Navigating Strange Worlds

[David Johnston2]

Originally Posted by lwcamp

If the orbital plane of the planet-moon system is in (or close to) the plane of the ecliptic, then once every day-month (which will be the same thing on this world) you will go into full eclipse. The planet will be entirely dark, and the moon will be in the planet' shadow with the sun behind the planet. You could still see where the planet is, because you'll have a spot in the sky without stars. The eclipse will last on the order of an hour or so (with considerable variation depending on orbital radius and planet size), after which the moon will go out of the planet's shadow and you will be able to see at least a crescent of light on the limb of the planet.

If the orbital plane of the planet-moon system is not particularly closely aligned with the plane of the ecliptic, eclipses will be much rarer. As long as you are not in eclipse, you will be able to see some sun reflecting off the planet somewhere from the moon, although it might only be a thin sliver.

Luke

As I understand it new moons don't happen because of eclipses. They just happen because it's night on the visible side of the moon. If the primary is optically large, then it wouldn't be a very precise position indicator. If it's small then it would be difficult to pick it out in the sky when it isn't illuminated.

[ericthered]

New moons will only happen during the day, because if the tidelocking.

[Johnny Angel]

Giving this further thought after a few Google searches...

From what I've read, the process of becoming tidally locked takes time (which makes sense).

While it was not part of my initial concept, I am now considering the idea of having the moon being in the process of becoming tidally locked, but not yet locked. More specifically, I was considering the idea that it takes the moon a month or so to turn in such a way that the primary planet would then be facing the opposite side of the moon. In essence, each "day" on the moon would have the same change in lighting than an hour here on Earth has. I'm not sure if what I'm trying to say makes sense.

As for the mass of the primary planet and the moon, I'm not sure. The idea is newly formed, and this discussion took my mind in a different direction than I had originally planned. For the purpose of the game, I doubt the players will care about the realism of the cosmology or how long days are, but I'm interested in figuring out what things look like.

What I had in mind for the primary planet was something larger than Earth, but roughly the same distance from its sun as Earth is from the sun. How much larger? I don't know. For sake of discussion, I'll say 5 times the size of Earth as the lower limit and 10 times as the upper limit (so roughly somewhere between Uranus-sized and Saturn-sized). Then perhaps a habitable moon -the game world- about the size of Jupiter's Ganymede or the planet Mars.

(I'm also considering the possibility of a second moon which could be traveled to via a magic gate during times when the two moons are near enough to each other during their orbits around the primary planet.)

I'm warming up to the concept of extended periods of darkness. I like the idea of the impending darkness bringing about a period in which nocturnal predators (as others mentioned) are more active.

[whswhs]

Originally Posted by Johnny Angel

What I had in mind for the primary planet was something larger than Earth, but roughly the same distance from its sun as Earth is from the sun. How much larger? I don't know. For sake of discussion, I'll say 5 times the size of Earth as the lower limit and 10 times as the upper limit (so roughly somewhere between Uranus-sized and Saturn-sized). Then perhaps a habitable moon -the game world- about the size of Jupiter's Ganymede or the planet Mars.

Do you mean five times the diameter or five times the mass? Is it a large terrestrial that's retained helium but not hydrogen, or a small jovian that has a lot of methane, ammonia, water, and so on in proportion to its hydrogen content?

I think the satellite would need to have a really high density to be habitable and earthlike at that size. You can probably tweak the blackbody temperature and density and see what's the smallest planet you can get that comes out as an ocean world.

[David Johnston2]

Originally Posted by ericthered

New moons will only happen during the day, because if the tidelocking.

I can not understand the mechanics of that given that we are not truly discussing a new moon but a "new Jupiter"

[ericthered]

Originally Posted by David Johnston2

I can not understand the mechanics of that given that we are not truly discussing a new moon but a "new Jupiter"

And that's exactly what makes the "new Jupiter" effect only happen during the day. You always point at the planet, which means the side of the planet you can see always points in the opposite direction you are. Only one of the two faces can point at the sun at a time.

Here is an illustration: https://docs.google.com/drawings/d/1...it?usp=sharing

[dcarson]

Planets can get in a resonance with what they are orbiting. Mercury is a 3:2 resonance so it rotates 3 times for every 2 orbits.

[Johnny Angel]

Originally Posted by whswhs

Do you mean five times the diameter or five times the mass? Is it a large terrestrial that's retained helium but not hydrogen, or a small jovian that has a lot of methane, ammonia, water, and so on in proportion to its hydrogen content?

I think the satellite would need to have a really high density to be habitable and earthlike at that size. You can probably tweak the blackbody temperature and density and see what's the smallest planet you can get that comes out as an ocean world.

I was thinking more in terms of size (diameter) than mass. Visually, what I imagined was a large obvious object, visible in the sky of the game-world (habitable moon).

With the little bit of Google-based knowledge I'm picking up, it seems that a large and solid rocky planet might be too much mass and might risk crushing (or adversely impacting in some other way) the habitable moon. Though, I suppose that could be solved by having the moon further away from the planet.

I like the idea of a mostly liquid planet, but most of the reading I've done indicates that tends to be quite rare, and it's debatable whether or not such a thing is actually possible at the size of something like Saturn.

In the end, the planet itself doesn't particularly matter. For the sake of the game, it's mostly just a visual effect. However, knowing what the visual effects are and how to explain them to my players is something which I feel adds to the game.

For the sake of making the conversation a little easier, I'll name the planet "Primus." The habitable moon which is the game world will be "Unshier."

Originally Posted by dcarson

Planets can get in a resonance with what they are orbiting. Mercury is a 3:2 resonance so it rotates 3 times for every 2 orbits.

That sounds similar to what I had in mind.


As I consider the idea more, I like the concept of Unshier's (the habitable moon) position and visual effects on Primus (the primary planet) being used in a manner similar to how various mythologies use moon phases and other astrological events to explain things here on Earth.

I started to consider that idea after thinking upon the red spot of Jupiter. Depending upon how close Unshier is to Primus, large planetary events could be visible. Magical traditions or rituals might attribute characteristics to the different faces of Primus and patterns on the surface of the planet.



My original question about whether or not navigation would be more difficult appears to have an answer of "no." ...and it may actually be a little easier to navigate using the large object in the sky.

[DAT]

How about a double planet - The pair orbit around the L1 point between them, which intern orbits around the star.

The Niven Smoke Ring is another interesting configuration.

[whswhs]

Originally Posted by ericthered

New moons will only happen during the day, because if the tidelocking.

Tidelocking or no tidelocking, new moons can only happen when the sun is illuminating the side of the moon that faces away from the planet. That requires the sun and moon to be in conjunction, which means they rise and set at the same time.