On our site we have the tag, which has 37 questions. We also have at least a few more questions beyond that that are also about celestial mechanics but have not been tagged.

I think it is clear that sometimes celestial mechanics is on topic here, at least when it is directly relevant to planning space missions or other space exploration topics (such as near-earth object detection and avoidance).

What is less clear and has always invoked controversy is whether celestial mechanics in general (when it does not directly relate to planning space missions) is on topic. This discussion was triggered by the recent re-closure of the following question:

Do the planets really orbit the Sun?

To prevent eternal open/close wars over old questions and to provide clear guidance to askers going forward, we need to make a firm determination as to what kinds of celestial mechanics questions are on topic. If you provide reasoning in your answer, please do not refer to what is on/off topic on other sites or what better expertise other sites may have. These factors may impact where a user chooses to ask a question but should not impact the discussion of topicality on our site.

So what kinds of celestial mechanics questions should we allow here?

As this is a controversial topic, I think we should set a high threshold of at least 7 upvotes and a score of no less than 5 for the answer that will be accepted as community policy. This can be up for discussion in the comments under this question, but I do think some kind of threshold is needed.

If the community decision results in old questions remaining/becoming closed, those questions also need to be locked as having historical significance.

Update: The accepted answer is the new community policy based on it being the first to reach the threshold.

  • $\begingroup$ Highly related: space.meta.stackexchange.com/questions/311/… $\endgroup$ – Machavity Mar 10 '20 at 15:29
  • $\begingroup$ @Machavity Yep, this is basically rehashing that discussion but about celestial mechanics instead of planetary science. We decided to include the latter here, but the former we never made a clear call on (aside from when it directly relates to planning space missions). $\endgroup$ – called2voyage Mar 10 '20 at 15:34
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    $\begingroup$ And I wholeheartedly support it. This is what I envisioned when I ran for mod. Good use of Meta [featured] $\endgroup$ – Machavity Mar 10 '20 at 15:53
  • $\begingroup$ "other sites or what better expertise other sites may have. These factors may impact where a user chooses to ask a question" - that's too bad because it will dictate the answer they want. Astronomy is too layman, and Physics is too expert. When WorldBuilding becomes the goto, something's wrong. $\endgroup$ – Mazura Mar 12 '20 at 4:15
  • $\begingroup$ Unfortunately, they're all off-topic (except as they relate to space travel, spacecraft and space exploration) : "The study of exosolar objects, except as they relate to space travel. Physical sciences, such as geology, astronomy, cosmology, meteorology, except as they relate to spacecraft and space exploration" $\endgroup$ – Mazura Mar 12 '20 at 4:16
  • $\begingroup$ @Mazura That's not a hard and fast rule. We already make an exception for planetary science. See the post Machavity linked. $\endgroup$ – called2voyage Mar 12 '20 at 10:16
  • $\begingroup$ The reason for the exceptions being that sometimes our community deems that a whole area of study does relate to space exploration enough to be in scope here even when spacecraft are not explicitly invoked. $\endgroup$ – called2voyage Mar 12 '20 at 10:18
  • $\begingroup$ I would much rather see all questions on terraforming and mining in space be deemed as off-topic than questions about celestial mechanics. $\endgroup$ – David Hammen Mar 22 '20 at 23:31
  • $\begingroup$ Has the What topics can I ask about here? page been changed lately? It explicitly lists "Trajectory design, orbital and celestial mechanics" as being on-topic. $\endgroup$ – David Hammen Mar 23 '20 at 6:12
  • $\begingroup$ @DavidHammen That is a good point. I do think that no one expected all celestial mechanics questions (e.g. extrasolar stuff) to be on topic here, but there certainly seems to be a history of allowing some subset of celestial mechanics questions. Whatever the results of this meta discussion, that topic guidance will need to be revised to be clearer. $\endgroup$ – called2voyage Mar 23 '20 at 17:34

I'll add a contra-position answer to permit voting.

I think any questions in the Space Exploration Stack Exchange should make it clear how they are related explicitly to the exploration of space, particularly by the use of space craft.

If we were to extend the remit to all topics that relate to knowledge of space, such as celestial mechanics, we end up including so many subjects that would be better suited to other areas of Stack Exchange. If carried too far then every single astronomy question could be regarded as related to space exploration.

In many questions that have been considered contentious, the questioner and the answerer were aware of the implicit connection between the subject of the question and space exploration but did not make that clear in the wording of the question. To avoid such situations, which may leads to other, less informed readers marking it as off-topic the original question should explicitly state its relation to space exploration or someone should edit it to add that explicit link in the wording.

Thus in some sense, celestial mechanics is a "meta-tag" and not an indicator of the prime subject of on-topic questions.

(I hope I worded it well enough to create a distinct policy position point)

  • $\begingroup$ How strict is "particularly by the use of space craft"? For example would any of these questions be off-topic based on this answer? One thing that I'm worried about is if a spacecraft has to be explicitly included in the question, people might start artificially adding one to the question in an attempt to avoid being off-topic, then a distracting discussion would ensue... $\endgroup$ – uhoh Mar 17 '20 at 6:22
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    $\begingroup$ A contrived "in space" or "by the use of a spacecraft" shouldn't make an off topic question on-topic, and attempts to game the rules are managed by the community voting a question on or off topic. It may take several rounds, but as there are generally less than 10 users who care either way a question will usually be closed, or closed and reopened $\endgroup$ – user20636 Mar 17 '20 at 9:28
  • $\begingroup$ @JCRM I'm thinking that it's best to refine a definition such that doesn't lead to "several rounds" scenarios in the first place. $\endgroup$ – uhoh Mar 18 '20 at 0:17
  • $\begingroup$ I think the system works well as designed @uhoh $\endgroup$ – user20636 Mar 18 '20 at 10:27
  • $\begingroup$ I also think "If it isn't obvious why the question is relevant to space explanation, be prepared to explain in the question why it is" isn't particularly onerous. While many on-topic questions should be tagged with celestial mechanics not every good question with good answers that can be tagged with celestial mechanics is on-topic $\endgroup$ – user20636 Mar 18 '20 at 10:37
  • $\begingroup$ @JCRM that's like saying a tool like a pair of vice grips, hammer or adjustable wrench/spanner works well as designed. While each of them does (as do SE's voting systems) we must be ever-vigilant to make sure they are used properly. Even a modest toolbox — if carefully stocked — will make the need to use of those particular tools a rarity. Better to craft a definition for scope that doesn't lead to lots of contention and close/open voting cycles than to craft one that does! $\endgroup$ – uhoh Mar 18 '20 at 11:20
  • $\begingroup$ OK @uhoh, lets define all celestial mechanics as off-topic then. No reopen cycles needed. $\endgroup$ – user20636 Mar 18 '20 at 11:27
  • $\begingroup$ @BrianTompsett-汤莱恩 in addition to those mentioned in my comment above, what about questions strictly about astronaut skill sets or physical requirements (e.g. vision accuity, intact appendix, dental health, etc.)? These don't necessarily involve any spacecraft. $\endgroup$ – uhoh Mar 18 '20 at 11:28
  • $\begingroup$ @JCRM proposing that as a solution is almost another instance of this. $\endgroup$ – uhoh Mar 18 '20 at 11:29
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    $\begingroup$ I prefer reductio ad absurdum @uhoh $\endgroup$ – user20636 Mar 18 '20 at 11:43
  • $\begingroup$ My point being, unless you can list all the questions that will ever be asked, and define them as on or off topic, then It will come down to the judgement of the community. A question being closed, modified and reopened is a good thing. A question being closed and reopened as-is demonstrates different parts of the community have different ideas of what is required to make a question on-topic. That's OK as well, and the community will reach a consensus, on a case by case basis. $\endgroup$ – user20636 Mar 18 '20 at 11:58
  • $\begingroup$ My only issue with this answer is that it kind of gives the impression that it would be reversing our community decision to render all planetary science questions on topic. If this is not the intention of the answer, I think it could stand to clarify that. $\endgroup$ – called2voyage Mar 18 '20 at 18:02
  • $\begingroup$ Actually, even more importantly, if that is the intention of this answer, then I think that needs to be stated in bold text so people are 100% sure what they are voting for. $\endgroup$ – called2voyage Mar 18 '20 at 18:03
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    $\begingroup$ I strongly disagree with the premise of this answer. Science fiction questions such as terraforming, asteroid mining, 12U cubesats communicating amongst one another between low Earth orbit and low Titan orbit, etc., ... are on-topic while relevant questions involving classical mechanics, the understanding of which is essential to space exploration as we currently know it, are not? That makes no sense. $\endgroup$ – David Hammen Mar 22 '20 at 22:56

Since it seems people are hesitant to stake out an answer, I would like to offer a suggestion. I didn't want to offer my own opinion because I wanted to give the community a chance to decide, but instead of offering an opinion I'll give an estimation of what the community wants based on what types of questions were closed.

With the exception of the recently closed question (which isn't even tagged with anyway), there are four celestial mechanics questions closed as off-topic. I want to look at them one at a time to try to get an idea of what the boundary is here:

Why is Uranus's rotation 98° to its orbital plane?

This is a question about the history of the solar system--specifically what caused the irregular rotation of a single planet. Can we find similar questions that are considered on topic?

  • Is the retrograde (clockwise) rotation rate of Venus speeding up or slowing down? - This question is actually asking about the present state of a planet's rotation, but the nature of this question requires knowledge of the past conditions that led to Venus's retrograde rotation, as well as present data and future projection.
  • Why is the rotation rate of Venus so slow? - This question is highly upvoted and firmly about the history of the rotation of a single planet. That said, it does have a comment from 2017 with one upvote that the question should be on Astronomy, not here. However, it also has a reply from a mod that the question is on topic because it concerns planetary science. The former question is also tagged planetary science. Given our ruling on planetary science, it makes sense that these questions were allowed. However, it raises an interesting dilemma. This specific question--to the layperson--may or may not have a planetary science answer. In fact, the Uranus question too is potentially ambiguous for someone who does not have the relevant experience to determine whether planetary science factors could cause such an irregular axial tilt.
  • Space and Newton laws - This question is about the orbital motion of Mercury. This is a history question involving celestial mechanics of a planet.

Taking all these factors into account, we see a slight inclination to perceive questions about the history of the orbits and rotations of planets in the solar system as off topic, but it seems to be generally accepted.

My suggestion: Leave all questions about the orbits and rotation of planets in the solar system on topic.

Does the Sun actually move on its own, or does it move with the Solar system?

This is a question about how the Sun moves within the solar system. Are there similar on topic questions to this one?

My suggestion: Questions about the Sun's motion unless directly relating to space missions or planetary motion should be off-topic.

Is Mercury's orbit still considered potentially unstable (in the very long term)?

This is a question about the orbit of Mercury. Per my first suggestion above, I don't think this should have been closed.

Why does the Sun track out a seemingly sinusoidal path on the celestial sphere?

This is a question about the Sun's motion as tracked on the celestial sphere. In addition to lending support to my second suggestion above, I think this was also ruled off topic because the celestial sphere, when not invoked in reference to the motions of artificial satellites, is really more of a concept for Astronomy than Space Exploration, in general. It is about Earth-based observation.

In Summary

I suggest that all questions about the orbits and rotation of planets in the solar system be on topic, while questions about the Sun's motions should be off topic unless directly relating to space missions or planetary motion.

This would be the position most consistent with our community's determinations to date about the scope of our site.

  • $\begingroup$ By the way, if my suggestions are accepted as the new policy, I think Why is Uranus's rotation 98° to its orbital plane? should remain closed, since it is old, has no answer, and has a comment pointing to an answered version of this question on Astronomy. $\endgroup$ – called2voyage Mar 11 '20 at 13:15
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    $\begingroup$ But I think Is Mercury's orbit still considered potentially unstable (in the very long term)? should actually be reopened because the user is still active on the site and the question does not appear to have been asked on Astronomy. $\endgroup$ – called2voyage Mar 11 '20 at 13:16
  • $\begingroup$ Also, this means the recently reclosed post mentioned in the question should be reopened again, since it relates to the orbits of planets in the solar system. $\endgroup$ – called2voyage Mar 11 '20 at 13:18
  • $\begingroup$ +n! for the methodical research and careful analysis. The Sun's movement about the solar system barycenter just reflects the other planets' movements. I can't yet feel comfortable with Jupiter's movements being on topic but the Sun's movement being off-topic. It feels a little bit like saying the left half of see-saws are on-topic but the right halves aren't. To me how solar system stuff is currently moving is all one topic because it can be explored using current technology, and current exploration requires we know how it all moves. $\endgroup$ – uhoh Mar 11 '20 at 18:29
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    $\begingroup$ How a planet moved million years ago seems much more off-topic than how the Sun is moving right now. $\endgroup$ – uhoh Mar 11 '20 at 18:31
  • $\begingroup$ @uhoh I think it makes sense to a degree. Since space exploration is so focused on exploring the solar system (since that's all we can explore right now) and planets are primary targets. It makes sense to include questions about the motions of the planets. But details about the sun itself when planets and space missions are not in scope are really just questions about a star, which is perfect astronomy territory. $\endgroup$ – called2voyage Mar 11 '20 at 18:31
  • $\begingroup$ @uhoh But I think you could also argue that maybe the present motions of the Sun (aside from questions like the celestial sphere one), should be on topic as well. $\endgroup$ – called2voyage Mar 11 '20 at 18:33
  • $\begingroup$ For 99% of the time the Sun is the single largest factor that determines the trajectory of exploration spacecraft. Where the Sun is seems kind-of central; computationally, geometrically, gravitationally and conceptually. $\endgroup$ – uhoh Mar 11 '20 at 18:33
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    $\begingroup$ @uhoh "when planets and space missions are not in scope" - but I get your point, thus my last comment. $\endgroup$ – called2voyage Mar 11 '20 at 18:34
  • $\begingroup$ @uhoh Oh, and if including present motions, that would be the motions of the Sun relative to the solar system, not the interstellar motions of the Sun, which I think should definitely remain off topic unless concerning a hypothetical interstellar mission. $\endgroup$ – called2voyage Mar 11 '20 at 18:44
  • $\begingroup$ I'm almost there but it's almost 3 AM here so maybe the "day shift" (other hemisphere) can take over the discussion ... so pretty much all of these (both groups) would be on-topic, right? $\endgroup$ – uhoh Mar 11 '20 at 18:49
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    $\begingroup$ @uhoh Yes, they would be on topic. $\endgroup$ – called2voyage Mar 11 '20 at 18:50
  • $\begingroup$ @downvoters - I'm not attached to this answer, but it would be helpful if you mentioned which parts you object to because it could help form an alternative answer that we can start voting on. $\endgroup$ – called2voyage Mar 13 '20 at 13:31

While it sounds great to allow questions that pertain to astronomy (celestial mechanics, to be precise) and do not have an explicit connection to space exploration, I really think this is calls for a lot of ambiguity.

There are highly upvoted questions from the past which have no reference to space exploration, but that hardly tells us if allowing it on the site was a good idea.

Here's my very delayed policy suggestion: we close questions which do not have the explicit theme of space exploration. I know most of the community members may respond with concerns like "it could be for future missions" or "We need to understand these bodies to send spacecraft to them". My only response to that will be: kindly have the OP make it explicit in their post. The OP can

This will not only save us a lot of confusion in the long run, but will permanently end this discussion with a consensus, and also prevent us from growing a mini-Astronomy tumor on our site.

Let us remember that we are one network. If someone has a question about Astronomy, let them be redirected there or asked to clarify the relevance of the theme. As for the quality of answers available on Astronomy, I'm sure most of you, in all your expertise, wouldn't mind answering those questions there.

  • $\begingroup$ I'm not sure how to tell what the "explicit theme" of a question is or isn't. This answer says the question should be "related explicitly to the exploration of space"; can you clarify how you see the "explicitness" test is applied in practice, and how is this answer different from that one? $\endgroup$ – uhoh Apr 3 '20 at 10:28
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    $\begingroup$ It isn't very different. I just wanted to put it in a better way, as the that answer's tone seems to suggest that it was posted just to place both options on the table to allow voting. As for the explicitness, I think that's fairly clear: the question must be centered around space exploration, and not a standalone astronomy question. Regardless of how this policy turns out, in my opinion, allowing purely astronomical (no pun intended) questions is against the very idea of having different sites for different topics. @uhoh $\endgroup$ – William R. Ebenezer Apr 3 '20 at 10:37
  • $\begingroup$ How would you rate the "explicit theme" of each of these questions as being space exploration or not? Are most of us likely to come to the same conclusion? $\endgroup$ – uhoh Apr 3 '20 at 10:43
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    $\begingroup$ I glanced through them, and yes, they appear to have the theme of space exploration. I wouldn't VTC. @uhoh $\endgroup$ – William R. Ebenezer Apr 3 '20 at 10:45
  • $\begingroup$ @uhoh, I see what you're trying to say here. Let me be clear though: the objection I have is not against such questions. Lagrange points are very obviously linked to the heart of spaceEx, and questions about orbits look fine to me as long as they are generic. But, for example, if the question asks about a planetary orbit, I would like the OP to tell me why they chose to post in SpaceEx instead of Astronomy. $\endgroup$ – William R. Ebenezer Apr 3 '20 at 10:55
  • $\begingroup$ and then Better to craft a definition for scope that doesn't lead to lots of contention and close/open voting cycles than to craft one that does! My concern is that everyone can have a different idea of what the "explicit theme" of a question really is, so we still don't have a test we can objectively apply. $\endgroup$ – uhoh Apr 3 '20 at 10:59
  • $\begingroup$ Well, as long as it makes sense, the OP can add whatever they want. But that would be silly. Wouldn't it be insane for them to post on SpaceEx.SE, and then add a fake spacecraft theme, when they could honestly post it on Astro.SE? Either way, it is not something that we should be concerned about in the making of this policy. @uhoh $\endgroup$ – William R. Ebenezer Apr 3 '20 at 11:02
  • $\begingroup$ My issue with this answer is it doesn't make an attempt to define what the domain of space exploration is. It's all well and good to say we want this site to be about space exploration, but what is that? Some would say planetary science inherently is a part of space exploration, some not. Some would say celestial mechanics is a part of space exploration, some not. That's the very nature of these scope discussions. Unless the answer can provide a suggestion of what space exploration is, it is merely arbitrarily deciding that celestial mechanics is not. $\endgroup$ – called2voyage Apr 3 '20 at 13:49
  • $\begingroup$ And as always, it is expected that there will be overlap between different domains. We must do more than simply say that a topic falls under Astronomy to demonstrate that it is not a part of space exploration. It is already known that the Venn diagram of these topics has an intersection. $\endgroup$ – called2voyage Apr 3 '20 at 13:51
  • $\begingroup$ @called2voyage Planetary science falls completely under space exploration, but only when there aren't any better sites in SE. And I don't think my definition of space exploration is of help. There could be a meta thread on this, and we might never reach a consensus. I also fail to understand why my answer needs to define what space exploration is. The question is "What celestial mechanics questions are on-topic here?" and my answer is "none, other than those with a theme of space exploration". I understand that you feel it to be an unquestionable part of SpaceEx. $\endgroup$ – William R. Ebenezer Apr 3 '20 at 15:05
  • $\begingroup$ I know that there could be overlaps, and that's why I suggest: play by the book. There are two ways of avoiding ambiguity: one, by being all-inclusive, and the other, by strictly evaluating what the OP mentions in the question. I think the former discards the fact that there are specific sites for specific questions. $\endgroup$ – William R. Ebenezer Apr 3 '20 at 15:10
  • $\begingroup$ Anyway, if my answer doesn't garner votes, I guess we'll be going with your policy :) $\endgroup$ – William R. Ebenezer Apr 3 '20 at 15:11
  • $\begingroup$ @WilliamR.Ebenezer I don't feel it to be an unquestionable part of Space Exploration, but there are experts on here who are saying that it is. My answer is not my opinion but merely a framing of a suggested policy based on our past determinations of scope. You're right, ultimately it is fine for an answer to simply say "No" and if it got enough votes it would be policy. But your answer and these last few comments attempt to rationalize using "relating to space exploration" and being strict vs. inclusive without defining what those relations are to. $\endgroup$ – called2voyage Apr 3 '20 at 15:18
  • $\begingroup$ Without that definition, I think your answer would be better without the rationalization at all. $\endgroup$ – called2voyage Apr 3 '20 at 15:18

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