Living in a earth-like moon orbiting a planet

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MrY
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Living in a earth-like moon orbiting a planet

Postby MrY » Fri Jul 26, 2013 10:55 am UTC

Hi everybody !

What are the implications of living in a earth-like moon orbiting a planet?

This question makes me dream. I looked into it myself but I really want to have your expertise!

About the day cycle: With tidal locking, the moon has to turn around the planet in 24h.
Also, the planet keeps the same location in the sky. So, for inhabitants of the inner face, they have a solar eclipse at the same hour every day.
As a result, the inner face spend less time in the sun than the other face, so we can imagine that winter occurs in the inner face and summer in the other face.
With hot air and cold air movement, maybe the two sides are separated by giant storms.

And what about the tidal heating? A high volcanic activity? Islands which emerge?

Maybe even other phenomena?
Last edited by MrY on Wed Jul 31, 2013 9:32 pm UTC, edited 3 times in total.

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EchoRomulus
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Re: Living in a earth-like moon orbiting a gas giant

Postby EchoRomulus » Wed Jul 31, 2013 1:18 am UTC

There would be two kinds of night.

The kind where the side you are on faces away from the sun

And the eclipsing of the giant which depending on the size and distance from the star, can last a long time aswell.

EDIT: If the planet is in a synchronous orbit with the giant, then the side facing away will have regular half and half sun and day, while the other side will only get sun once a day, for a few hours as the planet forms a 90 degree angle with the star as the giant as the vertex.
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LaserGuy
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Re: Living in a earth-like moon orbiting a gas giant

Postby LaserGuy » Wed Jul 31, 2013 6:02 am UTC

Tidal-induced volcanism is a possibility. There are several (about five, I think?) moons in the solar system that are volcanically active. Triton, for example, has liquid nitrogen volcanoes... In our current solar system, the moons of gas giants don't get enough tidal heating for livable surface temperatures, but could have habitable temperatures below ground level. Volcanism may be connected to lunar atmospheres... very unclear at this point.

One thing that would be interesting would be the skyline. On Jupiter's moon Io, for example, Jupiter spans about 20 degrees of arc. For comparison, the Sun takes up about half a degree on Earth. Jupiter would completely dominate the sky. When the moon is eclipsed by the planet, it would likely be near-total darkness at this distance. If sentient life somehow managed to evolve there, I'd expect that the gas giant would play a major, major part in their religions or mythologies.

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Re: Living in a earth-like moon orbiting a gas giant

Postby davidstarlingm » Wed Jul 31, 2013 12:38 pm UTC

I can imagine evolution taking two almost completely different paths on the inside and outside face of the moon. The transition shouldn't be impassable, but it would be noticeable. It's possible that two sentient species could evolve on opposite sides of the moon, capable of partially (but not entirely) sharing the same ecosystem, having some experiences in common but not others, etc.

Both sides would view the other side as an opportunity for conquest, obviously.

The near side would actually have a three-way cycle. When the moon eclipsed the sun with respect to the gas giant, the inhabitants would be experiencing a long twilight, as the light reflected off the gas giant would be quite bright. When the gas giant eclipsed the sun, it would be a very, very black night. Depending on which half of the near face you lived on, you'd have a glimpse of the sun for quite a while either on the incoming or outgoing leg of the orbit.

What would astronomy look like? The near side would not be able to make very many observations at all, I don't think.

I can foresee a very fantastic sort of setting, where one side of the moon was pretty normal and Earthlike, while the other side was dark and foreboding and mythical.

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Re: Living in a earth-like moon orbiting a gas giant

Postby Copper Bezel » Wed Jul 31, 2013 1:43 pm UTC

I'd think that the ecological differences between hemispheres would still be less than those between the equator and somewhere near the pole. Separate sapient species developing and conquering each other I'm not so sure about - I think you might need more separation than that - like, say, the biomes that produced us and dolphins = ] - and it still requires a big coincidence in timing. But it could be fun.

Good call on the gigantic perpetual full moon. That could well be at least as significant as the daily eclipse. Again, just how dominant those are on ecology is going to depend on how close the moon is to the planet.
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Re: Living in a earth-like moon orbiting a gas giant

Postby Schrollini » Wed Jul 31, 2013 2:26 pm UTC

davidstarlingm wrote:I can imagine evolution taking two almost completely different paths on the inside and outside face of the moon. The transition shouldn't be impassable, but it would be noticeable. It's possible that two sentient species could evolve on opposite sides of the moon, capable of partially (but not entirely) sharing the same ecosystem, having some experiences in common but not others, etc.

Both sides would view the other side as an opportunity for conquest, obviously.


This was a plot point in the novel Encounter with Tiber, by Buzz Aldrin (yes, him!) and John Barnes. But the two sides were separated by a global ocean, which played a significant role in separating the biomes. One point of trivia, scientists living on the far side of the moon got rather confused when trying to calculate the size of their world. Surveying gave a small number; parallax between sunrise and sunset gave a much larger number, since it was actually measuring the orbital diameter.

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Re: Living in a earth-like moon orbiting a gas giant

Postby EchoRomulus » Wed Jul 31, 2013 3:23 pm UTC

Two sentient species probably could not evolve together without a mountain chain or some serious geographic barrier. Competition would cause one to destroy the other in their equivalent of the stone age, like how we killed the Neanderthals.
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Re: Living in a earth-like moon orbiting a gas giant

Postby Xanthir » Wed Jul 31, 2013 3:31 pm UTC

Hmm. Using LaserGuy's numbers for Jupiter and IO, you'll get a daily eclipse that lasts about 10% of the day, right in the middle of the day (the hottest part). It would be in the hottest portion of the day, so you'll definitely get some significant cooling effects, and it would probably encourage crepuscular (morning/evening) life over diurnal.

I don't feel like doing the necessary math to figure out how much less solar energy you'd get, and what effect that'd have on temperatures (assuming that the far side ended up with roughly Earth-like amounts of sun).
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Re: Living in a earth-like moon orbiting a gas giant

Postby Copper Bezel » Wed Jul 31, 2013 4:08 pm UTC

It won't be much - equivalent to a few degrees of latitude. It'd be enough to make for some interesting equivalents to Hadley cells, though, like MrY noted.

EchoRomulus wrote:Two sentient species probably could not evolve together without a mountain chain or some serious geographic barrier. Competition would cause one to destroy the other in their equivalent of the stone age, like how we killed the Neanderthals.

Current theory is that we bred them out - our gene pools separated, some divergence happened, our gene pools rejoined in one geographic area, and selection happened. If the two species are too genetically diverged to interbreed, that doesn't happen. There's also only competition if both species are drawing on the same resources. I don't think sapience is a "niche."
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Re: Living in a earth-like moon orbiting a gas giant

Postby davidstarlingm » Wed Jul 31, 2013 6:07 pm UTC

Copper Bezel wrote:Good call on the gigantic perpetual full moon. That could well be at least as significant as the daily eclipse. Again, just how dominant those are on ecology is going to depend on how close the moon is to the planet.

For the purposes of speculation, of course we want the most dramatic effects possible. :D

It's actually quite complicated. Because the planet is not a point source, it will likely be visible from more than 50% of the moon at any time. The image provided is not to scale, but....yeah.

Image

There are actually six distinctly different lighting conditions possible. "Day" would include a period of highlight (light from both the star and the planet) as well as daylight (light from the star only) and twilight (light from the planet only). There would be two kinds of night; the outer hemisphere would have a normal night, whereas the inner hemisphere would have a "darknight" where the planet shadows you from even extrasolar light.

And because you're tidally locked, not every place on the moon can experience all these lighting conditions. The following image shows each point in the circuit from the perspective of an observer on the moon (outside half is up; inside half is down).

Image

The outside hemisphere experiences a fairly normal day-night cycle. The inside hemisphere does not. Closest to the planet, you would get a 5-cycle: twilight-highlight-daylight-darknight-daylight-highlight-twilight. On the inside hemisphere but not so close to the planet, you get twilight-darknight-daylight-highlight-twilight, reversed on the opposite side.

Quite complex; quite interesting. Four different zones, two of which are always mirrored, with six different possible light settings.

In the zone nearest to the planet, you would never be able to observe anything other than a narrow sliver of stars outside your solar system, because the planet eclipses most of the sky; moreover, the rest of the sky would only be sufficiently dark for a very short period.

The mirrored zones would have a better view of the sky during the eclipse.

The inner hemisphere would actually receive more light than the outer hemisphere, for brief periods. The brief period of highlight would be very close to the darknight period. Here's what a day would feel like on the inside of the planet.

  • Nearest the planet: The sun's emergence from behind the planet would be followed by the illumination of the planet, making the late morning very bright. The sun would set shortly after, making most of the day dim. The sun would rise again on the opposite horizon shortly before nightfall, making the end of the day very bright again; then the planet would become dark just before the sun disappeared behind the planet, plunging the world into night. Cosmological myths would probably have something to do with the big planet god "spitting out" the small bright sun god in the morning (which flees in terror into the underworld), then glowing with its light for most of the day before darkening and pulling the star out of the underworld to swallow again.
  • From 180-240: The planet would begin to shine for most of the day. Toward late afternoon, the sun would rise, shining more brightly into late evening; the planet would darken just before the sun disappeared behind it, entering a long night. Cosmological myths would focus on how the big planet god grows in power until the small sun god comes and conquers it, but is ultimately defeated when the planet god swallows it up and begins again.
  • From 300-0: The sun would emerge from behind the planet in morning, and then the planet would begin to grow, making the first part of the day very bright. Later, the sun would set, and the planetshine would continue until it dimmed at nightfall. This myth would likely be a very straightforward one, with the big planet god spitting out the small sun god every morning, which sets it on fire shortly thereafter, but dying after the sun god has disappeared into the underworld.

I'd think that the ecological differences between hemispheres would still be less than those between the equator and somewhere near the pole. Separate sapient species developing and conquering each other I'm not so sure about.

A single species would have to develop a very basic sapience, enough to become the dominant species on the moon, and spread out across it. Then, the species on the near side would specialize while the species on the far side did the same.

As far as seasons are concerned, an elliptical orbit for the planet would be the easiest way to pull that off. However, this means that there's no summer/winter balance; when it's winter, it's winter EVERYWHERE. This would mean much more drastic temperature differences. And the year need not be ~300 days; it could be longer or shorter.

The moon's orbit gives a day/night analogue; the eccentricity of the star's orbit gives a year/season analogue. Any variation that would give rise to something similar to months? (Anything from 10 days to a fifth of a year).

It should be noted that temperature differences are going to be a bigger deal because of the tidal locking. On Earth, it's always night when it's day on the other side; it's always summer when it's winter on the other side. Here, it's day everywhere or night everywhere; summer everywhere or winter everywhere. Not sure what that would do to weather.

Current theory is that we bred them out - our gene pools separated, some divergence happened, our gene pools rejoined in one geographic area, and selection happened. If the two species are too genetically diverged to interbreed, that doesn't happen. There's also only competition if both species are drawing on the same resources. I don't think sapience is a "niche."

Volcanism would be more intense on the near side of the planet, no? So lots of resources associated with volcanism would be available on the near side, making it a tempting destination for the farsiders.

Definitely a serious yin-yang thing with the dark side and the light side. What divergent adaptations would be likely?

Schrollini wrote:One point of trivia, scientists living on the far side of the moon got rather confused when trying to calculate the size of their world. Surveying gave a small number; parallax between sunrise and sunset gave a much larger number, since it was actually measuring the orbital diameter.

Haha! That's awesome. The lack of stars would probably cause problems for the near side of the world; with only a very narrow window and slice of the sky to survey, they wouldn't be able to make good star charts. You'd have better luck closer to the poles, but it would also be much colder closer to the poles. Science would not advance very quickly for the nearsiders.

Being closer to the hemisphere boundary would probably lead to rapid advances in cosmology. However, I doubt many would venture across it. The farsiders would notice that the sun rises and sets lower and lower, so they'd be loath to go to a place where it might not rise at all. The nearsiders would want to avoid venturing out of sight of the planet, since they are used to it providing them with most of their light.

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Re: Living in a earth-like moon orbiting a gas giant

Postby MrY » Wed Jul 31, 2013 7:49 pm UTC

Agree for two kinds of night!

I find this amazing video:
https://www.youtube.com/watch?v=pN7P1zZKlMM

About the tidal heating, is appearing when the moon orbit has an eccentricity (ground elevation of 100m for Io!).
I read that eccentricity is created by orbital resonance with other moons.
So tidal are when the planet is at apoapis?

But if there are other moons in resonance, at some times, the giant and moons are aligned.
I don't know if this alignment of moons is necessarily at periapsis or apoapsis.
Because if not, we have two periods of tidal effect (at the apoapis and when moons are aligned), creating small and big ground elevation when added!

I think the apparition of two predominant species is possible, especially if each side have its continent, not a connected territory.
One species with normal day like on earth, and the other with the view of the giant and an eclipse every day ...

For the astronomy, on the contrary!
For me, the more the sky is dynamic, the more the species want to understand the motion of the planets and stars!
For example, a solar system with only one inhabited planet shouldn't inspire a lot its dominant species because except the sun, nothing moves.

About the barrier, i'm interested to have some number about the power of storms for a temperature differential between the two sides.
The differential is the impact of an everyday eclipse on temperature.
I want to know if it's like "horse attitude" or perpetual huricanes?

So, we have an earth-like moon, the day must be 24 hours for correct temperature (at least on the outer side!).
With tidal locking, the moon must orbiting the giant in 24 hours.
The orbital period is related to the mass of the two planets and their distance.
Because the earth-like hypothesis, the moon mass shall be that of the Earth.
So, I have just an equation between the mass of the giant and the distance.
The mass comes into play in determining the diameter of the giant, which is seen smaller in the distance.
So the giant spans less degrees in the sky, and reduce the time of the eclipse.
a smaller eclipse implies a smaller temperature differential.

Who love math? :D
With one variable, we have all the others!!
(mass of giant, distance, degree in sky, time of eclipse, temperature differential)

(davidstarlingm, I will respond you immediately after reading in detail your post ;) )

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Re: Living in a earth-like moon orbiting a gas giant

Postby PolakoVoador » Wed Jul 31, 2013 8:00 pm UTC

Is it possible for the moon to not be tidal-locked to the planet? There would be even crazier night-day cycles, I guess.

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Re: Living in a earth-like moon orbiting a gas giant

Postby davidstarlingm » Wed Jul 31, 2013 9:09 pm UTC

MrY wrote:Agree for two kinds of night!

I find this amazing video:
https://www.youtube.com/watch?v=pN7P1zZKlMM

Okay, wow. That IS amazing. I loved the triple-layer eclipse. Pure brilliance and beauty.

In the version I had, the eclipse lasted a lot longer due to the moon being much closer to the gas giant, but we'll see whether that's feasible or not.

I think the apparition of two predominant species is possible, especially if each side have its continent, not a connected territory.
One species with normal day like on earth, and the other with the view of the giant and an eclipse every day ...

I don't know if any natural geographic features would form as a result of the tidal locking, or if such natural barriers would necessarily be coincidental. Regardless, I'd still argue that there would be a powerful psychological barrier once societies had formed. You want a divergence so that the two competing superspecies are related-yet-specieated.

For the astronomy, on the contrary!
For me, the more the sky is dynamic, the more the species want to understand the motion of the planets and stars!
For example, a solar system with only one inhabited planet shouldn't inspire a lot its dominant species because except the sun, nothing moves.

Well, our solar system only has one inhabited planet, and there's more moving than just the sun. ;)

But the question is how much of the sky is visible during the daily eclipse. In the video you showed, the vast majority of the sky is visible, providing unparalleled visibility. It's particularly neat because the whole moon is eclipsed, meaning that there's no pesky Rayleigh scattering to muddle things up. On Earth, a total solar eclipse doesn't obscure enough of the planet to make it really really dark.

In my example, so little of the sky was visible during the eclipse (particularly from the point nearest the gas giant) that observation is very limited, making astronomy more difficult.

i'm interested to have some number about the power of storms for a temperature differential between the two sides.

Eh. I don't think there would be a significant temperature differential. When it's hot, it's hot on both sides; when it's cool, it's cool on both sides. Any differential would be too gradual to produce noticeable weather, I think. Though I could be wrong.

So, we have an earth-like moon, the day must be 24 hours for correct temperature (at least on the outer side!).
With tidal locking, the moon must orbiting the giant in 24 hours.
With one variable, we have all the others!!
(mass of giant, distance, degree in sky, time of eclipse, temperature differential)

The only thing we really know so far is the mass and orbital period of the moon. How dramatic do you want the eclipse to be? The closer the moon is to the planet, the brighter the planet will be at "night" and the longer the eclipse will be.

The mass comes into play in determining the diameter of the giant, which is seen smaller in the distance.

Keep in mind that a gas giant will be about the same diameter as Jupiter regardless of its mass. Obviously, though, a different mass will mean a different orbit, which means a different angular diameter.

PolakoVoador wrote:Is it possible for the moon to not be tidal-locked to the planet? There would be even crazier night-day cycles, I guess.

I don't know whether it's possible. But yeah, there would be pretty crazy cycles. But how long is the orbit and how long is the rotation? Are they tied together in any way? If the orbit is much longer than the rotation, as with the Earth-Moon system, you'd have different "day" profile each day of the month. If they're close to the same period, things will be wack.

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Re: Living in a earth-like moon orbiting a gas giant

Postby LaserGuy » Wed Jul 31, 2013 9:12 pm UTC

PolakoVoador wrote:Is it possible for the moon to not be tidal-locked to the planet? There would be even crazier night-day cycles, I guess.


For a gas giant, not terribly likely, I don't think. Unless the moon was way farther away than what we're used to seeing. Or very early in its history, I suppose. Or maybe if it was one of those gas giants that is quite close to the star?

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Re: Living in a earth-like moon orbiting a gas giant

Postby MrY » Wed Jul 31, 2013 9:30 pm UTC

I modify my initial question:

What are the implications of living in a earth-like moon orbiting a planet?

So now the planet is not necessarily giant or exclusively made of gas!

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Re: Living in a earth-like moon orbiting a gas giant

Postby davidstarlingm » Wed Jul 31, 2013 9:36 pm UTC

MrY wrote:I modify my initial question:

What are the implications of living in a earth-like moon orbiting a planet?

So the planet is not necessarily giant or exclusively made of gas!

If it's Earth-like, then it won't be a moon unless it's orbiting something a great deal larger than itself. And anything much more than ten times the mass of Earth is going to end up turning into a gas giant (there are exceptions, but none of them are conducive to a situation where it could be in a stellar habitable zone).

The question of life on a binary planet pair is an interesting one, but it's not quite the same as the earth-like moon question.

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Re: Living in a earth-like moon orbiting a planet

Postby DanD » Wed Jul 31, 2013 9:56 pm UTC

I agree with all of the above with respect to lighting, but there is a caveat on thermal energy (and lighting if the inhabitants see well into the infrared). Jupiter is hot. It apparently radiates more of it's own energy than it takes in from the sun, largely due to adiabatic contraction. I can't find hard numbers, but don't assume the inner hemisphere is going to get less thermal energy just because it gets less sunlight.

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Re: Living in a earth-like moon orbiting a planet

Postby MrY » Thu Aug 01, 2013 7:30 pm UTC

To avoid misunderstanding, we need names!
I suggest calling the giant (more or less huge and gaseous or not) Alpha, and our earth-like moon Beta.
For your posts you can propose your own names!

I didn't think that Alpha could sent back so much light.
This depends mainly on its size in the sky and its albedo I guess.
Here you're talking about 50% of the sky anyway!
It is really notable? I can't imagine a more enlightened sky!

I like your analysis, and your mythological interpretations :o

For seasons, I think there are two types:
  • The orbit of Alpha is extremely elliptical, making enough vary the distance of the habitable zone, cause a global seasonal cycle.
  • Alpha or Beta has its axis tilted, but in this case, the time of eclipse will vary during the year (as well as the duration of the day, cause the seasons on Earth as we know).

Okay for the year, it can take as much time we want!
Except if it is longer, the star must be hot to keep the habitable zone at the same distance as Alpha.
Also, is a gas giant or a large planet may be on the current Earth orbit?
But I fear that change too parameters no longer meets the hypothesis of our earth-like moon.
Keep our Sun and at the same distance if possible!

For the temperature differential, if Beta and Alpha has a right axis, the outside hemisphere has 12 hour of day and 12 hour of night, but the inner hemisphere, with the eclipse, has necessarily more night than day.
If for an average of 12 hours of day there are 10°C, for an average of 10 hours of day there may be 5°C!
The differential would have nothing to do with an eccentric orbit of Alpha.

I also didn't think that Alpha could radiate energy and heat the side that spends less time in the sun. I don't also forget that the tidal heating can heat both sides.

Another question comes: what about the magnetosphere of Alpha?
I guess it can be variable. One can choose for it is not too powerful at the distance of Beta.
However, this will mean that Beta has maybe a great auroral activity, not necessarily at the poles?

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Re: Living in a earth-like moon orbiting a gas giant

Postby PolakoVoador » Thu Aug 01, 2013 9:12 pm UTC

LaserGuy wrote:
PolakoVoador wrote:Is it possible for the moon to not be tidal-locked to the planet? There would be even crazier night-day cycles, I guess.


For a gas giant, not terribly likely, I don't think. Unless the moon was way farther away than what we're used to seeing. Or very early in its history, I suppose. Or maybe if it was one of those gas giants that is quite close to the star?


But a gas giant very close to the star will not have a very habitable moon. But you're right, a not-tidal-locked moon for a gas giant is highly improbable

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Thu Aug 01, 2013 9:33 pm UTC

In estimating the various parameters initially provided, I've come up with a few additional points....

The OP specified that the position of Alpha does not move in the sky; I interpret this to also mean its angular diameter does not noticeably increase. Thus, the eccentricity of Beta's orbit can be assumed to be very low. If the eccentricity was not low, Alpha would appear to shrink and grow in size over the course of the day.

One thing that wasn't mentioned is Beta's apsidal precession. Thanks to perturbation from the star, any slight eccentricity in Beta's orbit will be pulled to one side; this will mean the orbit itself will slowly make a full circle around Alpha. For reference, our moon precesses around our earth once every 8.85 years.

Because of the very short orbital period (~24 hrs) of Beta, this precession will likely be much shorter (the precise period of apsidal precession requires a solution to the three-body problem, so don't ask). But this period can act as our "month" analogue. I'm not certain, though, what observable affect this would have other than slight changes in the angular diameter of Alpha.

I checked to make sure that we wouldn't be close to the Roche limit, but we aren't, so that's good.

It turns out, however, that my earlier estimate of a very high angular diameter was a little unrealistic. If Beta is the size and diameter of Earth and its orbit around Alpha is 24 hours, the maximum angular diameter you can get would be about 8.9 degrees (with a planet slightly smaller than Saturn and an orbital radius of 660000 km, about 72% farther than the distance to the moon). That's 18 times larger than the sun's apparent diameter, but it's nowhere near the "20-50% of the sky" estimate we were considering before. The daily eclipse would last for around 35 minutes....so, not really as much of a "night" as previously thought.

The problem is that if we get closer to Alpha, our orbital speed will increase, making the day shorter. To balance that out, you need to decrease Alpha's mass, but that will make it larger. Conversely, making Alpha larger will result in higher gravity, meaning that we would need to increase our orbital radius to keep the same 24-hour orbit. The only other thing you can do is keep the same mass and radius but decrease density; I think the largest angular diameter you could theoretically get with the minimum density (roughly what Jupiter's density was when it formed) would be about 15 degrees.

Anyone know how to estimate the light level reflected/generated by Alpha onto Beta at that particular obit?

MrY wrote:For seasons, I think there are two types:
  • The orbit of Alpha is extremely elliptical, making enough vary the distance of the habitable zone, cause a global seasonal cycle.
  • Alpha or Beta has its axis tilted, but in this case, the time of eclipse will vary during the year (as well as the duration of the day, cause the seasons on Earth as we know).

An axial tilt is unrealistic, as the tidal locking would prevent it from allowing any variation. An elliptic orbit and a global seasonal cycle is more probable. I'm not sure what a global seasonal cycle would look like.

Okay for the year, it can take as much time we want!
Except if it is longer, the star must be hot to keep the habitable zone at the same distance as Alpha.

Keeping years close to the length of those on Earth will make the seasons work fairly well. The longer the "year" is, the more extreme the temperature changes become through the global seasonal cycle. If you have a longer year, the apsidal precession will likely be longer; theoretically, one could imagine "years" being marked by apsidal precession cycles with the seasons taking 2-3 years each or even longer.

I also didn't think that Alpha could radiate energy and heat the side that spends less time in the sun. I don't also forget that the tidal heating can heat both sides.

The heat radiated by Alpha due to Kelvin-Hemholtz contraction will be almost entirely in the infra-red spectrum; while it will heat Beta somewhat, it won't provide visible light. Moreover, a slightly less massive planet (more Saturn-sized), which is necessary to maximize angular diameter, won't radiate nearly as much energy.

what about the magnetosphere of Alpha?

Beta's orbital radius is approximately the same as the radius of the Sun. Jupiter's magnetosphere is about the size of the sun. So the magnetosphere of Alpha would probably be just inside or just outside Beta's orbit. Either option could have cool effects.

But Beta will also have its own normal magnetosphere.

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Re: Living in a earth-like moon orbiting a planet

Postby Tass » Thu Aug 01, 2013 11:19 pm UTC

davidstarlingm wrote:The problem is that if we get closer to Alpha, our orbital speed will increase, making the day shorter. To balance that out, you need to decrease Alpha's mass, but that will make it larger [meant smaller?]. Conversely, making Alpha larger will result in higher gravity, meaning that we would need to increase our orbital radius to keep the same 24-hour orbit. The only other thing you can do is keep the same mass and radius but decrease density; I think the largest angular diameter you could theoretically get with the minimum density (roughly what Jupiter's density was when it formed) would be about 15 degrees.


Size of a gas giant actually depends surprisingly little on its mass. Saturn is almost as big as Jupiter, but much less dense.

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Fri Aug 02, 2013 1:00 am UTC

Tass wrote:
davidstarlingm wrote:The problem is that if we get closer to Alpha, our orbital speed will increase, making the day shorter. To balance that out, you need to decrease Alpha's mass, but that will make it larger [meant smaller?]. Conversely, making Alpha larger will result in higher gravity, meaning that we would need to increase our orbital radius to keep the same 24-hour orbit. The only other thing you can do is keep the same mass and radius but decrease density; I think the largest angular diameter you could theoretically get with the minimum density (roughly what Jupiter's density was when it formed) would be about 15 degrees.


Size of a gas giant actually depends surprisingly little on its mass. Saturn is almost as big as Jupiter, but much less dense.

80% as much diameter, 50% as much volume. But yeah, 15 degrees of angular diameter seems to be about the limit.

That's based on the minimum density of a gas giant being 1/8th of Jupiter's density. I'll have to run the numbers again tomorrow to make sure that's right. Any other sources which could suggest an even lower average density at least a billion years after formation?

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Re: Living in a earth-like moon orbiting a planet

Postby stianhat » Fri Aug 02, 2013 2:47 pm UTC

What an insanely cool discussion!

I just wanted to comment that what you all say about tidal locking and angles are correct for a stable system, that is usually not really a requirement, since the time scale is so absurdly large. It is perfectly fine to have a non-tidal locked moon at any angle or rotation - but tidal locked and in phase is where it is going to be in a couple of million years, depending on its inertia vs the forces it is subjected to. The sentient beings or whatever on that planet just happens to be there during the time when it is not yet tidally locked.

There is also a possibility to be locked in a 3/2 rotation around the mother planet. And the size difference does not have to be that large, but of course, the more equal they get the less moon / planet and the more binary planet they are going to be.

In fact, two binary, earth-like planets revolving around the common centre of mass would also be quite cool.

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Re: Living in a earth-like moon orbiting a planet

Postby PolakoVoador » Fri Aug 02, 2013 4:32 pm UTC

stianhat wrote:What an insanely cool discussion!

I just wanted to comment that what you all say about tidal locking and angles are correct for a stable system, that is usually not really a requirement, since the time scale is so absurdly large. It is perfectly fine to have a non-tidal locked moon at any angle or rotation - but tidal locked and in phase is where it is going to be in a couple of million years, depending on its inertia vs the forces it is subjected to. The sentient beings or whatever on that planet just happens to be there during the time when it is not yet tidally locked.

There is also a possibility to be locked in a 3/2 rotation around the mother planet. And the size difference does not have to be that large, but of course, the more equal they get the less moon / planet and the more binary planet they are going to be.

In fact, two binary, earth-like planets revolving around the common centre of mass would also be quite cool.


And both with space-faring civilizations warring each other. Hmmmmm, that's a nice start for a story...

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Fri Aug 02, 2013 6:06 pm UTC

Upon review, I came across this paper on the minimum densities of gas giant planets. As it turns out, the minimum possible density of a stable planet depends on several factors, including temperature. Because gas giants typically reflect more light than the Earth, it's unlikely for a gas giant in the Earth's habitable zone to be much above 200-300K, so we'll use the 300K equilibrium temperature from Page 4 as our guide.

For a gas giant with a 1 earth-mass core, I can get a pretty solid lineup of radii from 8 earth masses up to 30 earth masses. Using an Excel spreadsheet, I'm coming up with these values for the maximum possible angular diameter:

Alpha: 8 Earth masses, 10 Earth radii, 18 degrees of angular diameter (appears 36 times larger in diameter and 1300 times larger in area than the sun or moon).

Beta: Orbits every 24 hours; has a tangential orbit speed of 29 km/s, takes 1 hour and 11 minutes to pass through Alpha's shadow at high noon every day. 404,000 km away from Alpha, or just 5% farther than our moon is from us (though the distances from surface to surface is much less, only 334,000 km, or 89% of the Earth-moon surface separation).

Moonrise on Earth looks like this:

Image

While Alpha in the sky over Beta would look like this:

Image

From this view, Alpha would appear to take up about 10% of the horizon when facing any one direction.

Back to the discussion of the years and months and whatnot. Our moon's apsidal precession takes around 109 orbits; if we very roughly estimate a similar situation for the Alpha-Beta system but adjust for the mass ratio, that would be 13 Beta-orbits. We can potentially increase this to 365 days by moving the Alpha-Beta system to about 5.3 AU; in this scenario, a "year" would be the full growing-shrinking cycle of Alpha as seen from Beta's near side. Assuming only a very slightly eccentric orbit, this apsidal "year" would not have significant impact on the weather patterns of Beta.

However....and this is SUPER cool....if the eccentricity of Beta's orbit takes it just to the edge of Alpha's magnetosphere, the start of each apsidal year would be marked by a fantastic auroral light show visible all over Beta. Forget fireworks for New Year's Day....THAT would be amazing. I can't even imagine the kind of mythology that would spring up from that.

To provide a habitable zone at 5.3 AU, its star would need to have 28 solar luminosities. For a main-sequence star to have this luminosity (I'm assuming you want a main-sequence star, to provide stability), it would be need to be an A-type like Sirius. It would be much whiter and a little more than twice as heavy as the sun. As a result, the orbit of Alpha would take eight apsidal years, meaning that Beta would experience a four-year winter, a two-year summer, and spring and fall each one year long (this imbalance is because Alpha moves faster the closer it is to the star).

The bitterness of winter and the heat of summer will depend on how eccentric the orbit is. I'm not quite sure how to estimate just how eccentric the orbit should be to provide a winter and summer extremes slightly greater than those on Earth. Any ideas, guys? Complete glaciation of the Earth would probably occur around 1.4 AU due to ice albedo feedback and runaway greenhouse effect from water vapor would occur around 0.8 AU, so the maximum eccentricity is likely around 0.27 at those extremes. What would the seasonal weather patterns look like? Summer would cause a lot of evaporation, followed by condensation and a long rainy season ending in temporary glaciation, which would result in a spring flood season at the end of the cycle. Any guesses on how extreme the flooding would be, or what the inner-outer face interactions would look like?

stianhat wrote:It is perfectly fine to have a non-tidal locked moon at any angle or rotation - but tidal locked and in phase is where it is going to be in a couple of million years, depending on its inertia vs the forces it is subjected to. The sentient beings or whatever on that planet just happens to be there during the time when it is not yet tidally locked.

Unless we have some kind of panspermia going on, a few million years isn't going to be enough time for life to evolve to the level we're talking about.

There is also a possibility to be locked in a 3/2 rotation around the mother planet.

Boy, wouldn't THAT be an odd day.

PolakoVoador wrote:
stianhat wrote:In fact, two binary, earth-like planets revolving around the common centre of mass would also be quite cool.

And both with space-faring civilizations warring each other. Hmmmmm, that's a nice start for a story...

Isn't it?

For two Earthlike planets to be tidally locked in a binary pair with a 24-hour day, you'd need an orbital radius of 245,000 km. Each planet would see the other as being about 3 degrees of angular diameter, or six times the diameter of the sun (36 times the area of the sun). Each planet's inner face would experience a daily annular solar eclipse and a nightly lunar eclipse each lasting 12 minutes. Orbital eccentricity would again be necessary to get global seasons throughout the year, because you can't very well have any axial tilt.

Of course, the challenge here is that the species on the two planets would have to evolve almost completely independent of each other, which makes simultaneous emergence of sentience a little unlikely. The solution? Aliens seeded both planets with the same semisentient species and then allowed them to diverge. Since binary planets form under very specific conditions, this is perhaps one of the most likely scenarios in which the people from one world would be able to immediately survive on another world without needing life support (Star Wars is pretty unrealistic in this respect).

EDIT: I just realized how absolutely fantastic of a story premise this could be.

Why would the aliens do it?

They'd leave artifacts, naturally. Clues as to their identity. They might even appear on each planet to be worshipped as deities from time to time.

War would begin quite naturally. For one thing, there's the whole concept of conquest, along with the assumption that the enemy will destroy you if you don't destroy them. Given sufficient time for evolution, the cultures would have diverged; it would be an asymmetric warfare setup, but there would still be room for mutually assured destruction.

But the only possible reason for the aliens to do it, of course, would be as a grand social experiment. Posthumans who have long since left the Sol system seed life on the two planets using cloned 20th-century humans, then stand back to see how long it will take for them to destroy each other, leaving artifacts and installations to fuel the conflict. Of course, over the course of the story the two cultures uncover the truth and attack the aliens together....etc etc yada yada.

EDIT 2: Slight differences in the surface gravities of the planets would fuel asymmetric evolution.

Is there any possibility that the magnetic fields of the two planets could be somehow linked at that distance, allowing for an overall field that could be used to assist in orbital transfer? What about the possibility of large-scale contructs left by the aliens?

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Re: Living in a earth-like moon orbiting a planet

Postby MrY » Fri Aug 02, 2013 7:32 pm UTC

AMAZING

It was exactly what i'm searching on this forum!

meaning that Beta would experience a four-year winter,
a two-year summer, and spring and fall each one year long

Winter is coming ;) .

The goal to maximize the angular diameter is perfect!

I'm ok for excentricity in Beta orbit !
Making Alpha shrink and grow in size in the sky is a cool idea!

An axial tilt is unrealistic

Okay, I understand.

For the 3:2 rotation-orbit resonance, the orbit must be eccentric, the tidal effect must be weak and I don't know how Beta must be smaller than Alpha.
This second scenario can really be interesting, but don't forget to improve the first!

Do you think two binary earth-like planets revolving are common in the universe?
Less than an earth-like moon I guess!
In fact, I'm much for adding others (earth-like) moons around Alpha ;) .

It is possible to have all your calculations and data?
Because, for example, if I want to know the effect of matching the apsidal year for an earth month and not an earth year, what are the implications?
I don't think is just proportional, making the Alpha-Beta year take 8 apsidal years (so 8 earth months).

Thank you again, your posts are very interesting!

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Re: Living in a earth-like moon orbiting a planet

Postby Fire Brns » Fri Aug 02, 2013 8:32 pm UTC

Maybe i missed it skimming but I have some questions concerning development conditions. 1. Radiation, gas giants put off a lot. A magnetosphere has to be involved to protect the development of life or it remains beneath primordial seas. The distance of Beta from Alpha also comes into play?

2. Tectonic activity, how much is necessy to maintain a moderate magnetosphere? Is it a plausible amount or would evolution have to overcome the gap and produce thick shelled invertibrate life to survive on land. Even chitanous plated vertibrates?

And to the development of sentience, assuming the individualistic conciousness, toolmakers, ect. they would spread across the entire moon. We did so going from equatorial regions to artic regions and to distant islands. Expecting a curious race to not do so is odd.

Natural disasters have a big play in mythology. Middle east had gods threatening earthquakes and fires, whole region is hot, dry, and on fault lines.
I would assume that the tidal forces would have a big impact on mythology. Gas giants tend to catch a lot of space debris as well so impact events would likely play a big part in mythology. especially so to cultures on the outer side of beta.
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Re: Living in a earth-like moon orbiting a planet

Postby MrY » Fri Aug 02, 2013 9:02 pm UTC

I just noticed that the longitude of a place on Beta can infact be expressed at the time when the eclipse is at half its length:

Image

For people at 12h, the sun is at zenith, like Alpha!



Well thought for the fact of high meteorites activity on the outer side!
And yes, we lack information on amplitude and therefore impacts of the tidal heating.

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Fri Aug 02, 2013 10:20 pm UTC

MrY wrote:I'm ok for excentricity in Beta orbit !
Making Alpha shrink and grow in size in the sky is a cool idea!

The way I had phrased it before wasn't exactly correct; apsidal precession would just change the time of day at which the growing-and-shrinking happened. It would have to be a slightly different sort of interaction, where resonance with the star causes Beta's orbit to become more and then less eccentric on a cycle.

It is possible to have all your calculations and data?
Because, for example, if I want to know the effect of matching the apsidal year for an earth month and not an earth year, what are the implications?

Unfortunately, the orbital eccentricity resonance would require a highly specific solution of the three body problem. According to Wikipedia, the solar tide causes a resonant precession of the Moon's elliptical orbit every 3233 days (109 months). If the sun's action can produce this cycle every 109 orbits at 1 AU, then a system eight times heavier would have it happen every 13 orbits, and 5.3 times farther away would extend it to 365 orbits, and so forth. These are all just estimates. I'm not quite sure what other orbital resonances are possible.

Which other calculations are you specifically interested in?

For the 3:2 rotation-orbit resonance, the orbit must be eccentric, the tidal effect must be weak and I don't know how Beta must be smaller than Alpha.

It would be very difficult to get a solid 3:2 rotation-orbit resonance with Alpha being only 8 times heavier than Beta, which is required for the maximum angular diameter.

I just noticed that the longitude of a place on Beta can infact be expressed at the time when the eclipse is at half its length:

I'm not sure what you mean. Can you elaborate?

we lack information on amplitude and therefore impacts of the tidal heating.

I've provided the orbital radius above....

Fire Brns wrote:I have some questions concerning development conditions. 1. Radiation, gas giants put off a lot. A magnetosphere has to be involved to protect the development of life or it remains beneath primordial seas. The distance of Beta from Alpha also comes into play?

Beta would almost certainly be inside Alpha's magnetosphere, except when increases in its orbital eccentricity take it briefly outside the magnetosphere to mark the apsidal "year", accompanied by fantastic solar aurora (since Beta's magnetosphere would be weak). With Alpha having such a low density, its radiation output isn't nearly as high as Jupiter's.

I would assume that the tidal forces would have a big impact on mythology. Gas giants tend to catch a lot of space debris as well so impact events would likely play a big part in mythology. especially so to cultures on the outer side of beta.

A heavier gas giant farther out in the solar system could help with that, just like Jupiter does for us. Remember, Alpha only has 8 Earth masses, so it won't attract too much space debris.

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Re: Living in a earth-like moon orbiting a planet

Postby zenten » Sat Aug 03, 2013 4:48 am UTC

Wouldn't having Beta's orbitial plane around Alpha be tilted compared to Alpha's orbitial plane around the sun give Beta more typical Earth like seasons?

Also, could another small body (Gamma?) orbit Beta? Or would Alpha keep that from happening?

How prominent would Alpha's other moons look from Beta?

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Sat Aug 03, 2013 2:19 pm UTC

zenten wrote:Wouldn't having Beta's orbitial plane around Alpha be tilted compared to Alpha's orbitial plane around the sun give Beta more typical Earth like seasons?

Also, could another small body (Gamma?) orbit Beta? Or would Alpha keep that from happening?

How prominent would Alpha's other moons look from Beta?

A tilted orbital plane would negate the nice daily eclipses. I don't think that a tiny tilt (enough to preserve the eclipse alignment) would do enough.

I'll estimate the Hill Sphere of Beta when I get a chance.

Other moons would likely be very small. Not sure how close they could be.

Still curious as to whether there could be any sort of magnetic interaction between the binary planet also being discussed.

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Re: Living in a earth-like moon orbiting a planet

Postby Fire Brns » Sat Aug 03, 2013 7:00 pm UTC

davidstarlingm wrote:
Fire Brns wrote:I have some questions concerning development conditions. 1. Radiation, gas giants put off a lot. A magnetosphere has to be involved to protect the development of life or it remains beneath primordial seas. The distance of Beta from Alpha also comes into play?

Beta would almost certainly be inside Alpha's magnetosphere, except when increases in its orbital eccentricity take it briefly outside the magnetosphere to mark the apsidal "year", accompanied by fantastic solar aurora (since Beta's magnetosphere would be weak). With Alpha having such a low density, its radiation output isn't nearly as high as Jupiter's.

I would assume that the tidal forces would have a big impact on mythology. Gas giants tend to catch a lot of space debris as well so impact events would likely play a big part in mythology. especially so to cultures on the outer side of beta.

A heavier gas giant farther out in the solar system could help with that, just like Jupiter does for us. Remember, Alpha only has 8 Earth masses, so it won't attract too much space debris.
I meant as far as radiation put off by Alpha. I mised that it was only 8 earh masses so it's a bit less than I was originally anticipating.

Impacts are still going to be an issue, moons tend to protect what they orbit, beta is a moon. The additional gravitational forces in the neighborhood is going to be a magnet to space garbage as well. actually now i'm curious as to ther moons, gas giants have a lot: kappa, delta, gamma, theta, epsilon, ect are going to play a big role in astronomy and mythology.

And I see the tilted orbit was brought up as well, that intrigued me. The orbit could hardly be perfectly reglar in a similar manner as Luna's especially considering gravitational interference from other moons. Eclipses I would believe would still happen frequently enough that life would have adaptations for longer "cold" nights.
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Re: Living in a earth-like moon orbiting a planet

Postby EchoRomulus » Sat Aug 03, 2013 11:45 pm UTC

With all the gravitational weirdness, accurate clocks and calendars might be a problem for these people until they discover astronomy and calculus and become able to predict these changes.
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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Sun Aug 04, 2013 12:26 pm UTC

Fire Brns wrote:Impacts are still going to be an issue, moons tend to protect what they orbit, beta is a moon. The additional gravitational forces in the neighborhood is going to be a magnet to space garbage as well. actually now i'm curious as to ther moons, gas giants have a lot: kappa, delta, gamma, theta, epsilon, ect are going to play a big role in astronomy and mythology.

Other moons would fill the same role as the planets did for our early astronomers -- loads of epicycles and whatnot -- but these astronomers would probably figured out the truth more quickly because the distances involved would be smaller and the orbits wouldn't take so long to manage. Also, the relative size of Alpha in the sky would likely help to unseat geocentrism (Betacentrism?) earlier than with us. Especially because it doesn't appear to move.

And I see the tilted orbit was brought up as well, that intrigued me. The orbit could hardly be perfectly reglar in a similar manner as Luna's especially considering gravitational interference from other moons.

Going to really be negligible. The other moons will be tiny compared to Beta.

Eclipses I would believe would still happen frequently enough that life would have adaptations for longer "cold" nights.

Well, the daily eclipse would only be a little over an hour anyway. But I'm pretty sure any tilt large enough to produce seasons would make eclipses all but impossible (bi-annual at the most frequent).

EchoRomulus wrote:With all the gravitational weirdness, accurate clocks and calendars might be a problem for these people until they discover astronomy and calculus and become able to predict these changes.

Hmm, I need to calculate whether Alpha would make g noticeably higher on the outside than on the inside.

....nope. Alpha's gravitational field is a miniscule 0.2% of Beta's on Beta's surface.

I'm not sure what would be so difficult to predict, though. They have a light show every New Year's Day, and seasons in perfect resonance. Why would that make clocks tricky?

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Re: Living in a earth-like moon orbiting a planet

Postby Elmach » Sun Aug 04, 2013 6:26 pm UTC

I might be talking out of my ass here, since I haven't done the math yet, but what if it was part of a 1:2:4 resonance a'la io, Europa, Ganymede? Would that help with the asteroid problem? Would that cause more interesting dynamics?

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Mon Aug 05, 2013 6:28 pm UTC

Originally, I had said the apsidal precession could cause a slightly elliptical Betan orbit to have its maximum outside of Alpha's magnetosphere at certain times, providing a cycle. Then I said that wasn't possible, and it would have to be a change in eccentricity to move in and out of the magnetosphere.

But I forgot that the magnetosphere of a gas giant is not a sphere. It's shaped like this:

Image

Thanks to the solar wind, the magnetosphere is far larger on one side than on the other. In this way, a resonant apsidal precession could alter the tilt of Beta's orbit on a cycle of 300-400 days, producing the previously referenced apsidal year heralded by a fantastic auroral light display visible over almost the entire planet.

At one extreme of the apsidal year, Beta's daily apoapsis, or furthest excursion from Alpha, is opposite the sun. As a result, it is thoroughly inside Alpha's magnetosphere for the entirety of the orbital day.

Image

However, as the orbit precesses around Alpha in its resonant cycle, Beta's daily apoapsis happens when it is nearest the sun, and briefly outside Alpha's magnetosphere. Thus, the planet receives a brilliant auroral light show for a few days. It will, of course, be brightest for the near side, as it will take place during the near side's night, but it should be visible during the day as well.

Image

The above images are to scale, by the way.

One other thing -- this slight eccentricity can also be responsible for producing the tides on Beta; the slightly varying gravitational pull of Alpha will produce daily high tides depending on where you are on the planet. The longitude of tidal maximum will progress with the apsidal year, aligning with the nearest and farthest points on the planet during New Year's and at the half-year point. Unlike with the Earth-moon system, however, the tide will progress in a circle around the planet, so there will be a constant east-west current.

For the double-planet system previously discussed, a slight eccentricity can be the source of tides, and apsidal precession can cause local high tide to coincide with high noon (shall we call it falsenight?) in a periodic fashion, so we have a month analogue that way.

Fire Brns wrote:And I see the tilted orbit was brought up as well, that intrigued me. The orbit could hardly be perfectly reglar in a similar manner as Luna's especially considering gravitational interference from other moons. Eclipses I would believe would still happen frequently enough that life would have adaptations for longer "cold" nights.

You could have up to a 17.5-degree axial tilt and still have daily eclipses, but they wouldn't be nearly as long. Earth's axial tilt is 23.4 degrees, or a full third larger. So the seasons would be less significant and you'd reduce the hour-long daily eclipse to a few minutes with only seconds of total eclipse.

Elmach wrote:what if it was part of a 1:2:4 resonance a'la io, Europa, Ganymede? Would that help with the asteroid problem? Would that cause more interesting dynamics?

Because of its mass, Beta will dominate nearly all orbital resonances (however, it should be noted that the Alpha-Beta barycentre is still just under the surface of Alpha). One could imagine any conceivable set of orbital resonances involving other moons, but none of them will significantly alter Beta's orbit. They may or may not be able to protect Beta from asteroid impacts.

The Hill radius of Beta is 140,000 km, but we can't expect stability past half of this, and so the effective orbital radius would be 70,000 km. Beta can have no stable satellites with orbits longer than 51 hours, or Alpha will perturb them into an orbit that moves farther and farther out until they begin orbiting Alpha.

It should be noted that the Hill radius of the two co-orbiting planets we were also discussing is even smaller. The longest-period satellite that could stably orbit one of these planets would have a period of just 42 hours. Given the dual orbit configuration, I'm not certain that geostationary satellites would even be possible.

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Re: Living in a earth-like moon orbiting a planet

Postby DanD » Mon Aug 05, 2013 9:32 pm UTC

davidstarlingm wrote:

I also didn't think that Alpha could radiate energy and heat the side that spends less time in the sun. I don't also forget that the tidal heating can heat both sides.

The heat radiated by Alpha due to Kelvin-Hemholtz contraction will be almost entirely in the infra-red spectrum; while it will heat Beta somewhat, it won't provide visible light. Moreover, a slightly less massive planet (more Saturn-sized), which is necessary to maximize angular diameter, won't radiate nearly as much energy.


The question then becomes: are we talking about humans living in this environment, or life forms that evolved in that environment? If the latter, I would expect them to evolve vision that extended significantly further into the infrared, at least in the equivalent of rods for low light/night time vision.

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Mon Aug 05, 2013 10:01 pm UTC

DanD wrote:
davidstarlingm wrote:The heat radiated by Alpha due to Kelvin-Hemholtz contraction will be almost entirely in the infra-red spectrum; while it will heat Beta somewhat, it won't provide visible light. Moreover, a slightly less massive planet (more Saturn-sized), which is necessary to maximize angular diameter, won't radiate nearly as much energy.


The question then becomes: are we talking about humans living in this environment, or life forms that evolved in that environment? If the latter, I would expect them to evolve vision that extended significantly further into the infrared, at least in the equivalent of rods for low light/night time vision.

Most noctural animals get along well enough on Earth using ambient nighttime light without needing specifically infrared vision.

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Re: Living in a earth-like moon orbiting a planet

Postby Copper Bezel » Mon Aug 05, 2013 10:42 pm UTC

Even the ones that do make use of IR don't exactly have infrared vision per se. The visible spectrum for most animals apparently starts no higher than 300 nm and goes not much lower than our lower limit of 700 nm - for the range around 8 µm where "room temperature" infrared radiation happens, you apparently need very different apparatus. This is one interesting possible explanation why.
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Re: Living in a earth-like moon orbiting a planet

Postby Elmach » Tue Aug 06, 2013 4:19 am UTC

Let me rephrase my earlier question:
1) Is it possiblelikely for another 'large' body to be in orbit around Alpha? ('large' meaning big enough to have an appreciable effect on Beta's orbit)
2) If so, what would be the consequences of the second large body?


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