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Marth
2009-Dec-25, 06:16 AM
Here's the gist:

I'm writing a book, I've created a world on the inside, for certain maxims and axioms to be plausible I need to create a world on the outside. And to do that I need some brainpower. I'm usually the type that does my own research, but I'm out of my league on this.

I wanted something different, so what I did, in my limited understanding, was made almost no wobble to the axis, so there would be an almost uniform climate around the globe. I think that's what might happen. But what I'd really like is there to be different climates only in different regions of the globe. Similar to what we have, minus the seasons.

I then added 3 moons, one is invisible, or dark. I don't know what the mechanics would be to do this.

Atmosphere is similar, I guess, I don't want to get too ridiculous. But I did want either a 20 hour day or a 30 hour day. I wondered if the speed would affect the gravity, ie a 20 hour day would make things, to us, seem lighter and the longer day would objects would be heavier. And would subsequently change how the organisms adapt.

Another confusing thing is the length of the year, I want it unique, but the more unique I ponder, the more confused I become. I was also wondering if I could have my sun rotate around my planet, for giggles...there I go again.

Let's begin there;
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To summarize:

1) In what way would creating a minimal wobble on my planet's axis affect the seasons?

2) How could I have 3 moons around my planet at different distances and one remain eternally hidden?

3) In what way would increasing or decreasing the hours in which my planet would make one rotation on its axis affect the gravity or what might it affect?

4) What changes would occur by changing the time in which my planet would rotate around the sun? Or is it plausible to have my planet fixed and the other astral bodies rotate around it?

I know this is asking much, and I appreciate all the help everyone willing can give. Thanks!

Drunk Vegan
2009-Dec-25, 08:58 AM
1) AFAIK the less wobble a planet has, the smaller the variations in its seasons.

2) Maybe a bit farfetched, but could one moon be behind the other, orbiting much further out but at the same orbital velocity, and is smaller than the moon that is visible to the planet?

3) Faster spin equals a higher perceived gravitational pull (centripetal force). So a planet with longer days has less perceived gravity than the same mass planet with shorter days.

4) Essentially the length of year is a product of the planet's distance from its sun. So a planet with a longer year is colder (less sunlight) and a planet with a shorter year is hotter (more sunlight).

Temperatures aren't always colder the further away from the star though, Venus has a mean temperature almost double Mercury's due to the greenhouse effect in its atmosphere.

As for whether it's plausible to have other astral bodies orbit the planet - not really, barring some very unlikely plot device, like for instance a dark matter halo or sphere surrounding the planet. The planet would orbit the center of gravity of the dark matter, and the star and other planets would orbit around the dark matter from further out, appearing, at least, to orbit the planet.

eburacum45
2009-Dec-25, 11:56 AM
Climate
The absense of a tilt doesn't mean that the planet has the same climate all over its surface. A tiltless planet will be permanently cold at the poles, despite the fact that the local star is permanently just above the horizon there. At the equator the local star is directly overhead at mid-day, and the climate is hot. There will be temperate zones in mid-latitudes and so on, but no summer or winter (unless the planet has an eccentric orbit).

Moons
You could have three moons around a single planet; some planets such as Jupiter have dozens. In order for a moon to be permanently hidden, it could be in a geosynchronous orbit, with a month the same length as the planet's day. It could then be hidden on the far side of the planet (in other words, it would only be visible from half the planet). This would not be a permanently stable situation, I think, but it might persist for many millions of years.
Drunk Vegan's idea of hiding one moon behind another is a good one. A very small moon might occupy the small region of stability behind a larger moon (the so-called L2 point). But this would be even less stable than a geosynchronous moon, I think, especially if there is a third moon in the system to perturb it.

Orbit
A planet orbits its star, not the other way around; but moons orbit the planet. If you want your planet to have an Earth-like temperature you need to ensure it is a suitable distance from its star; this distance also affects the length of the planet's year. If you have a small, dim local star your planet must be closer to the star to get warm, and the year will be shorter too; if you have a hotter, brighter star the planet must be further away, and the year will be longer.

Marth
2009-Dec-25, 05:23 PM
Wow, big thanks guys, this is just the info I needed. :)

On the climate and moons, I've got enough of an idea to sell it. The orbit I was being silly, I do like the idea of a larger star with a longer year.

So I'll use the 30 hour day model, therefore having less perceived gravity and therefore springier humans? LOL Sounds fun!

One more thing I need help with. I want a mild collision with another astral body at an undetermined interval. For now lets say 1000 years. What I would like is for this astral body to swing by my planet and wreak a little havoc. Increase natural disasters or whatever it does. Namely by the proximity of the gravitational orbits.

Perhaps you could better imagine the destructive scenario and lend it to me? I would like to keep the homo sapiens alive, for the most part though. What would the basic mechanics of something like this be? And how would it affect my planet? Is it even plausible? Would it even cause destruction?

Thanks again!

eburacum45
2009-Dec-25, 07:59 PM
So I'll use the 30 hour day model, therefore having less perceived gravity and therefore springier humans? LOL Sounds fun!


Actually it is the other way round. The faster the planet spins, the lower the gravity at the equator. But unless your planet spins very fast indeed (say a couple of hours or less) the gravity wouldn't be very different between the equator and the poles. Unless you want a very short day I'd not bother with the effect of spin on gravity.

Marth
2009-Dec-25, 09:18 PM
Actually it is the other way round. The faster the planet spins, the lower the gravity at the equator. But unless your planet spins very fast indeed (say a couple of hours or less) the gravity wouldn't be very different between the equator and the poles. Unless you want a very short day I'd not bother with the effect of spin on gravity.



So would a 20 hour basically be the same as a 24 hour day in terms of gravitational effect? If so, I might as well keep the 24 hour day so as not to confuse the reader.

Any opinion on an astral body coming close to my planet and causing damage? From what I understand, two planets coming in close proximity could exchange gravitational orbits, affect the polar alignment, and even cause land waves similar to waves from a sea, but buckling and shifting massive strata. I wonder how much is true?

Drunk Vegan
2009-Dec-25, 09:49 PM
Actually it is the other way round. The faster the planet spins, the lower the gravity at the equator. But unless your planet spins very fast indeed (say a couple of hours or less) the gravity wouldn't be very different between the equator and the poles. Unless you want a very short day I'd not bother with the effect of spin on gravity.

Ah, yeah, I'd forgotten about that. Whenever I hear about spinning spaceships to create artificial gravity, there's usually some mention of the center of the craft having little or no gravity, getting heavier the further away you get from the center axis.

But yeah, on something the size of a planet, I'd imagine the spin rate would not have a large impact on gravity.


Perhaps you could better imagine the destructive scenario and lend it to me? I would like to keep the homo sapiens alive, for the most part though. What would the basic mechanics of something like this be? And how would it affect my planet? Is it even plausible? Would it even cause destruction?

Your planet could have a highly eccentric orbit that would intersect an asteroid field in the system every 1,000 years (this would make some seasons pretty mandatory though over the course of a year.)

Robert Tulip
2009-Dec-26, 01:01 AM
1) In what way would creating a minimal wobble on my planet's axis affect the seasons?
The wobble of earth's axis is not the cause of the seasons but of the long term precession of the equinox. Earth's axis has stable obliquity of about 23 degrees, and its annual orbit around the sun cannot properly be described as a wobble.

Marth
2009-Dec-26, 04:22 AM
Your planet could have a highly eccentric orbit that would intersect an asteroid field in the system every 1,000 years (this would make some seasons pretty mandatory though over the course of a year.)


I'm thinking it would be nice to have a large planet engulf the horizon, like a daunting orb of death. LOL I'd need something visible like this for my background to assist some plot points.

I was wondering what would happen if a large planet moved close to mine, I wonder if it would even do damage. It doesn't, however, need to cause damage or even turn people into zombies. But it would be nice :lol:

Romanus
2009-Dec-26, 04:24 AM
Can only answer two:

1) In what way would creating a minimal wobble on my planet's axis affect the seasons?
--It would probably be considerably colder than Earth, despite the lack of seasons. For instance, on an Earth with no axial tilt, the Sun would never have an altitude at my city higher than 60 degrees, the same maximum as at ~55 N on the "real" Earth.


3) In what way would increasing or decreasing the hours in which my planet would make one rotation on its axis affect the gravity or what might it affect?
--It wouldn't affect gravity, but a longer day would mean significantly colder nights and hotter days.

Marth
2009-Dec-26, 05:45 AM
The wobble of earth's axis is not the cause of the seasons but of the long term precession of the equinox. Earth's axis has stable obliquity of about 23 degrees, and its annual orbit around the sun cannot properly be described as a wobble.
Reply With Quote

I'm a tad dense, could you please explain how the precession of the equinox affects seasons? I'm not joking, I really don't know.



1) In what way would creating a minimal wobble on my planet's axis affect the seasons?
--It would probably be considerably colder than Earth, despite the lack of seasons. For instance, on an Earth with no axial tilt, the Sun would never have an altitude at my city higher than 60 degrees, the same maximum as at ~55 N on the "real" Earth.


Good point...But I could just make my sun really large perhaps, and my planet farther away. But would that radically affect the nighttime temperatures?



3) In what way would increasing or decreasing the hours in which my planet would make one rotation on its axis affect the gravity or what might it affect?
--It wouldn't affect gravity, but a longer day would mean significantly colder nights and hotter days.

I guess I viewed gravity more as a centrifugal force, but would it be that significant? Surely a warmer day would carry over into nighttime. Wouldn't the overall planetary temperature be higher in some parts of the world, ie the jungle? And especially hot and cold in the desert? Maybe changing the length of day is a bad idea...

Romanus
2009-Dec-26, 05:04 PM
Good point...But I could just make my sun really large perhaps, and my planet farther away. But would that radically affect the nighttime temperatures?

You could change the insolation, but the resultant planet would still be cooler than one with an axial tilt in the same orbit.

Surely a warmer day would carry over into nighttime. Wouldn't the overall planetary temperature be higher in some parts of the world, ie the jungle? And especially hot and cold in the desert? Maybe changing the length of day is a bad idea...

Not really. How warm or cold a certain place on Earth gets depends on several factors, such as humidity, ground cover, the presence or absence of large bodies of water, solar incidence, and much else.

However, when it's all said and done, the duration of sunlight is very important to any daily temperature swing, especially in the temperate zone. The highest temperature of the day occurs well after noon, after which the air and ground lose heat faster than the decreasing solar angle can induce it. Lengthen the day, and the air/ground would have correspondingly longer to heat up. Likewise, the coolest temperature of the day is typically reached right around sunrise; lengthening the night would allow the temperature to fall further.

The bottom line: a longer day would mean both hotter days and colder nights; they would not balance each other out. Things get more interesting with tidally-locked worlds, but that's another kettle of fish.

Marth
2009-Dec-26, 09:38 PM
Thanks for the help guys, I've decided to keep it similar to the Earth in most things. I originally wanted one moon to take up a large portion of the sky, but I thought the tide would be too affected if it was too close.

I think I'll make another planet come close every 1000 years or so and bring tidal waves and land waves instead. I think it will sell. I just wonder about such an orbit wherein that would work....

baric
2009-Dec-29, 08:15 PM
Another thought for an "invisible" moon is simply one that has a very low albedo (dark & non-reflective).

It's there, but very difficult to see at night. If it were distant and/or small enough, it would also be difficult to see during the day.

chornedsnorkack
2009-Dec-30, 04:48 PM
Thanks for the help guys, I've decided to keep it similar to the Earth in most things. I originally wanted one moon to take up a large portion of the sky, but I thought the tide would be too affected if it was too close.

Not necessarily.

If a moon is in a double tidal lock with the planet AND on exactly circular orbit with very little eccentricity, then it will cause no tides at all, and no libration. This does not remove the tides caused by Sun, or other moons.

And there is an excellent, and pretty compact, dwarf planetary system with a massive secondary in our own Solar System.

Pluto, Charon, Nix and Hydra. Exactly 3 moons.

All 3 orbit very exactly circular, low eccentricity orbits at very low inclination to each other.

Pluto and Charon are doubly tidelocked to each other, with period of 6 days. And Charon is as massive as 1/9 mass of Pluto, compare Moon which is 1/81 Earth.

Somehow Charon manages not to perturb the orbits of Nix and Hydra, which are so close as 25 and 38 day orbits. Nix and Hydra also manage not to perturb each other.

An observer on the far side of Pluto could never see Charon, but Nix and Hydra would be rising and setting, waxing and waning, and can be seen together (as well as eclipsing Hydra).

The 1/9 ratio of Earth and Moon could compare to the mass ratio of Earth and Mars. So imagine Earth with a Mars sized satellite fixed in sky AND other satellites orbiting further beyond...



I think I'll make another planet come close every 1000 years or so and bring tidal waves and land waves instead. I think it will sell. I just wonder about such an orbit wherein that would work....

There is a known extrasolar planet which is a hot Jupiter on eclipsing extremely eccentric cometlike orbit.

1000 years... Suppose that you had a solar system with Jupiter-like gas giant on an orbit with 1000 years, mean semiaxis 100 AU, perihelion 1 AU or slightly more or less and 199 AU aphelion. What perturbations should be expected on each approach?

Techist
2010-Jan-08, 07:33 AM
Another thought for an "invisible" moon is simply one that has a very low albedo (dark & non-reflective).

It's there, but very difficult to see at night. If it were distant and/or small enough, it would also be difficult to see during the day.

Low albedo is the only way; but if your civilisation is at all advanced, it would still be detectable by IR. If it were that dark, it would absorb a lot of sunlight.

Jens
2010-Jan-08, 08:01 AM
The wobble of earth's axis is not the cause of the seasons but of the long term precession of the equinox. Earth's axis has stable obliquity of about 23 degrees, and its annual orbit around the sun cannot properly be described as a wobble.


I'm a tad dense, could you please explain how the precession of the equinox affects seasons? I'm not joking, I really don't know.


I think you misunderstood. The statement meant, "the wobble is not the cause of the seasons; the wobble is only the cause of the long-term precession of the equinox." So the precession of the equinox does not affect the seasons. The seasons are caused by the tilt.

neilzero
2010-Jan-09, 05:51 PM
Eburacum as usual has the details correct. Length of day has only a tiny effect on the gravity near the Equator, unless we are thinking a one or two hour day.
Seasons are mostly the result of the the 23 degrees tilt of Earth's axis. A more eliptical orbit than Earth would produce different seasons near the equator as well as other latitudes. A planet could have 8 seasons per year instead of 4, but they would synchronize at perhaps 1000 year intervals producing alternately, severe seasons and mild seasons. Wobble and tilt change, change climate over thousands of years for Earth, but I supose the cycle could be centuries instead of mileniums. I don't think there is any likely mechanism to produce uniform temperatures planet wide, other than a very thick atmosphere, with the help of small tilt and near circular orbit.
The climate at each local will change very little if the the orbit is close to circular and the tilt tiny to none. A tide locked planet in a circular orbit would also have constant climate at each local, but be much hotter where the sun is always over head as compared to other locations. Extreme temperature differences are avoided by a very thick atmosphere.
As the others suggested, a small moon, can hide behind a larger moon in the L2 position, but it would not stay there more than a few thousand years, if there was a massive 3d moon or other planets or suns that came even as close as in our solar system. A geo synchronous moon would also not stay there long without station keeping. The only practical way to have an almost invisible moon, long term, would be for the surface to be dull black.
For animals, the oxygen partial pressure needs to be about 4 PSI, so 8 psi with 50% oxygen will work and so will 30 psi at 10% oxygen. 300 psi at 1% oxygen, works briefly for deep sea scuba divers, but some genetic modifications would likely be necessary, for long term.
Without seasons, the inhabitants will hardly notice the length of the year, but half as long a year is about right for a class K sun = dimmer than our G5 sun. A double length year is about right for a class F star = brighter than our sun.
An imitation sun could rotate around a planet like our moon, but perhaps in 1/15th day instead of 28 days = LEO = low earth orbit. It would be a flat disk, perhaps 3 miles in diameter with billions of lasers pointed at the planet. It could appear about the same color, as Sol, but would need lots of infrared lasers to warm the planet sufficiently. How about a less noticeable sun to provide heat and the artificial sun to provide mostly light? Neil

EDG
2010-Jan-10, 12:41 AM
Low albedo is the only way; but if your civilisation is at all advanced, it would still be detectable by IR. If it were that dark, it would absorb a lot of sunlight.

I should point out that the albedo of our own moon is 0.11, which means it reflects only 11% of the sunlight that falls on it (thus, it absorbs 89% of the sunlight) - and yet a full moon still looks pretty darn bright in our sky ;).

I would suspect that a body with even lower albedo would still be quite visible in the sky - the darkest asteroids and cometary nuclei have albedos around 0.05. Not sure what kind of natural materials would have albedoes lower than that.