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Developing writeer needs two moons

  1. Jul 18, 2012 #1
    I have taken a look over these forums and various other sites to get answers to this question. However, I want to re-frame it to get information which may be more useful. So, here it goes.........

    If you have an Earth size planet, where would the best placement for two moons be?

    In other words I am looking for the ideal orbit and size of these bodies. I understand that having an addition moon would have such effects as differing tides, seasonal effects and lighting. In addition to added cosmic events such as double eclipses or "grand" alignments. From that point I will be looking at developing a standard calendar, similar to our own, which shows the moon phases. I am sure to have questions on that as well, in addition to determining the timing of such cosmic events.

    Any advice or input is appreciated. If you have suggestions as to where I might find some additional information on developing a realistic solar system I would be grateful.
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  3. Jul 18, 2012 #2


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    I'd guess that a good place would be if both moons orbits were in orbital resonance with each other. http://en.wikipedia.org/wiki/Orbital_resonance

    The exact orbital characteristics would depend on each moon's mass. (and more)
  4. Jul 18, 2012 #3
    The Laplace orbit given for Juipter seems to be a good fit. So if give the example of using an Earth like planet and using the laplace orbit.

    what would be the given characteristics of the two orbiting bodies? Such as mass, orbital distance and path of orbit?
  5. Jul 19, 2012 #4


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    You have some freedom in those choices. Our own moon is quite large, compared to the planet. For one earth-like moon, I would probably add a smaller moon closer to the planet. Same orbital plane, 2:1 resonance? The distance can be calculated from the rotation period with Kepler's laws.

    As a minimal requirement, it has to be in the hill sphere.
  6. Jul 21, 2012 #5


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    Depends on the mass ratio to the mother planet and orbital distance. Jupiter, for example, supports numerous fairly massive moons. An earth mass planet would be a poor choice for multiple moon mass size satellites.
  7. Jul 21, 2012 #6
    I fully agree with MFB, that you have here quite big amount of freedom.

    However, he gave only upper limit for orbit, I think that there two lower limits:
    -one full orbit of a moon is shorter than a day - the moon would suffer from orbit decay and would be doomed to crash in to planet
    -Roche limit (limit under which tidal forces would tear moon in to pieces and turn it in to a ring) For Earth and the Moon it's about 9500 km, so it does not limit you.

    -Retrograde orbit - the moon would suffer from orbit decay

    Seasonal effects - you can achieve them in two ways:
    -usual axis tilt (however, you can select to have it straight (perpendicular to the orbit plane) practically no season, a bit higher difference in temperature between pools and equator and very often eclipses if moon size & distance allows for them)
    -elongated elliptical orbit (orbit with high eccentricity) It is so on Mars, keep in mind that length of astronomical seasons would be seriously different.

    If you want to play even more you can have these two system somewhat out of sync.

    And what would you like to have?

    (If you are curious - I thought about quite many crazy but acceptable from physical perspective planets for purpose of pen and paper RPGs)

    From funny ideas that I could think of - binary star system in which the other star is as seen from the planet roughly as bright as our full moon.
  8. Jul 21, 2012 #7


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    Something else you might consider is a "horseshoe orbit" configuration, something like Epimetheus and Janus share.
  9. Jul 26, 2012 #8
    Re: Developing writer needs two moons

    Just noticed the double "ee" in my subject line, gods what a long day that was.

    Ok, so you guys have given me some great information and I thank you all. However please understand that I am NOT a physics guy. If I understand "resonance" the ratio represents the number of orbits around the main body in the system compared to each other?

    So I will re-frame the question again taking the information I have into consideration.

    Using the planet earth as the basis of my system design. Using the Laplace orbit of Jupiter and a 2:1 resonance Where within the hill sphere would the placement for a second moon be?

    Now I know that you guys may want some more intricate details for the math equation. However, for my purposes exactness is not needed. Just a good amount of believability. So, as was pointed out, the exact mass and makeup of the second moon is not necessary for this project. However, knowing that it is either larger or smaller than our existing moon is and what the distance from the earth would likely be.

    As far as what effects the two moons would have on the earth will come later after I settel on this information.
  10. Jul 27, 2012 #9


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    2:1 resonance would mean that one moon performs 2 revolutions while the other moon does just one. As T^2 (squared orbital period) is proportional to a^3 (cubed orbital radius for circular orbits), this leads to a radius ratio of 4^(1/3)=~1.59

    One example:
    A moon with an orbital radius of 400,000km and the size of our moon, a second one with an orbital radius of 252,000km, smaller by some good fraction (maybe a diameter smaller by a factor of 2) to reduce issues with tides. With the mass of earth as the central body, the outer moon is just like ours, and the inner moon has a period of ~15 days (in terms of moon phases). I would assume a bound rotation, so it has a visible side and one invisible side (as seen from the planet), like the other moon.
    From time to time, the smaller moon could pass in front of the larger moon. Lunar eclipses would be more frequent with it. Partial solar eclipses would be more frequent, too. Details here depend on the precise orbitals of the moons.
  11. Aug 16, 2012 #10
    Thanks for the reply. I have been doing a little research on a few other topics as of late. Also a bit of cartography. Would anyone know of a program that I might be able to use to give some rough modeling of the solar system?

    For example;
    [/PLAIN] [Broken]
    Last edited by a moderator: May 6, 2017
  12. Aug 16, 2012 #11
    Have a look at the one mfb just introduced me to in this post and which I then used a bunch of times downthread. Is that the sort of thing you're looking for?
  13. Aug 16, 2012 #12
    That is the basics of what I am looking for. For some reason my link did not appear in my post. I was looking at this one;

  14. Aug 19, 2012 #13
    Also the smaller the moons the more realistic. The earth could easily support asteroid sized moons like Deimos and Phobos. If the moon is larger then it would start to effect the orbits of the earth and the moon, and that would make things complicated.

    For realistic orbital dynamics complicated is bad since it means that it's like to be an unstable configuration.

    One other thing for "realism". The moon seems to have originated through some very odd processes that would seem to preclude two large moons from forming. However for small moons you can imagine that the alternative earth just captured an asteroid which is what I think people figured happened to Mars.

    As far as "realistic" solar systems, that's a work in progress and for fiction purposes you can let your imagination run pretty wild. It's known that the star systems that we've observed around other stars look nothing like our solar system, so it's not clear right now what a "typical" solar system looks like.

    If you want the second moon to be noticable you can make it out of ice. That way it's going to be really bright even if it is tiny.
    Last edited: Aug 19, 2012
  15. Aug 19, 2012 #14
    Also one thing that you should be aware of is anthropic effects on solar system formation. One major surprise is how different exoplanet systems are from our solar system, and one popular idea is that our solar system has nice boring stable orbits because any solar system without nice boring stable orbits would not develop life.

    Depending on what you are trying to do this may affect your story. If you have a earth like planet in a lifeless solar system or one in which life developed through "non-traditional" means (i.e. something that was seeded), you have a bit more freedom to have wild orbits. Also this idea is new and somewhat controversial enough so that you can reject it in your story (i.e. you can have two people mention this idea and they laugh at how it turned out to be wrong.)
  16. Aug 23, 2012 #15
    The earth-type planet will be orbiting a star, much more massive than it is. This sun, and the two moons of the planet, will be the most noticeable objects in the sky as seen from this planet.
    If you are making up a calendar for your world, you will want the dynamics of the two planets to be a simple as possible. The simplest hypothesis would be that the moons are fixed in the sky relative to each other, and to the sun that the planet orbits. I will assume that you aren’t interested in having one moon eclipse the other or the sun.
    There will be Lagrangian points determined by the positions of both the earth-type planet and its suns. If you place the two moons on sun-planet Lagrangian points, the sun and moons will look stationary relative to each other. You can build a mythology for the inhabitants of this planet which is based on the three celestial objects being stationary with respect to each other. Of course, this means that there are no eclipses.

    “The Lagrangian points ( /ləˈɡrɑːndʒiən/; also Lagrange points, L-points, or libration points) are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be part of a constant-shape pattern with two larger objects (such as a satellite with respect to the Earth and Moon). The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them.”
  17. Aug 23, 2012 #16


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    Trojans in the stable Lagrangian points (L4 and L5) would be far away from the planet - they would be hard to spot at all (similar distance as venus and mars, but much smaller), and appear similar to stars.
    Apart from that, it is not possible to have fixed relative positions for moons and sun.

    The 2:1 orbital resonance would give a situation comparable with earth: day and year are independent of the moon, and you can define something like a month based on the moons, with 1/2 month as natural partition.
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