What makes a moon a moon?

[soap]

What makes a moon a moon? <br /><br />I attempted to answer this question by searching for a reasonable definition. One definition I found called a moon any natural satellite of a planet. This seems a little too ambiguous to me and opens the door to any rock circling a planet. I later found that what makes a moon a moon has not been defined, and there seems to be disagreement on what sort of criteria needs to be met. Will there ever be defined criteria or is this just a topic best avoided? <br />

 

[silylene old]

I would suggest the following requirements:<br />1) some minimum mass. Clearly we don't want to be calling ring particles "moons".<br />2) the moon must orbit a planet or planetoid (not the sun, e.g. Cruethne).<br />3) The center of mass of the two bodies (barycenter) must be located below the surface of the planet. Otherwise the "moon" is too massive to be a moon, and the system should be called a double planet system. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[vogon13]

And what of objects like Telesto, in a Lagrange relationship of another moon?<br /><br />And possibly, they may have formed (in some cases) orbiting other objects that remain today as bonifide moons. (tidal evolution having in the past moved them to their current Lagrange position)<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[bonzelite]

sily, good points. <br /><br />a question: is our earth/moon really a double planet? <br /><br />vogon, do you mean trojan moons? <br /><br />

 

[waxy]

Nice try sily, but...<br /><br /><i>I would suggest the following requirements:<br />1) some minimum mass. Clearly we don't want to be calling ring particles "moons". </i> (maybe minimum volume instead?)<i><br />2) the moon must orbit a planet or planetoid (not the sun, e.g. Cruethne).</i> (or planetoid? a moon orbiting a moon?)<i><br />3) The center of mass of the two bodies (barycenter) must be located below the surface of the planet. Otherwise the "moon" is too massive to be a moon, and the system should be called a double planet system.</i><br /><br /><br />Let's see how i do at this challange:<br />1) Planet - a body massive enough that it COULD hold a <br />decently thick gasious atmosphere and who's main orbit<br />revolves around a star (or stars in the case of a binary star <br />system). <br />(this leaves the door open for Pluto to be a planet because it <br />does have an atmosphere most of the time and it can support <br />an atmosphere based on it's mass. likewise, a planet the size of<br />Pluto that is not massive enough to hold a decently thick <br /> atmosphere would fail to be a planet.)<br /><br />2) Moon- must orbit a planet and be large enough to see from <br />that plant's surface using the naked eye. (when considering so, <br />use an atmospheric condition similar to earth.)<br /><br />3) Sub-Moon - a moon that orbits any body that is not a planet<br />and cannot qualify as a planet.<br /><br />4) Rouge Planet - any object qualifying as a planet but does not<br />hold it's orbit around a star.<br /><br />5) Multi-Planet - any planet who's main orbit is a star but has a <br />codependent wobble shared with other planet(s) due to a <br />barycenter. <br />(maybe new horizons will determine that by this definition pluto<br />would be a multi-planet with Charon and it's other unnamed <br />'moon'.)<br /><br /><br />Ok, what did i not cover? i'm sure i missed something. there's <br />allways a hole when trying to define fuzzy lines.<br />

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>And what of objects like Telesto, in a Lagrange relationship of another moon? <p><hr /></p></p></blockquote><br /><br />I would argue that Telesto is orbiting Saturn for the same reason that Cruithne is generally regarded as orbiting the Sun. A case can be made that it is also orbiting Tethys, but scientists don't generally speak of it that way. They describe it as orbiting the larger body, since this is a simpler description of its movement. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[vogon13]

Still, though, in the distant past, had such a body formed around an object we all agree today is a moon, what would we have called it then?<br /><br />And my musings on the Asgard and Valhalla craters on Callisto, possibly the result of a binary Trojan?<br /><br />What are they to each other in that scenario?<br /><br />I expect the outer asteroidal satellites of all the gas giant planets to have satellites in similar proportions to the main belt asteroids. It is just a matter of time till we see a real satellite of a satellite. Maybe NH will be the craft to make the observation.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[Philotas]

I think we should label things after what they are, rather than what they were. Problem solved. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[tfwthom]

Currently.....any natural satellite orbiting a planet is defined as a moon.<br /><br />From the IAU....At the IAU General Assembly in Sydney in July 2004, the WGPSN* has assigned names to fully 35 satellites as listed below. The number of known planetary satellites now stands at 101, with almost 30 additional satellites pending recovery and naming. The CCD technology has made it possible to discover satellites down to 1 km in size. At some time in the future it may be advisable to stop naming very small satellites. <br /><br /> <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[tony873004]

How about the moons of asteroids, such as Ida's Daychtl (sp?). Or the moons of KBOs. They don't orbit planets, but they're still referred to as moons.<br /><br />Saturn could have trillions of moons if ring particles count.<br /><br />Telesto and its Lagrange companions are all satellites of Saturn in their own right. If one disappeared, the others would continue to orbit Saturn almost as if nothing happened.<br /><br />Cruithne is not a satellite of Earth. Earth's gravity locks it into a 1:1 resonance with Earth, but it does not orbit Earth. It orbits the Sun. If Earth disappeared, it would continue to orbit the Sun in an almost identical orbit.

 

[vogon13]

Toro is in a weird little resonance with Venus sometimes, and at other times with earth.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[tfwthom]

Since they have the only opinion that matters anyway, I just go with what the IAU calls a moon, a planet, a ring, etc.<br /><br />Let the big brains with now 9,040 Individual Members and 63 National Members worldwide, make the call. <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p>Cruithne is not a satellite of Earth. Earth's gravity locks it into a 1:1 resonance with Earth, but it does not orbit Earth. It orbits the Sun. If Earth disappeared, it would continue to orbit the Sun in an almost identical orbit.<p><hr /></p></p></blockquote><br /><br />Actually, it's orbit definitely would change if Earth were suddenly made to magically vanish. There's a peculiar flattening in the orbit due to Earth's influence which would go away, leaving it pretty much a typical ellipse. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[tony873004]

<blockquote><font class="small">In reply to:</font><hr /><p>Actually, it's orbit definitely would change if Earth were suddenly made to magically vanish. There's a peculiar flattening in the orbit due to Earth's influence which would go away, leaving it pretty much a typical ellipse. <p><hr /></p></p></blockquote><br />It's a typical ellipse now. Its orbit has a period that is slightly less than 1 year. So each year, the part of its orbit that comes closest to Earth advances a little. In hundreds of years from now, it catches up to Earth on the other side. Energy is exchanged making its orbital period slightly more than 1 year. Now each year, the part of its orbit that comes closest to Earth advances in the other direction for hundreds of years until it approaches Earth from the other side again. This repeats over and over. But this whole time, Cruithne is in a typical elliptical orbit. In a rotating frame, the orbit appears to be kidney-bean shaped. Here's a web page I made about Cruithne. It has diagrams that illustrate what I'm saying.<br />http://www.orbitsimulator.com/gravity/articles/cruithne.html

 

[nexium]

Smaller won't work as planets, asteroids and comets can be bigger than than the black hole or neutron star that they orbit. Less massive, perhaps. Neil

 

[willpittenger]

Sorry. Dactyl is considered a moon. What does it orbit? Ida. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

If the Earth were to disappear, even the orbits of dust particles in the Virgo supercluster would change -- slightly. Einstein proved that. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

I once heard that a key factor in deciding whether to call an object a moon or not was its size relative to the object it was orbiting. Hence, that source quoted the saying "Moons are satellites, but the Moon is not a moon." Both the Moon and Charon were too large relative to the Earth or Pluto. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[CalliArcale]

The Moon is a moon. You can't beat thousands of years of linguistic precedence, no matter how hard you try. <img src="/images/icons/wink.gif" /> In fact, other moons are only called moons because they are, in some ways, similar to the Moon.<br /><br />However, situations like the Earth-Moon and Pluto-Charon pairs are sometimes called "double planets". The smaller body is often described as a moon anyway. But there are more ambiguous situations. I don't recall the name, but radar studies of one asteroid revealed that it is probably a double asteroid -- two bodies of nearly equal size orbiting a common center of gravity.<br /><br />I would consider an object a moon if it is a natural satellite of a larger body, where the common center of gravity is within the average surface of the larger body, and where the object itself is not piddlingly small in comparison to the parent object. I don't know where the cutoff for "piddlingly small" ought to be, alas. I want to exclude tiny fragments such as most of the particles in Saturn's rings. I suspect Cassini is going to heat up the debate about the small end of the discussion, as it finds more and more moons in Saturn's rings. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[kmarinas86]

Moon = Any object that can be seen with a crescent (however faint) from a known planet.<br /><br />Any object not associated with a major/minor planet cannot be a moon.

 

[willpittenger]

<blockquote><font class="small">In reply to:</font><hr /><p>Moon = Any object that can be seen with a crescent (however faint) from a known planet. <p><hr /></p></p></blockquote><br />Nice. Your definition would include Venus. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[kmarinas86]

<font color="yellow">Nice. Your definition would include Venus.</font><br /><br />ook<br /><br />And on top of that it must orbit that same planet.<br /><br />So<br /><br />Moon = Any natural object formed during the solarsystem or other similiar event that can be seen with a crescent (however faint) from the known planet which it orbits.

 

[CalliArcale]

In other words, the natural satellite has to be resolvable into something other than a point with the unaided eye from a human observer standing on the surface of the parent planet? That's an interesting and clever approach, because if we adopt a certain standard for human resolution (obviously it varies; a few rare individuals can see Venus' crescent with the unaided eye) then it should be a simple matter of math to work it out. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>