Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How would Earth be affected by an additional smaller moon?

  1. Jan 31, 2016 #1
    I have been looking for an answer for so long. Everywhere I look I find answers to the question how two lunar sized moons would affect the earths tides and so on. But I want to know, if we had our own moon, with its normal orbit, and an additional smaller moon, (or just further away from earth.) Would the impact have to be significant? Are there any circumstances in which there are no serious differences?
    My second question is, what would that smaller moon have to be made of or contain to look red and reflect the sun in a reddish light?
    Hopefully people will understand my English, it's not my first language.
  2. jcsd
  3. Jan 31, 2016 #2


    User Avatar
    Science Advisor
    Gold Member

    Well, the question has an obvious answer. All the satellites orbiting earth are tiny moons that have no significant effect on the earth or moon. It all depends how small you want to allow.

    A surface rich in ferric oxide should appear reddish (unless someone went and painted it black :wink:)
  4. Jan 31, 2016 #3
    Well I guess what I have in mind is a moon approximately half or a quarter of the size of our moon. How small does it have to be to have no or very little impact?
  5. Jan 31, 2016 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To minimize the tidal effect you would want the moon to orbit as far as possible from the Earth. The furthest truly stable orbit is ~ 2 times as far away as the Moon. Tidal forces decrease by the cube of the distance, so if the second body orbiting at that distance was 1/4 to 1/2 as massive as the Moon, it would have 1/32th to 1/16th the tidal effect as the Moon does (16th to 1/8 that of the Sun). It would have an orbital period ~2.828 times longer or ~77.27 days and the time between it being full would be 98 days. It and our present Moon would be in conjunction every 42.26 days. If its density were equivalent to our Moon then it would appear as being ~32% to 40% as wide as the Moon does.
    The tide tables would become a bit more complicated, but whether or not you would call it a significant effect depends on what you consider as being significant.
  6. Jan 31, 2016 #5
    My guess is you want something like a scaled down Mars-like object, about half the size of the present Moon, orbiting at about twice the Moon's distance.
    Multiple body systems are hard to simulate, but probably there would be some tidal locking, so sea tides on Earth might still be predictable, but less so than with a single moon.
    There may be exceptionally low or high tides on Earth sometimes when things line up in a particular way.
    Last edited: Jan 31, 2016
  7. Feb 1, 2016 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Why do you think tidal locking will have an effect on sea tides?
  8. Feb 1, 2016 #7


    User Avatar
    2016 Award

    Staff: Mentor

    I don't think something orbiting at twice the Moon distance would be stable. The Hill sphere is the outer edge where orbits are possible at all - to be stable, objects have to be significantly inside (~1/2 to 1/3 the radius). The moon is at 1/4 the radius of the Hill sphere already, and its orbit is complicated due to the strong influence of sun. At twice the distance, the motion should get quite chaotic. To make it worse, the Moon itself would lead to perturbations as well.

    A moon with half the distance and 1/4 the apparent diameter of Moon would have 1/8 actual diameter, or ~1/500 the mass of Moon, which leads to ~1/64 of tidal influence. You can easily have a small moon nearby that has negligible impact on tides. 1/2 to apparent diameter leads to 1/64 the mass and 1/8 of tidal influence - still fine. It would be included in predictions, but just as smaller correction, even smaller than the influence of the sun.
  9. Feb 1, 2016 #8
    I was almost asleep when I wrote that, but what I had in mind was more to do with orbital resonance than tidal locking, although I guess the two things could be related.
    If there is orbital resonance with two or more bodies in a multi-body system, which seems to be a surprisingly common result, there will be times when one body is experiencing a maximum combined gravity force from the others.
    Sea tides on parts of Earth would be highest (and in other places lowest) during the resonant phases, (Since the Sea is part of the Earth, but unlike most of the Earth it's not fixed in place and can easily be persuaded to move (relative to the rest of the surface), by a fairly weak force of gravity).
    Last edited: Feb 1, 2016
  10. Feb 1, 2016 #9


    User Avatar
    2016 Award

    Staff: Mentor

    This is independent of orbital locking. The moons have different periods, this is sufficient to have them align frequently. See moon/sun for an example, the amplitude of tides varies with a period of roughly a month.
  11. Feb 2, 2016 #10
    How close could a natural satellite get to GTO without destabilizing GTO, for a range of possible masses? Would it be possible to have any appreciably-sized natural satellite between LEO and GTO without destabilizing one or both?
  12. Feb 9, 2016 #11
    What if the second moon had the same mass as the current moon and the two moons orbited each other as well as the Earth?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook