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[[image:Moon2.jpg|right|thumb|350px|The Moon, age 21 days]]  

[[image:Moon2.jpg|right|thumb|350px|The Moon, age 21 days]]  

The '''Moon''' is a natural satellite orbiting the [[Earth]] at an average distance of about 238,000 miles (380,000 km).  

The '''Moon''' is a natural [[satellite]] orbiting the [[Earth]] at an average distance of about 238,000 miles (380,000 km).  

Although the Moon is often described as revolving around the Earth, this is slightly inaccurate: the Earth and the Moon ''both'' revolve around their ''barycenter'', their common [[centroid|center of mass]]. The combined motion is like an athlete performing an [[Olympics| Olympic]] hammer throw: one object swings in a large circle, the other almost pivots around a fixed point but actually performs a small opposing circle of its own. <ref>The Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 of its radius from its center. It is not the Earth, but the barycenter of the Earth-Moon system, that orbits the [[Sun]] in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref>

Although the Moon is often described as revolving around the Earth, this is slightly inaccurate: the Earth and the Moon ''both'' revolve around their ''barycenter'', their common [[centroid|center of mass]]. The combined motion is like an athlete performing an [[Olympics| Olympic]] hammer throw: one object swings in a large circle, the other almost pivots around a fixed point but actually performs a small opposing circle of its own. <ref>The Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 of its radius from its center. It is not the Earth, but the barycenter of the Earth-Moon system, that orbits the [[Sun]] in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref>

When speaking of things having to do with the moon, the adjective ''lunar'' is used. The Greek-derived prefixed root ''selen-'' followed by the morpheme ''-o-'' is used as well; for example, the science of mapping the moon is called ''[[selenography]]'' (morphemes: ''selen-o-graph-y'').

When speaking of things having to do with the moon, the adjective ''lunar'' is used. The Greek-derived prefixed root ''selen-'' followed by the morpheme ''-o-'' is used as well; for example, the [[science]] of mapping the moon is called ''[[selenography]]'' (morphemes: ''selen-o-graph-y'').

The gravitational force of the Moon causes the ocean [[tides]]; these tidal forces have other effects as well. These tidal forces have the effect of creating resistance which slows down the rotation of both objects. In the case of the Earth, the result is that the Earth's rotation slows on the order of one millisecond per century. In the case of the Moon, over time the Moon has entirely lost its rotation relative to the Earth. The Moon's rotation is now locked in to its period of revolution, and the same side of the Moon always faces the Earth.

The gravitational force of the Moon causes the ocean [[tides]]; these tidal forces have other effects as well. These tidal forces have the effect of creating resistance which slows down the rotation of both objects. In the case of the Earth, the result is that the Earth's rotation slows on the order of one millisecond per century. The Moon's rotation is locked in to its period of revolution, and the same side of the Moon always faces the Earth.

The phrase "''dark'' side of the Moon" is inaccurate because the Moon does rotate relative to the Sun, with the same period as its period of revolution, once every 29 days. Our view of the Moon from the Earth always shows the same side of the Moon, with the same markings and features. When the Moon is full, the entire side that faces us is experiencing lunar "day" and is lit by the Sun. Two weeks later, when the Moon is new, the side that faces us is experiencing solar "night" and is unlit (and virtually invisible).

The phrase "''dark'' side of the Moon" is inaccurate because the Moon does rotate relative to the Sun, with the same period as its period of revolution, once every 29 days. Our view of the Moon from the Earth always shows the same side of the Moon, with the same markings and features. When the Moon is full, the entire side that faces us is experiencing lunar "day" and is lit by the Sun. Two weeks later, when the Moon is new, the side that faces us is experiencing solar "night" and is unlit (and virtually invisible).

&nbsp;&nbsp;&nbsp;&nbsp;I fear we'll come to harm."<br>

&nbsp;&nbsp;&nbsp;&nbsp;I fear we'll come to harm."<br>

The effect known as "the old Moon in the new Moon's arms" can often be seen when the Moon is a narrow crescent. Within the intensely bright crescent, a dim and mysterious view of the entire circular moon is seen. The effect is also known as ''earthshine'' because it is light from the Sun reflected off the Earth to the Moon and back that lights up the darker portion of the Moon. The bright crescent is lit by the Sun and is in full daylight. The rest of the Moon is not lit by the Sun; but an astronaut standing in this area would see a "full Earth" dominating the nighttime sky, lighting it up by its earthshine. The brightness of Earthshine, and hence the visibility of the old Moon in the new Moon's arms, varies. When the Earth is covered with clouds, or when there is a lot of snow-pack on the side of the Earth facing the Moon, the Earth is a white and bright and the Earthshine is intense. When the Earth looks like a "big, blue marble," it is less intense.  

The effect known as "the old Moon in the new Moon's arms" can often be seen when the Moon is a narrow [[crescent]]. Within the intensely bright crescent, a dim and mysterious view of the entire circular moon is seen. The effect is also known as ''earthshine'' because it is light from the Sun reflected off the Earth to the Moon and back that lights up the darker portion of the Moon. The bright crescent is lit by the Sun and is in full daylight. The rest of the Moon is not lit by the Sun; but an [[astronaut]] standing in this area would see a "full Earth" dominating the nighttime sky, lighting it up by its earthshine. The brightness of Earthshine, and hence the visibility of the old Moon in the new Moon's arms, varies. When the Earth is covered with clouds, or when there is a lot of snow-pack on the side of the Earth facing the Moon, the Earth is a white and bright and the Earthshine is intense. When the Earth looks like a "big, blue marble," it is less intense.  

"The Old Moon in the New Moon's Arms" is the title of a poem by [[Ella Wheeler Wilcox]], in which she interprets the effect as the New Moon wishing that her lost love, the Old Moon, be in her arms one last time; Venus grants the wish, and:

"The Old Moon in the New Moon's Arms" is the title of a poem by [[Ella Wheeler Wilcox]], in which she interprets the effect as the New Moon wishing that her lost love, the Old Moon, be in her arms one last time; Venus grants the wish, and:

The months of the Muslim calendar are determined by observations of the [[hilal]], the moment at which the thin crescent moon first becomes visible. This marks the start of a month. Traditionally, and today in Pakistan and Jordan, hilal is determined by actual observation and can be affected by weather and other factors. When following the traditional method, it is not actually possible to be certain of when future months will begin. In most Muslim countries, actual observation has been replaced by astronomical rules.

The months of the [[Islam|Muslim]] [[calendar]] are determined by observations of the [[hilal]], the moment at which the thin crescent moon first becomes visible. This marks the start of a month. Traditionally, and today in [[Pakistan]] and [[Jordan]], hilal is determined by actual observation and can be affected by weather and other factors. When following the traditional method, it is not actually possible to be certain precisely when future months will begin. In most Muslim countries, actual observation has been replaced by [[astronomy}|astronomical]] rules.

==Unusual characteristics of the Moon==

==Unusual characteristics of the Moon==

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|The Moon presents the same side to Earth at all times.  That means that the rotation of the Earth offsets the separate effects of the revolution of the Moon. Physicists explain this phenomenon by [[tidal locking]]; tidal forces on Earth caused a bulge on the Moon, thereby creating a drag on the rotation of the Moon until its same face always presents to the Earth.  It was once believed that the planet [[Mercury]] had a similar locked rotation due to gravitational forces from the sun, such that the same side of Mercury always faces the sun. In fact, Mercury's tidal locking is incomplete; it does not always present the same face to the sun.   

|The Moon presents the same side to Earth at all times.  That means that the rotation of the Earth offsets the separate effects of the revolution of the Moon. Secular physicists explain this phenomenon by [[tidal locking]]; tidal forces on Earth caused a bulge on the Moon, thereby creating a drag on the rotation of the Moon until its same face always presents to the Earth.  It was once believed that the planet [[Mercury]] had a similar locked rotation due to gravitational forces from the sun, such that the same side of Mercury always faces the sun. In fact, Mercury's tidal locking is incomplete; it does not always present the same face to the sun.   

Current tidal forces exerted by the Earth on the Moon cannot account fully for its bulge or egg shape, though accounting for greater tidal forces in the past may explain this discrepancy.<ref>http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT308/HTML/AT30803.HTM</ref>

Current tidal forces exerted by the Earth on the Moon cannot account fully for its bulge or egg shape, though accounting for greater tidal forces in the past may explain this discrepancy.<ref>http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT308/HTML/AT30803.HTM</ref>

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|There were several [[Moon Theories|theories]] about the origin of the Moon. Most of the older theories had been discarded in the face of observations (for example based on lunar samples).<ref>Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Moon]</ref> The currently accepted theory is that the Moon originated from a collision early in Earth's history which created an orbiting ring of debris from the iron-poor surface of the planet which eventually coalesced into the moon.<ref>http://www.astronomy.com/asy/default.aspx?c=a&id=2198</ref> This theory explains many things that spoke against the older theories, but others have pointed out that the theory currently doesn't explain all observations of the Moon. Additionally, tests have been suggested, but the results neither supported or disproved the theory.<ref>J. H. Jones: [http://www.lpi.usra.edu/meetings/origin98/pdf/4045.pdf Tests of the Giant Impact Hypothesis]</ref>

|There were several [[Moon Theories|theories]] about the origin of the Moon. Most of the older theories had been discarded in the face of observations (for example based on lunar samples).<ref>Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Moon]</ref> The theory currently accepted by mainstream scientists is that the Moon originated from a collision early in Earth's history which created an orbiting ring of debris from the iron-poor surface of the planet which eventually coalesced into the moon.<ref>http://www.astronomy.com/asy/default.aspx?c=a&id=2198</ref> This theory explains many things that spoke against the older theories, but others have pointed out that the theory currently doesn't explain all observations of the Moon. Additionally, tests have been suggested, but the results neither supported or disproved the theory.<ref>J. H. Jones: [http://www.lpi.usra.edu/meetings/origin98/pdf/4045.pdf Tests of the Giant Impact Hypothesis]</ref>

 

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