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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 dsdfsedscribed as revolving around the Earth, this is slightly inaccurate: the Earth and the Moon ''both'' revolve around their ''barycenter'fd', their common [[centroid|center of mass]]. The combined motion is like an athlete performing an Olympic hammer throw: one object ssdfwasdsaffsings in a lfgarge circle, the other almost pivots around a fixed point but actually performs a small opposing circle of its own. <ref>Tdfsdfhe Earth and the Moon orbit each fsadfsafThe phrase "''dark'' side of the Moon" is inaccurate because the Moon does rotate relative to thed SunSA, with vfafsafGVFAFDSGAS

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 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>

The effect knodaSADwn as "the old Moon in the fnsadfasdew Moon's arms" can often be seen when the Moon is a narrow crescent. Within the intensely bright crescent, a dasim and mysterious view faosf the entire circular moon is seen. The effect is also known as ''earthshine'' because it is light from the Sun asdsareflected dc/lsdncksadfasdsaDJFC/lsm c, XMZCsadc

 

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

 

&nbsp;&dfnbsp;Swift tsahro' the vapors and the golden mist&mdash;<br>  

&nbsp;&ndfbsp;&nbspsa;&nbsp;The Full Moon's shadowy shape shone on the night,<br>  

 

&nbsp;&nbspsaasAfsa;The New Moon reached out clasping arms and kissed<br>  

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;&nbdsp;fas&nbsp;&nbsp;Her phantom lover in the whole world's sight.<br>

 

 

sdThe months of the Muslim calendar are determined by observations of the [[hilal]], the moment aDast which the thin crescent moon first befscdomes visible. This marks the start of a month. csadfc.ksajdcofdaSCin Pakistan and Jordan, hilaFDldaWSs is determined by actual observfsdation and can be affected by weather and other factors. When following the trSADaditional method, itasd aSis not actually possible to be cerfstdain of when future months will begin. In most Muslim countries, actual observation VAhFVGBDSGMJLA/FSDAs been replaced by astronomical frules.

sf==Unusual characteristics of the Moon==

 

Unalsdfiasdfksde most planets in the solar system, Earth has only one moon, and it has several striking characteristics:   

s{|

|dfsa-valign="top"

 

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:

 

&nbsp;&nbsp;Swift thro' the vapors and the golden mist&mdash;<br>  

&nbsp;&nbsp;&nbsp;&nbsp;The Full Moon's shadowy shape shone on the night,<br>  

&nbsp;&nbsp;The New Moon reached out clasping arms and kissed<br>  

&nbsp;&nbsp;&nbsp;&nbsp;Her phantom lover in the whole world's sight.<br>

 

 

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.

 

==Unusual characteristics of the Moon==

Unlike most planets in the solar system, Earth has only one moon, and it has several striking characteristics:   

 

{|

|-valign="top"

|1.

|1.

df|asdfaDuring recent history the Moon has had roughly same size as the [[Sun]] when viewed from Earth.  This creates a situation which permits sdfphenomenal solar eclipses to occur. The odds of this symmetry occurring by chance are very small. The symmetry will not last forever.asd

|During recent history the Moon has had roughly same size as the [[Sun]] when viewed from Earth.  This creates a situation which permits phenomenal solar eclipses to occur. The odds of this symmetry occurring by chance are very small. The symmetry will not last forever.

|-valign="tasdfop"

|-valign="top"

|2.fs

|2.

|The dfMoon presents the same side to Earth at all times.  That means that the rotation of the Earth offsets the separate effects of the revoluasdtion of the Moon. Physicists explain this phenomenon by [[tidal locking]]; tidal forces on Earth caused a bulge on the Moon, thereby crfedsafting a drag on the rotation of the Moon until its same face always presents to the Earth.  It was once believed that the planet [[Mercurysf]] 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, Mdfssdercury'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. 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.   

 

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

|-valisdfgn="top"

|-valign="top"

|3.ssd

|3.

|Theredsfd were several [[Moon Theories|theories]] about the origin of the Moon. Most of the older theories had been discarded in the face of observasdffatsdfions (for example based on lunar samples).<ref>Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Msdoon]</ref> The currently accepted theory is that the Moon originated from a collision early in Earth's history which created an orbiting rfiasdfng of debris from the iron-poor surface of the planet which eventually coalesced into the moon.<ref>http://www.astronosdmy.com/asy/default.aspx?c=a&id=2198</ref> This theory explains many things that spoke against the older theories, but others have poinfted out that the theory currently doesn't explain all observations of the Moon. Additionally, tests have been suggested, but the results neitfdher supported or disproved the theory.<ref>J. H. Jones: [http://www.lpi.usra.edu/meetingssdfs/dorigin98/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 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>

|-valign="topf"sd

|-valign="top"

|4.f

|4.

|Thesdf Moon currently recedes from the Earth at a rate of 1.5 inches (3.8 cm) per year<ref>NASA [http:/s/sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html]</ref>. The Moon could never have been closer than about 11,000 miles or it would hadvfe been broken up by tidal forces. If the rate of recession stayed constant at 3.8 cm per year, it would take 9.6 billion years for the lusndfar distance to migrate from 11,000 miles to the present distance of about 240,000 miles.  However, the tidal torque between Earth and the sdMoon would not be constant if the [[continental drift|continents drift]], and it also depends on the distance between the Earth and Moon fasdnd other factors, so a simple linear extrapolation is not valid.{{fact}}

|The Moon currently recedes from the Earth at a rate of 1.5 inches (3.8 cm) per year<ref>NASA [http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html]</ref>. The Moon could never have been closer than about 11,000 miles or it would have been broken up by tidal forces. If the rate of recession stayed constant at 3.8 cm per year, it would take 9.6 billion years for the lunar distance to migrate from 11,000 miles to the present distance of about 240,000 miles.  However, the tidal torque between Earth and the Moon would not be constant if the [[continental drift|continents drift]], and it also depends on the distance between the Earth and Moon and other factors, so a simple linear extrapolation is not valid.{{fact}}

|}f

|}

 

==fs dSee also ==

== See also ==

*[[Orifgsdin of the Moon]]

*[[Origin of the Moon]]

 

sd== Notes and References ==

== Notes and References ==

 

d<references/>

<references/>

 

sd[[Category:Astronomy]]

[[Category:Astronomy]]