I was one of those who voted to close the question as off-topic, and also the one who initially voted to close it (thus pushing it into the close votes queue), so I figure I should explain my reasoning for doing so.
The information sought is certainly potentially useful in the efforts of building a fictional world, and you could certainly make a valid argument based on that line of reasoning that it should be on topic here because it has worldbuilding applications.
However, pretty much every single answerable question imaginable could pass that litmus test. I can conjure up a reason why almost any question that can be answered has applications in worldbuilding. Worldbuilding SE is not meant to be the kitchen sink, and in fact, as the site summary says (my emphasis):
Worldbuilding Stack Exchange is a question and answer site for writers, artists and others using science, geography and culture to construct imaginary worlds and settings.
This applies elsewhere on the network as well. For example, I could want to know how an AC-DC rectifier works, and ask about some detail of that on Super User, arguing that it should be on topic there because it's a part of how computer power supplies work, and Super User's scope is computers, thus it fits within their scope. But it's still not within their subject scope; such a question would be more likely to be on topic on Electrical Engineering or possibly (depending on its focus) Amateur Radio, which is also where people who are more likely to be able to answer it well focus their efforts. Just because something has applications to one field doesn't make it a part of that field, or pretty much every question on the network would reduce to being on-topic on either Physics or Mathematics:
Thus, I feel that by default for a question to be on topic on Worldbuilding there needs to be something imaginary about it, and generally speaking, the question should somehow be asking about that imaginary portion. A question can be off topic despite asking about something imaginary, and a question can include something imaginary that is completely unrelated to the question being asked, but a question that asks about something that is not imaginary is almost certainly off topic here.
There is nothing imaginary (note: imaginary, not imaginative!) about asking how to figure out how mundane celestial bodies interact in a normal solar system that could just as easily appear in real life. That's one of the things that the science field of astronomy is all about. Astronomy already has a subject-specific site, populated by experts in that particular field. An expert in astronomy is far more likely to be able to provide a good answer to how celestial bodies interact in general, than an expert in worldbuilding.
Asking how bodies in a specific, described solar system will interact would, if the solar system is constructed for the world being asked about, meet the bar of something imaginary because it postulates a situation that doesn't exist in the real world and asks how it would work out. But that's not what the OP in this particular case asked for help with.
Also note from our what topics can I ask about here? page:
For example, Worldbuilding SE welcomes questions on the following: /.../ as long as they are not about: (...among other things...)
- Historical events of or historical facts about the real world, except when provided as examples or comparisons in the construction of an imaginary world (consider the History or respective subject-specific Stack Exchange sites)
While I feel that "historical" in that bullet point may be a somewhat unfortunate phrasing these days (it originated at a time when we were getting a number of questions about the history of our world, and reflects that background), I feel that bullet point is sufficiently generally applicable to be able to state that for the most part, questions about our own real world are off topic on Worldbuilding. Add something to it that is imaginary, and ask a question that focuses on the imaginary part, and it might very well become on-topic for us, as well as quite possibly becoming off-topic for the otherwise appropriate subject-specific site.