What celestial mechanics questions are on topic here?

Question

On our site we have the celestial-mechanics 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.

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

Answer 1

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 celestial-mechanics 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?

• Which of the planets could we detect today from just the movements of the sun? - This seems to be the only one. This kind of question seems to be the exception rather than the rule. If I had to guess it was accepted because it was about detecting solar system planets, which lends further support to my first suggestion above.

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.

Answer 2

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)

Answer 3

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.