Given a "parallel" Earth-like world with 9 days to our own 7 days (ie, each day is around 18 hours, 40 minutes), what other differences would one expect/require to be otherwise "normal" and habitable?

Would such a world necessarily need to be of different diameter, mass, orbital distance, etc? Or could one assert that everything else is completely Earth-like?

One other exception I had in mind was having dual, larger moons with inherently more varied tidal patterns, as well, but I'm unsure if I'm overlooking something about how the length of a day might influence seasons, water cycles, or imply something like a greater or lesser year length or orbital distance.

PS. Safe to assume native species will have suitable adaptations, ie, not transplanted Earth life that would have to adapt.

  • $\begingroup$ You did mention different plants, but if you were planning to transplant things from Earth, there would be less daylight each day for the plants to grow and a different cycle. This might have an effect on the growth of the plant or the harvesting season. $\endgroup$
    – ozone
    Mar 8, 2016 at 4:01
  • $\begingroup$ Wouldn't the total daylight, say, at equinox, be the same, though? ie, 7×12 = 9×9⅓ ? $\endgroup$
    – BRPocock
    Mar 8, 2016 at 4:50
  • $\begingroup$ You could be right. I don't really know much about plants. I assume the differences wouldn't be tolerated. On the other hand they might thrive on it. Just thought I would point out something he might consider. $\endgroup$
    – ozone
    Mar 16, 2016 at 2:00

2 Answers 2


No differences at all. The length of day is based entirely on the rotational speed of the planet (well the orbit has a tiny effect as well but it's negligible in most earth-like scenarios). None of the other factors have any influence on it.

Seasons depend on axial tilt and length of year so again are not effected.

Multiple moons does change the tides, we've had some questions about that in the past. See here for example: How would having multiple moons affect tides?

Rotation of the planet would have some effect there as it interacts with the tides, but it just makes them happen faster.


As Tim stated, there aren't really going to be any catastrophic problems from spinning a bit faster - although you might want to consider that the Earth slows down slightly each year due to the friction of the Moon dragging the oceans around it. The effect of two moons will be somewhat more pronounced based on the amount of water moving around each day.

You tagged this weather, so to address weather specifics, you'll have a stronger Coriolis effect which will cause wind bands to be more pronounced. In particular, there will be less mixing of polar and tropical air, and much more pronounced climate bands at each latitude. Wind-related weather in general will be more severe, though I don't have any guess as to how much. Knock-on effects will make things like evaporation happen faster, but also more intense rains from the more powerful storms.

In general, everything will be stockier to deal with the more intense winds, which might be the most noticeable difference to a traveler, as wind speeds and storms will probably trend only toward the top of normal ranges.

Of course, you might be able to adjust the wind speeds to a normal range by increasing the atmospheric pressure by increasing the size of the planet...but that could get really complicated.

In general, just saying "it's windier" should be sufficient for weather.

As far as effects on people, there have been studies that vary the length of the day. I can't find any of the short-day studies, but attempts to live according to Martian sols suggest that humans are pretty well wired for 24 hour days.

  • $\begingroup$ Thanks, I added a PS that the natives will presumably have native Circadian rhythms, although I'm pretty sure the "jet lag" effect of the 24:40 Mars Time has more to do with being on Earth while keeping Martian time rather than an inherent lack of adaptability. $\endgroup$
    – BRPocock
    Mar 8, 2016 at 3:23
  • 1
    $\begingroup$ @BRPocock That is of course the billion+ dollar question. The other studies I read years ago suggested that completely isolated people under 20 are somewhat adaptable, while people over 20 don't seem able to adapt. Unfortunately I can't find those studies now... $\endgroup$
    – Josiah
    Mar 8, 2016 at 17:43
  • $\begingroup$ Accepting Tim's answer because the direct answer (“no”) he got first, but greatly appreciate the Coriolis insight in particular. $\endgroup$
    – BRPocock
    Mar 25, 2016 at 16:31

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