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#71
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Pluto is out from planet dictionary
Margo Schulter wrote:
snip text The one thing that might be added -- especially if you or others can improve on my first quick attempt -- is a set of diagrams to show visually the levels of definition we're discussing. Here's that crude first try; note, by the way, that after the diagrams of 5A and 5B, there's a diagram for the kind of compromise I've proposed, where the term "planet" actually appears twice, once in a generic meaning (like the defeated 5B) and once in a specific meaning (like the adopted 5A), allowing a choice between the two usages. Margo, having had a longer look myself, I think your diagrams don't quite reflect the linguistics. The following shows how I see the intended outcome of 5B and illustrate how your version is really the same other than the choice of name. I have also indicated the priority of the 'satellite' criterion. [composed with monospaced font] ----------------------- Resolution 5A (Adopted) ----------------------- Solar system bodies | --------------------------------------- | | Is not a Satellite satellite of another | body [other ---------------------------------- than Sun] | | | Hydrostatic equilibrium, Not enough mass for | self-gravity produces hydrostatic equilibrium; | "nearly round" shape; and not a satellite of another | not a satellite of another body [other than Sun] | body [other than Sun] | | | | | -------------- | | | | | | Clears Doesn't | | orbit clear orbit | | | | | | | | | | "Planet" "Dwarf Planet" "Small Solar System Body" "Satellite" (e.g. most Solar System (e.g. Moon, asteroids, TNOs, comets) Titan) TNO = "Trans-Neptunian Object" ------------------------ Resolution 5B (defeated) ------------------------ Solar system bodies | --------------------------------------- | | Is not a Satellite satellite of another | body [other ---------------------------------- than Sun] | | | Hydrostatic equilibrium, Not enough mass for | self-gravity produces hydrostatic equilibrium; | "nearly round" shape; and not a satellite of another | not a satellite of another body [other than Sun] | body [other than Sun] | | | | | "Planet" | | | | | -------------- | | | | | | Clears Doesn't | | orbit clear orbit | | | | | | "Classical" "Dwarf" "Small Solar System Body" "Satellite" (e.g. most Solar System (e.g. Moon, asteroids, TNOs, comets) Titan) -------------------------------- Possible compromise proposal for future consideration -------------------------------- Solar system bodies | --------------------------------------- | | Is not a Satellite satellite of another | body [other ---------------------------------- than Sun] | | | Hydrostatic equilibrium, Not enough mass for | self-gravity produces hydrostatic equilibrium; | "nearly round" shape; and not a satellite of another | not a satellite of another body [other than Sun] | body [other than Sun] | | | | | "Planet" | | | | | -------------- | | | | | | Clears Doesn't | | orbit clear orbit | | | | | | "Major" "Dwarf" "Small Solar System Body" "Satellite" (e.g. most Solar System (e.g. Moon, asteroids, TNOs, comets) Titan) George |
#72
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Pluto is out from planet dictionary
Margo Schulter wrote: In sci.astro George Dishman wrote: Margo Schulter wrote: more trimmed OK, here's my alternative. Consider first Ceres, Pallas and the other largest main belt obects. If say the top tem had merged and were collecting the rubble then they would approach being classed as a planet albeit of very low mass. We already have a name for objects which subsequently merge to form planets, that being "planetesimal". As a result of our discussion, I would suggest that Ceres etc. should be classed as remnant planetesimals. That's certainly one usage of "planetesimal" with precedent. Personally the term tends to suggest for me more specifically something maybe around 1-10 km that's assumed to be one of the bodies serving by accretion to form some kind of larger planet -- in my terms, it could be a major planet (the "Big 8" in our Solar System); or a minor planet (dwarf or smaller). However, I certainly agree that there's a usage where any kind of minor planet (notably an asteroid) can be called a planetesimal. Having looked into it a little more, I think I agree with your criticism. From Wiki, the term "protoplanet" would be more fitting, though the "proto" prefix is perhaps not optimum. Here we get to the question of set or subset definitions versus the names to give. To me, "planetesimal" could be both evocative and "cosmogonically correct" for lots of the smaller minor planets -- "here we're seeing a living fossil, as it were, of those 1-10 km planetesimals that were the elementary building blocks of the larger planets, major or dwarf, etc." It suggests to me something smaller than a "gravito-spheroidal" planet, or "spheroidal" for short, which has likely accreted from lots of planetesimals in the narrower sense -- whether a dwarf planet, a major planet, or even a larger Small Solar System Body (SSSB) or "microplanet" as I call it, say Vesta, which isn't quite massive enough to be (gravito-)spheroidal. However, your usage seems to have lots of company, and I realize that from a certain dynamical view, anything other than a major planet (IAU "planet") could be viewed as "uncleared rubble." "Planetesimal" has the advantage of being clearly a single word. "Protoplanet" could be written "proto-planet" implying a subtype the definition of planet again leaving the ambiguity unresolved. That is unfortunate as the term is more accurate. If I wanted to propose some term other than "dwarf planet" or "mesoplanet" for the spheroidal minor planets, maybe it would be "planetoid" (carrying some science fiction associations, as has been pointed out) or possibly "planetino." Then people who wanted could view this term as referring to a type of planet, and others could argue the analogy that a neutrino is certainly not a type of neutron. On that basis, they could also argue that an asteroid is a type of star ;-) Again, it's a matter of taste -- and it seems that "planetesimal," like "planet," can evoke lots of distinct semantic preferences. I doubt there can be any existing term that doesn't and really this whole exercise is 'damage limitation'. If Ceres and Pallas at some time came close and became a binary, that doesn't change their individual nature so I would further suggest they should then be classed as a binary planetesimal system. An interesting question, indeed! If the barycenter (my provincial spelling, just to let everyone know I'm aware) is outside the radius of either body, then a binary planetesimal system -- or in my lingo "binary dwarf planet or mesoplanetary system" -- would indeed seem correct. My concerns with the barycentre argument are first that it sets a size limit that depends on the ratio of separation to diameter. A simple mass ratio limit, while arbitrary, could make the distinction more consistent. Secondly, it requires some knowledge of the density (to convert mass to radius) which will be a problem for extra-solar objects where size will not be directly measurable other than under fortuitous circumstances (e.g. transit detections). If the barycenter is within the radius of one of the bodies, then we have the "not a satellite" question -- does this apply to satellites of minor as well as major planets? Ah, well spotted, I had missed that. It's going to mess up my flowcharts :-( Yes, the test needs to apply at all levels so you can get a binary asteroid just as we already have a few asteroids with satellites. I probably need to include "not a member of a binary" as well as "not a satellite". If I were to propose a distinction, maybe based in part on precedent (catalogued minor planets, it seems to me, should stay minor planets, although they might also be satellites of another minor planet), I might argue that maybe for one minor planet to be a satellite of another in a belt environment is a bit more subtle of a relationship than the contrast between major planet and satellite. However, there's a problem there, too: with minor planet (and more specifically mesoplanet or dwarf planet) Pluto, Nixie, for example, is in a role much like that of a satellite of a major planet. Do we maybe use a mass ratio test, with Ceres-Pallas a "binary" but Pluto-Nixie a planet plus satellite? For the reasons given above, I prefer the mass ratio test but whichever we use I have no doubt that Nix and Hydra should be seen as satellites (or moons?). The question then is whether there are two satellites of the Pluto/Charon binary system or are there three moons of Pluto? If the barycenter is between the two bodies, of course, then I'd say "binary" is the right answer. This is a subtle line of questions. I think mass ratio versus barycentre is one question and moon versus satellite is another. snip - agreed Similarly the distinction between a satellite and a moon is unclear but let me suggest as a minimum that a planetesimal in orbit around a planet should be called a moon. The terms satellite and moon relate to orbital configuration rather than mass and shape so it would be both a moon and a planetesimal. Yes, a "moon" would then be what I recall that Stern/Levison (2002) call a "planetary-mass satellite" or the like, or what I might call a "spheroidal satellite." That is consistent since we now have a definition of "planetary mass" that is based on "nearly round". It also mirrors my own view that a "moon" should be a substantially larger object than the generic term satellite. By the way, I'd guess that the major/minor distinction might not apply for a satellite, since it's "circumstantial" as Basri would put it -- unless someone wants to estimate whether the satellite, if a planet in a comparable orbit, _would_ have sufficient mass to "clear its neighborhood" (if the major planet it is orbiting weren't there!). That would get into "what-if" cosmogonic scenarios, I guess. I think the question is "not applicable", an object too small to achieve a round shape is almost certainly incapable of clearing its orbit. I'm developing a new typology which does address extrasolar objects, as do your new ones, so maybe I'll have more coherent views to present soon -- _relatively_ more coherent, anyway grin. Excellent, I look forward to seing it. The criteria used by the IAU would still be applicable, hence there would be eight planets in the solar system. The definition could be easily rationalised to allow for extra-solar planets by replacing "the Sun" by "a star", even with the proviso that the current definition is limited to the solar system since AFAIK there is only one star in it ;-) True, unless we want to get into the "Nemesis" hypothesis of the 1980's (as I recall) when the whole mass extinction connection with asteroid or comet impacts (especially Cretaceous-Tertiary at around 65 Ma or "Mega-anni ago," to use a geological style) led to the hypothesis of an "invisible companion" to the Sun which every 26 My ("million years" as duration or interval rather than distance from present) or so was diverting Oort Cloud objects or the like toward the inner Solar System, including Earth. That theory wasn't found persuasive, I guess -- the "Nemesis" part, as opposed to the impact theory of the Cretaceous-Tertiary or "K-T" mass extinction, which now seems generally accepted as at least one main cause of the extinction (with the discovery of an impact crater that fits the geological timing and the scale of the hypothesized event). I believe there was another paper in the last year or so, it hasn't entirely gone away yet. I will try to find time to draw this up as a flowchart, but I have very limited opportunity over the next week, and perhaps also add a test for fusion to identify stars (including brown dwarf stars), free-floating 'planemos' and binary planemo systems. Those are neat charts! I am preparing a page of notes to explain the flowchart but it's not in a state where I can upload yet. For reference for anyone following previous posts, my flowchart is here (but needs some work to incorporate the changes discussed above): http://www.georgedishman.f2s.com/astronomy/GAD.png I would appreciate your views on the this proposal, in particular the criteria for distinguishing binary from object/satellite and what should qualify a satellite to be raised to the status of a moon. For a major planet and a satellite, or for two minor planets where the barycenter is between the two bodies, we might both be comfortable with the usual tests, although tending toward different names for some of the categories. With two minor planets like Pluto-Nixie, object/satellite seems fine. The harder situation might be when we have two minor planets not too different in size with the orbit of one within the radius of the other. Maybe we say, "Once a minor planet, always a minor planet," and put a code like the letter "S" for "satellite" after its minor planet number. Thus it seems that we both find mass ratio relevant if the barycenter test doesn't indicate a binary system. Note that this would mean that many of the moons of the planets would also be classified as planetesimals so the precedence of being a satellite and other definitions might be contentious. How about planetisimo-satellite or the like? I would say "spheroid satellite," or "gravitospheroid satellite" if we want to make it explicit that the "near-roundness" must result from self-gravity approximatinng hydrostatic equilibrium. I think we need to keep the names simple and understandable for the wider public appreciation while the criterion should be explicit, unabiguous, scientific and measurable. A press release saying "New gravitospheroidal satellites discovered!" just doesn't have the impact of "New moon discovered!" The more I consider this, the more I lean toward a convention where minor planets systems with the barycenter within the radius of one of the bodies get classified as "planet/satellite," but the satellite still keeps or gets its minor planet number, with a code like "S," as I described above. I would say "protoplanet/moon" if both are "nearly round" or "protoplanet/satellite" if the smaller is a rocky peanut or "asteroid/satellite" if both are irregular. Existing small body numbering should be undisturbed by any of this, though perhaps a few new numbers could be allocated for consistency. George |
#73
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Pluto is out from planet dictionary
"Jeff Root" wrote in message oups.com... George Dishman replied to Jeff Root George Dishman replied to Margo Schulter: My point on that is that (d) is redundant because an object cannot be a satellite and also meet criterion (a) that it be in orbit around the Sun. All known satellites are in orbit around the Sun. There is nothing redundant about (d). You see no difference in the gravitational binding of Ceres and Moon, both are orbiting the Sun? The sentence "The Moon orbits the Earth and the Earth- Moon system orbits the Sun while Ceres orbits the Sun." appears to recognise a significant difference regarding the hierarchy of gravitational binding IMO. Yes, there is a significant difference, but Ceres, the Earth, the Moon, and the Earth-Moon system all orbit the Sun. I think that depends on your understanding of orbit and perhaps you are raising a significant point. The IAU needs to define "satellite" to clear this up. Saying that a body is in orbit around the Sun does not determine whether it is a satellite or not. On the other hand, saying that a body orbits another body which in turn orbits the Sun means that the first body orbits the Sun. Well by the same understanding that says the Moon orbits the Sun (i.e. its path encompasses the Sun), Ceres orbits the Earth and Pluto orbits almost everything! I don't think that is a helpful definition of the term. Try putting "define: orbit" into Google and let me know what you think (you can ignore references to chewing gum!) best regards George |
#74
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Pluto is out from planet dictionary
On 11 Sep 2006 00:47:18 GMT, Margo Schulter
wrote: In sci.astro Cardman wrote: On 05 Sep 2006 21:22:22 GMT, Margo Schulter wrote: I should point out though that the IAU have already rejected Dwarf Planets being classed within a collective planet group, which means that this idea you will find the hardest to gain general support for. Hi, and I'd tend to suspect also that the issue was likely not just the term "Classical" but the "collective planet group" concept. Thus, indeed, people who supported 5A (and rejected 5B) might not find "major" vs. "classical" as a distinction making a difference in their votes. The use of the word "Classical" was clearly unhelpful in 5B, but if the removal of this word would have changed support for 5B remains to be seen. I would have assumed that these IAU members would have well considered the implications for rejecting 5B with or without this word. My honest view would be that this was a meeting to kick out Pluto and leave everything else exactly the same. More and more, I'd favor "planet" as an even wider umbrella covering everything customarily called a "major planet" or "minor planet," and subdivide from there, with the IAU concepts readily applied. I'm developing some formal definitions and more informal presentations on how such a scheme might be applied. I see the major fault in the IAU definition of "planet" is in their concept that a Dwarf Planet is not a planet. This is not just due to the contradiction in terms, but that I am sure that the vast majority of people would have been happy with Pluto being a Dwarf Planet had it still been a planet. The mistake that a lot of astronomers are making is in saying that Pluto should not be a planet when this 2306km diameter object cannot compare to the likes of Mars or Earth. The mistake in this logic is that Mars and Earth hardly compare to Jupiter and Saturn either. So if you are running a singular planet definition then only Jupiter, Saturn, Uranus and Neptune would be planets, where all the smaller objects would be given a different suitable name. And so once you entertain the concept that the "planet" word is currently defined to include two very different classes of objects can you see how this could be extended to three classes. And there should be no comparison between members of these three sub-groups when they are after all vastly different. The second excuse I hear is that they would be upset if there was like 47 planets in our solar system. This I find odd when I was under the assumption that astronomers were scientists. So the number will be the number there are and not the number that you want there to be. This usually trails on into what you would teach school children. That should be obvious enough, when you teach them about the 4 inner planets, the 4 gas giants, and then the teacher points out how many more dwarf planets there are with picking out a few choice examples. My Very Educated Mother Just Served Us Nachos, and a whole load more! It is now a sad day to see that our solar system, in the eyes of school children, has not got 1/9th smaller. I am sure that our solar system must go nearly half way to the next star and it seems a worthwhile classroom exorcise for the teacher to point out how BIG that actually is followed by "and this little section is what we have explored to date". The next problem I see with this IAU planet definition is that it is very warm planet biased, meaning that it only covers the area of our solar system that was subject to a collapse in the gas cloud. The evidence for that is obviously in the nice circular motions of the inner 8 planets. So I am in fact doubtful that Earth has cleared anything much when this "clearing" seems more a product of our local star and our closest gas giant neighbour. The trophy in how badly our planet clears local objects can be clearly seen in that planet-sized sphere above our heads. Also I suspect that it is true to say that had even Earth been located out at about 150 AU, or better yet 120 to 180 AU, then even Earth would have been unlikely to have cleared the region. The reason for this is that you no longer have your easy record shaped disk to vacuum up, when now you work in the 3D. So has anyone actually calculated the odds of two objects going head to head out in this vastness? And so this act of clearing could simply be an on-going event due to simply there not being enough time since our solar system began. So I am quite looking forwards to them finding something nice and big out there and seeing them try to label this a "dwarf". I would in fact say that the whole clearing concept is flawed and should be removed as such. Or at minimum this is clearly an attribute of the gas giants, and maybe smaller planets, and this attribute should be contained to the definition of these two groups. I can see why they want to strip Pluto of planethood in the belief that these objects are the left-overs from the creation of the solar system. I even read a lot of astronomy books saying exactly that, but time did not stop and there are planets formed out there as well. So one side of these astronomers needs to accept that the likes of Pluto and "Xena" are indeed planets, then the other half need to accept that these are dwarf planets are not as big/important as the larger planet classes. I think that is where you will find your agreement. Curiously, while "planet" for everything larger than a meteroid and smaller than a brown dwarf might be a bit radical, it could also in effect "neutralize" or at least play down the differences between people who have proposed usages where "planet" is defined so as to imply that either "orbit-clearing" or "hydrostatic equilibrium" is _the_ proper criterion for "planethood." Astronomers are mostly split into two groups. Both groups seem rather biased and hot-headed in defending their position. You can add a third group into this containing the majority who wisely avoid the entire debate. The 300 plus astronomers who next year will hold their own meeting to redefine "planet", in the way that they are happy with, is a clear indication that both side desire to remain in complete opposition. This seems fair proof that the third group was right after all. If everything traditionally called a "major planet" or "minor planet" qualifies for the generic sense of planet, then the arguments about what _kind_ of planet Ceres or Pluto or 2003 UB313 is might become a bit calmer and less charged. True. Neither side will easily give ground though. Also having real planets out there waiting to be found will create increased interest in the exploration of this region. The IAU already know that they have the public interest, so why not milk it for all its worth? That public can help get them increased budget to explore this region after all. And we could do with a few more fly-by probes, or better yet ones that can survive a high speed impact. Thus "major planet" would be equivalent to "planet" in 5A, and "minor planet" to what it means now, an object in a belt population ranging from a 100m asteroid to a spheroid minor planet like Ceres or Pluto, etc. The IAU desires anything 800km or above to be ranked for dwarf-planethood. Anything less is simply a solar system object. If a minor planet is spheroid (by self-gravitational forces), it would also be a "dwarf planet" -- in effect, a definition identical to 5A, but under the umbrella of "planet" along with smaller minor planets and the major ones too. The "minor planets" term has been killed off for being obsolete and has now been replaced with "solar system objects". The idea of "planet = major planet or minor planet" occurred to me in the course of a discussion with a planetary scientist who brought this generic definition to my attention -- easy to document in the OED and some astronomical reference books also. That sounds exactly like the old system, which explains why the minor planet center keeps track of all these lesser objects. They obviously have had to do some recent renaming as well. You are overlooking that being round has importance over non-round objects in the sense that the two should be independent groups. Also while I am about it I consider all three diagrams flawed when there is no size split in the satellites section, even though the satellite grouping is fine. What I mean is that I always hate it when some astronomer goes that they found another moon around a gas giant and it turns out to be a oddly shaped pebble. Maybe this usage is so widespread that it would be hard to change, especially if we take "moon" as a more informal term -- but there have been proposals to say things like "a planetary-scale satellite" to indicate that it approximates a spherical shape brought about by hydrostatic equilibrium. Yes, I prefer a three level system, but if the IAU wants to go as low as 800km, and possibly lower, then I won't argue with them. Or, as I'd say, a "spheroid satellite" or "gravitospheroid satellite" if we want to make it explicit that the near-roundness is gravitationally constrained (I'm not sure what the probability is of finding a "near-round" satellite of insufficient mass for self-gravity to be the constraining factor). You should check out 2003 EL61. This is a dwarf planet sized object that got deformed by its rapid rotation. So maybe that is just my personal gripe. Still, once you have finished playing "What is a planet?" then you can start on "What is a moon?". As if they don't stop their current method then soon enough your next moon of Saturn will be the size of a football. Yes, this satellite question is mentioned in some of the recent literature on defining a planet, but could well deserve a focus in its own right, both on its own merits and for a creative change of pace. I can only hope. Cardman http://www.cardman.org http://www.cardman.com http://www.cardman.co.uk |
#75
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Pluto is out from planet dictionary
Margo Schulter wrote: In sci.astro Paul Schlyter wrote: In article . com, Jeff Root wrote: http://www.bautforum.com/showthread.php?t=34542 The questions I asked: Were the Sun and Moon commonly referred to as "planets" in ancient times? When was Earth first called a "planet", or described as a body comparable to a planet, even if it wasn't thought to wander? When were the stars first recognized as being like the Sun? Probably around Copernicus' time. One prevailing argument against a heliocentric solar system was that the Earth's orbital motion around the Sun ought to cause a quite visible yearly parallax among the stars --- unless of course the stars were extremely distant and not just a little farther away than Saturn, as was commonly believed in that time. "Show us the stellar parallax" was also one of the lines of rebuttal to Galileo -- since if one followed the view expressed by St. Roberto Bellarmino (1615) that the traditional geocentric interpretation of certain Bible passages should be altered only on the basis of indubitable proof, the "missing" parallax could still be a reason for doubt. As I recall, the Church gradually grew more and more reconciled to Galileo's perspective as the 18th century progressed, but didn't make it fully "official" until the early 19th century, when parallax was confirmed for one or more of the nearest stars to our Sun. When the stars did turn out to be that vastly distant, they must also be very bright --- like the Sun. Herschel was one of the first trying to find the actual distance to some stars, and he thought Sirius was some 3 light years away. Interesting! Maybe that's about the same degree of accuracy as Roemer's estimate of the speed of light (1676) using the moons of Jupiter. It was an awesome leap -- I'm trying to find a worthy adjective -- from the state of things earlier in the century, when Galileo had tried the experiment of having two people a few miles away show lamps to each other and try to estimate any delay in seeing them at that distance. I found two speeds for Roemer's expeirment depending on the distance of an AU. Initially the time for light to travel an AU was thought to be 11 minutes, which is 11/8 or 1.375 of its actual time. Which is clearly beyond 8.6 LYs. However, by using Roemer's estimate of an AU (See: http://en.wikipedia.org/wiki/Ole_R%C3%B8mer), the speed is 135,000 km per second compared to the roughly 300,000 km per second it is. So, Sirius at 8.6 LY, we get 135/300 = .45 * 8.6 = 3.87 LY, so Roemer and Herschel (using 3 LY) were not that far off. Eric |
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Pluto is out from planet dictionary
George Dishman replied to Jeff Root: All known satellites are in orbit around the Sun. There is nothing redundant about (d). You see no difference in the gravitational binding of Ceres and Moon, both are orbiting the Sun? The sentence "The Moon orbits the Earth and the Earth- Moon system orbits the Sun while Ceres orbits the Sun." appears to recognise a significant difference regarding the hierarchy of gravitational binding IMO. Yes, there is a significant difference, but Ceres, the Earth, the Moon, and the Earth-Moon system all orbit the Sun. I think that depends on your understanding of orbit and perhaps you are raising a significant point. The IAU needs to define "satellite" to clear this up. No it doesn't. We don't need definitions. We can talk about these things just as well without definitions as with. Descriptions usually work better than definitions. Saying that a body is in orbit around the Sun does not determine whether it is a satellite or not. On the other hand, saying that a body orbits another body which in turn orbits the Sun means that the first body orbits the Sun. Well by the same understanding that says the Moon orbits the Sun (i.e. its path encompasses the Sun), Ceres orbits the Earth and Pluto orbits almost everything! I don't think that is a helpful definition of the term. Try putting "define: orbit" into Google and let me know what you think I haven't done that yet, but just a couple of hours before I saw this reply from you yesterday morning, I wrote something on the same subject in another forum, trying to explain to someone that "revolving" and "orbiting" are different things in astronomy: Orbiting is in general a synonym for "revolving", but in astronomy it means being in a trajectory which is primarily determined by a single gravitational source. It is possible for a body to be in several different orbits simultaneously, with those orbits determined by different gravity sources. An Apollo spacecraft orbited the Moon; the spacecraft and the Moon orbit the Earth; the spacecraft, the Moon, and the Earth orbit the Sun; the spacecraft, the Moon, the Earth, and the Sun orbit the center of mass of the Milky Way galaxy. I didn't put a lot of thought into that paragraph before posting it on the other forum, because I wasn't attempting to *define* the term "orbit", but it seems pretty good. The "primary" of the Apollo spacecraft as it orbited the Moon was the Moon. The Moon was the body at the center of mass of the Moon-Apollo system. Likewise, Earth is the primary of the Earth-Moon system, and the primary of the Earth-Moon-Apollo system. The Sun is the primary of the Sun-Earth system, the Sun-Earth-Moon system, and the Sun- Earth-Moon-Apollo system. And the Milky Way galaxy as a whole is the primary of the Galaxy-Sun system, the Galaxy- Sun-Earth system, the Galaxy-Sun-Earth-Moon system, and the Galaxy-Sun-Earth-Moon-Apollo system. You can say that Ceres orbits the entire inner Solar System, which comprises the Sun, Mercury, Venus, Earth, Mars, and lots of asteroids. That isn't much different from saying that it orbits the Sun. The Sun, being the most massive body in the system, and the body closest to the center of mass of the system, is the primary body of the system. The Earth is not the primary body of any system that Ceres belongs to. -- Jeff, in Minneapolis |
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Pluto is out from planet dictionary
"Jeff Root" wrote in message oups.com... George Dishman replied to Jeff Root: All known satellites are in orbit around the Sun. There is nothing redundant about (d). You see no difference in the gravitational binding of Ceres and Moon, both are orbiting the Sun? The sentence "The Moon orbits the Earth and the Earth- Moon system orbits the Sun while Ceres orbits the Sun." appears to recognise a significant difference regarding the hierarchy of gravitational binding IMO. Yes, there is a significant difference, but Ceres, the Earth, the Moon, and the Earth-Moon system all orbit the Sun. I think that depends on your understanding of orbit and perhaps you are raising a significant point. The IAU needs to define "satellite" to clear this up. No it doesn't. We don't need definitions. We can talk about these things just as well without definitions as with. Descriptions usually work better than definitions. OK, let's see how we get on. Saying that a body is in orbit around the Sun does not determine whether it is a satellite or not. On the other hand, saying that a body orbits another body which in turn orbits the Sun means that the first body orbits the Sun. Well by the same understanding that says the Moon orbits the Sun (i.e. its path encompasses the Sun), Ceres orbits the Earth and Pluto orbits almost everything! I don't think that is a helpful definition of the term. Try putting "define: orbit" into Google and let me know what you think I haven't done that yet, It would help. but just a couple of hours before I saw this reply from you yesterday morning, I wrote something on the same subject in another forum, trying to explain to someone that "revolving" and "orbiting" are different things in astronomy: I think "revolving around" and "orbiting". Orbiting is in general a synonym for "revolving", but in astronomy it means being in a trajectory which is primarily determined by a single gravitational source. It is possible for a body to be in several different orbits simultaneously, with those orbits determined by different gravity sources. However, those are generally split into the primary influence you describe below and perturbations of that orbit. An exception might be the phrase "chaotic orbit" which is almost self-contradictory but in a sense that merely emphasises the normal meaning. An Apollo spacecraft orbited the Moon; Yes, the craft was revolving around the Moon. the spacecraft and the Moon orbit the Earth; The craft/Moon system as a whole revolved around the Earth but I think it would be inaccurate or at least misleading to say the craft was revolving around the Earth while the LEM was on the surface. the spacecraft, the Moon, and the Earth orbit the Sun; the spacecraft, the Moon, the Earth, and the Sun orbit the center of mass of the Milky Way galaxy. I didn't put a lot of thought into that paragraph before posting it on the other forum, because I wasn't attempting to *define* the term "orbit", but it seems pretty good. I don't think it helps. If I draw the locus of the Moon over a year, the path encompases the Sun, however the Moon moves in a nearly Keplerian orbit around the Earth with Solar gravity producing only a perturbation of that orbit. On the other hand Ceres orbits the Sun with the Earth/Moon system being one perturbing influence. The "primary" of the Apollo spacecraft as it orbited the Moon was the Moon. And to me that is precisely what it means to say that the Moon orbits the Earth, not the Sun. The only exception I can think of to that rule would be possibly where a planet orbits a binary system at much larger radius than the separation of the binary components. The Moon was the body at the center of mass of the Moon-Apollo system. Likewise, Earth is the primary of the Earth-Moon system, and the primary of the Earth-Moon-Apollo system. But it is not the primary of the Earth-Apollo system since over a single orbit the Earth is not enclosed by the path of the craft. The Sun is the primary of the Sun-Earth system, the Sun-Earth-Moon system, and the Sun- Earth-Moon-Apollo system. And the Milky Way galaxy as a whole is the primary of the Galaxy-Sun system, the Galaxy- Sun-Earth system, the Galaxy-Sun-Earth-Moon system, and the Galaxy-Sun-Earth-Moon-Apollo system. You can say that Ceres orbits the entire inner Solar System, which comprises the Sun, Mercury, Venus, Earth, Mars, and lots of asteroids. You can if you define 'orbit' as meaning that the path of Ceres encloses that paths of those bodies but not if you define it as indicating which is the primary gravitational influence as I do. That isn't much different from saying that it orbits the Sun. The Sun, being the most massive body in the system, and the body closest to the center of mass of the system, is the primary body of the system. The Earth is not the primary body of any system that Ceres belongs to. No, but Earth is the primary influence on the path of the Moon, not the Sun, hence "The Moon orbits the Earth, not the Sun." is an accurate statement by my definiton of "orbit". The reason I mentioned Google is that I think my version better mirrors common usage based on the numerous dictionaries that the "define: " prefix searches. If I were to get technical I would suggest something along the lines of saying that the volume swept by the satellite is bounded by a surface on which the satellite would have zero kinetic energy relative to the primary. Simply put, the Moon cannot get too far from the Earth even though, if it were to be displaced nearer to Venus and in solar orbit, it might have the same total energy, because there is a peak of gravitational potential separating the volumes. That's difficult to explain but do you see what I am trying to convey? George |
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