help with astronomy?

[Corporea]

Sigh, so I'm working on a writing project and I've created a world without a moon, with lesser tidal effects.  But, up until recently I hadn't really thought about the climate aspect.  How concerned should I be about how our moon stabilizes our tilt and the whole season thing?  Would it be unrealistic to have a habitable world without a moon?  Would the climate and axial tilt vary over millions of years, or within a year? Or something else entirely.  Any astronomers, etc out there with ideas?  thanks in advance!!

 

[Arc 724]

Earth without a Moon

 

Mercury and Venus do not have moons.. if that tells you anything about stability. Look into them a bit you might get a good idea. I think you're pretty safe with your idea.

Edited January 27, 2015 by Arc 724

[Doug Sundseth]

Planets are huge gyroscopes. Over time the axis of rotation will precess as a result of tidal forces, but you needn't worry too much about precession on the scale of most campaigns, since it takes thousands of years for the axis to describe the full course and there's nothing violent about it.

 

Nutation is much smaller in scale if the planet is mostly spherical and related to the same tensors that cause it to be hard to flip a hammer without it tumbling. Again, you don't really need to worry about that, since it's unlikely to even be noticed except by very careful astronomers.

 

The lack of a dual planet system like the one we have might cause life to develop more slowly or not at all (we don't really know), but given an inhabited planet, it's likely that things would be more stable, not less. Seasons are caused by a combination of axial tilt and orbital eccentricity (virtually all by the former in the case of the Earth). Neither of those is notably affected by the presence or absence of a companion planet.

[ttuckerman]

But on the other hand, the tides that our moon produce have led to land creatures.  Some of the sea creatures were used to times they were left stranded out of the water and ones that could be amphibian had  a leg up.  This led to ones that were only land creatures.

[seej]

Top 5 things we'd miss without a moon

 

Short version is the rotation would speed up resulting in shorter days, and either no seasons (like Mercury) or dramatically varying seasons (like Uranus).

[Doug Sundseth]

I'll note that the referenced article assumes that the planet in question started with the same spin ours did, is of the same age ours is, is the same distance from the primary ours is, and has the same axial tilt ours does. The first might be inherent in the total system mass and distance from the center of mass at which the planet coalesced. But even the sun alone will slow a planet and a higher solar gravitational force (caused by a smaller star, incidentally, because of life zone differences based on mass and stellar temperature) and an older planet (also easier with a smaller, closer star) will have much the same effect as a huge moon like the one we have. (Note that our moon is far larger relative to its primary than that of any other moon in the system, so we think it's anomalous. But the data set is tiny.)

 

Axial tilt is effectively random (again on the basis of that small data set). And a lower axial tilt will result in less change from changes in axial tilt than the relatively high tilt we have. Further, nutational effects again occur mostly over the course of many centuries, so unless it's important to your campaign, not noticeable on any reasonable time scale. (They're mostly an evolutionary pressure source, frankly.)

 

Nutation is largely a result of unevenly distributed mass. Larger planets (Mars, for instance, is a very small planet) will tend to squash themselves into more spherical shapes; in fact that's one of the criteria used to determine whether a body is called a "planet" or something else.

 

Part of what keeps the core of our planet liquid is the friction energy from tides in the rock, but the last estimate I saw had much more of the energy coming from radioactive decay. It's been a while though, and the state of that art was changing fairly quickly when last I looked.

 

Short answer: It would not be unreasonable.

 

Longer answer: You might want to make the primary a dimmer star than ours (maybe G7V or so instead of G2V) and move the planet closer to help explain the slower rotation, precession, and nutation. You might want to have lower median axial tilt so that the worst changes aren't all that bad, which would mean generally less violent seasonal changes and less life at higher latitudes.

 

Or you might want to say, "It's reasonable" and then ignore it within the story. After all, when's the last time you talked about axial tilt and nutation with anyone before today? 

[Corporea]

Awesome!  Thanks everyone!!!  I think I can make it work. I just worry since I'm more of a fantasy person and am afraid of alienating the scifi folks by creating something too unbelievable.  But, this gives me great ideas and I really appreciate it!!!

[Pingo]

I've been reading up on the Deep History of the Earth. It's marvelous stuff.

 

One thing the moon has done is slow down the Earth's rotation by friction. The Earth's year has always been about the same length, but the number of days in it has gotten fewer and fewer. (One way of checking this is by counting daily growth rings in ancient corals.)

 

When the Earth first acquired the moon, about 4.5 billion years ago, a day was about six hours long, so there were 1460 days in a year! Thanks to the moon's friction effect the Earth slowed and slowed, until by 620 million years ago a year was 400 days long. 350 million years ago, when the coral rings grew, a year was 385 days long.

[Loim]

I wanted to do dual moons for my homebrew, but I'm not edumacated enough, and I couldn't find anyone to help. So I just said "magic" and called it good.

[Magnus Mercury]

I wanted to do dual moons for my homebrew, but I'm not edumacated enough, and I couldn't find anyone to help. So I just said "magic" and called it good.

 

I'm not even sure you'd have to call "magic" on that, unless there's a seriously crazy orbit or something.  If it's just two moons orbiting around the planet, I'd just state it as such and that's that.  Unless the players are doing something on a cosmic scale (like trying to boot a moon out of orbit or something), I'm not sure any kind of technical details matter...

[Doug Sundseth]

Two moons that are as large and close relative to the primary as ours would be tricky. The interaction of all of those gravitational fields could be problematic.

 

But more typical sized moons, or one moon close and one far, are pretty straightforward.

[Sanael]

I know of various binary planet systems in fiction (Star Wars' Correllia, Trek's Binar, etc), but am not studied enough to know of any in non-fictive space...I also don't know of any (in reality or otherwise) trinary planet systems...that's some gravitational shenanigans, I would expect.

 

I suppose Tatooine is vaguely trinary, in that it has two moons, but I'm thinking of roughly-equal-in-mass (and presumably in size) bodies.

[Corporea]

I'd read somewhere while doing research that a binary star would be unstable for planetary life and reasonable orbit, so I threw that out, even though I thought it would be really pretty.  Sigh. I also really want rings on a planet not in a single plane, but I get that this would not make sense either.  But it would be so cool!   I might do it anyway and hope no one notices... maybe with lots of moons as gravitational influence so it looks like a little atom?

[Pingo]

Um.  Atoms don't actually look like that.

 

They look way more interesting, mind you (this is some of the orbits of electrons on a single atom -- not at all in little rings).

[Corporea]

Sigh.  Alas.  But I like the Rutherford model...    It's so pretty!  I am such a girl...