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I'd just like to preface this with the fact that I'm super new and I apologise if this has been discussed in some way previously (I have looked at similar topics but they haven't quite given me the answers I'm looking for).

So, I'm writing a story that is set in the Jovian system; Jupiter and it's primary moons Io, Callisto, Ganymede & Europa. Of course, the story takes place a long time in the future.

Perhaps it might be worth mentioning that I don't plan on adding fictional systems or moons here. I'm working with the information available on the mass and orbital velocity of the moons and planets I want to include - so my questions relate to the reality of the situation (a little ironic, maybe!).

My problem is working out time. I think tidal locking means that either the day and year or the day and month are equal? I need a little clarification on that one (forgive me for being a bit dim!).

But how would I work out seasons and days/months/years? And perhaps more importantly, how would tidal locking affect the lives of people living on these moons?

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  • $\begingroup$ Welcome, Sian! When you get a moment, please take our tour and visit our help center to lean more about us and how to write the best questions. Thanks! $\endgroup$ – JBH Dec 29 '18 at 18:42
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    $\begingroup$ Tidal locking means one face of the (e.g.) moon is always facing the planet. Earth's moon is tidally locked to Earth. From a definitional standpoint, a year is one orbit around something and a day is one rotation (but, thanks to tidal locking, that's equal to one year). However, marking celestial time on a moon is very subjective because light (the essence of what a "day" is) comes from both the sun and reflection from the planet. On top of this is the fact that humans have a built-in diurnal rhythm that takes a whomping large effort to change. So we impose our own bilogocial definitions. $\endgroup$ – JBH Dec 29 '18 at 18:47
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    $\begingroup$ After digesting what I just said, what's your question, really? What is the problem you're trying to solve with the answer to this question? $\endgroup$ – JBH Dec 29 '18 at 18:47
  • $\begingroup$ (1) You say, "Of course, the story takes place a long time in the future."  OK, but why do you say "of course"?  Sure, if you're talking about human colonization, that's a ways off, but there could be indigenous life there today.  (2) You say, "... I don't plan on adding fictional systems or moons here.  I'm working with the information available ..." — but note that things can change naturally over time.  (And, in the time frame where humanity can colonize Jupiter, we may also be able to do some manipulation of the orbits of moons, etc.) $\endgroup$ – Peregrine Rook Dec 29 '18 at 20:18
  • $\begingroup$ @PeregrineRook Indigenous life is something I am working on right now so yes, I was referring to human colonies. Apologies if I was a bit unclear. Manipulation of orbits (or even planet moving as we see in Michael Cobley's 'Ancestral Machines') is not something I have looked at in any particular detail. That said, I am working along the lines of terraforming and similar technologies. $\endgroup$ – Sian Wheeler Dec 31 '18 at 13:53
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I'm assuming that you're talking about people living on the moons of Jupiter rather than the atmosphere of the planet itself, since you mention tidal locking.

The way days/months/years would work on a tidally locked moon would make days and orbits around the planet the same amount of time, so you would probably just end up calling a year an orbit of Jupiter around the Sun. This would make a "year" ~12 Earth years long.

Months aren't really an obvious unit of measurement like days and years are (Rotate once, orbit once), so you've got a lot of creative freedom with how you set them up. Months on Earth were originally based on the Moon's cycles, (Moon, Month, cycle= ~29 Days, Month= ~30 Days) but seeing as you have no moon orbiting around you, you don't exactly have that option. You could just find a nice number that divides out the number of days per year, or maybe you even use the cycles of your fellow moons. In the case of the Galilean moons, probably Callisto so you have more than a just couple days per month. Or you could do combinations of cycles of multiple moons. Like I said, lots of freedom with months, really any stable and repeating pattern that feels like a good length of time to you could work.

I'm fairly sure that seasons on the moons are basically nonexistent, because they return to exactly the same state in their orbit only a handful of days. Not even the long and frequent solar eclipses from Jupiter would make a practical difference because the same side of the moon is locked facing the planet. This would however cause the "Inner Face" to have a shorter day, interrupted by an eclipse, where the "Outer Face" would have equal times of daylight/darkness. Not exactly seasons, but possibly interesting!

Sorry if this answer is extremely long, it's my first one ever, but I just really enjoyed your question :D

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  • $\begingroup$ Nice first answer. $\endgroup$ – Dan Dec 30 '18 at 7:21
  • $\begingroup$ I know that our year is divided into quarters for ease in some fields, so is it possible a similar system would be used on the Galilean moons? The dating could go by day, quarter, year? It would be an interesting and self-evident divide between life on a terrestrial planet vs a moon. Time zones regularly impact our lives (and we're all on the same planet) so it isn't too hard to think of the same thing governing life between the Galilean moons. Having to convert from one zone to another etc... $\endgroup$ – Sian Wheeler Dec 31 '18 at 14:11
  • $\begingroup$ What you say about seasons make sense actually - my bad! I suppose it would be like the jokes we have about England being dark and murky all the time - the Inner face of the moon would be known for its darkness. Travellers would probably prefer to visit/stay on the Outer face due to its more proportional light cycle. It would impact industry too... Thanks for the great answer Leo! $\endgroup$ – Sian Wheeler Dec 31 '18 at 14:15
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You ask, "But how would I work out seasons and days/months/years?"

  • Years, on the planet, are what they are — revolutions around the Sun.  "Years" on moons is not well defined, unless you take it to mean simply revolutions of the moon around the planet (i.e., months) or planetary years.
  • Months (on Earth) are defined by the orbit of the Moon (Earth's natural satellite) and are not well defined on a planet with multiple moons.
  • Seasons on Earth are a somewhat man-made concept, and are based on the Earth's orbit around the Sun and the tilt of the Earth's rotational axis relative to the planetary orbital plane, and how that affects energy received from the Sun at the surface of the planet.  I don't know offhand whether Jupiter's axis is tilted.  But it receives very little energy from the Sun, and so the lifeforms that live there may be oblivious to such issues.

    For that matter, Jovian folks might be oblivious to the passage of years.  Earthlings living there would probably keep track of time based on Earth's calendar.

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  • $\begingroup$ Given the fact that the Galilean moons are all (I think?) tidally locked, I think I am in agreement with Leo that seasons are pretty much non-existent. It makes a nice change of pace between tidally locked celestial bodies and those with revolutions like our Earth - they will be very different places. The primary source of energy for these moons is Jupiter itself because, as you say, they recieve little energy from the sun. Seasons are probably a thing fo the past for people in the Jovian 'system'. $\endgroup$ – Sian Wheeler Dec 31 '18 at 14:23

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