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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] hotmail com> 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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Among other things, Darren New saw fit to write:
>> 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".
Yes, but we "notice" tides in relation to the solid earth, so the liquid
sphere being deformed while the solid one remains rigid is what we call
tide, isn't it (in a broad sense)?
Anyway, I agree my explanation may not be the best one, and the differential
rotation speed could be a "more true" one. I recall this was beautifully
explained by Isaac Asimov (back to science fiction) in one of his writings.
Also, it is the tidal force (wich in truth also works on solid bodies),
which has made the rotation periods of the Moon around itself and around
the Earth to match, so that we see always the same face of the Moon. Which
could be a bit harder to explain is the marvelous chance of the Moon not
only having almost the same apparent size as the Sun from Earth, but being
sometimes a bit larger and sometimes a bit smaller, so that we can see
total as well as annular eclipses. We are so lucky!
--
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 wrote:
> Yes, but we "notice" tides in relation to the solid earth, so the liquid
> sphere being deformed while the solid one remains rigid is what we call
> tide, isn't it (in a broad sense)?
Cetainly, speaking informally, the tide cannot "go out" if there's no
solid beach from it to go out from.
But if you measure the shape of a world made entirely of liquid, you
still get two bulges. And if you have a world entirely of solids, you're
still have less apparent gravity at two places instead of one. And if
you're tidally locked (like the moon is with the earth) but with liquid,
you get "tides" that don't move.
That's generally the sort of thing people mean by "tides" when they're
talking about the astrophysics of things.
> Also, it is the tidal force (wich in truth also works on solid bodies),
> which has made the rotation periods of the Moon around itself and around
> the Earth to match, so that we see always the same face of the Moon.
It's also what makes the gaps in saturn's rings, makes the Lagrange
points stable, and lots of other effects as well.
--
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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Darren New <dne### [at] sanrrcom> wrote:
> Jellby wrote:
> > Yes, but we "notice" tides in relation to the solid earth, so the liquid
> > sphere being deformed while the solid one remains rigid is what we call
> > tide, isn't it (in a broad sense)?
>
> Cetainly, speaking informally, the tide cannot "go out" if there's no
> solid beach from it to go out from.
>
> But if you measure the shape of a world made entirely of liquid, you
> still get two bulges. And if you have a world entirely of solids, you're
> still have less apparent gravity at two places instead of one. And if
> you're tidally locked (like the moon is with the earth) but with liquid,
> you get "tides" that don't move.
And here we go back to Larry Niven's work, specifically, Jinx... where it's
tidally locked, entirely solid, and pretty damn close to the Roche limit,
so you've got the two "tides" poking out of the atmosphere...
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