The planetary system I'm creating is a large Jupiter-like planet with four moons. I've based them on the Galilean moons.

The story initially takes place on the outer most moon. The magic system is based on the other moons. So, while standing on the moon, I will see the three other moons above me, either coming towards me or moving away from me in their orbits. The calendar would show the daily movements of the moons. I need this to be as accurate as possible so that when I move the story to the third moon, or second moon, I can calculate the calendars there.

All of the calendar systems I've seen are based on being on a planet. I have not found any references as to how to even begin calculating what's seen from a moon with multiple systems.

Here is the data about the planetary system:

Planet diameter is 142,800 km mass is 1.9 x 1027 kg. 9.8 Earth hours to revolve around its axis 11.86 Earth years to orbit the star once 778,330,000 km from the star.

Moon1, innermost diameter of 3,636 km mass of 8.9319×1022 kg distance from planet is 422,000 km It takes Moon1 1.77 days to orbit planet

Moon2 diameter is 3,138 km mass of 4.7998 ×1022 kg distance from planet is 670,900 km It takes Moon2 three and a half Earth-days to orbit the planet

Moon3 diameter of 5,268 km mass of 1.4819 x 1023 kg distance from planet 1,070,000 km It takes Moon3 7.15 Earth days to orbit planet

Moon4 diameter of 4800 km Mass. 1.076 x 1023 kg distance from planet 1,883,000 km It takes Moon4 16.7 days to orbit planet

  • $\begingroup$ Welcome to the site, LAnne! Please edit your question to provide details about the moons. We can't be "scientifically accurate" if we don't know the details. We need to know mass of the planet, diameter of the planet, mass and diameter of the moons, distance of each moon from the planet center. It would help if we knew the planet's distance from its sun and it's rotation speed. (As you just learned, calendars are not at all simple). Finally, be prepared for more requests and remember to edit your question, not just answer in comments. $\endgroup$
    – JBH
    Jun 14, 2020 at 16:56
  • $\begingroup$ Conversley, you could tell us the lunar-calendar details that you're looking to set your story to (we don't need to know the story per se), then we can try to help fit the moons to that. Either way - we need more details. $\endgroup$ Jun 14, 2020 at 17:06
  • $\begingroup$ I’m voting to close this question because its an researchable using orbital mechanics data available online from wikipedia or NASA, or in most encylopedia. $\endgroup$
    – EDL
    Jun 15, 2020 at 2:49
  • $\begingroup$ Creating a calendar requires an origin point. The modern calendar uses the estimated birth of Christ (because Holy Roman Emperor and all that). I might recommend something like alignment of the all the moons and the planet as a "cosmic event" of some sort. You can easily use that as a sort of (0,0) point for all planets in their rotation. You then simply need to turn the clock the amount of time you expect the story take place and the math will give you a set number of rotations for each body. Then with the numerical number of rotations you can plot a literal map of the solar system to date. $\endgroup$ Jun 15, 2020 at 20:46
  • $\begingroup$ Also, on a note about necessary information and time calculation: mass probably isn't needed, but it may be worth noting that we are assuming a stable, perfectly (or semi-perfectly) circular orbit. Most objects actually orbit in a sort of oval or egg shape, but that's hard to explain and involves a hell of a lot more math. Also, what do you consider a day or a year? Our day is based on the Earth's spin on it's axis and the year on its rotation around the sun. Is your year the planet around the sun or the moon around the planet? Do the moons spin at all? Is a day based on the planet or moons? $\endgroup$ Jun 15, 2020 at 20:57

1 Answer 1


Earth people keep time with three natural heavenly cycles: the day, the month, and the year. Each is important. The day influences short-term rest and activity periods. The month determines the tides and strongly influences medium-term biological cycles in many species (nobody is quite sure how, but it's probably by illuminating the nighttime). The year measures not so much the background stars as the seasons, which influence growing cycles, hibernation, etc.

Your planet orbiting a gas giant will enjoy the same three heavenly cycles as Earth, with the giant taking the place of our nearby moon.

Your planet will probably receive practically all of its radiant input from the giant's sun, with a tiny portion of infrared from the giant, so the day/night influence will be similar to Earth. Of course solar eclipses will be more common and the longest could last several days.

The effect of your monthly cycle will be much more pronounced than on Earth, with that huge giant lighting up the night for almost half the month. Life will have no trouble synchronizing its medium-term cycles, such as estrus, with such a strong natural cue. Sentient females there will be much easier to live with than Earth women. You'll still have spring tides and neap tides, with spring tides at new giant and full giant and neap tides at the first and last quarters, but all tides will be much higher.

However, if you planet is tidally locked to the gas giant, then life is quite different. The day and the month are the same. On the giant-facing side, the nights are never very dark, with a sort of brightness peak as the giant passes through its full phase. The only tides will be those caused by the sun, so their magnitude will be about equal to the difference between spring and neap tides on Earth.

Your annual seasons will depend on either or both the eccentricity of the gas giant's orbit around its sun, and the axial tilt of your planet. The axial tilt of the giant, the eccentricity of your planet's orbit, and inclination of your orbit will not contribute to the seasons.

There is no compelling reason why your seasons should be any different than those of Earth, even if your planet is tidally locked to the gas giant. You can make seasons more or less pronounced by declaring the orbital eccentricity and axial tilt. It's your story and it's your planet.

The four other planets orbiting the gas giant would certainly be visible in the sky, but they would not compel attention the way Earth's moon does, or the way visible rings would. They would be more apparent than the five classical planets are from Earth, being so much closer, but I can't see how they would be prominent enough to add cycles to the main calendar. The main guiding periods for the inhabitants will remain the day, month, and year.

The magical calendar cycles will be overlaid onto the main calendar. They will be calculated and consulted by magic practitioners. They will be largely ignored by the laiety during their ordinary activities, which revolve around farming, sailing, mating, hunting, gathering, and celebrating the results of the foregoing.


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