tag:theconversation.com,2011:/id/topics/moon-series-5041/articlesMoon series – The Conversation2013-04-03T03:30:53Ztag:theconversation.com,2011:article/115022013-04-03T03:30:53Z2013-04-03T03:30:53ZSatellite of love: our on-off relationship with the moon<figure><img src="https://images.theconversation.com/files/19928/original/wxdsgz6v-1360021308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Astronaut Buzz Aldrin and his colleagues on the Apollo 11 mission inspired generations to be interested in lunar exploration.</span> <span class="attribution"><span class="source">EPA/NASA</span></span></figcaption></figure><p>Like all relationships, our association with the moon has had its ups and downs. </p>
<p>In <a href="https://theconversation.com/topics/moon-series">this series</a> we’ve talked about the <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">nature of the satellite</a> and how we think it was formed - in a <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">giant collision</a> that tore Earth asunder. </p>
<p>We’ve seen how the moon <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">interacts with Earth</a>, raising tides as it gradually recedes into space. And we’ve been through some of the ways that the moon may have <a href="https://theconversation.com/with-or-without-you-the-role-of-the-moon-on-life-11501">helped to shape life on Earth</a>. </p>
<p>But what about human interaction with the moon - past, present and future? That’s the topic of this final article on our nearest neighbour.</p>
<h2>Moon exploration</h2>
<p>Since we first looked at the sky, the moon has played a prominent role in <a href="http://lunar.ksc.nasa.gov/history/mythologyh.html">our stories and mythology</a>.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=507&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=507&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=507&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=637&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=637&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19923/original/fvcfc9s3-1360018984.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=637&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The first image of the far side of the moon as captured by the Luna 3 shuttle in 1959.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>It’s only natural that, once humans learnt how to escape Earth’s gravitational pull, the moon would be the first place we’d visit. Stories of trips to the moon date back <a href="http://hauntedtavern.proboards.com/index.cgi?board=conspiracies&action=print&thread=2153">at least 2,000 years</a>.</p>
<p>The first successful mission to the moon was the Russian spacecraft, <a href="http://en.wikipedia.org/wiki/Luna_2">Luna 2</a>, which crashed into the moon on September 14, 1959 - less than two years after the launch of <a href="http://en.wikipedia.org/wiki/Sputnik_1">Sputnik 1</a>, Earth’s first artificial satellite. </p>
<p>Less than a month later, the Soviets launched <a href="http://en.wikipedia.org/wiki/Luna_3">Luna 3</a>, which returned the first images of the far side of the moon, revealing that the two sides of our satellite look hugely different: something current theories of lunar formation are still <a href="http://theconversation.com/was-our-two-faced-moon-in-a-small-collision-2659">striving to explain</a>. </p>
<p>Of course, this being the Cold War, the Americans were not to be outdone, and in May 1961 John F. Kennedy committed the US to the space race, with the promise of:</p>
<blockquote>
<p>Before this decade is out, landing a man on the moon and returning him safely to the Earth.</p>
</blockquote>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/TUXuV7XbZvU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">JFK’s “moon speech” to Congress in 1961.</span></figcaption>
</figure>
<p>The mid-1960s to mid-1970s marked a golden age in lunar exploration. The first probe to soft-land on the moon (the Russian <a href="http://en.wikipedia.org/wiki/Luna_9">Luna 9</a> in February 1966) was quickly followed by the first lunar orbiter (<a href="http://en.wikipedia.org/wiki/Luna_10">Luna 10</a>, April 1966).</p>
<p>The first men to orbit the moon, the crew of <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo8.html">Apollo 8</a> (<a href="http://en.wikipedia.org/wiki/Frank_Borman">Frank Borman</a>, <a href="http://en.wikipedia.org/wiki/Jim_Lovell">James Lovell</a> and <a href="http://en.wikipedia.org/wiki/William_Anders">William Anders</a>), entered lunar orbit on Christmas Eve, 1968, and set the stage for what remains one of mankind’s greatest achievements - on July 20 1969, man first walked on the moon. </p>
<p><a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo11.html">The Apollo 11 mission</a>, crewed by <a href="http://en.wikipedia.org/wiki/Neil_Armstrong">Neil Armstrong</a>, <a href="http://en.wikipedia.org/wiki/Buzz_Aldrin">Buzz Aldrin</a> and <a href="http://www.guardian.co.uk/science/2009/jul/19/michael-collins-astronaut-apollo11">Michael Collins</a> (who remained on board the lunar orbiter while Armstrong and Aldrin got to walk on the surface) was a worldwide sensation.</p>
<p>Around the planet, people watched as, for a couple of hours, Armstrong and Aldrin explored the lunar surface, planting the US flag, gathering rocks and soil samples, and bounding around like kangaroos in the low gravity. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/RMINSD7MmT4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Giant leaps indeed.</span></figcaption>
</figure>
<p>Five more Apollo missions (Apollo <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo12.html">12</a>, <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo14.html">14</a>, <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo15.html">15</a>, <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo16.html">16</a> and <a href="http://www.nasa.gov/mission_pages/apollo/missions/apollo17.html">17</a>) carried men to the moon.</p>
<p>Apollo 13 almost went <a href="http://science.ksc.nasa.gov/history/apollo/apollo-13/apollo-13.html">catastrophically wrong</a>, and later became the subject of a <a href="http://www.imdb.com/title/tt0112384/">Hollywood blockbuster</a>, but after 1972, things stopped.</p>
<p>When <a href="http://www.jsc.nasa.gov/Bios/htmlbios/cernan-ea.html">Eugene Cernan</a> stepped back into the lunar lander on December 14 1972, he said:</p>
<blockquote>
<p>I’m on the surface; and, as I take man’s last step from the surface, back home for some time to come — but we believe not too long into the future — I’d like to just [say] what I believe history will record. That America’s challenge of today has forged man’s destiny of tomorrow. And, as we leave the moon at <a href="http://www.hq.nasa.gov/alsj/a17/a17.site.html">Taurus-Littrow</a>, we leave as we came and, God willing, as we shall return: with peace and hope for all mankind. Godspeed the crew of Apollo 17.</p>
</blockquote>
<p>I doubt anyone would have expected that, more than 40 years later, we would not have returned.</p>
<h2>Modern and future moon exploration</h2>
<p>In the <a href="http://en.wikipedia.org/wiki/Exploration_of_the_Moon#Recent_exploration">past two decades</a>, lunar exploration has begun anew, with Japan, the European Space Agency, China and India joining the US and Russia as nations to have explored our satellite - albeit with <a href="http://en.wikipedia.org/wiki/List_of_lunar_probes">unmanned probes</a> rather than human explorers. </p>
<p>These lunar orbiters continue to make exciting discoveries about our nearest neighbour - including the <a href="https://theconversation.com/blowin-in-the-solar-wind-how-the-moon-got-its-water-10160">presence of water ice on the lunar surface</a>. They have <a href="http://www.nasa.gov/mission_pages/grail/news/grail20121205.html">mapped the moon’s gravity</a>, allowing the structure and composition of the moon to be studied in great depth. They have even obtained images showing the <a href="http://lunarscience.nasa.gov/articles/lro-images-apollo-12-landing-site/">Apollo landing sites</a>.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=504&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=504&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19975/original/rmxtcfs3-1360043884.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=504&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">China’s space program continues to grow, with an unmanned lunar landing planned for later this year.</span>
<span class="attribution"><span class="source">EPA/Michael Reynolds</span></span>
</figcaption>
</figure>
<p>Much of the current exploration of the moon is focused on the idea it might be possible, one day, to return, and <a href="https://theconversation.com/lunar-boom-well-soon-be-mining-the-moon-7031">perhaps even mine there</a> for precious resources such as <a href="http://www.wired.com/science/space/news/2006/12/72276?currentPage=all">Helium-3</a>, which is incredibly rare on Earth, but thought to be much more common in the lunar <a href="http://en.wikipedia.org/wiki/Regolith">regolith</a> as a result of its continuous bombardment by the solar wind. </p>
<p>To that end, the Chinese lunar lander and rover mission <a href="http://en.wikipedia.org/wiki/Chang%27e_3">Chang'e 3</a> is currently scheduled for launch late this year. If successful, it will be the first time we have successfully made a soft landing on the moon’s surface since <a href="http://en.wikipedia.org/wiki/Luna_24">Luna 24</a> in 1976.</p>
<p>Further in the future, it is almost certain that we will return to the moon - whether to <a href="http://www.smh.com.au/technology/sci-tech/moon-mining-looks-to-uncover-hidden-gems-20130220-2eroa.html">mine the surface</a>, or perhaps <a href="http://www.independent.co.uk/news/world/americas/spacetourism-company-to-offer-two-seats-to-the-moon-8392388.html">just as tourists</a>. </p>
<p>After all, it would be a tragedy if Eugene Cernan’s excursion on the lunar surface back in 1972 was to remain our last steps on another world.</p>
<p><strong>This is the fifth and final part of our series on the moon. To read the other instalments, follow the links below:</strong></p>
<p><strong>Part One: <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">I see the moon: introducing our nearest neighbour</a></strong><br>
<strong>Part Two: <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">Crash – a-ah! Our moon has a history of violence</a></strong><br>
<strong>Part Three: <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">Out on the pull: why the moon always shows its face</a></strong><br>
<strong>Part Four: <a href="https://theconversation.com/with-or-without-you-the-role-of-the-moon-on-life-11501">With or without you: the role of the moon on life</a></strong></p><img src="https://counter.theconversation.com/content/11502/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Like all relationships, our association with the moon has had its ups and downs. In this series we’ve talked about the nature of the satellite and how we think it was formed - in a giant collision that…Jonti Horner, Post Doctoral Research Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/115012013-03-27T19:03:48Z2013-03-27T19:03:48ZWith or without you: the role of the moon on life<figure><img src="https://images.theconversation.com/files/19864/original/c6fk5wv7-1359952270.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How does the moon affect Earth's inhabitants?</span> <span class="attribution"><span class="source">shutterstock.com</span></span></figcaption></figure><p>From encouraging the first steps of <a href="http://www.scientificamerican.com/article.cfm?id=moon-life-tides">life migrating from the oceans to the land</a>, to <a href="http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=2507">stabilising Earth’s axial tilt</a> against chaotic excursions, the moon is often put forth almost as a magical ingredient - a prerequisite for life.</p>
<p>Of course, the question of the origin of life on Earth has long puzzled scientists. To date, no one has come up with a totally convincing origin story - and there are even suggestions that life may have originated elsewhere, and been delivered to Earth on the comets and asteroids that bombard our planet.</p>
<p>This theory, <a href="http://www.astrobio.net/pressrelease/5051/the-probability-of-panspermia">panspermia</a>, is attractive, since it removes the burden of life having had to form on our planet, giving us an infinitely greater range of origins, mechanisms, and times. </p>
<p>It is certainly the case that biologically-viable bacteria can be transferred between the planets.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/_TPQUgUOPhY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Richard Dawkins talks panspermia.</span></figcaption>
</figure>
<p><a href="http://www.spacedaily.com/news/life-02zx.html">Numerous studies</a> (typically involving firing bacteria in small projectiles out of guns at speeds of several kilometres per second to crater a target) have shown that bacteria can survive the shocks involved in the collision between asteroids and planetary surfaces, while <a href="http://www.bbc.co.uk/news/science-environment-11039206">experiments</a> on the International Space Station have shown how bacteria can survive and remain viable for long periods of time in space. </p>
<p>But for now, let’s just consider those theories that suggest life originated here, rather than being brought from elsewhere.</p>
<p>For theories that suggest life originated on Earth, it has often been proposed that the moon may have played in important role in its origin. </p>
<p>The youthful Earth was a vastly different place to the planet we observe today, but <a href="http://www.biog1105-1106.org/demos/106/unit04/1e.ancientoceans.html">current thinking</a> holds that our early oceans contained a plethora of organic molecules of varying complexity.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=395&fit=crop&dpr=1 600w, https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=395&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=395&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=496&fit=crop&dpr=1 754w, https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=496&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/21806/original/qd3pg5br-1364360028.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=496&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">kevin dooley</span></span>
</figcaption>
</figure>
<p>Many mechanisms have been proposed for the origin of those molecules, ranging from delivery by <a href="http://www.sciencedaily.com/releases/2013/03/130305131412.htm">cometary and asteroidal impactors</a> to a huge variety of <a href="http://geology.gsapubs.org/content/33/11/913.abstract">chemical</a> processes, or even production through radioactive decay! </p>
<p>The question of how those molecules went on to become life is another thing entirely. Again, a number of different <a href="http://en.wikipedia.org/wiki/Abiogenesis#From_organic_molecules_to_protocells">theories</a> have been put forth, but one particularly relevant to our story relies on the presence of the moon driving vast tides, which created huge tidal areas, in which complex chemistry would occur.</p>
<p>We know that, shortly <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">after its formation</a>, the moon was very, very close to Earth, and that therefore the <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">tides it would raise</a> on the oceans would be far, far greater than those we see today.</p>
<p>Since the days were far shorter, tides washed in and out again with great frequency, creating vast tidal areas on the boundaries of any continents that existed at the time.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=410&fit=crop&dpr=1 600w, https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=410&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=410&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=515&fit=crop&dpr=1 754w, https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=515&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/21805/original/df3hm793-1364359836.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=515&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tidal regions create high-energy environments.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Some authors have suggested these tidal regions <a href="http://www.telegraph.co.uk/science/science-news/3321000/Life-on-Earth-began-on-a-radioactive-beach.html">helped to concentrate radioactive materials</a> near the high-tide line, which would in turn have helped to make the building blocks of life. </p>
<p>Others point out that the constant <a href="http://www.evolutionfaq.com/articles/probability-life">grinding of the tides</a> would have created sands and small grained sediments, greatly increasing the surface area available to catalyse chemical reactions, again facilitating the development of life’s building blocks. </p>
<p>Although it is far from certain, it’s definitely feasible that the moon may have played a role in the origin of life. But what about its influence on life since?</p>
<h2>The moon and Earth’s habitability</h2>
<p>In their book <a href="http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387952896">Rare Earth: Why Complex Life Is Uncommon in the Universe</a>, published in 2000, Peter Ward and Donald Brownlee argued that planets, such as Earth, hosting complex life are likely to be incredibly rare throughout the universe.</p>
<p>Among many other arguments, the moon played a central role in their <a href="http://btc.montana.edu/ceres/astrobiology/files/RareEarth.htm">“Rare Earth” hypothesis</a>. </p>
<p>Ward and Brownlee argued that the presence of an <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">over-sized moon</a>, such as ours, is most likely a key component to making a planet habitable.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=451&fit=crop&dpr=1 600w, https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=451&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=451&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/21803/original/vg69c3fz-1364359040.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A moonglow image of the Earth and Moon.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Part of their argument was based on the fact that, were the moon non-existent, the tides raised on Earth (solely due to the influence of the sun) would be smaller, and that this might have inhibited the development of life. </p>
<p>Beyond this, they argued a key component of Earth’s habitability has been its remarkably stable axial tilt by the presence of the moon.</p>
<p>Without the moon, they argue, the Earth’s axis would vary hugely, and chaotically, on timescales of millions of years - ranging from almost no tilt at all, through the planet’s current tilt (just over 23 degrees), to our planet being tipped over on its side, <a href="http://www.universetoday.com/18955/tilt-of-uranus/">like Uranus</a>, causing all locations on the planet to experience six months of daytime, followed by six months of night, every single year. </p>
<p>While such an idea sounds somewhat outlandish, it is based on observations of the planet Mars.</p>
<p>At the current epoch, the <a href="http://en.wikipedia.org/wiki/Astronomy_on_Mars#Seasons">axial tilt of Mars</a> is almost identical to that of Earth - 25 degrees against our 23 and a half.</p>
<p>However, while Earth’s axial tilt varies by only a degree or so in either direction, that of Mars is far more chaotic.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/21804/original/d49xpd7c-1364359417.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Our relatively stable axis is apparent in long exposure photos.</span>
<span class="attribution"><span class="source">Brandon Townley</span></span>
</figcaption>
</figure>
<p>In fact, it is thought that Mars’ axial tilt varies between 0 degrees and around 60 degrees, over a few million years, as a result of perturbations from the other planets.</p>
<p>Thus, this would render the planet hugely inhospitable for complex life.</p>
<p>This hypothesis (that the moon is required to stabilise Earth’s axis) has unfortunately not stood up to scientific scrutiny. A <a href="http://online.liebertpub.com/doi/abs/10.1089/ast.2004.4.460">study</a> by <a href="http://pure.rhul.ac.uk/portal/en/persons/dave-waltham_02cb9186-0aaa-4dcc-9809-43f51180168b.html">Dave Waltham</a>, of Royal Holloway, University of London, revealed something far more interesting.</p>
<p>The Earth’s axial tilt is actually remarkably stable for a wide range of Earth-moon configurations (even for scenarios without the moon).</p>
<p>But, were the moon just slightly larger (by around ten kilometres, just a third of 1% of its diameter), it would force Earth’s axis to become unstable, driving a hugely chaotic motion.</p>
<p>More recently, other researchers have reached the <a href="http://www.space.com/12574-moonless-earth-life-habitable-alien-planets.html">same conclusion</a> - a giant moon is not needed for Earth’s axial tilt to be stable.</p>
<p>In other words, had the <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">“big splash”</a> that formed the Earth-moon system created a satellite just slightly larger, Earth would likely be a far less pleasant place for life to develop and thrive.</p>
<p>And perhaps “Earths” aren’t quite as rare as we might otherwise have thought.</p>
<p><strong>This is the fourth part of our series on the moon. To read the other instalments, follow the links below:</strong></p>
<p><strong>Part One: <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">I see the moon: introducing our nearest neighbour</a></strong><br>
<strong>Part Two: <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">Crash – a-ah! Our moon has a history of violence</a></strong><br>
<strong>Part Three: <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">Out on the pull: why the moon always shows its face</a></strong><br>
<strong>Part Five: <a href="https://theconversation.com/satellite-of-love-our-on-off-relationship-with-the-moon-11502">Satellite of love: our on-off relationship with the moon</a></strong></p><img src="https://counter.theconversation.com/content/11501/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>From encouraging the first steps of life migrating from the oceans to the land, to stabilising Earth’s axial tilt against chaotic excursions, the moon is often put forth almost as a magical ingredient…Jonti Horner, Professor (Astrophysics), University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/115002013-03-25T19:04:58Z2013-03-25T19:04:58ZOut on the pull: why the moon always shows its face<figure><img src="https://images.theconversation.com/files/21710/original/dp4y8w4j-1364187824.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The moon has no choice but to show its good side.</span> <span class="attribution"><span class="source">Nuranna</span></span></figcaption></figure><p>Technically, Pink Floyd had it wrong. The space-facing side of the moon isn’t dark (except at full moon when the Earth is between the sun and the moon). Not that you’d know that, given we always see the same side of our nearest neighbour.</p>
<p>To understand why we only see that one side, we need to explore the relationship between the moon and Earth, and the forces that will slowly, but inexorably, sling the moon from our orbit into space.</p>
<p>As the moon orbits Earth, its gravitational pull raises “tidal bulges” on our planet. Both solid ground and oceans respond to this pull, causing the moon to raise <a href="http://www.scientificamerican.com/article.cfm?id=letters-june-2008">land</a> and ocean tides. </p>
<p>At the same time, <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/tide.html">the sun</a> also raises tides on Earth which, while noticeably weaker than those caused by the moon, adds a level of complexity to the tides we experience.</p>
<p>When the moon and sun are aligned correctly (either at <a href="http://en.wikipedia.org/wiki/New_moon">new moon</a> – when the moon is approximately between Earth and the sun – or at <a href="http://en.wikipedia.org/wiki/Full_moon">full moon</a>, when Earth is approximately between the moon and the sun), the tides induced by the moon and sun add together, and we get extra-high and extra-low tides. These are commonly known as “spring tides”. </p>
<p>Equally, when the moon and sun are pulling at right angles to one another, their influence cancels out, to some extent, and we get “<a href="http://www.wisegeek.com/what-is-a-neap-tide.htm">neap tides</a>” - high tides are at their lowest, and low tides their highest.</p>
<h2>Slow spin-down – the long-term influence of tides</h2>
<p>Beyond just causing the daily <a href="http://www.thefreedictionary.com/ingress">ingress</a> and <a href="http://www.thefreedictionary.com/egress">egress</a> of the ocean onto land, tides raised by the moon on Earth have another interesting effect - they are <a href="http://en.wikipedia.org/wiki/Tidal_acceleration">slowly causing Earth’s rotation to slow</a>. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=690&fit=crop&dpr=1 600w, https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=690&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=690&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=867&fit=crop&dpr=1 754w, https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=867&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/21698/original/vxmb29p5-1364185172.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=867&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Walraven</span></span>
</figcaption>
</figure>
<p>As we all know, Earth spins on its axis once a day, but the moon takes almost a whole month to orbit our planet. As a result, the location of the tidal bulges from the moon move around our planet significantly more slowly than Earth’s surface spins. </p>
<p>Friction causes the bulges to be pulled along with Earth’s motion, to some degree, and they end up slightly ahead of the location directly beneath the moon.</p>
<p>While friction with Earth tries to pull the bulges ahead of the moon, the moon’s gravity tries to keep the bulges aligned beneath it. The end result of this conflict is to cause Earth to slowly spin down, losing rotational energy to the drag from the tidal bulges. </p>
<p>That energy is transferred to the moon, causing it to speed up in its orbit, and therefore gradually swing away from Earth.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19947/original/jhjtnxq3-1360027087.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The moon’s gravitational pull is what causes tides of “high” - as seen here at Bondi Beach - and “low” tide.</span>
<span class="attribution"><span class="source">AAP/James Horan</span></span>
</figcaption>
</figure>
<h2>We’re slowly growing apart - the moon’s recession</h2>
<p>The rate at which the moon is receding from Earth is relatively small, but easily measured (using the retroreflectors <a href="http://science.nasa.gov/science-news/science-at-nasa/2004/21jul_llr/">left on the Moon’s surface</a> by the Apollo astronauts, among other techniques). </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/-mBvBdGTkR8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The moon’s effect on tides, explained.</span></figcaption>
</figure>
<p>Currently, the recession is only around 22 millimetres a year, causing one Earth day to lengthen by about 23 microseconds a year.</p>
<p>While that doesn’t sound like much, it means <a href="http://curious.astro.cornell.edu/question.php?number=699">the moon was once much closer to Earth</a>, and Earth was spinning far faster than its current 24-hour rotation. </p>
<p>Again, these are both properties best explained by the “<a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">big splash</a>” that created the Earth-moon system.</p>
<h2>Synchronous rotation</h2>
<p>Interestingly, that same tidal evolution is the reason the moon now keeps one face continually pointed towards Earth. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/OZIB_leg75Q?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">1:1 spin-orbit resonance, or synchronous rotation, of the moon.</span></figcaption>
</figure>
<p>Earth exerts tides on the moon, just as the moon exerts tides on Earth. Since Earth is comparatively massive, the tides it raises on the moon are much greater than those raised by the moon on Earth. And those tides long ago slowed the moon’s rotation so that it spins on its axis exactly once in the time it takes to orbit our planet once. This is called “<a href="http://en.wikipedia.org/wiki/Tidal_locking">1:1 spin-orbit resonance</a>” or synchronous rotation.</p>
<p>As the moon recedes from Earth, its orbital period will increase, but the strength of Earth’s tides will ensure its spin slows, so it will always continue to show the same face to our planet.</p>
<p><strong>This is the third part of our series on the moon. To read the first and second instalments, follow the links below:</strong></p>
<p><strong>Part One: <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">I see the moon: introducing our nearest neighbour</a></strong><br>
<strong>Part Two: <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">Crash – a-ah! Our moon has a history of violence</a></strong><br>
<strong>Part Four: <a href="https://theconversation.com/with-or-without-you-the-role-of-the-moon-on-life-11501">With or without you: the role of the moon on life</a></strong><br>
<strong>Part Five: <a href="https://theconversation.com/satellite-of-love-our-on-off-relationship-with-the-moon-11502">Satellite of love: our on-off relationship with the moon</a></strong></p><img src="https://counter.theconversation.com/content/11500/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Technically, Pink Floyd had it wrong. The space-facing side of the moon isn’t dark (except at full moon when the Earth is between the sun and the moon). Not that you’d know that, given we always see the…Jonti Horner, Post Doctoral Research Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/114032013-03-21T19:29:46Z2013-03-21T19:29:46ZCrash – a-ah! Our moon has a history of violence<figure><img src="https://images.theconversation.com/files/19836/original/6tp8khhh-1359939881.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What do we know about how the moon was created?</span> <span class="attribution"><span class="source">EPA/Ali Ali</span></span></figcaption></figure><p>The more we learn about the formation and evolution of our solar system, the more we realise it was far from a sedate, gentle process. Everywhere we look we find evidence the final stages of planetary formation were punctuated by giant collisions - planet-sized bodies ploughing into one another with predictably catastrophic consequences.</p>
<p>The planet Mercury, <a href="http://www.astronomy.com/en/sitecore/content/Home/News-Observing/News/2006/04/Mercury%20blast%20splashed%20Earth.aspx">unusually over-dense and under-sized</a>; the Martian <a href="http://www.spacedaily.com/reports/Giant_Impact_Explains_Mars_Dichotomy_999.html">“hemispherical dichotomy”</a> (the marked difference between Mars’ northern lowlands and southern highlands); and Uranus’ <a href="http://www.space.com/13231-planet-uranus-knocked-sideways-impacts.html">extreme axial tilt</a> are just three examples of odd features in our solar system best explained by such catastrophic collisions.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19837/original/vp9dtw23-1359940915.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Part of the surface of Mercury that had never been imaged in detail before as photographed by the spacecraft Messenger in 2008.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Planetary satellites in the solar system are thought to have formed in one of two ways. The <a href="http://en.wikipedia.org/wiki/Regular_moon">“regular” satellites</a> (those that orbit their host on circular paths, in the plane of their equators) are thought to have formed orbiting their planets, much as the planets are thought to have formed orbiting the sun - from a disk of material around their youthful host.</p>
<p>This does a very good job of explaining their low masses (relative to their host) and the way their orbits are aligned with their host’s equators. </p>
<p>The <a href="http://en.wikipedia.org/wiki/Irregular_moon">“irregular” satellites</a> - a large population of typically tiny satellites, orbiting at vast distances from their hosts, on highly eccentric and tilted orbits - by contrast, are thought to have been captured during the latter stages of their host planets’ formation. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=512&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=512&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=512&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=644&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=644&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19842/original/vh2jkdy6-1359942561.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=644&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A map of the orbits of the irregular satellites of Saturn.</span>
<span class="attribution"><span class="source">The Singing Badger</span></span>
</figcaption>
</figure>
<p>Neither of these models works for our moon. Instead, the current leading theory for the moon’s origin is that it formed in a “big smash” – a <a href="https://theconversation.com/was-the-moon-formed-by-a-planetary-hit-and-run-9027">collision between the proto-Earth and a Mars-sized body</a>, a few tens of millions of years after the start of planetary formation.</p>
<p>The most widely accepted version of this theory holds that a Mars-sized object formed in the inner solar system, on an orbit very similar to that of the Earth. Eventually, the two bodies collided at a relatively low velocity (in astronomical terms).</p>
<p>The collision greatly disrupted the proto-Earth, but fell short of destroying it completely. Instead, vast quantities of material, primarily from the mantle and crust, were sloughed off into space around Earth, with a significant fraction of that material going on to form the moon.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/2msFzwil5HM?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The evolution of the moon in all its violent glory.</span></figcaption>
</figure>
<p>The beauty of this theory is that it explains some of the otherwise challenging things we know about the Earth-moon system. The moon is chemically and isotopically <a href="http://www.space.com/55-earths-moon-formation-composition-and-orbit.html">very similar to the Earth</a> - but it is depleted in heavy elements such as iron.</p>
<p>That’s really hard to explain if you think the moon formed alone, then was captured in our orbit (since it would then have accreted similar amounts of iron to Earth). But if you accept the moon is made primarily from material from the mantle and crust of the Earth, then its composition makes perfect sense - after all, the Earth is differentiated, so most of the heavy stuff has sunk to the core.</p>
<p>This theory also explains the unusually high mass of the moon relative to the Earth (and the associated <a href="http://www.lpl.arizona.edu/outreach/origin/">high angular momentum</a> of the system), the moon’s unusual orbit, and many other features.</p>
<p>A number of slightly different versions of this story have been proposed in recent years - suggesting the collision <a href="https://theconversation.com/was-the-moon-formed-by-a-planetary-hit-and-run-9027">could have been a bit faster</a>, and even that the moon itself was <a href="https://theconversation.com/was-our-two-faced-moon-in-a-small-collision-2659">involved in a second collision</a> later on. </p>
<p>But the core of the theory is now widely accepted: the moon was (most likely) formed in a big <a href="http://www.youtube.com/watch?v=LfmrHTdXgK4">smash</a>.</p>
<p><strong>This is the second part of our series on the moon. To read the other instalments, follow the links below:</strong></p>
<p><strong>Part One: <a href="https://theconversation.com/i-see-the-moon-introducing-our-nearest-neighbour-11499">I see the moon: introducing our nearest neighbour</a></strong><br>
<strong>Part Three: <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">Out on the pull: why the moon always shows its face</a></strong><br>
<strong>Part Four: <a href="https://theconversation.com/with-or-without-you-the-role-of-the-moon-on-life-11501">With or without you: the role of the moon on life</a></strong><br>
<strong>Part Five: <a href="https://theconversation.com/satellite-of-love-our-on-off-relationship-with-the-moon-11502">Satellite of love: our on-off relationship with the moon</a></strong></p><img src="https://counter.theconversation.com/content/11403/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The more we learn about the formation and evolution of our solar system, the more we realise it was far from a sedate, gentle process. Everywhere we look we find evidence the final stages of planetary…Jonti Horner, Post Doctoral Research Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/114992013-03-19T04:41:40Z2013-03-19T04:41:40ZI see the moon: introducing our nearest neighbour<figure><img src="https://images.theconversation.com/files/19816/original/73gpsxvx-1359935141.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The moon in total lunar eclipse as seen over Sydney in 2011.</span> <span class="attribution"><span class="source">AAP/Sydney Observatory</span></span></figcaption></figure><p>The moon. Our nearest neighbour. The main source of the ocean’s tides, and a beacon that drives the lives of animals across the globe. And also, to date, the only object beyond Earth on which humans have set foot.</p>
<p>Over the years, the moon has played a central role in the burgeoning understanding of our place in the universe. Yet despite its proximity to Earth, and the detail with which it has been studied, many of the moon’s secrets still elude us. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=606&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=606&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=606&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=762&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=762&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19819/original/fmrctzjr-1359935744.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=762&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Neil Armstrong’s “small step for man” onto the moon was a defining moment of the 20th century.</span>
<span class="attribution"><span class="source">EPA/NASA</span></span>
</figcaption>
</figure>
<p>In this, and another four articles, I’ll introduce you to some of the ways in which the moon has influenced life on Earth, how it has taught us about the formation of the solar system, and the past (and future) of our exploration of the solar system’s strangest satellite. But first, some initial thoughts on our weird neighbour.</p>
<h2>The moon: the stats</h2>
<p>The moon orbits Earth at a <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html">mean distance</a> of just over 384,000km. Its orbit is inclined to the plane of Earth’s orbit around the sun by just over five degrees. </p>
<p>If we were looking at the Earth-moon system from beyond Earth’s orbit, we would see the moon spends roughly half its time below the plane of Earth’s orbit around the sun, and the other half above that plane.</p>
<p>The location in its orbit at which the moon passes from below the plane of Earth’s orbit to above that plane is known as the <a href="http://astronomy.swin.edu.au/cosmos/a/ascending+node">“ascending node”</a>. Over a period of around 18-and-a-half years, the location of the ascending node <a href="http://en.wikipedia.org/wiki/Precession">precesses</a> around the moon’s orbit completely. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=409&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=409&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=409&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=514&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=514&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19824/original/62pj4nyj-1359936965.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=514&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Earth-moon dynamic, explained.</span>
<span class="attribution"><span class="source">infringer1</span></span>
</figcaption>
</figure>
<p>Because the rotation axis of Earth is tilted with respect to the plane of its orbit by 23-and-a-half degrees (<a href="http://www.crh.noaa.gov/lmk/?n=seasons">the cause of the seasons</a>), the moon’s orbital tilt, with respect to the Earth’s equator, varies over this 18-and-a-half year period between about 18 and 29 degrees.</p>
<p>The moon’s orbit around Earth is also slightly eccentric - at <a href="http://whatis.techtarget.com/definition/perigee">perigee</a> (its closest distance to Earth), it’s a little over 362,500km from Earth, while at <a href="http://whatis.techtarget.com/definition/apogee">apogee</a> (furthest from Earth), it is almost 405,500km away. </p>
<p>That might not sound like much - around a 10% variation in distance - but it means that the apparent size of the moon in the sky can vary quite markedly, as can be seen below.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=471&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=471&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=471&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=592&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=592&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19827/original/d3c56z4p-1359937381.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=592&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The moon at its most extreme states of distance from Earth.</span>
<span class="attribution"><span class="source">Anthony Ayiomamitis/NASA</span></span>
</figcaption>
</figure>
<p>The moon is 1,737km in <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html">mean radius</a> (compared to the Earth’s 6,371km), making it the fifth largest satellite in the solar system. But even though it is ¼ the diameter of Earth, it is significantly less dense than our planet (at <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html">roughly 1/81 the mass of Earth</a>).</p>
<p>Even stranger is the fact that the near- and far-sides of the moon look so different. The near-side, familiar to anyone who has spent any time looking at the night sky, is dominated by the “mare”, basalt outpourings that span a large fraction of surface. </p>
<p>The far side of the moon, by contrast, looks totally different - a dichotomy that has long puzzled researchers (although a recent study may have come up with <a href="https://theconversation.com/was-our-two-faced-moon-in-a-small-collision-2659">an explanation</a>).</p>
<h2>The strangest planetary satellite?</h2>
<p>In the four centuries that have passed since the discovery of the <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Display=Moons&Object=Jupiter">Galilean satellites</a> (the four moons of Jupiter), the number of satellites known through our solar system has grown to more than <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Display=Moons">170 moons</a>. And it’s not just the planets that have been found to host companions: the solar system’s smaller bodies are also <a href="http://www.johnstonsarchive.net/astro/asteroidmoons.html">regularly accompanied</a> by their own satellites.</p>
<p>The dwarf planet Pluto is now known to have <a href="https://theconversation.com/new-horizons-plutos-latest-moon-sets-the-stage-for-nasas-mission-8229">at least five satellites</a>, and companions abound in every population of solar system object we study.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=532&fit=crop&dpr=1 754w, https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=532&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/19834/original/ng6s57m4-1359938308.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=532&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The size of Earth’s moon as compared to other moons in the solar system.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>While there are several satellites larger than our moon (Jupiter’s <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Ganymede">Ganymede</a>, <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Callisto">Callisto</a> and <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jup_Io">Io</a>; and Saturn’s <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Titan">Titan</a>), those satellites are all dwarfed by their host planet. In stark contrast, when compared with Earth, the moon is huge. </p>
<p>The other large satellites in the solar system orbit their host planets almost perfectly in the plane of their equator. Yet our moon’s orbit is inclined by between 18 degrees and 28 degrees to our equator. </p>
<p>So why is our moon so unusual? Why is it so different to so many of the other satellites in the solar system? The answer lies in the moon’s origin - and that’s a story we’ll return to shortly.</p>
<p><strong>This is the first part of our series on the moon. To read the other instalments, follow the links below:</strong></p>
<p><strong>Part Two: <a href="https://theconversation.com/crash-a-ah-our-moon-has-a-history-of-violence-11403">Crash – a-ah! Our moon has a history of violence</a></strong><br>
<strong>Part Three: <a href="https://theconversation.com/out-on-the-pull-why-the-moon-always-shows-its-face-11500">Out on the pull: why the moon always shows its face</a></strong><br>
<strong>Part Four: <a href="https://theconversation.com/with-or-without-you-the-role-of-the-moon-on-life-11501">With or without you: the role of the moon on life</a></strong><br>
<strong>Part Five: <a href="https://theconversation.com/satellite-of-love-our-on-off-relationship-with-the-moon-11502">Satellite of love: our on-off relationship with the moon</a></strong></p><img src="https://counter.theconversation.com/content/11499/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The moon. Our nearest neighbour. The main source of the ocean’s tides, and a beacon that drives the lives of animals across the globe. And also, to date, the only object beyond Earth on which humans have…Jonti Horner, Post Doctoral Research Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.