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Deducing planetary properties from its calendar?

  1. Nov 15, 2016 #1
    I'm writing a story involving multiple sentient species, interstellar travel, and a long history of colonisation. My main character is a Human living on the planet "Tender", whose ancestors colonised it from the planet "Ardel" a few centuries ago.

    Is it possible to arbitrarily write up the calendar based info (length of day, length of year, etc.) and deduce from this such things as size of the planet, distance from its sun, etc? For that matter, if all we know is the timekeeping details of a fictitious world, is that enough to deduce any other factors?

    For reference, my tentative values are this:
    Planet Tender has a 20 hour day and a 300 day year.
    Planet Ardel has a 33 hour day and an 80 day year.

    Assume a 60 minute hour in both cases.

    If one can't deduce much from this, what extra information is needed? All the material I've found so far starts with the star's characteristics, and then working down to the details of its planets. I want to work in the opposite direction if possible so I can specify certain conditions (day length, gravity, etc.) and work out any matching info (star type, climate, planet size, etc.).
  2. jcsd
  3. Nov 15, 2016 #2


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    Staff Emeritus
    Science Advisor

    Day length and year length don't tell you anything specific about the planet's size, nor the orbit. You'd think that the planet with the shorter year would be closer to its sun but orbital speed also depends on the mass of the objects. In this case Ardel could be further away from its sun but have a fast orbit because its sun masses more. Don't fret though, it won't be that hard for you to decide on figures for your systems that will fit with what you have so far.

    I'd suggest watching this video, and other videos produced by artifexian. He's done some great, easily accessible guides on how to worldbuild planets.

    If you get stuck with any of the maths you can always post here.
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