I am writing a story that is set on a counter-earth, and I am debating the number of Moons this planet will have. I want to make the planet exotic, so I am thinking of having no moons or two moons, but am wondering what effect this will have on the axis, climate, tides, etc. Does any one know what the effects may be? Thank you.
Most moons are smaller relative to the primary than Earth's moon is. The tidal effects from each moon can generally be considered separeately and added together, but the tide tables will be more complicated with more moons. Tidal effects will be about the only significant factor unless you have an extrremely ubusual planetary system. (Jupiter's moons are VERY atypical - there are electrically charged plasma streams between the closest moons and the gas giant, but that wouldn't happen with a typical habitable ball-of-rock planet.) Earth's moon is VERY atypical in one other aspect as well. It is an amazing coincidence that it's size as seen from the planet is almost exactly the same as the size of the Sun from the same place. Solar eclipses on Earth are therefore more spectacular that can be expected from any othe rplanet in our solar system, blocking the vright disc of the sun while displaying the corona in its full glory.
I'm not a scientist, but if I remember correctly if Earth was on a straight axis there wouldn't be any seasons. The reason Earth's axis is tilted is because when the Earth was being formed it was hit by another planet which caused a large piece of both planets to later form the moon. So it might be good to have at least one moon for this reason, though having a planet without seasons is itself pretty interesting. As for the tides, I have no idea. Edit: just read Cogito's post and I think the Sun is 400 times bigger than the Moon but the Moon is 400 times closer to us than the Sun. Also it is slowly drifting.
The misalignment of the Earth's axis relative to its orbital plane has nothing to do with the presence of the moon. The angular momentum of the condensing mass of the planet was simply not perfectly alined with the angular momentum of the overall cloud that formed the entoire solar system
Sorry, but that is incorrect. In fact, the northern hemisphere is in winter when the Earth is at perihelion (closest proximity to the Sun). The shallower the angle of sunlihjt with the surface it is shining upon, the greater the area warmed by the same anount of solar energy, therefore cooler weather prevails.
My mother is a nurse and she has often complained about having to go to the hospital on nights when there's a full moon because it has a tendency to bring out fairly crazy behaviour in some members of the community. I can't even begin to guess at what kind of lunacy might erupt on a planet experiencing two full moons on the same night.
With more than one moon, I suspect the behavioral cycles would be less pronounced. Interestingly, the term 'lunacy' itself is based on behavioral anomalies associated with the lunar cycle. Biorhythms do tend to synchronize with external periodic phenomena. How much is psychological and how much is biochemical or biophysical is not entirely known. Would we have the concept of a week if that weren't the approximate duration of one phase of the lunar cycle? Would seven carry the significance it does in our mythologies if there were no week? Would twelve have special significance if there weren't approximately twelve lunar cycles per year (one orbit around the Sun)? Without a single, prominent moon, how much different would our lives and our ways of thinking differ? These are questions that have long fascinated me.
Thank you Cogito, that was very helpful. If I understand what you are saying, the only effect of having multiple moons would be the tides, which can be added separately. That was what I was most concerned about. As for the effects on behavior, I had not thought of that, and may try to incorporate that into my story. Do you know of any website that might have a program for calculating the tides. Thank you. SA Mitchell, a seasonless planet would be interesting, I might try that in another story, thank you.
Ocean tides are complicated by the shapoes of the bodies of water, which introduce oter forces than just the current tidal balance. The overall effect you need to know is that both the nearest and the farthest point from the attracting body will try to pull away from the center of your planet, and nearly equally. I don't know of any general calculators, and it has been years decades since I've had to derive the equations.
Wow, this was kind of weird. I was thinking about this today too and had no idea there was a post on this site about it. My science class is learning alittle about astronomy (I'm in 8th grade) right now so it got my mind wondering. Besides what affects the moon(s)(or having no moon at all) would have on the planet itself, it would also have an affect on the people that populate it. For centuries humans have looked at the moon, sun, and stars and have thought of them as gods. The moon also has to do with astrology along with the stars and sun. Our calendar is based on many events noted in the sky. Aproximately, it takes a month ("moonth") for the moon to complete 1 lunation. Unrelated to this, all moons do not look like Earth's moon. Europa is a moon of Jupiter's. It is completely covered in ice and supposedly has oceans beneath it. Life is expected to be found there. Io is also a moon of Jupiter's. It has frequent volcanic plumes and lava flow. ...Sorry if this didn't help at all. I'm not sure if all my information is correct (forgive me if it isn't)
If you still need help: - Some people belive organisms from the oceans would have never gone onto land and evolved into land organisms if it weren't for ocean tides. Tides -caused by the pull of gravity from the moon (and the sun). -High tide(or low tide) occurs every 12.5 hours (I think it's exactly 12.26 hours) therefore there are two high tides and two low tides a day. -The moon's gravity pulls the oceans on one side of Earth and the land on the other side. So, there are two high tides occuring on Earth at one time. -The space in between high tides are low tides. - Spring tides: Occur when there is a new moon or full moon (when the sun, Earth and moon form a straight line). The combination of the moon and sun's gravitational pull on Earth causes the high tides to be especially high and the low tide to be especially low. - Neap tides: Occur during first or third quarter moons (when the sun, Earth, and moon form a right angle). The sun and moon's gravity works on oposite sides so that there is the least amount of difference between high and low tides.
But note from Joe's comments that the absence of a moon does not mean an absence of tides. The solar tide due to the Earth revolving around the sun is less than the lunar tide, but it does exist, and the Earth's rotation causes it to rise and fall with respect to a given point on the Earth. The tidal forces affect every surface unit on the planet, but only liquids are free to move in response to it. Only larger bodies of water can generate enough flow between lower and higher tidal forces to produce a measurable rise and fall, so the sizes of your planetary bodies of water will also affect the difference between high and low tide. Just a couple more things to think about when you design your planet's behavior.
If you plan on having more than one moon, you need to consider their orbital paths and the speed at which the planet's rotation pulls the moons along. Smaller moons tend to orbit planets at a faster speed than larger moons therefore making it possible for the two moons to be in different phases on any given night. For orbital paths, the moons would have to be in positions where a collision accident wouldn't happen (or for fun, you could make this so and write about what happens to the planet when two moons collide). Either one could be farther away than the other, or the could be at the same position but one be lower or higher on the vertical axis relative to the other moon. The number of moons can also have an affect on the planet's inhabitant's mythology and beliefs. You could also make it that the moons are visable from the planet during daylight but they are just a little dull in color and then when night comes the moons glow.
Correction: it isn't the size of the moon that deterines orbital period, it is its average distance from the primary (what it is orbiting around), and the mass of the primary. Moons have to be in orbits that not only don't intersect, but also are far enough at all times to keep them from gravitationally disturb each other's orbits. The more massive the moon, the farther away other moons' orbits it must be in order not to destabilize their orbits.
