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Old 08-06-2017, 11:57 AM   #1
Johnny Angel
 
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Default Navigating Strange Worlds

Here on Earth, the concept of using the sun, stars, and various other objects in the sky to navigate is a common idea. On other similar planets and worlds, the same concepts and skills can still be used. The sky simply looks different on those worlds because you have a different point of reference. However, as I was doing some prep work for a campaign I will be running, it dawned on me that what I had in mind for the setting I was creating might make navigational skills and concepts, ones which I take for granted to be simple, might be more difficult.

What I had in mind was that the game's world would actually be a habitable moon which orbits a large planet. At first, I gave that little thought. I simply wanted the sky of my fantasy setting to look different, and I wanted to try something radically different than what I had used before. Upon further thought, I questioned whether or not night time navigation would be more difficult due to the proximity and/or size of the planet. I have seen pictures of what the Earth looks like from the moon, and I do not imagine any extra difficulty in that situation.

But does having something larger, such as Jupiter or Saturn (or perhaps even bigger), dominating the sky upon which you're looking add any difficulty to navigating? My gut feeling is "not really," but I wanted to check my thoughts against a community which often has a better understanding of celestial mechanics and concepts than I do.
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Old 08-06-2017, 12:05 PM   #2
lwcamp
 
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Default Re: Navigating Strange Worlds

A moon around a large planet will probably be tidally locked, meaning that the planet around which the moon orbits will always be in the same spot in the sky (ignoring minor wobbles from tidal libration forced by other moons around the planet). This makes navigation even easier - you can just reference your position compared to a huge big obvious planet hanging in the sky. Combined with a pole star (or group of stars near the pole, if no star is conveniently right on the pole like Polaris is for us) you get both latitude and longitude.

Obviously, this only works on one hemisphere of the moon. The hemisphere facing away from the planet doesn't let you see the planet, so you only get latitude from your sky observations, like on Earth.

Luke
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Old 08-06-2017, 12:24 PM   #3
Johnny Angel
 
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Default Re: Navigating Strange Worlds

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Originally Posted by lwcamp View Post
A moon around a large planet will probably be tidally locked, meaning that the planet around which the moon orbits will always be in the same spot in the sky (ignoring minor wobbles from tidal libration forced by other moons around the planet). This makes navigation even easier - you can just reference your position compared to a huge big obvious planet hanging in the sky. Combined with a pole star (or group of stars near the pole, if no star is conveniently right on the pole like Polaris is for us) you get both latitude and longitude.

Obviously, this only works on one hemisphere of the moon. The hemisphere facing away from the planet doesn't let you see the planet, so you only get latitude from your sky observations, like on Earth.

Luke

So, would that typically mean that (much like our own moon) that "night" and "day" would be measured in days instead of hours?
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Old 08-06-2017, 01:11 PM   #4
David Johnston2
 
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Originally Posted by lwcamp View Post
A moon around a large planet will probably be tidally locked, meaning that the planet around which the moon orbits will always be in the same spot in the sky (ignoring minor wobbles from tidal libration forced by other moons around the planet). This makes navigation even easier - you can just reference your position compared to a huge big obvious planet hanging in the sky. Combined with a pole star (or group of stars near the pole, if no star is conveniently right on the pole like Polaris is for us) you get both latitude and longitude.

Obviously, this only works on one hemisphere of the moon. The hemisphere facing away from the planet doesn't let you see the planet, so you only get latitude from your sky observations, like on Earth.

Luke
Also of course at regular intervals the planet would go entirely dark because you're on the night side of it.
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Old 08-06-2017, 02:05 PM   #5
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So, would that typically mean that (much like our own moon) that "night" and "day" would be measured in days instead of hours?
That depends on the mass of the primary (the planet your moon is orbiting) and the distance of the moon from the planet. For example, Io orbits Jupiter in 48 hours (2 days), and Amalthea orbits Jupiter in 12 hours.

A couple of examples of worlds that are moons from my own setting, using physics to find their orbits:
http://panoptesv.com/RPGs/Settings/V...s/Jiangluo.php
http://panoptesv.com/RPGs/Settings/V...lds/Zemyna.php

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Old 08-06-2017, 02:36 PM   #6
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Also of course at regular intervals the planet would go entirely dark because you're on the night side of it.
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.

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Old 08-06-2017, 03:07 PM   #7
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Default Re: Navigating Strange Worlds

I was wondering if there's a possibility of the moon's axis of rotation pointing directly toward the planet, so the planet was its "pole star," in effect. . . ?
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Old 08-06-2017, 03:37 PM   #8
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I was wondering if there's a possibility of the moon's axis of rotation pointing directly toward the planet, so the planet was its "pole star," in effect. . . ?
It would only be temporary. Conservation of momentum will keep the spin axis pointing in the same direction, like a top. So consider if you had a situation where the moon's spin axis was pointing directly at the planet. As the moon orbited its planet, the moon would move off to the side but the axis would continue to point in the same direction, so that it would no longer be pointing at the planet. A quarter orbital period later, in fact, the planet would be rising and setting over the projection of the moon's equator onto the celestial sphere, and a half orbital period later the planet would have switched poles.

You would also need the moon far enough away from the planet that it wasn't tide locked.

Luke
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Old 08-06-2017, 04:22 PM   #9
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It would only be temporary. Conservation of momentum will keep the spin axis pointing in the same direction, like a top. So consider if you had a situation where the moon's spin axis was pointing directly at the planet. As the moon orbited its planet, the moon would move off to the side but the axis would continue to point in the same direction, so that it would no longer be pointing at the planet. A quarter orbital period later, in fact, the planet would be rising and setting over the projection of the moon's equator onto the celestial sphere, and a half orbital period later the planet would have switched poles.
Okay, yeah, that would be like the sun's apparent position shifting to north and south as the earth orbits it. Only more so.
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Old 08-06-2017, 04:41 PM   #10
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Default Re: Navigating Strange Worlds

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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...
Luke
Regular eclipses like that, even if somewhat rare, would have some interesting effects on native wildlife. Mating flurries, pollen explosions, and nocturnal predators having a heyday from the deeper darkness.
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