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According to many places online, Jupiter's tiny axial tilt makes it have no seasons, and its huge size eliminates any variance possible due to its orbital eccentricity. All these considered, what would a Jovian calendar be based on - the Jovian tropical year or the Jovian sidereal year? After all, the tropical year's used when you have seasons to keep up with, and as most places say, Jupiter doesn't have any.

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  • $\begingroup$ "According to many places online". Can you mention those "many places"? $\endgroup$ Oct 12, 2021 at 10:15
  • $\begingroup$ @NilayGhosh here are a few examples: universetoday.com/121259/once-around-the-sun-with-jupiter/amp , sciencing.com/planets-seasons-8493952.html $\endgroup$ Oct 12, 2021 at 10:26
  • $\begingroup$ It would probably be based on the Earth calendar. The Gregorian calendar is aligned to the Tropical Earth Year, so seasons stay put (useful for farmers). But there aren't many farmers on the surface of Jupiter. Human Colonisers would probably keep the Gregorian calendar and other time keeping, not switch to a Jovian one. However I think any answer would be speculative . $\endgroup$
    – James K
    Oct 12, 2021 at 11:24
  • $\begingroup$ @JamesK well I asked the question cause on earth the Gregorian calendar's based on the seasons and keeps track of them, but apparently on Jupiter there aren't any seasons so the sidereal year can be used instead (correct me if I'm wrong) $\endgroup$ Oct 12, 2021 at 11:58
  • $\begingroup$ Another option is the anomalistic year. The eccentricity of Jupiter's orbit is almost 3× that of Earth's. $\endgroup$
    – PM 2Ring
    Oct 12, 2021 at 16:22

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Jupiter has no solid surface, and only the upper layers of its atmosphere should be transparent enough to see the Sun, the stars, and the larger moons of Jupiter.

If there should be intelligent beings floating around and flying around in the upper atmosphere of Jupiter, and if they could see the Sun, the stars, and the larger moons of Jupiter, they would probably base their calendar on the apparent motions of the Sun, the stars, and the larger moons of Jupiter.

As far as I know there would be no seasons on Jupiter, and possibly there would not be much temperature differences between day and night. But if thehypthetical atmospheric Jovians wanted or needed a calendar anyway, they would base it on the apparent motions of the Sun, the stars, and the larger moons of Jupiter.

They would note that the Sun was visible about half the time, and that the stars were visible when the Sun was not. They would also note that each night the stars would appear to move across the sky. They would also note that in the course of a jovian year, the stars visible at night would slowly change and then change back again. And they would note the motions of all the moons they could see, and their changing phases.

So with several moons visible, any possible Jovian calendar could be much more complicated than Earthly calendars.

None of the jovian moons has a atmosphere thick enough for biological purposes. But three Jovian moons, Europa, Ganymede, and Callisto have liquid oceans under many kilometers of ice.

So there possibly could be hypothetical lifeforms living in those subsurface oceans, and maybe some of those lifeforms could hypothetically be complex multicelled lifeforms, and maybe soom of those more complex lifeforms could be intelligent beings and people.

If intelligent beings exist in the interior oceans of any Jovian moons. they won't be able to see the Sun, Jupiter, the stars, and the other moons of Jupiter, not though many kilometers of solid ice. I am not aware of any periodic weather and climate variations which those interior oceans might have and which the hypothetical natives might need to keep track of with a calendar.

As the large moons of Jupiter, Io, Europea, Ganymende, and Callisto, orbit around Jupter, their distances and directions change. Thus the strength and direction of tidal forces between moons is constantly and periodically changing. And if any effects of the tidal forces such as quakes or volcanism, are important to the natives, they will keep track of those periodic changes, even though they can't see the orbital motions which cause them.

Human scientists and settlers on the moons of Jupiter would die instantly on the surface without spacesuits, so they will have to wear spacesuits whenever they leave their habitations. Those habitations will have to be totally self enclosed and cut off from the deadly outside environment, like bases on earth's moon.

So human colonists of the Jovian moons would have no need to keep track of any minor changes in the outside climate, which would be changes from instantly deadly to also instantly deadly.

They would have clocks and calendars which kept track of the date and time on Earth. They would probably have a 24 hour day period inside their bases, like Earth's.

And they would probably also have clocks and calendars based on the motions and phases of Jupiter, the big moons, and the Sun. People who went outside would want to know beforehand how light it would be, and people would want to know when Jupiter and the moons looked most spectacular.

Humans who ventured into the upper layers of Jupiter's atmosphere and stayed there for long might use a Jovian calender similar to one which hypothetical intelligent Jovains might make. In addition to using a calendar keeping track of Earth time, of course.

However, it takes a lot of energy to get out of Jupiter and back into outer space. And the surface gravity in the cloudtops of Jupiter is about 2.4 times that of Earth, which would probably cause health problems and certainly be very unfomfortable.

So I don't think that humans will venture into the atmosphere of Jupiter or need a Jovian calendar any time sooon.

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