What is Cidri?

[FireHorse]

Originally Posted by larsdangly

… A Cidri with ~50 continents needs something like 10x the surface area of the Earth, which calls for ~3x the radius. …

Except ITL says that Cidri is large enough to contain "thousands" of Earths — which could mean 108,612 Earths, or 57,915, or anything, but absolutely must mean at least 2000 times the surface area, bare minimum.

And that means a radius of over 177,000 miles (compared to Earth's ~3959).

Now, try to model a sphere that large, while maintaining not only the same surface gravity as Earth, but also the same rotational period (length of day) and the same orbital period (length of year), and without violating the Laws of Physics.

[larsdangly]

Originally Posted by FireHorse

Except ITL says that Cidri is large enough to contain "thousands" of Earths — which could mean 108,612 Earths, or 57,915, or anything, but absolutely must mean at least 2000 times the surface area, bare minimum.

And that means a radius of over 177,000 miles (compared to Earth's ~3959).

Now, try to model a sphere that large, while maintaining not only the same surface gravity as Earth, but also the same rotational period (length of day) and the same orbital period (length of year), and without violating the Laws of Physics.

I'm not sure what to make of the orbital period 'problem' because who's to say what the angular momentum of Cidri is. But the gravity problem is not as bad as it sounds. A psuedo earth with 50x the radius still has 1g so long as its bulk density is about 1-2g/cc (like Jupiter, Saturn, Neptune and Uranus). I'm not sure how you get a solid surface on such a body, but the point is surface gravity doesn't scale with planetary size in a way most people seem to assume

[FireHorse]

Originally Posted by larsdangly

I'm not sure what to make of the orbital period 'problem' because who's to say what the angular momentum of Cidri is.

Kepler…?

…the point is surface gravity doesn't scale with planetary size in a way most people seem to assume

I make no such assumption; I am familiar with the physics involved. The relevant numbers here are the total mass and the square of the radius (as measured from the surface). The density is irrelevant, at least with regard to calculating the surface gravity.

[larsdangly]

Originally Posted by FireHorse

Kepler…?



I make no such assumption; I am familiar with the physics involved. The relevant numbers here are the total mass and the square of the radius (as measured from the surface). The density is irrelevant, at least with regard to calculating the surface gravity.

We are obviously going deeper on this than is necessary, but...

1) As far as I know, Kepler's laws have nothing to say about the rotational period of a planetary body; they describe the orbital kinematics of one body about another. Also, note that Jupiter, Uranus, Saturn and Neptune all have rotational periods of less than an Earth day.

2) The density is (obviously) equal to mass/volume so we are in violent agreement about what factors matter here. In any case, if you know the maths then presumably you agree with me that natural bodies in the size range we're discussing can have surface gravities of 1g (e.g., Saturn, Uranus and Neptune all are within ~10 % of earth's surface gravity).

[larsdangly]

A science-minded gaming group could have a lot of fun addressing this question as a campaign goal.

[FireHorse]

larsdangly: My reference to Kepler was because you said "orbital period" before, not "rotational period". (And according to Kepler's Laws, Cidri cannot have a year the same length as Earth's unless it's sitting at the same distance from its sun as we are from ours.)

Originally Posted by larsdangly

… presumably you agree with me that natural bodies in the size range we're discussing can have surface gravities of 1g (e.g., Saturn, Uranus and Neptune all are within ~10 % of earth's surface gravity).

Indeed, they most assuredly can. But the size we're discussing is quite a bit greater than Jupiter, which only has about 120 times the surface area of Earth. We need 2000, minimum, so a Cidri that large would need to have a mean density much, much lower than a solid rocky planet like Earth. And then you have to explain why the Mnoren would bother to build a colossal eggshell.

I mean, you can certainly perform the creative Math necessary, if you really want to. (I have, and I am not persuaded.) Or you could just stop trying to force Square Science into Circular Magic holes.

Personally, I prefer to take the game's self-description of "Swords & Sorcery* as an indicator that orbital mechanics and gravitational computations aren't really the point.

[oldwolf]

While I personally don't use Cidri, those who like it may want to consider avoiding modern physics and cosmology altogether. You might instead consider the cosmology of Nehwon, espoused by the Grey Mouser and Fahrd in one of the several books in Leiber's Swords... series. It should particularly appeal to those mentioning Dyson spheres, since it postulates that all of Nehwon's continents as well as the sun moon and stars are spread over the inner surface of a bubble, rising through an infinite universal sea.

There is no reason a fantasy world shouldn't be fantastical.

[philreed]

Originally Posted by oldwolf

There is no reason a fantasy world shouldn't be fantastical.

Thank you. Nicely said.

[FireHorse]

Originally Posted by oldwolf

… it postulates that all of Nehwon's continents as well as the sun moon and stars are spread over the inner surface of a bubble, rising through an infinite universal sea.

I prefer the idea of the giant Space Beehive, but yeah, that works too. ;)

Anybody ever read Roger Zelazny's Roadmarks…? Maybe Cidri is just a big intersection.

[whswhs]

Originally Posted by FireHorse

My reference to Kepler was because you said "orbital period" before, not "rotational period". (And according to Kepler's Laws, Cidri cannot have a year the same length as Earth's unless it's sitting at the same distance from its sun as we are from ours.)

That part is true if its sun has the same mass as ours. Is there reason to conclude this?