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I'd like to know how to set up a secondary moon for a very much Earth-like planet that I could use for religious and time measurement purposes in a novel but that would not change general situation on the planet.

So the moon would need to be visible with naked eye, but too small/too far to significantly impact tides, night time light conditions etc.

What size it could have (maximum), how far and how long orbital period it should have?

How big, comparable to current Earth Moon, would it appear on the sky? And how would it affect various astronomical events? For example: How often "small" moon would disappear behind the big? Would the solar ellipse of the small moon be noticeable?

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You can combine a small body, like Mars satellites Phobos and Deimos, with high albedo because it happens to be made of metal or a highly reflective material if you want to make them more visible.

Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much further than Mars's other moon, Phobos.

Both Phobos and Deimos are visible with the naked eyes from Mars, therefore a higher albedo would simply make them more visible.

Their small size would ensure that they would have practically no appreciable tidal effect.

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Little known fun fact, but Earth actually has a second moon. It's significantly smaller than our central moon (about 164 feet across) and is essentially an asteroid. It corkscrews in a bizarre orbit around Earth, since the Earth's gravitational pull is playing tug-of-war with the sun's.

That to say...if you're wanting a second moon that doesn't massively affect the planet, you're going to need a pretty small moon, since the larger the mass, the more effect it has on tides, etc. But for it to be visible with the naked eye, it either needs to be insanely bright in the night sky, or be much, much closer to the planet than the "major moon." (I'd actually go with both bright and near).

We can see satellites in the sky already, so if you have a moon the size of a large satellite at about the same distance, you could potentially see it. To make it bright, it would have to be made of something reflective, or be capable of giving off its own energy. Currently I don't know of anything in the cosmos that can create such an energy at that size (quasars are abnormally bright for their size, but even they're massive compare to what you're looking for), and other small, bright objects like comets would be hard to find circling the earth (for a more in-depth reason for this, here's another Stack Exchange answer dealing specifically with comets).

Perhaps your second moon actually is a man-made satellite, but the planet's population doesn't realize—it could either have been space junk from another planet, or it was sent up by a previous civilization...all depends on the history of your world. But a satellite made of metal and reflective panels may be brighter than an asteroid or any other natural body orbiting.

For your secondary questions, their answers would depend on the size and how close your moon orbited to the earth, though I would think your small moon wouldn't go behind the large moon—instead it would pass in front.

For bonus reading: https://www.newscientist.com/lastword/mg24232301-200-has-all-the-hardware-sent-into-space-affected-earths-gravity/

And: https://www.sciencelearn.org.nz/resources/268-gravity-and-satellite-motion

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Quite a few of oportunities here:

  1. A body in an orbital resonance with the main moon - be it 1:2, 2:3, whatever - it may be both inside and outside the main moon's orbit.

  2. A body in L4 or L5 Lagrange point of Earth/Moon system

  3. A body in L4 or L5 Lagrange point of Sun/Earth system

A body with 1/100 mass of the main moon is still pretty much visible. The system may be stable over the Sun's lifespan just like our Earth/Moon system is.

The object will be resolvable by naked eye as more than a point (if in orbit around the Earth). Eclipses will be diverse (Moon1/Moon2, Moon1/Sun, Moon2/Sun, etc...) and visible as well.

The tidal effect will be negligible, compared to the tides created by the Moon1 and the Sun.

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  • $\begingroup$ Pretty sure you can go higher than 1% the moons mass and still have negligible effects on the earth. Also, if the item sits around geosynchronous orbit around the earth, it will have localized effects on the earth and not necessarily have global effects. Also, being at a orbital ration of 1:28, I do not think there is any resonance to worry about. $\endgroup$
    – Sonvar
    Oct 1 '21 at 21:50
  • $\begingroup$ The points L4 and L5 are not stable orbits if you put there a body much heavier than 1% of the main orbiting body. I am not sure about the exact limit of the stability but it is pretty much between 1% and 1.5%. $\endgroup$
    – fraxinus
    Oct 1 '21 at 22:26
  • $\begingroup$ You do not necessarily need to be in one of the Lagrange points. It is possible it could maintain its own orbit. I calculated that if this new moon was 1% the moons mass, with all other parameters equivalent, it would have an apparent diameter of 1/9th of the moon. Place it at geosynchronous orbit ~35000 km, it would be 1/10th the distance to the moon, thus appear almost the same in the night sky. Now place it a little higher up to achieve a stable orbit, minimize its impact on the earth, then you will still have a very visible moon in the sky. Not so much at L4 or L5 $\endgroup$
    – Sonvar
    Oct 4 '21 at 16:46
  • $\begingroup$ Lagrange point L4 or L5 means it is the same distance from the earth as the moon (or as the sun, if in sun-earth lagrange). Given the need for it to be 1/26th the mass of the moon or less for the lagrange position to be stable, the visual size will be very unimpressive. Put it in resonance with the Moon, or very near Geostationary. How about a moon that is just outside geostationary, and loses only 1 degree of position per day. hey presto, a moon that appears to orbit Earth once per year $\endgroup$
    – PcMan
    Oct 31 '21 at 8:17
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Moon made of aerogel with a total mass comparable to the miserably small asteroids that Mars has for "moons", painted in a high-tech reflective coating. The low-density material should make it big enough for the relatively small mass.

Preventing meteorite strikes from damaging, deforming, and ultimately knocking the ultra-light secondary moon out of orbit is left to the reader as an exercise.

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The moon's moon

A submoon orbiting the main moon might provide an interesting enough different view of the sky compared to ours while not affecting the planet supporting the main moon too drastically. The tides will be higher and lower than usual though, depending on the position of the submoon. I don't know how fast such a submoon, one that's visible to the naked eye from the planet's surface. would orbit our moon but I presume it would make a nice natural clock of sorts.

As for the religious side of things, people have always regarded the moon as a female entity as far as my knowledge goes, so perhaps the submoon would be considered the moon's daughter or something. submoons

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