Gliese 581-G (Just for fun)

[Meshon]

I forgot about gravity.

So many times I've wanted to do that.

[Azélor]

I'm not sure that your planet can have a satellite because it is tide locked. Think about it, the gravity pull form the star is strong enough to stop the rotation of the planet. What would happen to the satellite in that case?

[Podcreature]

I'm not sure that your planet can have a satellite because it is tide locked. Think about it, the gravity pull form the star is strong enough to stop the rotation of the planet. What would happen to the satellite in that case?

Really great feedback guys, thanks for sharing all your thoughts. It's truly helped my system become not only more realistic but more detailed.

Actually tidally locked planets rotate, they just rotate at the same rate that they make a revolution. That is why this planet does have a North Pole, just like our moon does. Where it is just depends on axial tilt. Orbit is indeed possible, though I'm sure it would be eccentric. I hadn't considered it too big of a problem... The real problem, I realized, after these theories got me doing more research, is that unless the moon was close enough in mass by a factor of 10 it would either fall into the planet or the sun at some point depending on which orbit was faster (which is in agreement with Raptori's theory except that larger moons actually are more sustainable.) The real problem here is just that a very big moon would probably cause the objects to become tidally locked to one another - a great idea for a different planet project, but alters this one too much.

I figured some kind of collision would have been the reason for the moons, but if they'd be falling into the planet eventually, I'd rather scrap them, however, it seems interesting that two similarly sized objects can be tidally locked to each-other and orbit a sun... I -could- go with my collision idea and say that once there were two worlds orbiting one another, which is a scenario that would give life an earlier chance to get started, too (wider habitable zone from day/night.) Let's say a disaster broke up the sister planet, creating a few moons, either destined to someday coalesce into a large enough object to hold orbit, or eventually fall into the sun. The recent-ness is really not a big issue, since in astrological terms "recent" can give or take hundreds of millions of years. My ancient sentient race is only a tenth of that age. Life could have survived such a bombardment and still be very diverse at present, (Die-offs actually promote evolution, by opening new niches for more and more complex organisms to fill.)

But I digress, if most of the debris was propelled at a trajectory that sent the largest chunks of the sister planet into the sun, leaving smaller bits to shower our host or coalesce back into the biggest remaining chunk of the sister, then it might work out for life. The moon might still be large enough to orbit for a good while if, like Raptori says, it's close enough.

This would cause some strange things to happen, but I think they might be beneficial. The tides would be very aggressive and stir up the sea a bit, good and bad... There might even be a noticeable change in gravity depending on where the moon is at, just fun... I assume the planet would stay locked to the sun but it would wobble distinctly. This would actually help create more dynamic weather, and keep hot spots from being too extreme - the desert in the center might not actually cook flesh off bones. (Otherwise, my world doesn't have a big enough liquid sea to have a lot of ocean current and distribute heat.) The moon would have to be just the right size, though I'm confident that size exists, whether I figure it out or not. In conclusion, it doesn't really matter if the moon will stay in orbit forever, because neither will our own, after all.

[Raptori]

Thinking about it you're right. The moon would have to be far closer to the planet for it to orbit the planet rather than the sun, so a smaller moon moving at extremely high speed and relatively low altitude orbit would be more plausible... o.O

[Azélor]

Thinking about it you're right. The moon would have to be far closer to the planet for it to orbit the planet rather than the sun, so a smaller moon moving at extremely high speed and relatively low altitude orbit would be more plausible... o.O

I don't think so but Universal sandbox indicate otherwise...

[acrosome]

Yes, a moon would be unlikely. Unless it were a recently captured vagrant.

With a giant ball of fusing gas in a predictable location in the sky, I doubt that the locals would care much about magnetic compasses. It would be pretty damned easy to fix your latitude, just by measuring the height of the sun above the horizon. As always, fixing longitude would be more of a challenge. In fact, my mind boggles on a way to do so, even with accurate clocks. Perhaps with very accurate stellar measurements, but that could only work near the ice zone where you can see the stars. The stars also wouldn't wheel around the pole star (if there is one) as quickly as they do on Earth- instead, one rotation would take one year.

More intersting is how they would define direction. North/South/East/West makes little sense. Perhaps they'd use a polar system instead? Clockwise, Widdershins, Axial, and Radial? (With Axial defined as "towards the spot where the sun is directly over head," and Radial as the opposite.)

Also, there's really no reason to expect the hot side to be all land, is there? There'd be no ice given the model you're using, true. But there might be a sea crossing the hotside pole. Land might run right up to the ice sheet in some spots. Etc. Because, yes, the planet would be pulled into a bit of an egg shape, but that tidal force is acting on the water too, so you can model it as a flat surface. High points are high, low points are low (and full of water, which is rapidly evaporating at the hotside pole).

[Podcreature]

Also, there's really no reason to expect the hot side to be all land, is there? There'd be no ice given the model you're using, true. But there might be a sea crossing the hotside pole. Land might run right up to the ice sheet in some spots. Etc. Because, yes, the planet would be pulled into a bit of an egg shape, but that tidal force is acting on the water too, so you can model it as a flat surface. High points are high, low points are low (and full of water, which is rapidly evaporating at the hotside pole).

Thanks for the thoughts on navigation. Ideas are welcome, as I have yet to settle on something that will work for future maps... should I work up the nerve to make one that's detailed, and I'm hesitantly tempted.

To answer your question, I wouldn't say I "expected" it to be all land when I made it that way. I guess it's just fun to embrace coincidences. Not to mention, I do prefer my land organisms to have more real-estate to compete over. I decided that this world looks this way specifically because I just imagined this large continent on the face, and erosion along the glacial melt zones made a cookie cutout of the warm region. But here's some reasons it might look the way it does... aside from coincidence/preference: Most of the ocean is frozen on the back side of the planet, keeping sea-levels low on the face. Rain falls frequently along the "green band" and erodes the earth there. When glaciers melt water pools in the depressions they made. So it's a cookie-cutter concept, basically, and water is the cutter.

[Nexis]

HI.

I was reading this thread and had a thought. Depending, of course, on the planet having a small or large elliptical orbit. This may cause sun watchers to see the sun "Beat like a heart" in a year. This may have more theological implications than navigational.

The other idea comes from Eratosthenes. The scholar who measured the circumference of the world by measuring the path of the sun by studying the shadow of a stick. If you place a stick directly under the sun at 90 degrees from the ground it would cast no shadow. Moving away from that stick and place another, it would cast a shadow directly away from the center under the sun. Do this in a circle and farther away you can have a very accurate navigation instrument and mapping. Carrying a "Sun Compass" will tell you how far away you are from the "Sun-ward" center of the world and the direction of the shadow will always point in that direction. That will create a compass if you standardize the height of the stick you can measure the distance away you are from "Sun-ward" by measuring the length of the shadow. Say If you want to reach the city of Randar you must travel North- East of sun-ward for seven shadow lengths. North south east and west generally being directions away from the Sun Center of the world.

[JefBT]

Also, orbiting at high speed on low altitude may impact the planet's rotation as well, after some time. Or at least the water level may change very often.

[sangi39]

You could also use a vertically-hanging weight and then measure the angle of the sun in relation to that vertical line to determine latitude. While the sun-shadow compass realised on a standardised length of the stick to make accurate calculations, this method relies on accurate and equal divisions of a circle, so they both have some drawbacks. Actually, thinking about it, does the sun-shadow compass need to be a standardised length? If you measure the length of the shadow, then the ratio of that length and the length of the stick would end up equal for any length stick at the same latitude. As Podcreature points out, however, the only problem is making sure it's exactly vertical to the ground, but several methods could be used to fix that issue.

As for determining longitude, I think your best best would probably be to use a magnetic compass. While magnetic north might not line up with axial north, the position of magnetic north is relatively stable over the space of centuries.

If you take the measurement of the suns position in the sky in relation to magnetic north, then you should be able to determine longitude (if the sun is directly behind you as you point the compass toward north, then you're north, while the sun being directly ahead of you would put you in the south while the sun being directly above would put you at the magnetic equator. If you're on the equator, but then as you move east the sun should the sun should be directly to your left while it should be directly to your right as you move west. Positions in the north west would then have the sun be behind you and to the right and so on and so on. You should, then be able to work out your exact position from there using either the sun-show compass or the weight-angle compass. I think