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I've enjoyed Robert's writing for over 40 years, but I haven't read everything he's written. My wife got "The cat who walks through walls" and as expected, I've enjoyed it, both for the enjoyable plot and descriptions that were scientifically accurate. I think he wrote Neutron star and that had a section using tidal forces that matched exactly what we know about physics if we had the equipment that could do what that spaceship was capable of doing.
Now our hero is on a space "ship?" with multiple rings that has multiple gravity levels, and all that is easily explained by centripetal forces. Now for the part where all things we know about physics get's thrown out the window.Heinlen wrote: What does it have to do with a tidal lock on Luna; the forward end points forever straight down at the Moon.
First off, on a spinning ship, where would the forward end be? To me, it would have to be on the center axis line that the ship is spinning on and forward would arbitrarily be one direction or the other. There is a way that the orientation of the ship wouldn't fight the tidal forces of gravity and that is with the axis of the the orbit and the axis of the spin being parallel. (Well sort of, then the moon would be using tidal forces to slow down the spin of the ship. Just like it is trying to do with the earth, but more successfully because of distance and mass.)
With a Top, it has a pointed end and a body you wrap a string around and throw it while holding on to the string. Immediately, earth's gravity tries to get the Top to topple, but it fails. If you throw it right it stays pretty stationary at first and then it starts to precess as it slows down. That speeds up the slow-down of the spin until the top does topple and quickly loses all its spin. But the spin has to slow significantly, to get it to topple.
Yes, tidal forces have slowed down the moon until it's rotation matches it's orbit around the Earth. The sun is putting tidal forces on the moon but they are ignored because the Earth's tidal force is MUCH greater. The moon is applying tidal forces to the Earth and it is slowing it's rotation down (about as much as a mach truck is slowed down when it hits an ant). It is much better at slowing the water on the surface of the earth down. (Creating tides)
The moon would try to slow the ship's spin, but if it succeeded in stopping the spin, the whole ship would be in zero gravity. If the axis wasn't parallel to the station's orbit, the station would precess like a top but the angle of forces would continue to change orientation and everything would be working to slow the station's spin, which would reduce the induced centripetal force, but would be negating the reason for having the spin. So you would have engines to speed the spin up again when the moon started to slow it down. (Negating the tidal force with engine force.)
If you could get the moon spinning every 24 hours with the current moon's face as the axis of spin, the moon's face would have the same orientation to the earth every 28 days instead of 100% of the time. We would eventually have a new "face on the moon" but it would take many centuries to get it.
Oh well, at least he rarely disappoints.
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First, Heinlein didn't write Neutron Star, Larry Niven did. It's one of his Beowulf Shaeffer stories - you can get his full collection of Beowulf stories, along with a story that ties them all together, in his book Crashlander.
Second, while, "front" of the ship may be a bit ambiguous, it seems pretty clear he meant one end of the center of the ship which is the axis of rotation for the rings. With the axis of rotation pointing directly towards the center of the moon while in orbit, it could be considered sort of "tidally locked". The rotation of the rings around that axis could be made dynamically stable in this configuration.
And lastly, I very much enjoy Heinlein's writing as well, though my personal opinion is that he peaked with Time Enough for Love - much of his work after that got pretty strange (though I still did enjoy To Sail Beyond the Sunset, but mostly for the portion of the story about Maureen's early life).
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Thanks for the correction of authors. Maybe H isn't as scientifically adept as I thought.
"With the axis of rotation pointing directly towards the center of the moon while in orbit" Again, this station is like a top. That top continues to point to the center of the Earth against Earth's massive gravity trying to upend it. We are talking short-term so the top's refusal to drop is because the top's axis of rotation starts directly pointed to the center of the earth.
If you put a gyroscope to spinning on gimbals with the axis originally pointed directly down and keep it spinning for 24 hours, that gyroscope would end up rotating close to 360 degrees relative to Earth's surface anywhere reasonably close to the equator.
You put that gyroscope spinning horizontally on the equator with the axis pointed to true north, over 24 hours it shouldn't rotate it's orientation at all.
Once a body is in motion, it tends to stay in motion. That rule is consistent even with rotating bodies.
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KP Lee wrote: Again, this station is like a top. That top continues to point to the center of the Earth against Earth's massive gravity trying to upend it.
But that doesn't sound right to me. the reason a top upends is because its point cannot move, but the rest of it can.
If you stand a pencil on a table it will fall over.
If you drop a pencil from a height it will land point down.
The Earth isn't trying to upend it!
The gyroscope effect will keep a spinning body oriented in the same direction, relative to the universe, as it moves - so you are right in that, if the axis is pointing toward the moon, and the ship is in orbit, then it would tend to rotate through a vertical plane through 360 degrees each orbit.
H suggests the tidal forces act on the ship - so is he assuming the tidal force is great enough to overcome the gyroscopic force?
PooperPig - Coming Soon
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Maxxx wrote: the reason a top upends is because its point cannot move, but the rest of it can.
Sort of right, but not quite. The top upends because the point is not a stable platform. You try to balance a top on it's point and let go, within seconds it will have tipped over. (Unless you stick it in sand, but that just widens the support base.) It doesn't immediately topple when you release the spinning top because the gyroscopic force overcomes the natural desire to topple and the point is a quite stable platform. In fact if the top lands unbalanced it will spin in smaller and smaller circles until the point comes to a complete stop (Relative to the floor location) and spins in place. The point hitting the floor is an anchor point that tries to keep the top in one place. The gyroscopic action is the stabilizing force. It finally becomes an unstable platform as the top slows it's spin.
Maxxx wrote: so is he assuming the tidal force is great enough to overcome the gyroscopic force?
I can't know what he is assuming. A spaceship with multiple rings ranging from 0.01 G's through 1 g levels would be massive, probably in the millions of KGms of material. Yes, the closer the orbit the bigger the tidal force, but the moon hasn't come close to stopping the Earth. I'm guessing putting a 100 Gm Top spinning in a 100 M orbit around the moon would take more than a month to stop spinning because of tidal forces. (It would fairly quickly start precessing.)
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Gjeltema wrote: he peaked with Time Enough for Love
Have to agree with you there, although Moon is a Harsh Mistress is my favourite among many. I have read a number of authors who are touted as the new Heinlein, Spider Robinson comes to mind, but they are not a patch on the RAH.
I think having someone pick at his scientific accuracy after he is dead would hugely amuse him.
Never underestimate the power of human stupidity
RAH
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Given your username, I'm not surprised that Moon is a Harsh Mistress is your favorite. 
I thoroughly enjoyed most of his books/stories before and after Time Enough for Love, I just personally found that to be his best book (it's one of my top 2 favorite books overall).
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Most think Mycroft comes from the detective guy, I always liked the idea of a self aware computer 
Never underestimate the power of human stupidity
RAH
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I must come to Heinlein's defence here.
The quote comes from Chapter 8 of The Cat Who Walks Through Walls. We have a cylindrical space station, spinning about the cylinder's axis. This gives you a space station made of cylindrical shells, each with its own level of gravity due to centripetal force.
The cylinder is quite long (at least a few hundred meters, judging by the presence of scooters inside), and was built with the axis of the cylinder pointing at the Moon. For a non-rotating cylinder, this position will be maintained because the gravitational pull on the near end of the cylinder is higher than that on the far end. This is your tidal lock.
As I see it, the problem here is that the cylinder is rotating. Forcing the axis to always point at the moon requires torque to be applied, and I am unsure whether the Moon's gravitational force provides enough. I have not read the book recently, but IIRC it doesn't give us enough information to do the calculation.
[Height over the Moon's surface - 300km (Chapter 9)
Length of the cylinder - ???
Radius of the cylinder - ??? <==> Rate of rotation - ???]
If Heinlein sinned here, it was not a great sin.
Gentlemen (and Ladies) of the Jury, give me your verdict. 
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
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Your Honour; he mentions not if the cylinders were counter-rotating either ... not sure if it makes a difference, anyway, but wouldn't counter-rotating areas cancel out the gyroscope effect and allow it to maintain its attitude?
PooperPig - Coming Soon
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You have to have all cylindrical shells rotating in the same direction, otherwise transferring from one shell to the next would require very precise timing (think about it...). The same reasoning applies to an extension of the cylinder.
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
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Think paternoster
PooperPig - Coming Soon
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If the cylindrical shells were counter-rotating, you would still have a problem of the lift going through the shells. I don't see how you would arrange this without the shells being in segments, broken where the lift goes through the shell.
I'm not a mechanical engineer, so perhaps there is a way to get this to work, but I can't see it.
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
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you don't need a lift - as far as you're concerned it's walking through a doorway from one to the other - might be a bit jerky on the old legs I suppose, but it's not vertical, it's horizontal (from a 'gravitational perspective' point of view)
PooperPig - Coming Soon
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I still can't visualise it; have you any virtual napkins to use for drawings?
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
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Imagine putting two hamster wheels side-by-side.
Spin one clockwise, the other anti-clockwise.
A well timed jump could take you from one wheel to the other.
So, now add 50 wheels.
The one at the far end spins fast
Next to it spins one slightly slower
And so on, until the middle, where it doesn't spin.
The *next* one spins slowly in the opposite direction.
then faster and faster until the ends.
Obviously, for docking purposes, it would be best to also have continually slowing rings out toward the ends so we can dock without having to match rotation - but as lonog as we duplicate the situation at each end, but in the opposite direction, then the rotations cancel each other out.
As I mentioned- I have absolutely no idea if this reduces the gyroscope effect at all!
PooperPig - Coming Soon
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I see your point, but this will not solve the prime problem of providing maximal living space. A rigid set of concentric cylindrical shells, all rotating together about their common long axis (same angular velocity for all shells), does so admirably - you can have full Earth gravity at the outer shell, going down as you get closer to the main cylinder's axis.
If you have a non-rigid set of shells (each shell rotating with a different angular velocity), you must have some sort of arrangement to keep the shells rotating smoothly past each other. Moving from one shell to another is much more difficult proposition, and God help the inhabitants if the "ball bearings" seize up...
The rigid set of shells can solve the docking problem very nicely. Docking is always at the axis. Either your spacecraft match rotation with the space station and dock, or there is a counter-rotating docking station at the axis, which (after the spacecraft has undocked) speeds up in order to let the passengers cross into the space station. IIRC, the second choice is the one used by Heinlein, Clarke, and others.
If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack.
--Winston Churchill
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I thought "Surely, it is the end of days" and "darkness triumphs in the end". Donning sackcloth and ashes was in order. Deities, whether above or below, would certainly now allow such perversion, such a festering canker, to erupt within universe.
Thank goodness it was a mere April Fools joke.
| "The difference between genius and stupidity is that genius has its limits." - Albert Einstein | | "As far as we know, our computer has never had an undetected error." - Weisert | | "If you are searching for perfection in others, then you seek disappointment. If you are seek perfection in yourself, then you will find failure." - Balboos HaGadol Mar 2010 |
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I'm trying to set a value to the maximum smalldatetime allowed in SQL, yet what's appearing is "6 June 1979". Nothing I do seems to work: ensuring Culture is OK, ensuring I'm passing dates around in a sensible format, ensuring the data I'm setting is the actual date I'm setting.
In the debugger I see _date with a value of "6/06/2079 12:00:00 AM" (see rant below). The date is fine. I pass this into a formatter to turn it into "dd-MMM-yy" format. That works perfectly and the output is correct. This string goes to the user, they hit submit, it comes back and is parsed fine. Everything works perfectly.
Except the date always comes back as 1979.
ARGH. And then I spot it.
15 years too late I've been hit by the Y2K bug.
6/06/2079 12:00:00 AM is 6 Jun 2079 -> "6-Jun-79" which is parsed as 6-Jun-1979.
/more coffee, then better formatting.
As a side note: The American date format (mm/dd/yyyy) is painfully and dangerously ambiguous so given that OS installs are often set with US as the region, surely debuggers should display dates and times as dd-MMM-yyyy or even better, yyyy-mm-dd to account for the Rest Of The World who wants to scream everytime they get an email saying "the date is 4/7/2015".
cheers
Chris Maunder
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The American date format of MM/DD/YYYY is simply idiotic, as is our ridiculous clinging to Imperial measurements, when the metric system makes so much more sense. Why? I believe we Americans are just too lazy to adapt to a new system, despite the fact that it just makes sense to do so!
OK, I have my coat and am long gone! 
How do we preserve the wisdom men will need,
when their violent passions are spent?
- The Lost Horizon
modified 23-Jul-15 7:14am.
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How does the metric system make more sense? It's no less arbitrary and the assumption that people will find the arithmetic less difficult is highly doubtful (shortly after we decimalised currency in the UK I was delayed by several minutes in a shop because the assistant needed to fetch a calculator to work out a 10% discount!) The imperial weights fit particularly well with the quantities of things that people might want to buy - I know exactly what I'm getting in a quarter of mushrooms, an ounce of yeast or a pound of mince and so that's what I continue to ask for despite the EU's attempts to homogenise us all with baffling amounts. Long live the pound, the pint and the mile, I say.
And to return the date. It is of course the departure from the Imperial norm made by Americans (when and why I have yet to discover) that causes all the problems. Who else could come up with a system that takes a unit from its rightful place in between the lower and the higher and sticks it at the front? You don't do mm:hh:ss so why MM:dd:yy Bewildering!
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I believe the US Navy uses dd/MM/yyyy. Of course, the correct usage should be yyyyMMdd. No ambiguity there at all.
Long live Imperial!
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True, and year should be yyyy, not yy only.
The sh*t I complain about
It's like there ain't a cloud in the sky and it's raining out - Eminem
~! Firewall !~
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Neither imperial nor metrics system makes sense. Check the very basic physics for this 
modified 19-Jan-21 21:04pm.
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