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I'm a writer, so I apologize in advance. On an earth like planet with and orbit double to ours in days, how long would days be? My planet would take aproximately 672 earth days to orbit around it's sun. it's similar to Earth in everything but the time it takes to orbit its star, which is slightly brighter than our sun.

I tried to ask an AI, but it wasn't very helpful.

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  • $\begingroup$ The ratio of the mean solar day length to the sidereal day length is smaller for a longer year length. So if you moved the Earth to a 672 day orbit without changing its spinning rate, the mean solar day would be reduced to ~23h 58m 12s. $\endgroup$
    – PM 2Ring
    Commented Apr 16 at 3:12

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The time for a planet's revolution (orbit) is completely unrelated to the time for its rotation (spin). There's no connection between the length of a day and the length of a year on any given body. Just as an analogy, imagine a basketball player spinning a basketball on his fingertip while he walks around a car. Now, if he walked around a bus instead, how fast would the basketball on his finger spin? Intuitively, we understand that it doesn't change; the ball doesn't care how far he's walking or in what direction.

The orbital distance that has a year that's just about twice ours is really close to where Mars is (Mars has a year of 687 earth-days, as compared to your requested 672). If you took Earth and magically moved it out to Mars's orbital distance and then increased the sun's output to compensate, that wouldn't change the Earth's rotation rate. The day would still be 24 hours long.

The only difference would be a reduction in the tidal force the sun puts on Earth. The sun has about 45% as much tidal force as the Moon, and if you double the distance, then you'd reduce the tidal force by the cube of that, which is to say it would be 1/8th as strong, or about 5.7% of the moon. Which is to say, spring tides -- when the sun and moon line up to produce bigger tidal bulges that make for extra-high and extra-low tides -- would be practically nonexistent, detectable to careful measurement but functionally gone. (That's all assuming the sun has somehow increased its output without changing in mass, which is impossible, but it's not significantly more impossible than casually moving Earth out to the orbit of Mars, so we'll just ignore that.)

Taken over the lifetime of the planet, a one-third reduction in total tidal force might have slowed the planet's rotation slightly less, resulting in a slightly shorter day... but on the other hand, having Earth form at Mars's distance requires making a lot of arbitrary changes to the initial conditions of the solar system that make the comparison kind of meaningless. If we can posit a quasi-earth that forms at Mars's orbital distance but with Earth's initial rotation speed, we could also posit a planet that starts at Mars's orbital distance with slightly less than Earth's initial rotation speed so that it ends up at 24 hours by 2000 AD.

As mentioned, Mars has a year of almost exactly twice ours, but the day is only 40 minutes longer than Earth's. Meanwhile, Mercury's year is 88 days, but its day is 1,408 hours, or nearly 60 days long. And if you want something really wild, Venus's year is 225 earth-days, but its day is even longer at 243 earth-days -- and it spins backward as compared to every other large body in the solar system! We think that in the early solar system, Venus got hit by a massive object that almost completely stopped its rotation, leaving it with just a tiny bit of backspin. (A similar impact on Earth didn't stop the spin, but gave us a ridiculously huge moon, so apparently anything can happen.)

If you're working on a story and want to know what day/year combinations are realistic -- pretty much go as crazy as you want. There's no rules. The only thing that would be unlikely is a fast retrograde rotation (that is, the sun rises in the west).

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  • $\begingroup$ Yeah, as PM Ring pointed out in a comment, technically if you just moved earth to Mars's orbit without any changes, the day length would actually change by a couple of minutes because of how the solar vs sidereal day works. $\endgroup$ Commented Apr 18 at 14:28
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The Earth's rotation period has been slowed to some extent by the tidal influences of the Moon (principally) and the Sun. The Sun's tidal influence is much smaller than that of the Moon.

Tidal torques diminish as orbital separation to the power of 6, which in turn (via Kepler's third law) is proportional to orbital period to the power of 4. Thus the Sun's tidal influence on the rotation of your hypothetical planet is smaller by a factor of 16.

If your Earth-like planet has an Earth-like Moon, then it is that Moon which will totally dominate any tidal interactions, independently of the orbital period around the Sun.

On the other hand, given that your planet could have started with almost any rotation rate, then it could conceivably have any rotation rate now.

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  • $\begingroup$ If you're sticking your toe in the science fictiom water, you should know that the ones you seem to aiming at could have answered your question - probably all your additional questions. Your hypothetical planet can't have an Earth-like climate. It would be colder. $\endgroup$
    – stretch
    Commented Apr 15 at 23:08
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The length of a day can be almost any number. For example, Mars' days are just slightly longer than Earth's. The planets beyond Mars have faster rotation rates. Its orbit would be somewhat beyond Mars.

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    $\begingroup$ There is a lower limit on the length of a day - if the planet spins too fast then gravity can't provide the necessary centripetal force, and the planet tears itself apart. The earth, for example, could not have a day of less than 1 hour, but planets with higher gravity and larger radius could. $\endgroup$ Commented Apr 15 at 18:53

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