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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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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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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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