Is there a possible scenario in which once a year a celestial body can be seen from the surface of the Earth-like planet for a short period of time?

  • With it being big enough for early civilization to consider it a second moon but much smaller than the actual one.
  • And occurring each year for one thousand years.

I thought two moons questions are similar enough, but I haven't found a proper answer for myself.

Thank you!

  • $\begingroup$ Humans considered all celestial bodies to be going around the Earth until a chap named Copernicus (and some of his contemporaries) suggested otherwise. I doubt the size would make a difference or periodicity. $\endgroup$ – StephenG Mar 17 at 21:43
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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. What do you mean by "occurring for 1,000 years"? Do you mean the moon is in the sky, visible and basically the same size, for a 1,000 year period, or that the inhabitants see it once every 1,000 years, like a comet? $\endgroup$ – JBH Mar 17 at 22:20
  • $\begingroup$ @StephenG true, but not all of them were considered moons. $\endgroup$ – shootshi Mar 17 at 22:27
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    $\begingroup$ Thanks, please edit your question with the clarification. Never depend on people reading through the comments to find clarifications. Cheers! $\endgroup$ – JBH Mar 17 at 22:50
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    $\begingroup$ Just to say, it is custom here to wait 24 hours after asking a question before awarding acceptance, otherwise it can discourage other, perhaps better answers - this policy benefits the comunity. $\endgroup$ – We are Monica. Mar 18 at 1:06

Yes. That would be a kind of quasi-satellite.

And Earth already has a small one - it's called Cruithne. At perigee, a larger Cruithne might well be visible, always in the same section of the sky.

enter image description here

A larger body would be less stable, but if you only need one thousand years, I think it could work out.

As rightfully pointed out by Ville Niemi, in this context "less stable" might have very, very dire consequences - you might be looking at a Theia-like endgame. Emphasis on the "end".

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    $\begingroup$ Just wanted to point out this despite it not being in the question. I think the "failure mode" for this solution would be pretty bad. I mean, if I had bought this solution from a world building agency I would want my money back or at least very good insurance for my civilization bundled in the deal. $\endgroup$ – Ville Niemi Mar 18 at 1:38
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    $\begingroup$ @VilleNiemi, is there such a real thing as a "world building agency", or is that just a Douglas Adams style joke? $\endgroup$ – EveryBitHelps Mar 18 at 16:04
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    $\begingroup$ @EveryBitHelps Obviously building planet scale objects to specification requires highly specialized equipment and expertise. Simple economics requires it is handled by an organization specialized in it. $\endgroup$ – Ville Niemi Mar 19 at 7:08
  • $\begingroup$ @VilleNiemi damn, here I though there where some ppl offering dedicated wb services to help with our geeky hobbies! Besides those of us here on wb.se. $\endgroup$ – EveryBitHelps Mar 19 at 16:15

A comet could fill this role. Comets are notoriously bright.

comet https://en.wikipedia.org/wiki/File:Comet_P1_McNaught02_-_23-01-07.jpg

I like the tail but you could have your celestial body be made of something more solid, equally reflective, but not falling apart and leaving the tail.

Comets also have very elliptical orbits and visit infrequently. In this list of periodic comets, 3200 Phaeton has a period of 1.4 years - very short by comet standards.


3200 phaeton orbit

So: your second moon is a very bright short period comet. That seems plausible.

  • $\begingroup$ This could work, but probably not for very long - a short period comet would have less volatiles, and likely not last for one thousand years (and a large comet would be less stable). But yes, especially if it came from outside the ecliptic, it would be plausible and is a much better solution than a 'quasi-satellite'. ( thenakedscientists.com/articles/questions/… ) $\endgroup$ – LSerni Mar 18 at 7:38

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