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ddude987
2015-01-20, 11:30 AM
Hello Giant,

I am currently world-building a campaign setting I've been working on for 2 years, and plan to finally run this fall. It is planned to be a low-magic campaign but will have plenty of locations and items of previous ages.

My question to you all is what are immediate thoughts, problems, occurrences, or downright interesting things to do from a fantasy world having 2 suns. Let's work under the assumption the world is fairly normal (Earth-like) and is survivable.

What I have thought of so far: (also may add peoples contributions here)

Have the suns be of perfect mass/size/density and have them orbit perfectly around each other

Semi-rare solar flares. Depending on which star or both flares, sky color changes, emp some magic, strange weather patterns.

Thanks!

Mark Hall
2015-01-20, 12:27 PM
Depends on how heavy into it you want to get. There's a few different sources on binary stars and planets that might orbit them, but for a sci-fi/fantasy feel, you might take a look at Fire Time (http://en.wikipedia.org/wiki/Fire_Time) by Poul Anderson. It's a bit old, but you might find it at a half-price bookstore or your library.

In Fire Time, there were actually three stars. There's the main sequence star that Ishtar orbits (Bel), the red giant that drives the plot a bit (Anu), and a far distant dwarf (Ea, who is more or less a footnote). In the Ishtarian system, the approach of Anu every thousand years results in the scorching of the northern hemisphere of Ishtar, driving the liontaur-like people into the southern hemisphere, and pushing northern barbarians against the civilized southern lands.

You might go with a Tattooine-like system, where two stars simply means a bright sky during the day, because the stars are close enough that your planet orbits outside their mutual orbits.

BWR
2015-01-20, 12:43 PM
You might also want to read Isaac Asimov's "Nightfall" (in either short story or novel version). There are actually multiple suns in that one, but you should read it anyway. Even if it had nothing to do with your game, you should read it.

hamlet
2015-01-21, 09:49 AM
You might go with a Tattooine-like system, where two stars simply means a bright sky during the day, because the stars are close enough that your planet orbits outside their mutual orbits.

Actually, this is a really good bet. Having the planet orbit a point of common gravity between the two parent stars is about the easiest, and only as far as I can tell, way to have a standard day/night cycle.

Otherwise, you're really going to have to hash out hte math a bit on which sun is up when and for how long. You'd end up with long periods where there was never a night (i.e., full dark). Such things tend to really mess with plants A LOT: plants on Earth have evolved specifically around a regular day/night cycle and often around a typical 4 season year. Too much sun can kill them. What kind of plant would evolve in such an environment? I have no real idea.

An idea that I might offer is to drop the idea of two stars and instead have the planet be an earth sized moon orbiting a really big gas giant which, in turn, orbits a star at relatively close range. The sun would provide much of the required radiant energy to create a viable ecosystem and the planet is going to be big enough in the night sky almost all the time that it will provide enough light to almost be a second sun but with some strange light. You could even add in a few other moons that travel around the sky. One, maybe, is close enough to the planet that every few decades it causes a disruption in the magnetic fields that hold the radiation from both planet and sun at bay resulting in mutations and monstrous entities (i.e., your various monsters explained in a semi-sciency way!). Or one that every thousand years passes through the ring system of the planet and brings down a few big rocks to impact the surface of your world.

Segev
2015-01-21, 10:44 AM
In many artistic depictions, the sun and the moon wind up being used as a pair of eyes belonging to some cosmic entity (usually divine or otherwise mystic).

If you go with a pair of suns around which the planet orbits as a pair (rather than having them rise and fall separately), you could have the mythology be that the two suns are the eyes of a god.

Storm_Of_Snow
2015-01-21, 10:59 AM
Actually, this is a really good bet. Having the planet orbit a point of common gravity between the two parent stars is about the easiest, and only as far as I can tell, way to have a standard day/night cycle.

Day/night is a factor of whether the planet itself rotates or is tidally locked with respect to the primary. In the latter case, you'd have a scorched desert day side, an arctic night side, and a habitable twilight zone around 90 degrees from the star.

For a binary system, you've got three possible locations for a planet.

1) Orbiting one or other of the stars. In this case, you would have periods of time where there is no night (when the orbit takes it between the two stars). And I guess it might theoretically be possible for the planet to occasionally switch which star it's orbiting, depending on the distance between each star, the relative positions of each star and the other orbiting planets at the time, their masses and so on.

2) Orbiting both stars. In this case, you'd have similar orbits to those around a single star, but, depending on the stars in question, it might be too far outside the life zone for life to have evolved.

3) Locked at the centre of gravity between both stars, so they orbit it. See tidally locked, but without the arctic side, and maybe not the twilight edge. Most likely, it'd be burnt to a crisp and completely inhospitable to native life. Given that the stars would have swept up any matter in their vicinity during their creation, about the only reasonable way for one to be here would be an object that was originally in orbit around one star, but became slowed and eventually captured at the barycentre by the gravity of the other.

Mark Hall
2015-01-21, 12:11 PM
Actually, this is a really good bet. Having the planet orbit a point of common gravity between the two parent stars is about the easiest, and only as far as I can tell, way to have a standard day/night cycle.


q.v. the bit about Ishtar in Fire Time; the orbit of Bel and Anu was elliptical and inclined, meaning that Anu spent about a thousand years away from Bel, and its presence tended to leave the southern hemisphere survivable.

Knaight
2015-01-21, 12:23 PM
3) Locked at the centre of gravity between both stars, so they orbit it. See tidally locked, but without the arctic side, and maybe not the twilight edge. Most likely, it'd be burnt to a crisp and completely inhospitable to native life. Given that the stars would have swept up any matter in their vicinity during their creation, about the only reasonable way for one to be here would be an object that was originally in orbit around one star, but became slowed and eventually captured at the barycentre by the gravity of the other.

If it's fairly far away from both, it isn't necessarily inhospitably hot. You might even have two areas directly facing each star that are habitable, plus a ring in the middle that is ridiculously cold at all times. The atmosphere is a big consideration here as well, something like Venus is just going to be hot in general, even over the areas that aren't being particularly heated, because when you have an atmosphere of greenhouse supercritical fluids that sort of thing happens.

(Un)Inspired
2015-01-21, 12:51 PM
It's fantasy right? The suns don't need to be hydrogen bodies.

Make them two deities constantly fighting in the sky. Have meteor showers regularly occur as they knock bits off of each other.

JusticeZero
2015-01-21, 01:02 PM
The question is which orbit the second star has.
Some settings have the second star in an inner orbit; i'm not actually sure if you can have stable orbits outside of that, but really it leaves you having two stars in more or less the same place, and two seasons out of the year you see two suns. (Which two will change.)
Sometimes, the other sun is in an outer orbit, likely even a very outer orbit; imagine Neptune being a star. In this case, you have one very very bright star that orbits with you.
A star will clear a HUGE amount of space around it, thus another reason why I have doubts about the 'two suns close together' system - in such a system, I imagine you would see things like "You have reached your first birthday - now you are an adult!" A bigger sun will clear more space, and require a habitable world to be further from it. The sun from Neptune approaches "And there is our sun.. or was it that one? Hmm, i'm not sure..", so a binary is probably going to mostly feature one very bright star in the night sky that you see for about half of the year.

Naanomi
2015-01-21, 02:12 PM
There are metaphysical considerations as well... Do they both effect vampires or other creatures vulnerable to the sun? Do spell effects that care about day and night only care about one sun?

ddude987
2015-01-21, 07:07 PM
Thank you all so much for all the input!


There are metaphysical considerations as well... Do they both effect vampires or other creatures vulnerable to the sun? Do spell effects that care about day and night only care about one sun?

This is a great question. I was thinking maybe tying the suns to different kinds of magic (arcane / divine) and having one be more powerful with the approach of its sun or something similar.

Rakaydos
2015-01-22, 02:51 AM
3) Locked at the centre of gravity between both stars, so they orbit it. See tidally locked, but without the arctic side, and maybe not the twilight edge. Most likely, it'd be burnt to a crisp and completely inhospitable to native life. Given that the stars would have swept up any matter in their vicinity during their creation, about the only reasonable way for one to be here would be an object that was originally in orbit around one star, but became slowed and eventually captured at the barycentre by the gravity of the other.

the L1 lagrage point is unstable. What you really want is a planet that formed in a similar way to jupiter's Trojan asteroids, in the L4 or L5 lagrange of the less massive star.

Like orbiting a distant binary, the sus would rise and set as a pair, never moving relative to eachther. But unlike tatoine, they'd be 60 degrees apart in the sky- if a 24 hour day, one would rise 4 hours after the other.

Segev
2015-01-22, 10:19 AM
the L1 lagrage point is unstable. What you really want is a planet that formed in a similar way to jupiter's Trojan asteroids, in the L4 or L5 lagrange of the less massive star.

Like orbiting a distant binary, the sus would rise and set as a pair, never moving relative to eachther. But unlike tatoine, they'd be 60 degrees apart in the sky- if a 24 hour day, one would rise 4 hours after the other.

Such a planet would either have no seasons, or seasons dependent only on the larger, more distant star (around which its closer star, the one it's in the lagrange point of, orbits).

If the two stars were of a size to provide roughly equal light/heat for the distance, and they were calibrated to provide "enough" but not "too much" when combined, you'd have a slightly cooler world than Earth for much of the time (since 0-1 suns would be in the sky for most of the day-night cycle), but it would be overall habitable. Longer "twilights" as one or the other sun was in the sky, but not both, and then sub-twilights when the sole sun in the sky is rising or setting, moving to or from "night."

If there is an axial tilt, and we designate "north" as the end tilted closer to the nearer sun, the northern hemisphere would be overall warmer, similar to how the equator is for the Earth, due to always having longer near-sun days and shorter near-sun nights. There would be seasons based on the orbit around the farther star, which would behave more like our own sun does as the year progressed.

For a wonkier revolution/orbit cycle, the axial tilt could be still different. Think of the various orientations it could have wrt the two stars.

If it points inwards towards the further star, for instance, the "north" (pointing that way) would have a perpetual half-lit day from that star, and a day-night cycle from the nearer star. Think of it as if the North Star were a second sun. The south would have something more like what we experience on Earth, though with no seasons. It would never see the second, further sun.

Rakaydos
2015-01-22, 01:30 PM
Such a planet would either have no seasons, or seasons dependent only on the larger, more distant star (around which its closer star, the one it's in the lagrange point of, orbits).

If the two stars were of a size to provide roughly equal light/heat for the distance, and they were calibrated to provide "enough" but not "too much" when combined, you'd have a slightly cooler world than Earth for much of the time (since 0-1 suns would be in the sky for most of the day-night cycle), but it would be overall habitable. Longer "twilights" as one or the other sun was in the sky, but not both, and then sub-twilights when the sole sun in the sky is rising or setting, moving to or from "night."

If there is an axial tilt, and we designate "north" as the end tilted closer to the nearer sun, the northern hemisphere would be overall warmer, similar to how the equator is for the Earth, due to always having longer near-sun days and shorter near-sun nights. There would be seasons based on the orbit around the farther star, which would behave more like our own sun does as the year progressed.

For a wonkier revolution/orbit cycle, the axial tilt could be still different. Think of the various orientations it could have wrt the two stars.

If it points inwards towards the further star, for instance, the "north" (pointing that way) would have a perpetual half-lit day from that star, and a day-night cycle from the nearer star. Think of it as if the North Star were a second sun. The south would have something more like what we experience on Earth, though with no seasons. It would never see the second, further sun.
.
I'm pretty sure it eould stil have seasons- jus because it's in a Lagrange doesnt mean it isnt in an orbit. it just has an orbit matching the smaller star. Spinning on an axis, that axis will go thrugh the full set of seasons in one year-of-the-smaller-star.

Now, you still can have fun with massive axal tilts, because there will be a season where the near star doesnt set, the far star doesnt set, the near one dloesnt rise, and the far one doesnt rise.

Jay R
2015-01-22, 02:49 PM
Based on modern physics, if you want two suns that look and feel like suns, and to have the world be habitable all the time, the planet must be at L4 or L5.

So if you don't want the suns parked at an eternal angle of 60 degrees from each other, blow off modern physics, at least as far as universal gravitation, and have the suns do what the gods want them to do.

Segev
2015-01-22, 03:26 PM
.
I'm pretty sure it eould stil have seasons- jus because it's in a Lagrange doesnt mean it isnt in an orbit. it just has an orbit matching the smaller star. Spinning on an axis, that axis will go thrugh the full set of seasons in one year-of-the-smaller-star.

Yes. Note that what you quoted said "or have seasons based on the orbit of the larger, more distant star." This would be the situation described in the paragraph following that opening.

Jay R
2015-01-22, 03:51 PM
Such a planet would either have no seasons, or seasons dependent only on the larger, more distant star (around which its closer star, the one it's in the lagrange point of, orbits).

L4 and L5 are points 60 degrees in front of or behind the smaller star in its orbit around the larger one. Therefore there is no "more distant star" or "closer star". The three bodies form an equilateral triangle, and are equidistant from each other.

Segev
2015-01-22, 04:20 PM
L4 and L5 are points 60 degrees in front of or behind the smaller star in its orbit around the larger one. Therefore there is no "more distant star" or "closer star". The three bodies form an equilateral triangle, and are equidistant from each other.

Okay, that, I did not know.

Still, there would only be seasons wrt the star which is being orbitted, not the one whose L4 or L5 point the planet is in.

Rakaydos
2015-01-22, 04:46 PM
Okay, that, I did not know.

Still, there would only be seasons wrt the star which is being orbitted, not the one whose L4 or L5 point the planet is in.

Not true. Consider the extreme example of the planet spinning entirely on it's side, like Uranus in our own system

In January, the north pole might be pointed at the primary star, with the secondary star, with the seondary star above the 30 degrees north latitude.
2 months later, after 1/6 of an orbit, the pole would be pointed at the secondary star, with the primary at 30 degrees north on the other side.
2 months after that, the axis will be equally distant from both stars- the primary has dropped to 30 degrees south, while the secondary is at 30 degrees north
and 2 months later, with the planet on the opposite side of it's orbit from where it began, the south pole would be pointed directly at the primary star with the secondary star trailing at 30 degrees south.
The other six months follow the suns bath back from south to north.

With a more normal axle tilt, the seasons wouldnt be quite so extreme, but there would be a definate spring, sunner, fall and winter based on both stars.

Segev
2015-01-22, 04:59 PM
Due to inclarity in my mind as to which is the "primary" star, I will simply say that the axial tilt can't be changing wrt the star in whose L4 or L5 point it is because it isn't orbiting that star. If the "axial tilt" is changing, it's actually rotating on another axis.

Rakaydos
2015-01-22, 09:32 PM
Due to inclarity in my mind as to which is the "primary" star, I will simply say that the axial tilt can't be changing wrt the star in whose L4 or L5 point it is because it isn't orbiting that star. If the "axial tilt" is changing, it's actually rotating on another axis.

You seem to be unclear as to the nature of Lagrange points.

Sufficent to say, there is spinning triangle in space, with a sun each in 2 corners and the planet in the third. As the triangle spins, the planet's axal tilt remains in one direction, presenting seasons to the -two- stars it orbits.

Segev
2015-01-23, 09:12 AM
You seem to be unclear as to the nature of Lagrange points.

Sufficent to say, there is spinning triangle in space, with a sun each in 2 corners and the planet in the third. As the triangle spins, the planet's axal tilt remains in one direction, presenting seasons to the -two- stars it orbits.

My understanding is that the Lagrange Point is a point that exists on the same circle swept out by the star orbiting the other star.

I'm going to label them Alpha (the "stationary" star for our reference frame) and Beta (the star orbiting Alpha in our reference frame).

The Lagrange Point we're looking at for our planet exists 60 degrees "behind" (or "in front of") Beta in Beta's orbit, and stays there in its own orbit around Alpha.

Assuming it has an Earth-like axial tilt wrt the plane in which its (and Beta's) orbit is embedded, what you're saying is that it will maintain axial orientation wrt Alpha?

So what's actually happening here is that it's not so much caught in a position wrt Beta as it is sharing Beta's orbit about Alpha. This also means, in practice (if we assume Alpha and Beta are tide-locked to each other), that if we shift reference frames to have Beta be stationary, Beta perceives the planet to be orbiting about it, after all. I had been envisioning Beta, from its reference frame, seeing the planet as stationary.

Rakaydos
2015-01-23, 03:38 PM
Rotational axises stay the same regardless of who's orbiting who. The earth's north pole points toward a fixed point in space, whch happens to have the star Polaris near it.

Seasons happen when (from the planets frame of reference) suns move closer or furthur from that fixed point. So, summer in northern hemisphere is winder in the southern hemispere.

Our hypothetical lagrage planet would have suns in two different parts of the sky at the same time, one "seasonally" trailing the other by 2 months. So "summer" might last a bit befre alpha solstie, through Alpha Summer Solstice, alpha-beta transitional summer, Beta Summer Solstice, and a bit after beta solstice. (by whicch point Alpha is almst at fall equinox.)

Segev
2015-01-23, 03:53 PM
Er, rotational axes remain fixed wrt some reference frame, yes, but what that frame is can vary considerably.

Though really, we're not discussing anything useful so much as getting lost in the weeds. I concede any argument that might be going on.

Rakaydos
2015-01-23, 04:16 PM
I dunno, the idea of 'summer" being from Alpha solstice to Beta Solstce is an interestng one. Especially once you throw ritual magic into the mix.

obryn
2015-01-23, 05:32 PM
I think if you're doing orbital calculations or worrying about Lagrange points, you're probably overthinking it. :smallsmile:

The social and world-building effects are where the interesting bits come in, which will interact with play in meaningful ways.

Jay R
2015-01-24, 10:20 AM
I think if you're doing orbital calculations or worrying about Lagrange points, you're probably overthinking it. :smallsmile:

Assuming Lagrange point suns:

The social and world-building effects are where the interesting bits come in, which will interact with play in meaningful ways.

The first monotheistic religion on earth identified their one god with the sun. I suspect that two-god religions would be much more common than one-god ones on planet Lagrange.

On earth, the lunar tide follows the earth, while the solar tide follows the sun, causing higher "spring tides" at new moon and full moon, with lesser neap tides at the quarters. The two solar tides would be at a constant 60 degrees from each other. Ignoring a moon, the highest solar tide faces directly towards and directly away from the most massive sun, with a secondary solar tide 60 degrees away, and the lowest tides about 60 degrees from that, more or less, depending on the relative difference in solar masses.

Add in a lunar tide, and navigation is a much more complex subject, and navigators are worth much more.

Daytime averages around 2/3 of the time, and nighttime 1/3. This is better for diurnal animals, and worse for nocturnal ones. Nocturnal ones are much better hunters than on earth, or they won't survive. If there is a significant difference between the apparent brightness of the two suns, predators might develop who only hunt by the light of the lesser one. It would be the only one up about 1/6 of the time.

On earth, the longest day (one one side) and longest night (on the other) occurs when the flat projection of the axis lines up with the sun (the solstices). On planet Lagrange, this happens four times a year, not two times. The actual longest days and nights would occur halfway between the ones that are 1/3 of a year apart.

The most average days and nights occur at ninety degrees to that (the equinoxes). On Lagrange, they would occur when the axis projection pointed at the spots 60 degrees behind the back star, and sixty degrees in front of the forestar. I suspect that cultures would define six seasons, not four. (Possible twelve, and you have our months even without a moon.) The obvious breakpoints are the solstices two months apart, and the false equinoxes defined above.

["False" because "equinox" actually means "equal night", and her it wouldn't be. It would be the day when the night was exactly 1/3 of a full cycle, or exactly half the length of the day. Maybe it would be called a "deminox."]

[And yes, I know the Latin plural is "equinoctes". But it's been "equinoxes" in English for a very long time.

The heat of the day does when you are under two suns. During summer, it would be brutal, and the summer siesta would be a very common custom in warmer climates.

A midnight sun would be much more common than a polar night. There is no polar night at all unless the planet has an axial tilt of more than 30 degrees (more than earth). So either seasons are much more variable, or there is no polar night.

Is this the kind of thing you're looking for?

ddude987
2015-01-25, 03:12 PM
I dunno, the idea of 'summer" being from Alpha solstice to Beta Solstce is an interestng one. Especially once you throw ritual magic into the mix.

I think that actually sounds really cool!


scrubbed
Is this the kind of thing you're looking for?

...I think so?

Just to make sure I am understanding what you are proposing here, there would be 2 "summer" solstices and 2 "winter" solstices as in 2 longest days and 2 shortest days?

Summer would be super hot but depending on humidity it might not be a problem. Look at places in America out West. Places like Texas (parts) and Las Vegas have low humidity. While extremely hot, it isn't uncomfortable as opposed to places of mid-high humidity where even a temperature of 90~ can be overbearing.

The nocturnal predator bit was actually really cool, and I think adds some interesting fluff.

Would be winters be equally as cold as the summers? Is it possible to have areas of permafrost?

For the standard in-world calendar, I decided for simplicity, 10 months 30 days. When exactly would the 2 longest and shortest days be?

Thanks for all the help from everyone! You're all wonderful. And I have lots of research to do about LaGrange points among other things.

Ravens_cry
2015-01-25, 05:02 PM
As much as I love getting science fiction in my fantasy, and I does so love that, maybe add a fantasy twist by having the suns be sun gods at war. When they are in the sky together, one can even see motes of light travel from one to the other, for example.

Jay R
2015-01-25, 07:28 PM
Just to make sure I am understanding what you are proposing here, there would be 2 "summer" solstices and 2 "winter" solstices as in 2 longest days and 2 shortest days?

You would have two "summer" solstices and two winter solstices in the sense of having four days in which the planar projection of the axis points directly at (and away from) a sun. Neither would be the longest day of the year. The longest day (on one side of the world) and the shortest day (on the other side) would occur when the axis projection pointed directly between the two suns.


Summer would be super hot but depending on humidity it might not be a problem. Look at places in America out West. Places like Texas (parts) and Las Vegas have low humidity. While extremely hot, it isn't uncomfortable as opposed to places of mid-high humidity where even a temperature of 90~ can be overbearing.

Relatively. You can make the hottest temperature whatever you want. Put the distance at Venus's orbit, for instance, and you'd always be at several hundred degrees.


The nocturnal predator bit was actually really cool, and I think adds some interesting fluff.

Have fun with it.


Would be winters be equally as cold as the summers? Is it possible to have areas of permafrost?

You can control this. Put it in Pluto's orbit, and it's all permafrost.


For the standard in-world calendar, I decided for simplicity, 10 months 30 days. When exactly would the 2 longest and shortest days be?

If you want simplicity, it should divide by 12.

Let's assume that the axis points at Sun1 at the start of the year (summer solstice 1)
1/12 of the year later the axis point between the suns, and there is the longest day.
1/12 of the year later, (1/6 of the year total) the axis points at Sun2 (summer solstice 2)
1/6 of the year later (1/3 of the year total) the axis points 60 degrees from each sun, at the first deminox (night is exactly half as long as day)
1/6 of the year later (1/2 of the year total) the axis points at Sun 1, but tilted the other direction (winter solstice 1)
1/12 of the year later the axis point between the suns (7/12 of the way through the year), still tilted away, and there is the longest night..
1/12 of the year later, (2/3 of the year total) the axis points at Sun2, tilted away (winter solstice 2)
1/6 of the year later (5/6 of the year total) the axis points 60 degrees from each sun, at the second deminox (night is exactly half as long as day)
At the end of the year, the axis returns to Sun1.

On the other half of the world it would be the same, but with summer and winter switched.


Thanks for all the help from everyone! You're all wonderful. And I have lots of research to do about LaGrange points among other things.

Have fun doing it.

Segev
2015-01-26, 10:15 AM
It gets even weirder if you consider the possibility that the two stars emit in sufficiently different spectra that one sun being up and the other down could lead to those whose day/night cycles are for the one that's down having a harder time seeing.

You could have Alpha-diurnal, Beta-diurnal, and both-diurnal creatures. The Alpha-diurnal ones see clearly by Alpha's light, and dimly if at all by Beta's (getting weird twilight effects akin to "darkness" and "night" as presented in TV shows). Beta-diurnal would be the other way around.

Those with both-diurnal systems would eitehr only see clearly when both were up (seeing things in spectra across both stars' distributions), or would have the ability to see so broad-spectrum that they can see clearly in all three conditions.

You could get "normal" predators of a nocturnal nature that preyed on those whose sight were for the other star. And then the night-night predators, who haveo nly half the night to hunt successfully for those who can see clearly in either, have to be teh big scary monsters.

ddude987
2015-01-26, 10:15 AM
So you're saying the axis is pointing at different places, but I'm curious, isn't the axis in a stable point? Earth's axis always points towards polaris, for example. Or is this more of 3d orbit as opposed to a 2d one.

Jay R
2015-01-26, 12:12 PM
So you're saying the axis is pointing at different places, but I'm curious, isn't the axis in a stable point? Earth's axis always points towards polaris, for example. Or is this more of 3d orbit as opposed to a 2d one.

It's exactly the difference between 3d and 2d.

The earth's axis always points at Polaris, 67 degrees from the plane of the ecliptic. (The circle of the earth's orbit around the sun). It always (in the short term) points in that direction.

Because it's not straight up and down, it has a projection onto the plane of the ecliptic. In short, from the point of view of that plane, it's leaning. The direction of that lean is what I mean by the projection onto the plane.

As the earth circles the sun, that lean never changes. But the direction from the earth to the sun does change.

At one point in its orbit, earth is closer to Polaris than the sun is. That means that the north pole is pointing away from the sun. That's the winter solstice (in the northern hemisphere). Six months later, the earth is further from Polaris than the sun, and the axis points over the sun on its way to Polaris. That's the summer solstice.

Lord Torath
2015-01-26, 12:16 PM
Yes, the axis is "stable" in that it always points in roughly the same direction relative to the rest of the universe. But the two stars and planet are arranged in a triangular arrangement, and that arrangement is spinning about an axis perpendicular to the plane of the triangle.

Assuming one of the stars is at each end, and the planet is at the point, here’s what the arrangement looks like: >

One quarter of a year later, it will look something like this: ^

Another quarter-year later, it will look like this: < Note that the star that was at the top of the triangle in the first case is now at bottom.

And the whole time, the planet’s axis of rotation is pointing (probably at an angle) out of the screen, say, directly at the center of the “enter” key on your keyboard.

The system is rotating about a point directly between the two stars (assuming they are both much more massive than your planet). Any planets farther out will also orbit the same point.

Does that clarify things for you?

Edit: Ninja'd by the excellent Jay R