Thomas de Groot wrote:
> "Mike Andrews" <nomail@nomail> schreef in bericht
> news:web.434fc6b84ff5b998c717c9af0@news.povray.org...
> 
>>For practicality I prefer Banks' 'Culture' ring model which is to have a
>>smaller ring orbiting in a star's life zone, spinning once per day to give
>>one g of centripetal accelleration and with a small rotation plane offset
>>from the sun. If I got my calculation right (a = lw^2) the radius is about
>>1.85 million km,  which still gives a huge surface area for a reasonable
>>ring width - and you can build several in one system :-)

There may be a problem with tidal stresses at that size.

>>(Wanders off, dreaming of how to reduce Jupiter to workable material ...)
> 
> And there you get the Ringworld described by Larry Niven!!

Not exactly.

What Mike described is a small ring, the center of which is at a 
distance (150Gm or so) from the star.  Ringworld is a huge ring (150Gm 
or so radius), with the star at the center of the ring.

Regards,
John

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Anton Sherwood wrote:
>> Larry Hudson wrote:
>>
>>> No, it's not a resonnance.  The way I've heard the two tides 
>>> explained is that the ocean is raised by the moon's gravity on that 
>>> side of the earth, but it also pulls the _earth_ away from the water 
>>> on the far side.
> 
> 
> Darren New wrote:
> 
>> Um, not really.
> 
> 
> I don't see why not.
> 
>> Tides are caused when any large body orbits a point. Consider two 
>> rocks on the moon, one on the ground very close to the Earth, one on 
>> the ground on the side we never see.  [snip orbital mechanics]
> 
> 
> This does not contradict Larry's version.
> 
 > ,,, [much quoting left out] ...

It's not really "my" version, I don't have any expertise on the subject. 
  And as I stated in my original post, this was the explanation I had 
heard.  It was quite some time ago that I heard this explained, and I 
don't recall what the source was (possibly a science program on TV, but 
it might have been a book or magazine.  Hmmm, thinking back again, it 
probably was TV, because I seem to vaguely recall it being illustrated 
with animated pictures.)  However, I did believe it was an authorative 
source at the time, and it did sound like a reasonable explanation, so I 
accepted it.

But I'm not going to argue about it...    ;-)

      -=- Larry -=-

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Alain <ele### [at] netscapenet> wrote:
> Gaea is a wheel space-station like living and centient hollow world orbiting Saturn.
It have a main
> "brain" in the central hub and 6 secondary "brains" around the rim. All have ther
own parsonalities.
> Tha main Gaea is obsessed with old movies. The interior environment is subtropical
earth like thank
> to the mirror pannels and large transparent areas on the inside ot the rim.

It would therefore appear that Hamilton plundered both this idea and the
original Rama, combining them for his bitek habitats*. They are closed
environments, like asteroids, with extensive cities on the exterior and
garden space on the interior. They are composed of a versatile polyp
material; almost everything is part of a huge single 'creature' - energy,
plumbing, recycling etc. They have superconductor spines which sweep
through their primaries' magnetospheres for energy. Their personalities can
be quirky too. One of them is even a main character!

*Not that this is a bad thing - Hamilton's work weaves virtually every large
sci-fi idea from the last 50 years into his universe, with a generous
helping of his own ideas.

I'll keep my eyes open for some Varley.

B

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Just got around to reading some of these earlier posts! A couple of
comments:

Regarding Niven's Ringworld -

"Anthony D. Baye" <Sha### [at] hotmailcom> wrote:
> > conceived as an intermediate step toward a dyson sphere, spun at 770
> > mi/h.

mi/s I think - mi/h is barely mach 1.

Regarding Dyson spheres -

> then the total inside surface area (Not accounting for
> variations in topography) would be 2(pi)(9.28e6)(1e6) mi^2 that's
> several thousand times the surface area of earth.

Again, I think this is a little off. As Niven attempted to convey, the sheer
scale of structures like the Ringworld (not to mention Dyson spheres)
defies the imagination. The Ringworld would have a livable surface area of
approx 2.8 million earths, and a Dyson sphere of similar radius would have
approx 510 million earths.

These numbers are ridiculous. I find it difficult enough to properly imagine
the surface area of the earth (trivialised as it is by cheap and fast air
travel) - trying to visualise Ringworlds and Dyson spheres just leaves me
agog and gently drooling!

Bill

PS for the record: "homogeneous".

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Among other things, Larry Hudson saw fit to write:

>>>>In other words, to
>>>>>simplify things, without extra masses...you weigh less at noon?
>>>>
>>>>Don't tides work this way?
>>>
>>>Not at all. If that was how tides worked, there would only be one tide
>>>per day instead of two. Tidal forces also make you lighter when the
>>>extra mass is directly below your feet.
>> 
>> The secon rise is due to a resonnance, an harmonic
> 
> No, it's not a resonnance.  The way I've heard the two tides explained
> is that the ocean is raised by the moon's gravity on that side of the
> earth, but it also pulls the _earth_ away from the water on the far
> side.  So the high tide on the far side is not that the water is higher,
> but that the earth is lower.

That's it. The solid earth can be considered rigid, while the water in
oceans obviously isn't. The portion of water close to the Moon is pulled
towards it more strongly than the rock, because it's closer to it, and it
can deform. The portion of water in the far side is pulled more weakly, so
in relation to the solid earth it looks like it's being pushed away.

To answer the original question: "you weight less at noon?" Yes, of course.
Whether that's measurable is something I don't know, but I've learnt there
are stronger tides at full and new moon, because the influences of Moon and
Sun combine. That would mean the effect is probably measurable, but the
Moon's effect is still more important, so you'll weight even less with the
Moon high up in the sky, no matter day or night.

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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Jellby <me### [at] privacynet> wrote:
> That's it. The solid earth can be considered rigid, while the water in
> oceans obviously isn't. The portion of water close to the Moon is pulled
> towards it more strongly than the rock, because it's closer to it, and it
> can deform. The portion of water in the far side is pulled more weakly, so
> in relation to the solid earth it looks like it's being pushed away.
I'm not sure this is right. The best (and definitely correct) explanation so
far was Darren's, from earlier in this thread:

"Tides are caused when any large body orbits a point. Consider two rocks
on the moon, one on the ground very close to the Earth, one on the
ground on the side we never see. The one on the ground close to the
Earth is going slower than it would if it were all by itself in the same
orbit without the moon. A lower orbit is a faster orbit, so the rock
there is going too slow, so it should fall down towards the earth. A
higher orbit is a slower orbit, but the rock on the far side is actually
travelling faster than the rock on the near side instead of slower, so
it would normally be "flung away" from the center. The smaller the
radius of orbit compared to the size of the orbiting body, the more
evident the effect. The stronger the gravity, of course, the more
evident the effect."

This is how it was taught to me during my Physics degree.

It's interesting to note that the only reason we notice tides is because our
moon is unusually large - relative to its parent body, it's the largest
moon in the solar system by some orders of magnitude. In light of the
sci-fi element to this thread, it's also worth recounting two of Larry
Niven's short stories, "There Is A Tide", and "Neutron Star", both of which
hinge upon understanding tidal effects. "Neutron Star" in particular
features a good explanation of tides.

Bill

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Jellby wrote:
> That's it. The solid earth can be considered rigid, while the water in
> oceans obviously isn't. 

Except you get tides even when a planet is entirely made of liquid. So 
that's not really it.  It took people a long time to explain why there 
are two tides per day instead of one (given that such was noticed 
thousands of years before Newton was alive), and many of the old 
incorrect explanations still float around as "lies to children".

-- 
   Darren New / San Diego, CA, USA (PST)
    Neither rocks nor slush nor salted rims
    shall keep us from our appointed rounds.

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"John VanSickle" <evi### [at] hotmailcom> schreef in bericht
news:43513849$1@news.povray.org...
>
> Not exactly.
>
> What Mike described is a small ring, the center of which is at a
> distance (150Gm or so) from the star.  Ringworld is a huge ring (150Gm
> or so radius), with the star at the center of the ring.
>
> Regards,
> John

Oops, yes of course! Thanks for correcting me

Thomas

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"Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
news:web.4350f4204ff5b998731f01d10@news.povray.org...
> Not heard of that. I shall have to keep my eyes open. Speaking of
Ringworld,
>
> http://www.infradead.org/~wmp/gallery4/ringworld.jpg
>
> One of my earlier attempts at world-building. It is exactly the dimensions
> laid down by one L. Niven - including the shadow squares! However, being
at
> the extreme of precision, it has a tendency to munge itself occasionally
if
> I change the viewpoint etc. One day I'll sort it out.
>
Yes, looks like my own attempt also! Was some years ago that I tried that in
Moray.

> (I was building a description-accurate model of the Lying Bastard, too,
but
> this project too has fallen by the wayside - although I was quite proud of
> my No.2 General Products hull!)
>
Well, you will have to revive that project one day! There is nothing like GP
hulls!

Thomas

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At:
http://www.larryniven.org/images/ringworldart/ringworld1.jpg

Is this image made with POV-Ray by Schuyler Horn in 1998.
Not much different from yours!

Thomas

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