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How would you achieve a 48 hour day? Or just ~48.*** hours. I'm writing a book that takes place on another planet. I've had trouble coming up with ways to get this, without disturbing an "Earthly equilibrium." Well, more of I'm not sure IF these things would disturb anything. I assume they would. These were my theories(being bigger than Earth is also a +):

If the planet was twice the size of Earth, maintaining the same speed of rotation. By the same speed of rotation, I guess I more mean that the exterior of the planet was to travel at the same speed as Earth's exterior, more or less making the axis spin at a slower rate. And, besides the theoretical complications of the planet collapsing into itself and its life span being dramatically shortened, how would this affect things like the weather and tides? And to maintain a Semi-Earthly relationship with the moon, would the distances and rotations and sizes of the sun(more or less) and the moon have to be proportionately changed?

Okay I guess this was my only "theory." Sooooo....(Note I wrote this next paragraph before the above theory)

So now I ask if these would achieve it, their complications, as well as other ways to do this with more/less complications. Obviously this is all theoretical, and any sources, reading you can offer would be helpful.

Thanks in advance.

I remember hearing about a study where they put a guy in a room with no clocks, no windows outside, basically no way for him to figure out what time it was. After a few days, the man started living on a 25-hour day instead of 24. That may come into play with your new planet if humans are involved. But as for how people would be able to survive living in a 48 hour environment is to split it up into seperate 24 hour days. For example, one day would be, well, day, and everyone goes to bed for 8 hours around dusk, then the next day would be night and everyone goes to bed around dawn. That's how I would do it, anyway.

As for the actual physics involve, this is where you need to do a bit of research into astronomy and planets. You have to realise that there aren't many planets much bigger than Earth, except of course for gas giants which aren't even solid anyway. You also have to take gravity into acount. An object's gravitational pull get stronger the more mass it has. A planet twice the size of Earth would mean its gravity would be twice as heavy. I don't think people would be able to live comfortably in such an environment, since their bones would most likely snap like twigs the moment they step foot onto the planet. The rotation of the planet itself plays a part in its gravity too, though I'm not too sure how. I'm a fantasy writer, not a sci-fi writer.

But that's the beauty of sci-fi, you can invent just about anything you want as long as you can make it sound believable. So, artifitial gravity reducement, or something. This is where your talent as a writer comes int play.

Haha, I can't believe I forgot about gravity. :/ Well, it's actually not quite sci-fi, and the humans on the planet have no relation to us humans. Space travel isn't possible effectively because of some anomalies on the planet, as well. It's actually just more that these intricacies concern me. :p Since the humans are naturally adapted, the actual daylight hours and such fit with them. And yes, that sounds right with the rotation/gravity relationship. Hm...I guess I need a lot more research. The planets gravity completely escaped me(if you can come up with a pun for this its unintended ).

I see. Well, instead of making the planet larger, why don't you just make its roatation slower? That'll solve the problem with the gravity, and you'll still be able to achieve your 48 hour day. After all, I doubt your story is going to take place on every part of the planet, so its size doesn't matter too much, at least not in the first book since you're just going to be establishing the series. Since it's not a sci-fi story, you don't have to let your readers know too much about the makeup of the planet itself.

Why do you need to explain? Unless this is hard sci-fi, you really don't need to go into any details about why the planet has a 48 hour day. If you do still want to go into detail, Dynamo's suggestion is the way to go. A planet with twice the mass of Earth could probably still support life, but it will also have twice the gravity. If this isn't hard sci-fi, you probably don't want to deal with how things would be affected at twice Earth's gravity.

With a 48 hour day, weather would probably be somewhat more severe. Things would cool down a lot more with a 24 hour night, and heat up a lot more with a 24 hour day. That increased difference between hot and cold would lead to more severe weather here on Earth.

As for tides, most places on Earth experience two high tides and two low tides every day. With a 48 hour day, my guess is that you'd actually have fewer tides assuming the moon accompanying your planet has the same rotational velocity as our moon. Here on Earth, there will eventually be no tides at all as the speed of the Earth slows down to the same speed as the moon.

Good point, Nate. I hadn't even thought about temperature. Although, other factors that would affect the temperature are also the planet's distance from the sun, and the thickness of its atmosphere. If the planet is further from the sun than our Earth is, then the days wouldn't be as hot, but at the same time if it has a thicker atmosphere it'll trap more heat inside, making the nights a little less cool.

But like Nate said, since this isn't going to be hard sci-fi you really don't need to go into detail about this stuff. If this is a fantasy setting, I doubt the people in your world would even know what an atmosphere is.

Since everything involving rotation is probably the most (arguably) theoretical considering we've only been on this planet for a few thousand years/2 million we haven't really had the ability to knowingly experience rotation nor do we still really have the ability to know what happens at the core so I guess rotation would be the best explanation(pardon the run on sentence).

And well, I am actually trying to set up more of a world and even if these intricacies don't matter in the first book, they could matter later. As well as I'm a fairly scientific person(at least one who has in interest in), so I just have a natural habit to ask the whys.

And as for tides, I think Nate got that right. It makes sense if the planet was bigger. So if the planet was the same size as Earth and had a slower rotation, then you'd still be dealing with heat but that's also in relation to the atmosphere. And the tides, since my first book is probably going to pertain to a fairly coastal nation/people, tides would be pretty important. I assume if you had a slower rotation then the gravity would be less and thus tides would be much/1.5x higher, approximately twice as high? I could also thicken the atmosphere and increase the distance from the sun. As I said earlier, the other planets and the solar system isn't important really because space travel is almost impossible due to some anomalies.

SUMMARY:
So now with slower rotation I get slightly decreased gravity(plus scifi factor/writer control=fairly normal) and an increased temperature variation. Slower rotation may also cause a loss of wind velocity possibly, so then you'd get some strange weather patterns, I guess. And I can fix heat variation by giving a stronger atmosphere to retain heat in the night and with an increased distance from the sun I could retain semi-unchanged temperatures during the day to balance out heat trapping/absorption. Can anyone think of what else would happen?

I think I can finally get something together now. Thanks guys.

PS: And if anyone else has anything to add I will be more than happy to hear. Plus if I find out more information on my own I'll post it here. Again, thanks everyone.

Glad I could help.

I assume if you had a slower rotation then the gravity would be less

Gravity would actually be more. Rotation results in a weaker effect of gravity because the people are essentially being flung away by the rotation (think a ball on a string being spun around), and the only thing keeping them on the planet is the gravity (the string). If the rotation is too fast, everything goes flying away.

And if its too slow you fall and get splattered on the ground. :/ Makes a lot of sense. Thanks for the info.

However, do you know how much?

PS: I love your signature. I love replies to my reviews.

ShadowKnight155 wrote:And if its too slow you fall and get splattered on the ground. :/ Makes a lot of sense. Thanks for the info.

However, do you know how much?

I wouldn't worry about it much (since you don't seem to be doing hard scifi). Just try to figure out what would happen if Earth's rotation would decrease, and factor in what happens on the larger planets to see what kind of results gravity can have. By using Earth as a base, you can more easily alter what's currently in existence to make the world realistic.

I would actually make the planet smaller than Earth to counteract the slow rotation increasing gravity. Smaller planet+ slower rotation= a comparable gravity to Earth.

And if its too slow you fall and get splattered on the ground.

Smaller planet+ slower rotation= a comparable gravity to Earth.

No. The amount of gravitational force an object can exert is directly tied to its mass (how much stuff it has in that space); not size or volume (how much space it is taking up). If the planet did not rotate, the force of gravity felt by a person would be slightly more, but not dramatically more, unless the planet was rotating crazy fast to begin with.

The formula for calculating the force of gravity an object can exert is G*m1*m2 divided by r^2 where G = roughly 6.67 x 10^-11, m1 is the mass of one object, m2 is the mass of the second object, and r = the distance between the center of masses for the two objects. In most cases, since we're talking people, the distance can be simplified to the radius of the planet.

Using the above formula nets you, on Earth, the acceleration of gravity, which comes out to about 9.81 m/s^2. And that is not taking into account the effect of rotation. However, the effects of rotation are pretty small, so the 9.81 m/s^2 is the value used in most calculations.

However, do you know how much?

You'd need speed of rotation, mass of the planet, and the planet's radius to calculate it. However, unless your planet is rotating at an insane speed (which it doesn't seem to be), there's no real need to calculate it.

This article on Wikipedia does a decent job of explaining it. If you want to calculate it yourself, the section on acceleration in an arbitrary direction has the appropriate formula.

Thanks Kyllorac! I'll hopefully remember to read that wikipedia article, but right now I'm pretty tired, so, later. But, thanks again!