# Calling Astronomy, Physics enthusiasts for help with planetary setting issues.



## mithril (Jul 11, 2011)

Hi

I've been doing some world-building for a mainly fantasy project. I need help resolving certain issues relating to skies on an extrasolar planet.

My world is an earth like planet (P) having a moon (M) orbiting a G1 star. The world and the moon can be taken to be the Earth and the Moon for all practical purposes. The world is habitable. Now G1 is part of a binary star system with the other star being a K2 star. The inspiration for the system is the Alpha Centauri system.

From searching the web and some discussions at other forums etc (notably bad astronomy (can not post links yet)), I've pinned down some details enough to work with. These are



 G1 and K2 are orbiting in elliptical orbits with a period of approx 80 years.
 For Alpha Centauri system, the distance between G1 and K2 varies from 11.2 AU to 35.6 A. My system might not have quite as much variation but for data calculations these are what we went with.
 G1 and K2 both have habitable zones where the orbits of the planets (if any) are NOT affected by the other stars approach.
 From the world, G1 looks slightly bigger and brighter than the Sun looks from Earth but not a lot.
 Earth's Sun is about 400,000 times brighter than the Moon.
 We calculated that from the world P1, K2's apparent brightness would vary b/w -18.18 to -21 (i.e. approx 150 times to 2000 times the brightness of the moon M). Full Moon has an apparent brightness of -12.74 mean.
 K2's apparent size as seen from P1 varies b/w 0.1 to 0.25 times its moon's size.
 Length of the year for the world can be taken as constant and is equal to time taken by the world to revolve around G1.
 
Since I can't yet post links, my discussion at BAUT can be found in the _Life in Space_ section under the title _Making some hypothetical worlds more realistic? Scenario One._

Now the Issues.

ONE

These deal with K2's path or times of rising and setting at different parts of the world's year.

Tentatively we came up with the solution that the K2 is visible for half the year at night and half the year during the day. (day meaning when G1 is visible)

I still need to get this confirmed/verified etc. In addition I need more details than just half the year during day and half the year during night. So if anyone's willing to take up that challenge, I'd be grateful.

However the one I'm finding more challenging is to do with brightness levels.

TWO

If it was night on the world and K2 was visible, we can assume that the brightness contributed by the moon would be negligible in comparison to K2. However saying that K2 is 150 to 2000 times as bright as the moon does not tell me anything in real terms. 

I want to know how much of the scenery would be illuminated. Full Moonlight for comparison is enough to make out the shape of things but there are a lot of shadows too. There is no colour. So at its dimmest, would night under K2 be as bright as sitting under a sodium vapour lamp? Under a floodlight at a sports stadium? What about K2 at its brightest?

I tried looking for ambient brightness/luminosity data on earth corresponding to sun at different apparent magnitudes during a solar eclipse but didn't really find anything. The only maybe useful thing was that there is not much difference in brightness till the sun is almost eclipsed and then it gets dark very quickly and at its darkness one can see a few of the brightest stars/planets/comets. Still not sure how that could be useful.

At 10% of moon's size (K2's maximum apparent size) It should be visible as a very small disc. Maybe like a pea? At its minimum I'm assuming I should treat it as just a very bright star.

Any help would be greatly appreciated.

Edit: If this thread needs to be moved to some other sub-forum, please do that. I posted here since I wasn't sure where such discussions belonged.


----------



## mithril (Jul 15, 2011)

Any help on this please....


----------



## mithril (Jul 22, 2011)

Additional Information.

_The secondary star would be visible during the day for half a year and during the night time for the other half. 

During the times it is visible during the day, it would be outshone by  the primary so that there is no real difference to the day's brightness.  However it would still be visible except at times when it's position is  too close to the sun in the sky. 

While the secondary star is visible during the night, it would be seen  as the brightest object in the sky (including the full moon), and would  be the primary light source for the night during this time of the year.  These nights will be termed as 'bright nights', with the night with just  the moon as normal nights. The moon of course changes phases normally.

_A big thank you to Vertigo for pointing out these omissions._ 
_


----------



## Vertigo (Jul 22, 2011)

You're very welcome Mithril. I'm afraid anything beyond the comments I've made already is really well out of my knowledge comfort zone.

As a refinement on the half year dark nights and half bright nights, it would probably be more gradual with K2 gradually shifting from only visible at night to only visible during the day. However, one thought (and I certainly don't have the math for this any longer), but I'm not sure with elliptic orbits whether that would actually be an even half year split.


----------



## Metryq (Jul 23, 2011)

Forgive me if this is redundant information, but this seemed an appropriate thread to mention the *Project Rho Atomic Rocket* pages. This is an excellent reference for sci-fi writers who strive for scientific accuracy in their way-out tales. Click "Show Topic List" in the upper right corner. There is even a *Weird Astronomy* page.


----------



## Sapheron (Jul 24, 2011)

As promised, and just taking the original questions rather than reading through things already discussed. Also, it may be added, off the top of my head. I’m not going into statistics and calculations because, to be perfectly honest with you, that would be a waste of time. No one reading would have the time or motivation to go into this except the odd one or two people who always obsessively look for scientific error, and they will, I repeat WILL, find an error somewhere if they want to. 

  Assuming the factors you’ve given:

  The second star is visible but compared to the closer star makes no real difference during the ‘double daytime’ (a phrase I shall use in this, referring to the number of stars in the sky during daytime); ie, the days are not brighter. The second star should be quite easily visible until it gets very close to the closer star, at which point it will fade, and when close enough probably be completely invisible. During the ‘double daytime’ when the stars line up the moon will pass through phases identical to those on real Earth. As the stars move more out of line, these become skewed, with more moon being lit up more of the time. 

  An interesting aside is that if the moon eclipses the first star while the second is also in the sky then the second star will briefly brighten up and become dominant, and if at an angle it could even mean that the moon is also visible while it eclipses the sun. In the same way, during the ‘single daytime’ the second sun could illuminate the moon from the other side, meaning the moon is even fully visible while it eclipses the primary star. Could make for a very interesting event, and would definitely be included in primitive culture and religion, I should think. An even more spectacular (albeit very rare) event would be if all three bodies lined up, giving the occupants of the planet the exceedingly rare experience of being unable to see either star or the moon. 

  In the other half of the time, the already mentioned ‘single daytime’ when the two stars are at roughly opposite sides of the planet, then one side shall be light as normal by the primary sun, the other will be partially illuminated by the second star. This second star, in a night sky, will become quite prominent at the distances you talk about, basically being a mini sun in the sky. It will be far brighter than the moon, which may even appear duller due to illumination from behind when it is close to the second star. This would create a sort of twilight, I should think. Think of a particularly dull day, or a reasonably clear late evening. You can see, yes, but distances become hazy, colours more difficult to distinguish. A major factor here is the elliptical orbit of the stars while the planet circulates. If they are close together during the single daytime then the nights will be very bright indeed, as described above. While at their furthest points, the nighttimes on the planet will be considerably darker (though still lighter than our full moon; you’d not have too much trouble doing things at night still, but you might not want to try and navigate a walk through the woods). 

  For the appearance of the second star. Look up pictures of Jupiter, Mars and Venus and compare them to the moon for scale. Aside from giving out light, I’d think its size would be considerably bigger than the planets but quite a bit smaller than the moon. This is probably my shakiest conclusion, but I’ll stick out the neck and say perhaps a 10-20% size of the moon? That might be a way off. It’s as much gut instinct as any real scientific input. 

  Whoops, just found you’ve worked it out. The last paragraph is pointless now, but stream of consciousness style essays are best unedited, I sometimes think, so I’ll leave it in. If nothing else I feel happy I was around the right sort of ballpark. 

  Appearance-wise, I would think the second star will look more like the sun, in miniature, than the moon or stars.

  As for other things I have considered:

  The seasons will be right out of whack. During double daytime there will be a rather larger than normal input of heat into whichever hemisphere faces the suns, so a proportionately hotter summer and colder winter on the other side. During the single daytime, however, when they are at opposite sides, the effects of seasons will be more evened out by the lesser but present input of the second star to what should be the winter side of things. Again, this will be massively changed by the effect of the ellipticallity of the second stars orbit. At its closest (10AU, wasn’t it?) it will be quite a presence, noticeably contributing to double summers or cancelled out winters and giving severe cases of confusion to countless kinds of plant, animal, ocean current and wind or rain systems. At its furthest (30AU?) this will be dramatically lessened.

  A much lesser effect would be on the tides. Luckily, I think this would be very minor, perhaps moving them the odd few cm’s here and there, so it can be discounted. 

  A big one though, I think, is the effect on the visibility of planets and stars. All the other planets (if any), their moons and stars will be severely dimmed by the presence of the second star in the sky during the single daytime. This could have a religious and social impact as well, as all these were often a key part of things, and not seeing them half the year might have an effect.

  Errr… I think I’ve run out of contribution for now. A late night at the pub has perhaps lessened my abilities to help. Did that clear anything up? If it’s a bit vague or useless I can try to clarify.


----------



## goldhawk (Jul 24, 2011)

One thing about having two suns is that any slight difference in colour will be exaggerated.  This is because humans have colour invariance, which can have strange effects.  If only one sun is in the sky, it will appear yellow in a blue sky and all the colours will be normal.  If both are in the sky, they will appear different colours and the shadows they cast would be of different colours too but the colour of objects would be normal.  Oh, the sky would remain blue.

Their illumination of the moon would be strange: one side being one colour; the other side, the other; with a white patch between them.


----------



## Vertigo (Jul 24, 2011)

Excellent post Sapheron, I hadn't considered the effect of how the moon would be lit.

Metryq, that is one handy (if a little erratically laid out) website that is now in my bookmarks!

And Goldhawk I certainly hadn't considered the way our brains interpret colour and the impact on that of two different coloured light sources.

There's some neat stuff coming out of these threads


----------



## Metryq (Jul 24, 2011)

goldhawk said:


> One thing about having two suns is that any slight difference in colour will be exaggerated. This is because humans have colour invariance, which can have strange effects.



I don't understand what you mean by "color invariance." Anyone who has done a little photography (or videography) knows about color temperature. That is, the color of sunlight varies throughout the day due to the amount of atmosphere it passes through. Mankind also has a wide variety of light sources that run the gamut of the color spectrum. For example, incandescent lamps (those with a filament) and fluorescent lamps (gas discharge tubes) are both nominally "white" and used for illuminating homes and work places. Yet incandescent lamps tend to be yellower, while the gas lamps tend towards the green/blue end of the spectrum.

These color differences can be seen with the unaided eye, and without the two side-by-side for comparison. Since perception is in the brain and not the eye, many people do not notice the variance. However, photographers who do not compensate for these differences will produce images with an obvious color cast. I've run into people who cannot distinguish the color differences (sunlight, incandescent, gas, LED, etc.) when they are side-by-side and the color cast has been pointed out to them. Either these people are color blind, or simply not very discriminating. 

What the average human considers "white" was influenced by the star we evolved under. (Again, remember that sunlight varies in color temp over the day.) Bees evolved under the same star, yet they can see into the ultraviolet. 

Sometimes colors we can't see can affect the colors we do see. For example, many laundry detergents fluoresce (glow) under "blacklight" (ultraviolet). Grandma used to put "bluing" into the wash to offset the yellowing of white shirts. Detergents with fluorescing compounds achieve the same effect when the fabric is viewed in light with strong UV (sunlight or fluorescent lighting).

CCDs are inherently sensitive to infrared light, so they must be filtered. Some photographers remove the IR filter and produce bizarre images. Since plants reflect a lot of IR light, *landscapes look like snowscapes*. Even when filtered, a CCD camera can still "see" IR light directly. (Turn on your Web cam and point a TV remote at it.) Is this the way cats and other predators see the world?

(There is a big difference between IR photography and *thermography*.)

But back to human vision—humans can definitely see the differences in color temp. And traveling to another star will make the color differences very apparent. The videogame _Quake II_ is set on another planet under a K-type star. Inside the artificially lit buildings, the lighting appears "white," while the environment outdoors is very obviously orange. Wild.


----------



## goldhawk (Jul 24, 2011)

Colour invariance is when your brain assigns the same colour to different shades.  The dark brown of a branch in the shade becomes the same colour as the light brown one in the sunlight.  Your brain then joins the two together to form one branch.  A technique to determine what is a branch and what is not.  Very useful for when our ancestors were jumping from tree to tree.

Both suns would look yellow in a blue sky if they are there alone.  And the shadows they cast would be dark.  If both are up at the same time, they would look different colours, one more greenish, the other more orange and their shadows would be different too.  Only where they overlap would they just be darker.


----------



## Vertigo (Jul 24, 2011)

It has been shown that if you light something with red green and blue lights then, as you would expect you can see the correct colour. Now supposing there is a blue object being lit and you remove the blue light. A camera will no longer see a blue object and yet our eyes will. This is all down to how our brain manages to interpret the colour information from our eyes.

Consider a room lit by flourescent tubes and a room lit by incandescent bulbs. If you take photographs in each, one will have a green colour cast and the other a red one. We can detect that difference when we quickly move from one room to the other but very quickly our eyes cancel that colour cast and all seems normal to us.

I've not heard it called colour invariance but then I don't know what it is called. So despite the "wrong" colour sun being around and giving a cast to everything on the planet, we would simply not see that cast except where it is side by side as in the two different coloured shadows or two different colours on the different sides of the moon.


----------



## Metryq (Jul 24, 2011)

Vertigo said:


> We can detect that difference when we quickly move from one room to the other but very quickly our eyes cancel that colour cast and all seems normal to us.



"Normal" is a slippery term. Our brains learn to compensate. However, I can see the color temp of different light sources without comparing them side-by-side, and prolonged exposure does not diminish my ability to see the color casts. (This is both a blessing and a curse.) I can also see the "flicker" of fluorescent lights (at 60 Hz), but it does not bother me the way the red-blue strobing of PAL video does. If I had grown up in the UK, I would never have taken a career in video. Thankfully, the old Standard Definition formats are being replaced by digital video with very high refresh rates. Now if we can just ban interlacing altogether...


----------



## TheEndIsNigh (Jul 24, 2011)

Mithril:

Now I'm not a picky person: ask anyone, but I do think you are in danger of getting bogged down, or even worse in that respect.

You say you're writing fantasy. I don't now much about K2, G2 or the correct orbits for a binary system. To be frank I couldn't give a toss. Write the damn story. It's fantasy. If you say the sun is turquoise with yellow spots then that'll do for me.

To be even more frank, if I started to read the details you seem to want to include the book would probably end up executing a hyper parabolic:

 X^4 + 3*x*y - PI^-(i+x) = 0 curve into the bin.

Basically if someone wants to pull you up on the things you assert, then you will have sold the book to millions of people including every weirdo on the planet: so you should care if they want to moan about it.

Light another cigar, ask the young lady's/men (delete as required) to peel you another grape and another tequila sunrise/scotch on the rocks, and laugh all the way to the bank.


----------



## goldhawk (Jul 24, 2011)

TheEndIsNigh said:


> Now I'm not a picky person: ask anyone, but I do think you are in danger of getting bogged down, or even worse in that respect.



In other words, A Wizard Did It.


----------



## mithril (Jul 25, 2011)

Oh wow... So many responses over thew weekend. It might take more than a single post to reply...

Here goes..

*Tien* - Thanks for the heads up. I know reading the details make it seem like i'm in the danger of getting bogged down. However I'm not going to be including most of these in the actual stories/novel. I come from an engineering background so it just helps me if I have the details written down in the shortest and most succinct form I know  And I've noticed that when you want answers, it's better to give all the info you have rather than not give enough. This is all a hypothetical situation and I _will_ change and simplify the details to suit me and my story. Nothing's set in stone. But this way I do get to see ideas I'd never have considered and complications I wouldn't have thought about. I know that it's totally up to me to include them or not. Everything need not line up to a T and I will definitely keep your advice in mind about writing the best story I can and not worry about nit-pickers who want to find faults in something or the other.

*Goldhawk* - That was a fascinating link  I did not know they had so many terms for such things.

*Metryq *- Again great link. 

*Metryq *and *Goldhawk *- Unfortunately I haven't really been able to understand your discussion about color invariance. So picking up the information that seems most relevant to me for the world, (I hope that's ok) would it be correct to say that if both stars are visible during the day, they would both seem slightly differently coloured, however if only one of them is in the sky it'll seem white? That sounds interesting


----------



## mithril (Jul 25, 2011)

Thanks a lot Sapheron for taking the time and the effort to reply. You've raised certain very interesting points. I shall have to decide exactly how complex I want to make it soon..



Sapheron said:


> I’m not going into statistics and calculations because, to be perfectly honest with you, that would be a waste of time.



That is totally okay  Just gave the numbers so that if someone wanted to play with hard facts, they could.



Sapheron said:


> During the ‘double daytime’ when the stars line up the moon will pass through phases identical to those on real Earth. As the stars move more out of line, these become skewed, with more moon being lit up more of the time.
> 
> An interesting aside is that if the moon eclipses the first star while  the second is also in the sky then the second star will briefly brighten  up and become dominant, and if at an angle it could even mean that the  moon is also visible while it eclipses the sun. In the same way, during  the ‘single daytime’ the second sun could illuminate the moon from the  other side, meaning the moon is even fully visible while it eclipses the  primary star. Could make for a very interesting event, and would  definitely be included in primitive culture and religion, I should  think. An even more spectacular (albeit very rare) event would be if all  three bodies lined up, giving the occupants of the planet the  exceedingly rare experience of being unable to see either star or the  moon.



The effect of the secondary star on the phases of the moon was something I hadn't even considered... Ditto for the idea that the moon itself might be visible when it eclipses the primary sun if the secondary is still there in the sky...



Sapheron said:


> A major factor here is the elliptical orbit of the stars while the planet circulates. If they are close together during the single daytime then the nights will be very bright indeed, as described above.



Unfortunately didn't really understand this part much...



Sapheron said:


> The seasons will be right out of whack. During double daytime there will be a rather larger than normal input of heat into whichever hemisphere faces the suns, so a proportionately hotter summer and colder winter on the other side. During the single daytime, however, when they are at opposite sides, the effects of seasons will be more evened out by the lesser but present input of the second star to what should be the winter side of things.
> 
> Again, this will be massively changed by the effect of the  ellipticallity of the second stars orbit. At its closest (10AU, wasn’t  it?) it will be quite a presence, noticeably contributing to double  summers or cancelled out winters and giving severe cases of confusion to  countless kinds of plant, animal, ocean current and wind or rain  systems. At its furthest (30AU?) this will be dramatically lessened.



I was under the impression that the amount of heat received due to the secondary sun would be in proportion to its apparent magnitude. ie, at its closest, it should be supplying roughly 1/200 of the sun's heat, and 1/2700 at its farthest. If this is correct, could we say that during the times of the year when there was double daytime, the summers and winters would be slightly harsher and the part of the year with single daytime, they would be slightly milder. However the difference should not be so extreme as to cancel out winters entirely and make it seem like mid spring/autumn? If so I'll need to increase the closest approach of the secondary star from 10 AU to maybe 15 




Sapheron said:


> A big one though, I think, is the effect on the visibility of planets and stars. All the other planets (if any), their moons and stars will be severely dimmed by the presence of the second star in the sky during the single daytime. This could have a religious and social impact as well, as all these were often a key part of things, and not seeing them half the year might have an effect.



Thanks for clearing that up as well as suggesting that the single daytime nights might be as bright as late evenings... And it's a good thing tidal effects from the secondary can be ignored.

You've given me quite a lot of material to think over  Thanks heaps


----------



## Metryq (Jul 25, 2011)

mithril said:


> would it be correct to say that if both stars are visible during the day, they would both seem slightly differently coloured, however if only one of them is in the sky it'll seem white? That sounds interesting



Mid-day both stars would appear "white" simply because they'd be too bright to look at directly. When lower in the sky, or when the sky is hazy, the stars might appear to have a color. 

The environment would look "white" to natives because their eyes are adapted to their primary sun. I don't know how much light the secondary would give; perhaps as much as a full moon? Granted, our Moon has an albedo of only 0.11, but the distance might counter the brightness. (How bright would Jupiter or Saturn be as "lesser" suns farther from Earth than Sol, even at opposition?)

Assuming it is only about as bright as a full moon on Earth—which can be bright enough to cast shadows—would the environment really have a different color? The secondary star in the sky "at night" might have a more pronounced color, but would the locals see color at night any more than we do? At best, there might be partial color, as during our twilight. Locals might be able to see shadows of different colors towards primary sunset. (The primary shadow might be orangish, and the secondary shadow might look bluish—assuming the usual atmospheric effects didn't make the primary sunset yellower. In that case, there might be double shadows, but they'd be nearly the same color.)

Assume too much photo flux, and it will begin to change the weather.


----------



## goldhawk (Jul 25, 2011)

mithril said:


> *Goldhawk* - That was a fascinating link  I did not know they had so many terms for such things.



Bah, ah, ah, ah.  Trapped another one.  



mithril said:


> *Metryq *and *Goldhawk *- Unfortunately I haven't really been able to understand your discussion about color invariance. So picking up the information that seems most relevant to me for the world, (I hope that's ok) would it be correct to say that if both stars are visible during the day, they would both seem slightly differently coloured, however if only one of them is in the sky it'll seem white? That sounds interesting



In a blue sky, they would appear yellow, just like our sun.  Our sun is actually white but the atmosphere diverts the blue rays away from it to other parts of the sky.  When you look at the sky, you see the blue rays from the sun that have been diverted by that part of the sky, way over there.


----------



## mithril (Jul 29, 2011)

Thanks Metryq, goldhawk. It's miuch clearer now.


----------

