting the Sun. The phases are not created by the shadow of the Earth on the moon (that would be a Lunar eclipse); instead, they are a result of our seeing only part of the illuminated half of the Moon. Diagram of lunar phases, created by Minesweeper and donated to Wikipedia. ... Diagram of lunar phases, created by Minesweeper and donated to Wikipedia. ... The Galileo spacecraft took this composite image on 7 December 1992 on its way to explore the Jupiter system in 1995-97. ... Traditionally, the lunar phase new moon begins with the first visible crescent of the Moon, after conjunction with the Sun. ... The dark moon is the time when the Moon is so close to the Sun in the sky that it cannot be seen even near sunset or sunrise. ... Syzygy can refer to any of several different things: Astronomy In astronomy, a syzygy (Greek: yoked together) is a situation where three celestial bodies are positioned along a straight line. ... An eclipse occurs whenever the Sun, Earth and Moon line up exactly. ...

• Dark Moon - Not visible
• New Moon - Not visible, or traditionally: first visible crescent of the Moon
• Waxing crescent Moon - Right 1-49% visible
• First quarter Moon - Right 50% visible
• Waxing gibbous Moon - Right 51-99% visible
• Full Moon - Fully visible
• Waning gibbous Moon - Left 51-99% visible
• Third quarter Moon - Left 50% visible
• Waning crescent Moon - Left 1-49% visible
• New Moon - Not visible

In the southern hemisphere, the above is reversed. For example:

• Waxing crescent Moon - Left 1-49% visible
• Waning crescent Moon - Right 1-49% visible

When the Sun and Moon are on opposite sides of the Earth, the Moon appears full: the moon appears as a whole disc. As the Moon orbits the earth, the moon wanes, as the amount of illuminated lunar surface reduces, until the moon effectively disappears at the New Moon, when the Moon is between the Earth and the Sun and thus the illuminated half cannot be seen at all.

The different phases of the moon have different names. As the moon waxes (the amount of illuminated surface is growing), the moon moves through the New Moon, Crescent Moon, First-Quarter Moon, Gibbous Moon and Full Moon phases, before returning through the Gibbous Moon, Third-quarter Moon, Crescent Moon and Old Moon phases. Old Moon and New Moon are interchangeable, although New Moon is used in preference, and Half Moon is often used to mean the First- and Third-Quarter Moons.

Note that the plane of the Moon's orbit around the Earth is tilted by about 5 degrees with respect to the Earth's orbit around the Sun. An eclipse can only occur when the moon is positioned at, or very nearly at, one of its nodes (or crossing points between the moon's orbital path and the ecliptic). Since this normally does not occur, lunar eclipses (which can only occur at full Moon) and solar eclipses (which can only occur at new Moon) are rather rare and newsworthy events. Total solar eclipse in Zambia, 2001 An eclipse (Greek verb: ecleipo = cease to exist) is an astronomical event that occurs when one celestial object moves into the shadow of another. ... The lunar nodes are the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). ...

Casual observers will not typically notice a waxing crescent Moon until about 60 hours after it has passed conjunction with the Sun; but some individuals have crafted a hobby out of attempting to view the Moon after a much shorter interval than this. Informal "records" and their confirmability vary; some have claimed to have seen a Moon in as little as 12 hours after the moment of conjunction. Three factors increase the likelihood of spying a very "young" Moon: First, the angle the Moon makes with the ecliptic must be favourable for the applicable side of the Earth — the optimum scenario for this would be a new moon that falls in mid-March in the Northern Hemisphere or mid-September in the Southern Hemisphere; second, the Moon should be at or near perigee, causing it to appear to move faster (and hence draw away from the Sun sooner); and third, the new Moon must be at or near its maximum separation from the node in a favourable direction based on the hemisphere of the observer. These same principles can be applied to sight a very "old" Moon just before conjunction (with the best time of year being very early autumn for that side of the Earth), but this is far less commonly pursued. The plane of the Ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... March is the third month of the year in the Gregorian Calendar and one of seven Gregorian months with the length of 31 days. ... The Northern Hemisphere is the half of a planets surface (or celestial sphere) that is north of the equator. ... September is the ninth month of the year in the Gregorian Calendar and one of four Gregorian months with the length of 30 days. ... The Southern Hemisphere is the part of Earths surface that is south of the equator. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... The lunar nodes are the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). ...

A first-quarter Moon follows a daily path in the sky corresponding to that of the Sun after three months. Hence it comes at the highest altitude — or "runs high" — at or near the vernal equinox. Similarly, a full moon comes highest at the winter solstice, a last quarter Moon at the autumnal equinox, and a (almost) new Moon at the summer solstice (the opposite of "runs high" is "rides low" — a first-quarter moon at or near the autumnal equinox, a full moon at the summer solstice, etc.). This also means that a first-quarter Moon will not necessarily set at midnight, nor must a last-quarter Moon rise at midnight; both would do so at the equator, but north or south of this the time of rising or setting will vary by a progressively wider margin as the latitude increases; indeed, a first-quarter Moon in the late winter or early spring would remain constantly above the horizon in the polar regions (as would a last-quarter moon in the late summer or early autumn). Altitude is the elevation of an object from a known level or datum, called zero level. ... Illumination of Earth by Sun on the day of equinox In astronomy, the vernal equinox (spring equinox, March equinox, or northward equinox) is the moment when the sun appears to cross the celestial equator, heading northward. ... Illumination of Earth by Sun on the day of winter solstice on northern hemisphere In astronomy, the winter solstice is the moment when the earth is in a point of its orbit at which the northern or southern hemisphere is most inclined away from the sun. ... In astronomy, the autumnal equinox signals the beginning of autumn in the northern hemisphere: the moment when the sun appears to cross the celestial equator, heading southward; the equinox occurs around September 22–September 24, varying slightly each year according to the 400-year cycle of leap years in the... Illumination of Earth by Sun on the day of summer solstice on northern hemisphere The summer solstice is an astronomical term regarding the position of the Sun in relation to the celestial equator. ... The equator is an imaginary line drawn around a planet, halfway between the poles. ...

For an animation of how the Moon appears from Earth over the course of an orbit, see libration. Although the Moon keeps the same side towards Earth, careful observations will reveal you can actually see 59% of the Moons surface. ...

Mnemonics

In the northern hemisphere, if the left side of the moon is dark, the light part is growing, that is, the Moon is waxing (moving towards a full Moon). If the right side of the Moon is dark, the light part is shrinking: the Moon is waning (moving towards a new Moon). The acronym mnemonic "DOC" represents this ("D" is the waxing Moon; "O" the full moon; and "C" the waning moon). In the Southern hemisphere, this is reversed, and the mnemonic is "COD". A French mnemonic is that the waxing moon at its first "premier" quarter phase looks like a 'p', and the waning moon at its last "dernier" quarter looks like a 'd'. The southern hemisphere equivalent for 'p' and 'd' is that the moon is 'past it', or 'doing it'. One more (Northern hemisphere) mnemonic, which works for most Romance languages, says that the Moon is a liar: it spells "C", as in crescere (Italian for "to grow") when it wanes, and "D" as in decrescere ("decrease") when it waxes. For Polish it is easy to remember that C stands for "cofa się" ("is going back") and D - for "dopełnia się" ("is filling up"). In German, one mnemonic uses the cursive forms of the capital letters A for "abnehmend" (waning) and Z for "zunehmend" (waxing). In Russian, the 'C' stands for "Стареющая" or "[a moon] becoming old", while a line is added to the waning crescent to form 'P', which stands for "Рождающаяся" or "[a moon] getting born" The Northern Hemisphere is the half of a planets surface (or celestial sphere) that is north of the equator. ... The Mnemonic acronym system is a technique which uses a stored phrase to recall a list of items, often in a certain order. ... The Southern Hemisphere is the part of Earths surface that is south of the equator. ... The Romance languages, also called Romanic languages, are a subfamily of the Italic languages, specifically the descendants of the Vulgar Latin dialects spoken by the common people evolving in different areas after the break-up of the Roman Empire. ...

Lunar phase calculation

, where t = [UT] - [12AM, January 1, 2001], days

such that new moon=.0, first quarter=.25, full moon=.5, last quarter=.75

or a C program.

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Patent

• U.S. Patent 508467 - Moon dial for clocks - Clark - Nov., 1893
• U.S. Patent 4711583 - Astronomical wrist-watch - Oechslin - Dec., 1987
• U.S. Patent 5245590 - Display of changing moon on watch face - Galison - Sept., 1993

See also

• Month
• Syzygy
• Dark moon
• New moon
• Full moon

In Egyptian mythology, Month is an alternate spelling for Menthu. ... Syzygy can refer to any of several different things: Astronomy In astronomy, a syzygy (Greek: yoked together) is a situation where three celestial bodies are positioned along a straight line. ... The dark moon is the time when the Moon is so close to the Sun in the sky that it cannot be seen even near sunset or sunrise. ... Traditionally, the lunar phase new moon begins with the first visible crescent of the Moon, after conjunction with the Sun. ... The Galileo spacecraft took this composite image on 7 December 1992 on its way to explore the Jupiter system in 1995-97. ...

Links

• Virtual Reality Moon Phase
• US Naval Service on Moon Phase