Orbital Mechanics and stuff: Check my understanding of spatial relations

[Kraven Kor]

This is actually for more of a "fantasy" than "Star Hero" thing, but figured this might be a better place to ask this.

 

I am writing for my setting, and wanting to make sure I am describing something correctly.

 

The planet of the setting orbits one sun of a binary system; the two suns fairly distant, the host sun of the planet (a star much like our own, a yellow dwarf) trailing the larger, blue giant at over a light year away.

 

The planet has a (likely impossible) rotational axis / orbit - the south pole of the planet always faces the sun, at about a 40 degree axial tilt. So, the south polar region never sees night, the north never sees the light of the closer sun. The planet wobbles a bit on its axis as it revolves around the host star, such that you have some variation in night / day in the middle latitudes, but not much.

 

And, YES, I know this is likely if not actually impossible using real physics.

 

My rudimentary thought experiments on how this would look at various latitudes are as such:

 

Starting at what would be the southern 'arctic' circle, you would have a similar setup as Alaska - about half the year being constant daylight, and then the other half having very short nights. As you go further north, the day / night cycle becomes more constant, with short nights and long days until about the equator where you would have day / night cycles similar to our own. Above the equator, nights become typically longer than day, until about the northern 'temperate' meridian, where you start having a reversal of the southern cycle - half the year of no daylight, the other half having short periods of daylight per rotation.

 

At the southern polar region, the sun would simply spin in a circle in the sky, rarely if ever setting. Most of the southern hemisphere would see the sun rise in the southeast, make an elliptical arc upwards and towards the east, then further up and back west, then down and west, then down and back east, setting in the southwest. Above the equator, this would be a more typical winter-time arc like we see - rising in the southeast, setting in the southwest.

 

The planet's 'winter' is when it is pulled farthest from the host star, towards the larger but distant second star, and the axis leans a few more degrees 'away' from the host star - making day longer in the south, shorter in the north. During the "summer" the planet is on the opposite side of the host star from the larger second star. During summer, the second star would appear as a very bright star on the southern horizon, and make a circular path through the sky, mostly below the horizon for the northern hemisphere. During spring / fall, the second star would be slightly brighter, and rise in the east, set in the west, tracking almost directly across the sky. In winter, the second star would be much brighter, enough to provide a kind of 'twilight' when in the sky alone, and would mostly be seen in the northern sky, dipping below the northern horizon for much of the southern hemisphere.

 

Brain hurt yet?

 

Does that sound, more or less, accurate?

 

Any other cool 'fluff' descriptions I might be missing?

[Beast]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

so the planet is tidally locked to the main sun like the moon is to the earth

the weather will pretty much be the same all the time

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

so the planet is tidally locked to the main sun like the moon is to the earth

the weather will pretty much be the same all the time

 

Well, the way I am thinking is that you will have constant hot, wet air moving up from the southern polar region into the north, and then cooler, dry air from the north. So not "always the same" but yes, very distinct climate zones.

 

There is actually a perpetual storm over most of the southern pole, and that sends nearly constant storm systems into the lower southern hemisphere. Like "a storm a day" on average. These storms break up on the mountains along the southern edge of the main continent, so I have one small-ish desert to the north of these mountains. The northern hemisphere will be very dry, particularly in 'winter,' when the storms coming from the south become a little less consistent and severe.

 

The planet is a touch further from its host sun than we are from Sol, more of a Mars-distance orbit, so the planet is overall colder; but much warmer than earth in the south (average yearly temp over 100 degrees ferenheit,) and much colder in the north (average yearly temp below freezing once you are above the northern 'temperate' meridian.) The more elliptical nature of the orbit gives it the seasonal weather changes - the seasons are more due to distance from the host star than due to axial tilt.

 

And it isn't exactly tidally locked - the south pole does not point *directly* at the host star, and it does wobble a bit (by about 10 degrees total) due to the gravitational influence of the larger, primary sun of the system.

[Captain Obvious]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

If the orbit is elliptical enough that the planet is much closer to the sun at some times than others, it would in effect cause seasons. I don't think a planet can be tidally locked without a largely circular orbit though. I don't think it can be locked if the axis is far off perpendicular to the plane of orbit either.

 

However, if this is a fantasy planet, anything goes. I think KK is mostly looking for confirmation of his ideas of how this setup would affect the surface of the planet (weather, day length, apparent motion of the celestial bodies), and angling for any cool or important effects that he may have overlooked.

 

It seems to work, to my mind. I don't mess around a whole lot with non-Earth-like fantasy worlds, though, and don't GM enough sci-fi to get too deep in the weeds on unusual mechanics there either.

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

However' date=' if this is a fantasy planet, anything goes. I think KK is mostly looking for confirmation of his ideas of how this setup would affect the surface of the planet (weather, day length, apparent motion of the celestial bodies), and angling for any cool or important effects that he may have overlooked.[/quote']

 

Exactly - much like many others before me, I wanted a "cool and unique" setting in whatever fashion, and as my ideas solidified, the idea of two suns, and the light / dark side of the planet became some of the key descriptive fluff. I don't care if it isn't 'scientifically accurate,' but I do want the descriptions of how things look and some basic mechanics of the setting to at least show that things were well thought out. I have always felt the best fiction was written by someone who put a lot of thought into the implications of their ideas.

 

I do want it to be "very different from Earth" in about the same fashion that the Game of Thrones world is.

 

Naturally, the very fact that humans and lizard people and cat people and dinosaurs populate the same planet, where magic is as real as the laws of physics, puts it directly into the realm of fantasy, regardless of how it is explained.

[Lucius]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Orbital Mechanics: Mechanics 11-, Ranged (+1/2), MegaScale (1m = 100,000 km; +2 1/4) (11 Active Points)

 

Lucius Alexander

 

Animal Handler (Other Palindromedaries) 11-, Ranged (+1/2), Transdimensional (Related Group of Dimensions; +3/4) (4 Active Points)

[McCoy]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

The planet has a (likely impossible) rotational axis / orbit - the south pole of the planet always faces the sun, at about a 40 degree axial tilt. So, the south polar region never sees night, the north never sees the light of the closer sun. The planet wobbles a bit on its axis as it revolves around the host star, such that you have some variation in night / day in the middle latitudes, but not much.

 

And, YES, I know this is likely if not actually impossible using real physics.

Isn't that Uranus? South pole always points toward the sun?

[McCoy]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

the idea of two suns' date=' [/quote']

Um, check with Cancer, but I believe at a light year distance, the blue star is going to be a very, very bright star, no disk naked-eye visible. Might be a light bright enough to cast shadows, but I think less than a full moon.

[Vulcan]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Yeah, at a light-year distant I don't even know if it really qualifies as a binary system. The orbital period of the system would be on the order of billions of years, I'd bet, assuming there was enough gravitational influence between the two at that distance.

 

Remember, Alpha Cenutari is four light-years away from the Sun, and we're not classified as a binary system with them (Isn't Centauri a trinary system? I forget, my last astronomy class was a loooooonng time ago).

[McCoy]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Yeah, at a light-year distant I don't even know if it really qualifies as a binary system. The orbital period of the system would be on the order of billions of years, I'd bet, assuming there was enough gravitational influence between the two at that distance.

 

Remember, Alpha Cenutari is four light-years away from the Sun, and we're not classified as a binary system with them (Isn't Centauri a trinary system? I forget, my last astronomy class was a loooooonng time ago).

The blue star probably masses more than Sol and the Centari system put together. Binaries can have a great separation, light years even, as long as there is enough mass that they are gravitationally connected.

 

With an average orbital radius of 1 light year, arbitrarily assuming Yellow and Blue add up to ten solar masses, I get Yellow orbits Blue every half million years [edit:Woops! make that every 5 million years!]. But with any eccentricity in Yellow's orbit, Blue might not affect the seasons but it would affect the ice ages.

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Um' date=' check with Cancer, but I believe at a light year distance, the blue star is going to be a very, very bright star, no disk naked-eye visible. [i']Might[/i] be a light bright enough to cast shadows, but I think less than a full moon.

 

And that was generally the idea - the primary blue giant is visible on the planet as an intensely bright star, bright enough to still be seen during the day (for the half or so of the year that it isn't "behind" the planet and thus below the horizon during the day) and bright enough to give near-full moon "twilight" when alone in the sky. I won't have to put the words "Light" and "year" together in the setting information or fiction; the world this is set on doesn't understand the 'speed of light' or such things yet. So the exact distance is unimportant, and I'm giving the approximate for the sake of understanding how things would look from the planet's surface.

 

The blue star probably masses more than Sol and the Centari system put together. Binaries can have a great separation, light years even, as long as there is enough mass that they are gravitationally connected.

 

With an average orbital radius of 1 light year, arbitrarily assuming Yellow and Blue add up to ten solar masses, I get Yellow orbits Blue every half million years. But with any eccentricity in Yellow's orbit, Blue might not affect the seasons but it would affect the ice ages.

 

I read somewhere about a binary system where the smaller, dwarf star is actually just trailing along behind the primary giant, not exactly orbiting at all. So I was using that as a basis, but again, it doesn't terribly matter at the distance involved, and, like us, the cultures on the planet have only really been around 10,000 years or so at best.

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Yeah, at a light-year distant I don't even know if it really qualifies as a binary system. The orbital period of the system would be on the order of billions of years, I'd bet, assuming there was enough gravitational influence between the two at that distance.

 

Remember, Alpha Cenutari is four light-years away from the Sun, and we're not classified as a binary system with them (Isn't Centauri a trinary system? I forget, my last astronomy class was a loooooonng time ago).

 

http://en.wikipedia.org/wiki/Alpha_Centauri, http://www.astro.wisc.edu/~dolan/constellations/hr/5459.html,'>http://www.astro.wisc.edu/~dolan/constellations/hr/5459.html, and http://www.astro.wisc.edu/~dolan/constellations/hr/5459.html - There is the brown dwarf, Proxima Centauri, that is the third star of the system along with Rigel Kentaurus / Alpha Centauri A and B. Proxima is the closest star to Sol at about 1.3 LY, but can't even be seen in the sky at all, probably couldn't be seen with the naked eye even if the two brighter stars of the system weren't there to outshine it.

[McCoy]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

I read somewhere about a binary system where the smaller' date=' dwarf star is actually just trailing along behind the primary giant, not exactly orbiting at all.[/quote']

That's -- interesting.

 

So I was using that as a basis' date=' but again, it doesn't terribly matter at the distance involved, and, like us, the cultures on the planet have only really been around 10,000 years or so at best.[/quote']

Even if climate change isn't part of you storyline, there will be folklore of warmer and cooler times. If your "elves" are living to a thousand years old, they will be observing the changes in their lifetime. It's a throwaway detail, but can add depth to the worldbuilding if you chose to use it.

 

Likewise, if any of your cultures have astrologers, they would see the Bright Star moving slowly against the "fixed" stars a degree every 14000 years.

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

That's -- interesting.

 

And could very well be mis-remembered BS

 

Even if climate change isn't part of you storyline, there will be folklore of warmer and cooler times. If your "elves" are living to a thousand years old, they will be observing the changes in their lifetime. It's a throwaway detail, but can add depth to the worldbuilding if you chose to use it.

 

Likewise, if any of your cultures have astrologers, they would see the Bright Star moving slowly against the "fixed" stars a degree every 14000 years.

 

I had not linked it to the second (primary) star, but 'climate change' and such are a major part of the history of the world. And there is already some fluff written about this world's version of Copernicus or whatever who discovered that the primary star was more than just a bright star, using the old astrological charts that date back to those early civilizations with incredibly and impossibly accurate solar calendars and such (the 'stonehenges' of this world and the like.)

[Xavier Onassiss]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

That's -- interesting.

 

 

Even if climate change isn't part of you storyline, there will be folklore of warmer and cooler times. If your "elves" are living to a thousand years old, they will be observing the changes in their lifetime. It's a throwaway detail, but can add depth to the worldbuilding if you chose to use it.

 

Likewise, if any of your cultures have astrologers, they would see the Bright Star moving slowly against the "fixed" stars a degree every 14000 years.

 

Brian Aldiss' Helliconia trilogy describes a world with some similarities to the one in the OP; it's in a binary system with a dwarf star and a giant star, and has a seasonal cycle lasting centuries. (Not tide-locked, though.) If you'd like to check it out, go here:

 

Helliconia Spring

[Cancer]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Ok, things that have already been mentioned:

 

The idea of the rotation axis pointing always sort of toward the yellow dwarf ... yeah, that's purest fantasy; you can't do that with real orbital mechanics. The real planet Uranus has its rotation axis fixed in space, but with a long orbital period (84 years) for extended times one pole is pointed sort of toward the Sun and the other pointed away; it alternates sunward poles, with episodes of "barrel roll" rotation in between. FWIW, the Voyager spacecraft passed through the Uranus system at one solstice (when one pole was pointed most nearly toward the Sun) and the planet was utterly weatherless at the time: it was a featureless blue ball. Since that time (it went through an equinox in 2007) weather has clearly developed ... we have seen storms and cloud features come and go with HST, albeit with MUCH poorer resolution (you can see what I'm talking about by looking up Uranus images at the NASA Planetary Photojournal site). On that basis, who knows what the planet's climate is like in terms of storms? We've only been able to monitor Uranus's weather in any way for 15 years or so, plus the one-shot early peek in 1986: much less than a full Uranian year. So make up what you want.

 

You can have a tidally locked body in a non-circular orbit, but it takes regular perturbations by a (so far unmentioned) third (or more) body in the system. See Io: it is tidally locked, but its orbit and rotation are continually perturbed by the other Galilean satellites. However, the planet rotational motion you specify is NOT tidal locking, and there's no way to get that kind of motion with strict orbital mechanics, unless you are applying tidal torques to the planet so great that those torques should be causing irreparable structural damage to the planet itself.

 

The blue giant ... it isn't a binary as you are used to thinking of that. With a separation that wide (about a third of a parsec), the orbital period is immensely long (a couple of million years or more). That said, there are lots of "common proper motion" pairs out there, stars which are widely separated so there is no hope of ever getting enough information to characterize any sort of binary orbit, but the two have the same space motion through the Galaxy and are probably physically related. So that part isn't out of the question.

 

What does get you back toward the fantasy regime is that blue giants are rather short-lived stars. At 8 solar masses for the B2III blue giant Bellatrix, the main sequence lifetime is really short ... again, a few million years, and with it being a giant the end of that lifetime is not far away. If there really is a physical association between the blue giant and the yellow dwarf, well, the dwarf could in fact still be a pre-main-sequence star at this stage. (8 solar masses is sort of the break point for initial mass of stars that go core-collapse supernova and those that become white dwarfs, to look into the future, but whichever end happens, it is not far off.) In any case, it's a really really young solar-type dwarf (and such stars have lots of flares and magnetic activity and strong stellar winds and rapid rotation and so one) and a really really young planet (as in, it could reasonably be expected NOT to have suffered the giant impact that made the Moon yet!).

 

Physically, you're much closer to reality if you assert a "close pass" of the dwarf through the part of space near the blue giant at this stage in its history. That, incidentally, could put you in a star forming region, with lots of nearby thick dark clouds and bright nebulosity, so that the night sky could have a relatively small number of stars in, lots of amorphous pretty colors, and no hint of the structure of the Galaxy around it. You have a more or less free hand if you posit this, actually.

 

At 1 l-y (about a third of a parsec) that blue giant will have an apparent magnitude of about -10. This is not quite Full Moon brightness, but it is within the range of brightness (and toward the brighter end, actually) that the Moon goes through over the course of its monthly phase cycle. It will still be a point source, however (that is, a star, not a disk).

[Cancer]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

Likewise' date=' if any of your cultures have astrologers, they would see the Bright Star moving slowly against the "fixed" stars a degree every 14000 years.[/quote']

 

If you take that 14000 year period at face value, with a total of 9 solar masses to the system, the orbital size would be about 1200 AU. And that would make the blue giant have an apparent magnitude of about -19, which is MUCH brighter than the Moon; in fact, closer to the Sun than the Full Moon in brightness. At that distance the eye could not tell it was a disk, and neither could Galileo's telescope, but Herschel's could, as it would appear comparable to (slightly larger in angular size than) Uranus.

[Cancer]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

The planet's 'winter' is when it is pulled farthest from the host star' date=' towards the larger but distant second star, and the axis leans a few more degrees 'away' from the host star - making day longer in the south, shorter in the north. During the "summer" the planet is on the opposite side of the host star from the larger second star. During summer, the second star would appear as a very bright star on the southern horizon, and make a circular path through the sky, mostly below the horizon for the northern hemisphere. During spring / fall, the second star would be slightly brighter, and rise in the east, set in the west, tracking almost directly across the sky. In winter, the second star would be much brighter, enough to provide a kind of 'twilight' when in the sky alone, and would mostly be seen in the northern sky, dipping below the northern horizon for much of the southern hemisphere.[/quote']

 

You have other options, remember: star systems are three-dimensional things. There's no reason why that blue giant has to be anywhere close to the orbital plane of the planet. If you wanted, you can put it near the ecliptic pole (that is, "above" the planet's orbital plane), which would mean it is more or less always visible for half a day from the planet.

[McCoy]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

The real planet Uranus has its rotation axis fixed in space' date=' but with a long orbital period (84 years) for extended times one pole is pointed sort of toward the Sun and the other pointed away; it alternates sunward poles, with episodes of "barrel roll" rotation in between. [/quote']

OK, I got that wrong.

 

The blue giant ... it isn't a binary as you are used to thinking of that. With a separation that wide (about a third of a parsec)' date=' the orbital period is immensely long (a couple of million years or more). [/quote']

I said 5 million, I got that right.

 

At 1 l-y (about a third of a parsec) that blue giant will have an apparent magnitude of about -10. This is not quite Full Moon brightness' date=' but it is within the range of brightness (and toward the brighter end, actually) that the Moon goes through over the course of its monthly phase cycle. It will still be a point source, however (that is, a star, not a disk).[/quote']

And i got that one right! Three-for-Two!

 

If you take that 14000 year period at face value' date=' with a total of 9 solar masses to the system, the orbital size would be about 1200 AU. And that would make the blue giant have an apparent magnitude of about -19, which is MUCH brighter than the Moon; in fact, closer to the Sun than the Full Moon in brightness. At that distance the eye could not tell it was a disk, and neither could Galileo's telescope, but Herschel's could, as it would appear comparable to (slightly larger in angular size than) Uranus.[/quote']

I said about a degree every 14000 years, or an orbital period of over 5 million years.

[Kraven Kor]

Re: Orbital Mechanics and stuff: Check my understanding of spatial relations

 

You have other options' date=' remember: star systems are three-dimensional things. There's no reason why that blue giant has to be anywhere close to the orbital plane of the planet. If you wanted, you can put it near the ecliptic pole (that is, "above" the planet's orbital plane), which would mean it is more or less always visible for half a day from the planet.[/quote']

 

Unless the blue giant's influence being above or below the orbital plane of the planets can somehow legitimize the "not quite tidally locked" orbit, I would think it would make more sense for the orbits of the planets to be aligned between the two suns. But again, at some point, the exact details of the solar system are of less importance than the "appearance" from the ground

 

And, again, cannot thank you all enough.