World-building for fantasy story

[snorkack]

Weather will be fairly extreme.

Why?

With two suns and a gas giant nearby, there will be some complex and intense seasonal patterns.

Complex, sure. Intense, why?

If the moon goes into the shadow of the gas giant, it will get cold and dark. This might replace the concept of night, but it could last much longer.

No, it couldn´t. Night is necessarily half the orbit. Eclipse necessarily less than half. Can be much less than that. Moon has a night of half a month (360 hours), but total lunar eclipse is under three hours.

If the rotation is tidally locked, then the side facing the gas giant will be in shadow most of the time.

Literally, yes. Half the time is night, i. e. shadow of ground, and a small fraction of the rest is eclipse/shadow of the gas giant, so a slight majority of time is in shado of something and not in direct sunlight.

But with two suns, you'll have periods illuminated by one sun but not the other, and periods of illumination by both or neither.

And illumination by the gas giant. So you have 3 illumination sources.

It will be much more extreme than Earth.

No. More complex, but why more extreme?

 

[Czcibor]

Why?

Complex, sure. Intense, why?

No. More complex, but why more extreme?

Two suns should make the climate a bit complex.
But one risk getting the climate extreme if the moon orbit is not very short.

No, it couldn´t. Night is necessarily half the orbit. Eclipse necessarily less than half. Can be much less than that. Moon has a night of half a month (360 hours), but total lunar eclipse is under three hours.

Literally, yes. Half the time is night, i. e. shadow of ground, and a small fraction of the rest is eclipse/shadow of the gas giant, so a slight majority of time is in shado of something and not in direct sunlight.

One may play here with plains. In perfect case you may have eclipses each time. In more typical case - not often.

It's possible to have an eclipse each midday or a two eclipse seasons, pending on angle

And illumination by the gas giant. So you have 3 illumination sources.

For heat purposes one may ignore one source. Just for story reasons there would be light enough to see something during night because of reflected light.

 

[snorkack]

Two suns should make the climate a bit complex.
But one risk getting the climate extreme if the moon orbit is not very short.

True. Long orbit means long nights.

One may play here with plains. In perfect case you may have eclipses each time. In more typical case - not often.

It's possible to have an eclipse each midday or a two eclipse seasons, pending on angle

Since the moon´s axis of rotation is necessarily aligned with its orbital plane, it ties into the moon having or not having season. If the moon has darkness every day, it likely does not have seasons either. If the moon has seasons then there are necessarily two eclipse seasons, namely spring and autumn.

For heat purposes one may ignore one source. Just for story reasons there would be light enough to see something during night because of reflected light.

Oh, sure.
Earth has Moon.
Full Moon is an appreciable source of light. A night lit by full Moon is hundreds of times brighter than a moonless night lit by stars alone. Man sees not only something in moonlight, but also possesses some colour vision. The sky is noticeably brighter in full moonlight... though its colour (objectively blue) is not visible.
Earth´s rotation is not locked to Moon, so Moon rises and sets. It´s not visible through all nights.
Moon is locked to Earth. For an observer on the near side of Moon, Earth is always in sky, by day and throughout night.
And since Earth is reflecting sunlight, the phases of Earth are fixed to visibility of Sun. At midnight (new Moon), Earth is necessarily full. At sunrise (half Moon) Earth is necessarily half. Earth does wane to crescent... but only by day. Nights on Moon near side are always illuminated by full or at least gibbous Earth.

Now, Earth is bigger than Moon. Almost 4 times by length. Roughly 14 times by area.

And Moon is black. Earth is about 3 times more reflective than even full Moon.
In total, full Earth is estimated at 43 times brighter than full Moon.

Also, Moon fades dramatically when not full. Half moon shows half the illuminated surface of full Moon - yet only 1/9 of the light!

Earth does not fade so badly. Half Earth should be slightly less than half the brightness of full Earth (who knows exactly how much?) but still much brighter than full Moon.

In terms of lux, the illumination of full Moon is about 0,27 lux. Illumination of full Earth should then be about 12 lux.

For comparison, the suggested targets for street lighting seem to be 5...10 lux.

When you build a town in the middle of near side of Moon, you can save money by omitting streetlights. Earth will provide that level of lighting all night. You would still need lamps inside the homes and on working tables, though.

 

[Czcibor]

Since the moon´s axis of rotation is necessarily aligned with its orbital plane, it ties into the moon having or not having season. If the moon has darkness every day, it likely does not have seasons either. If the moon has seasons then there are necessarily two eclipse seasons, namely spring and autumn.

Yes. Assuming of course that orbit of the gas giant is not seriously eccentric. In such case there would be another source of seasons. (I'm not sure how to calculate that)

I wonder that whether frequent and long eclipses in midday would have some impact on one of hemispheres.

When I think about it if the planet is supposed to be Jupiter mass, then it should be bigger. Why? Jupiter contracts because of loosing heat, while it this case the planet would get more heat from its stars so should contract slower. (or reach an equilibrium?)

Oh, sure.
Earth has Moon.
Full Moon is an appreciable source of light. A night lit by full Moon is hundreds of times brighter than a moonless night lit by stars alone. Man sees not only something in moonlight, but also possesses some colour vision. The sky is noticeably brighter in full moonlight... though its colour (objectively blue) is not visible.
Earth´s rotation is not locked to Moon, so Moon rises and sets. It´s not visible through all nights.
Moon is locked to Earth. For an observer on the near side of Moon, Earth is always in sky, by day and throughout night.
And since Earth is reflecting sunlight, the phases of Earth are fixed to visibility of Sun. At midnight (new Moon), Earth is necessarily full. At sunrise (half Moon) Earth is necessarily half. Earth does wane to crescent... but only by day. Nights on Moon near side are always illuminated by full or at least gibbous Earth.

Now, Earth is bigger than Moon. Almost 4 times by length. Roughly 14 times by area.

And Moon is black. Earth is about 3 times more reflective than even full Moon.
In total, full Earth is estimated at 43 times brighter than full Moon.

Also, Moon fades dramatically when not full. Half moon shows half the illuminated surface of full Moon - yet only 1/9 of the light!

Earth does not fade so badly. Half Earth should be slightly less than half the brightness of full Earth (who knows exactly how much?) but still much brighter than full Moon.

In terms of lux, the illumination of full Moon is about 0,27 lux. Illumination of full Earth should then be about 12 lux.

For comparison, the suggested targets for street lighting seem to be 5...10 lux.

When you build a town in the middle of near side of Moon, you can save money by omitting streetlights. Earth will provide that level of lighting all night. You would still need lamps inside the homes and on working tables, though.

Such moon would get light from gas giant. (Jupiter - 11 times Earth diameter) So local civilization after inventing fire to survive long nights would need much more time to invent candles? :D

 

[snorkack]

Such moon would get light from gas giant. (Jupiter - 11 times Earth diameter) So local civilization after inventing fire to survive long nights would need much more time to invent candles? :D

Man depends on fire even in broad daylight, because man is an omnivore whose guts rely on good quality cooked food.

If they have cold nights or winters and need fire to warm themselves, they have a reason to invent lamps anyway. Lamps are easier to invent than clear window glass.

 

[Xyooj]

I've been wondering why so many wants to go into other planets when we still have so much to do to improve Earth and our own part of the universe. why not a story about building Earth in your vision, then we have nine more planets in our solar system to your world-building imagination, plus countless moons, etc?

if you want it to be a game, perhaps try Simcountry, http://www.simcountry.com/cgi-bin/cgip?ad&miSite=promo231142
I've played this game for awhile now, there are still lots of things I think could be done better, but the owner don't ever listen to anyone.

personally, I think the Earth is the perfect place to start a world-building alternative version :)

 

[|Glitch|]

Thanks for the quick reply. Using Astrosynthesis v2.0, I get the following information:

Multistar system
Star 1: Mass 0.55, Radius 0.62, Luminosity 0.13 sols (all relative to our sun)
Star 2: Mass 1.57, Radius 1.44, Luminosity 4.8 sols (all relative to our sun)
Total of 12 primary satellites, with the gas giant in question the 5th planet

Gas Giant
Distance from Star 2 (assuming it orbits only one star) 125,695,818km
Eccentricity of orbit 0.18
Inclination (in degrees) 1.75
Ascending node (deg) 50.31
Periapsis Angle (deg) 34.78
Time past Periapsis (days) 7

Moon
Distance from gas giant 1,252,540km
Radius 6,504km
Gravity 1.02g
Retrograde orbit
Rotation 28hrs
Eccentricity of orbit 0.0185
Inclination (deg) 5.36
Ascending node (deg) 76.34
Periapsis angle (deg) 107.09
Time past Periapsis (days) 0

Using your values I was able to calculate a few things that may interest you.

Star 1 would have an effective surface temperature of 4,406°K, which would make the star a spectral Type K star. Not factoring in albedo or radiative forcing, the habitable zone range for Star 1 would be between 0.200 and 0.375 AU.

Star 2 would have an effective surface temperature of 7,127°K, which would make the star a spectral Type F star. The habitable zone for Star 2 would be between 1.22 and 2.28 AU.

You mentioned in your original post that you wanted both stars to be in a close binary orbit, but you did not specify the distance between the two stars. Since both stars orbit their common center of mass (barycenter), they would have to be in a fairly close orbit in order to keep the habitable zone relatively stable.

The distance you placed the gas giant (125,695,818 km) is only 0.84 AU from Star 2. That is far too close, and would give the gas giant (and any moons in orbit around the gas giant) a mean surface temperature closer to Mercury or Venus than Earth. The gas giant should have a semi-major axis closer to ~261,792,290 km (1.75 AU) from Star 2. At 1.75 AU from Star 2, a black body object would have a mean surface temperature of ~50°C. While that is well within the liquid water range, that is too hot for complex life to exist. Ideally, the mean surface temperature should be half that or slightly less, somewhere between 20°C and 25°C (based upon Earth's life forms). You can accomplish this by: 1) moving the gas giant further away from the barycenter of the two stars; or 2) significantly increasing the albedo of the moon; or 3) a combination of the two - moving the gas giant further away and increasing the albedo of the moon.

My calculations did not factor in albedo or radiative forcing. Depending on the chemical composition and density of the atmosphere on the moon, the greenhouse gases in the atmosphere may also add a significant amount of warming. On Earth, the amount of sunlight reaching the planet is 1,367 W/m² with 1,050 W/m² of direct sunlight reaching the surface of the planet, and another 70 W/m² of indirect sunlight also reaching the surface for a total of 1,120 W/m². Radiative forcing due to the greenhouse gases on Earth accounts for an additional ~72 W/m².

Obtaining a retrograde orbit of the moon would be extremely difficult to accomplish. There is only one moon that has achieved hydrostatic equilibrium with a retrograde orbit in our solar system - Triton. Nobody is sure how Triton obtained its retrograde orbit around Neptune. Triton is spectroscopically similar to Pluto, and it is therefore thought that Triton may have been a captured Kuiper Belt object. A naturally forming moon, made of the same accretion disk that formed the gas giant, should not have a retrograde orbit.

In order to determine the orbital characteristics of the moon, you will need to include the mass of the gas giant. The moon's distance of 1,252,540 km from the gas giant would most likely mean that the moon is tidally locked to the gas giant (similar to the moon Triton), but without knowing the mass of the gas giant that is impossible to determine. Keep in mind that the gas giant mass must be less than ~14 Jupiter masses, otherwise it will start fusing deuterium and be considered a brown dwarf star and no longer a planet.