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Working on a world for a fantasy novel. The planet is Earth-like and for all intents and purposes, tidally locked. It has a dark and light side and most human inhabitants live in a verdant twilight realm. It orbits a very large planet, and there are one or more moons made from debris that orbit the small world. I am going to say the moon and the giant provide light on the dark side. I want the giant to be visible from somewhere on the small planet.

My questions are: how to tell time in any way on this world? What would the sky look like in the twilight realm? As you traveled, would you see the light change? How would the giant neighbor planet look as you traveled? It would only be visible on the dark side, correct?

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    $\begingroup$ Could you clarify to which body the 'earth' is tidally locked? Eternal day/night would imply that your earth is tidally locked to its host star, but that would be impossible if the earth is orbiting another larger planet. Also, see this for some suggestions on timekeeping. $\endgroup$ – cinnamon18 May 30 '16 at 2:00
  • $\begingroup$ It would be, in my concept, locked to the large planet. But basically it's a planet that does not rotate and it may have stopped for non-scientic reasons (i.e. magic) and not a gradual process $\endgroup$ – Septab May 30 '16 at 2:56
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    $\begingroup$ If you're okay with pretty soft science feel free to disregard the following, but if your big planet doesn't rotate a tidally locked earth won't revolve around the big planet, and will thus fall into the planet. Furthermore, because the big planet would revolve around the host star, your earth would actually have one day on earth (in the "sun moves across the sky" sense) per year. $\endgroup$ – cinnamon18 May 30 '16 at 3:35
  • $\begingroup$ Your setup seems to bring a lot of problems with it. The three-bodies orbital mechanics are quite complex. $\endgroup$ – Nico May 30 '16 at 9:27
  • $\begingroup$ I think I fell short in my wording. The small planet is the one that stopped rotating, so perhaps it could be considered a moon. The big one is the one the small one orbits, and does rotate and revolve around its sun. So that means the small planet would have one day per year still? $\endgroup$ – Septab May 30 '16 at 14:58
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You are right that telling time on a tidally-locked planet is a little tricky. A few comments:

First, here is a simple article about tidally-locked planets if you are interested. http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets

Second, beware that the L1, L2 and L3 points are unstable. L4 and L5 are stable (and indeed, Jupiter has large populations of asteroids lurking there) but, in the limit of strong tides (which would lock a planet's spin to its orbit), they are generally unstable on long (billion year) timescales although it depends on the details. Moons are also unstable in the limit of strong tides. If your system is old then the most reasonable bet is a planet on a nearby orbit (see here for details: https://planetplanet.net/2014/05/21/building-the-ultimate-solar-system-part-3-choosing-the-planets-orbits/)

A nearby planet can appear very large. For instance, in the Kepler-36 planetary system, the larger planet appears as big as the full moon viewed from the smaller one when the two planets have their closest approaches.

Third, on a tidally-locked planet the stars move at the same rate as the planet orbits the star. In terms of what the sky would look like, I built up a much more complex system and went through how the sky would change. It should be of some help: https://planetplanet.net/2016/03/23/earth-with-five-suns-in-the-sky-when-would-night-fall/

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As a general rule for a satellite tidally locked to the planet, in the short run the orbital period can by anything you want. The Earth's Moon is an example. In the long run, the orbital period of the small planet must equal the length of the larger planet's day, since tidal-locking mechanisms work both ways, although for an Earth/Jupiter type system the long run may be very long indeed. In the case of the Earth/Moon system the difference is driving the Moon outwards at a slow rate. In our case, the Sun is expected to go red giant before the Earth's day gets locked to the Moon, so you don't really need to worry about this for your larger system.

At any rate, if the small planet (technically, a satellite) shows no rotation with respect to the sun, this establishes its orbital period as being the same as the primary. It would probably make sense to use the L1 point (as Hohmannfan pointed out) simply because this would keep the dark side from being too dark and cold. Planets with a face fixed toward its sun will either have ferocious winds carrying hot air (at high altitude) to the dark face and low altitude cold winds carrying the air back to the day side, or the atmosphere will freeze out on the dark side, leaving the rest of the planet airless. You might eventually want to incorporate some magic to keep the satellite from wandering away from the L1 point, since it's not dynamically stable.

If the primary has not yet been locked to the satellite, timekeeping will be easy as long as the primary has fixed observable features. The appearance of the primary will change as it rotates, and this will provide a basis for primitive timekeeping.

If the primary is locked with respect to the satellite, the appearance of the primary will not change with time, and timekeeping will not be obvious. However, in this case the primary will actually rotate once per year, and will have a day equal to one year.

If the primary actually has no rotation, it will appear to the satellite to rotate once per year, and this rotation is probably too slow to be useful for telling time on a short (hours or less) scale.

If the primary is a gas giant (which seems probable) it won't have any fixed features, so timekeeping won't be simple even if the primary appears to rotate. You might conceivably claim the primary as a very large rocky body with a thin or no atmosphere, but this also seems like you'd need magic. Natural formation of the satellite would cause the primary to hog the gasses, leaving none for the satellite.

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Either, the moon is tidally locked to the big planet, or it is tidally locked to the Sun, as it cannot be tidally locked to both. That is because that would require the moon to have the exact same orbital period as the big planet has around the star. That is not possible, except in the five special cases of the Lagrangian points, two of which are stable. As you are looking for a solution where both conditions can be true, I assume you are interested in these points:

Lagrangian points

L4 and L5 (the stable ones):

One hemisphere is always facing the star, which is the brightest light in the sky, and exactly 60 degrees away from the sun, a weaker light is the planet, looking like the Moon in its first quarter forever. Astronomers on this world can observe that the distance to the star, the big planet, and between the big planet and the star are exactly equal, forming a triangle. This may cause some strange views on orbital mechanics.

L3:

Boring. The planet is hidden behind the star, so the inhabitants of the world may think they are just on a regular planet.

L1:

One hemisphere is always illuminated by the star, and the opposite hemisphere by "planet light". What may be visible from the dark side is a small black dot on the big planet, caused by our world blocking some sunlight.

L2:

Case 1:

If the big planet has a low density, such as a gas giant, the L2 point lies inside the penumbra, causing an eternal solar eclipse and total darkness.

Case 2:

If the big planet has a higher density, the planet is only blocking some of the light from the star, causing a "ring of fire" in the sky.

Note however that none of these points are actually orbits around the planet, but rather border-case scenarios. The closest one can get to what you describe is having a Moon which orbit is tilted 90 degrees in relation to the orbit of the big planet, and combined with a tidal lock, both the big planet and the star appears to stay almost still in the sky, with the exception of the star making a full revolution for each of the big planet's orbits.

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  • $\begingroup$ This helps a lot actually. I think your L1 scenario is most like what I'm after. In your diagram, I see a noon around the big planet. Would it be visible sometimes from the small L1 dark side? Thank you, everyone, for helping me out. This is intended to be a fantasy novel with SF elements, so I am ok with soft science. But it needs to make some sense for me to be able to run with the idea. Been trying to get this novel going for a decade and I am not going to give up! $\endgroup$ – Septab May 30 '16 at 15:08
  • $\begingroup$ @Septab The only soft science aspect with utilizing L1 is the fact that a planet would not naturally end up there, as you would slowly drift away from it. But the concept itself is hard science. $\endgroup$ – Hohmannfan May 30 '16 at 15:15
  • $\begingroup$ How could it unnaturally end up there? A large impact? Could be time to employ the "magic" card...Been reading about Lagrangian points and wrapping my head around all possibilities. $\endgroup$ – Septab May 30 '16 at 15:21
  • $\begingroup$ You could orbit the planet in the wrong plane, it would still catch the disbelief as it doesn't quite work but would allow you to maintain a single face to the sun. $\endgroup$ – Separatrix May 30 '16 at 15:35
  • $\begingroup$ Hmmmm. In L1 could the moon eventually be thrown out and collide with either planet? $\endgroup$ – Septab May 30 '16 at 16:29

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