How to detect if you are in orbit?

[Ursa major]

Good set up. (I almost wish I hadn't asked, now.)

[RJM Corbet]

Originally Posted by Ursa major

Good set up. (I almost wish I hadn't asked, now.)

Well, we know certain agents do lurk.

Yes, Vertigo, busy (good) plot ... phew: lot of work ahead of you.

[Vertigo]

Originally Posted by Ursa major

Good set up. (I almost wish I hadn't asked, now.)

It was the current 75 worder that got me thinking about it!

My big problem is to come up with anomalies that the 'adults' can explain away but just implausibly enough to eventually make the older children suspicious.

[Vertigo]

Ah I've found the passage! It was another SF Masterworks story: Babel 17 from Samuel Delaney. Here it is, a bit long I'm afraid even though I've cut some incidental stuff out:

"A watch," saidRydra, "and a—bag of marbles!"
"Huh?" asked Calli.
"Mar-bles?" articulated Mollya wonderingly. "Marbles?"
"One of the kids in the platoon must have brought along a bag of marbles. Get it and meet me in G-center."
She jumped over the ruined skin of the bubble seat and leapt up the hatchway, turned off at the radial shaft seven, and launched down the cylindrical corridor toward the hollow spherical chamber of G-center. The calculated center of gravity of the ship, it was a chamber thirty feet in diameter in constant free fall where certain gravity-sensitive instruments took their readings. A moment later the three Navigators appeared through the diametric entrance- Ron held up a mesh bag of glass balls.
" What we've got to do is arrange the marbles around the wall of the room in a perfect sphere, and then sit back with the clock and keep tabs on the second hand."
"What for?" asked Calli.
"To see where they go and how long it takes them to get there."
"I don't get it," said Ron.
"Our orbit tends toward a great circle about the Earth, right? That means everything in the ship is also tending to orbit in a great circle, and, if left free of influence, will automatically seek out such a path."
"Right. So what?"
"Help me get these marbles in place," Rydra said. "These things have iron cores. Magnetize the walls, will you, to hold them in place, so they can all be released at once." Ron, confused, went to power the metal walls of the spherical chamber. "You still don't see? You're mathematicians, tell me about great circles."
Calli took a handful of marbles and started to space them—tiny click after click—over the wall. "A great circle is the largest circle you can cut through a sphere."
“The diameter of the great circle equals the diameter of the sphere," from Ron, as he came back from the power switch.
"The summation of the angles of intersection of any three great circles within one topologically contained shape approaches five hundred and forty degrees. The summation of the angles of N great circles approaches N times one hundred and eighty degrees." Mollya intoned the definitions, which she had begun memorizing in English with the help of a personafix that morning, with her musically inflected voice. “Marbles here, yes?"
"All over, yes. Even as you can space them, but they don't have to be exact. Tell me some more about the intersections."
"Well," said Ron, "on any given sphere all great circles intersect each other—or lie congruent."
Rydra laughed. "Just like that, hey? Are there any other circles on a sphere that have to intersect no matter how you maneuver them?"
"I think you can push around any other circles so that they're equidistant at all points and don't touch. All great circles have to have at least two points in common."
'Think about that for a minute and look at these marbles, all being pulled along great circles."
Mollya suddenly floated back from the wall with an expression of recognition and brought her hands together. She blurted something in Kiswahili, and Rydra laughed. "That's right," she said. To Ron's and Calli's bewilderment she translated: "They'll move toward each other and their paths'll intersect."
Calli's eyes widened. "That's right, at exactly a quarter of the way around our orbit, they should have flattened out to a circular plane."
"Lying along the plane of our orbit," Ron finished.
Mollya frowned and made a stretching motion with her hands. "Yeah," Ron said, "a distorted circular plane with a tail at each end, from which we can compute which way the earth lies."

It's possibly a bit complicated for even older teenagers to have worked out but then on such a ship astrophysics is certainly something they would be studying. Also since the book was written in the 60s I'm not sure if the technique is a valid one.

[Ursa major]

Imagines:

"You need marbles?" asked Jill.

Bill explained what he intended to do.

Jill smiled. "So you have lost yours."

[Vertigo]

Yes, I also like that fact that Delaney assumed marbles would still be played with the same enthusiasm as in the 60s - "One of the kids in the platoon must have brought along a bag of marbles." I remember back then no self respecting kid was without their little bag of marbles (sane or otherwise!).

[Bowler1]

I can remember the name of the book I'm going to discuss, maybe someone else will remember but is one of the SF Masterworks series.

A generational star ship traveling between planets but its traveling so long the passengers regress and have different tribes between the levels. Eventually one of the characters discovers a control room and the plot quickly descends into a nice quick ending, in orbit around earth I think which felt like a cheat.

Anyway, sealed in or not there would be a control room somewhere. The discovery and finding of said room could be interesting. Just try and have a better ending than the one above Veritgo, I hate lazy endings.

Do they still make marbles?

[RJM Corbet]

Cool. You found it.

Reads quite well, at first.

But I faded at: 'The summation of the angles of intersection ...'

Nevertheless, there you are.

I think ...?

[Vertigo]

Yes I am rather daunted by that Delaney's explanation (I started glazing at the same point RJM and seem to remember doing so first time I read it as well) and any attempt to simplify it without fully understanding it would probably be... unwise. However I do quite like the idea that they would need to make their way to the habitat axis (zero centrifugal force) to test that one out. Problem would be setting the marbles (or whatever) up without any intial motion. Delaney's 'iron cores' struck me as a bit of a deus ex machina; I don't think I've ever come across marbles with iron cores. Ball bearings might be a bit more plausible and surely something that would be knocking around somewhere in the machine shop of a spaceship.

Bowler once they have figured out they are in orbit I thought a little pilfering of an 'adult's' logon details followed by hacking of the system to get the real sensor data...

I'm sure you could still get marbles somewhere...

[RJM Corbet]

Originally Posted by Vertigo

... I'm sure you could still get marbles somewhere...

Oh, you can.

A marble is a good way of getting a shelf or table level, if you haven't got a spirit level handy ...

[chrispenycate]

So, is the thing in free fall, or rotating to simulate gravity? If weightless you could, with a sufficiently large block of fluid detect tides in it geometrically, or use a 'Forward mass detector', (yes, invented by the same Robert L. Forward who wrote Dragon's egg, and Rocheworld) http://www.gravityresearchfoundation...ell_morris.pdf
which can detect mountain ranges from orbit, let alone planets.

It would still be possible to detect tiny differences in weight between the 'bow' and 'stern' of your orbiting cylinder, or use the Forward detector if it were spinning on its axis to keep people stuck to the walls, but be much more difficult, the centripetal force masking the minute variations. Obviously, you'd know you were spinning, but that would be the case whether you were next to a planet or lightyears from anything.

Bowler, I suspect 'Non-Stop', by Brian Aldiss. And marbles are still for sale in toy shops, despite the obvious H&S risks, of swallowing them or trading on them and falling over…

[RJM Corbet]

Originally Posted by chrispenycate

... despite the obvious H&S risks, of swallowing them or trading on them and falling over…

Shhh ... they'll hear you!

[Vertigo]

Yes, it's definitely a spinning habitat Chrispy and I agree that does complicate things, but it seems too implausible to have it otherwise. This is why I thought it would make a nice plot excursion for the children to head off to the axis (at either bow or stern) to get to a weightless environment. Actually that might not be such an 'adventure' as they would almost certainly have such weightless chambers in the ship for anything from manufacturing special materials to a play area.

Unfortunately that document comes up as corrupt when I download it and try to open it

Actually I do still like the Delaney idea. Let's see if I can simplify it for my purposes since I don't need to figure our position only that we are in orbit around something.

So anything orbiting another body will have it's centre of orbit at the centre of gravity of the other object, the planet in this case. At least that is true (as in this case) where there is a massive difference in the masses of the two bodies. So you get to the axis area of the ship and so have no relative gravity. Suspend several objects reasonably far apart and motionless with respect to each other (that could be difficult but a long electromagnet and several steel ball bearings 'stuck' to it might work when the electromagnet current is turned off. Now assuming we are orbiting a planet each object will naturally 'fall' in a great circle centred on the same point of the planet. These orbits must intersect twice each time they (and the ship) orbits around the planet. So their paths will tend to converge and cross. They won't necessarily collide since it is highly unlikely they have been placed so each orbit is exactly in phase. However the converging and diverging should be visible over the period of an orbit, which would likely only be an hour or so. Whereas if they are not in orbit around a planet they will never converge.

What do you think? I'll need to work on it to make it clearer - a drawing would make it so much easier

[RJM Corbet]

Originally Posted by Vertigo

... What do you think? I'll need to work on it to make it clearer - a drawing would make it so much easier

You can do that you know Vertigo, in an appendix, although I know you've said this is a story, not a book.

I've got two appendices already, one contains drawings and tables of figures, another scans of old letters ...

[Vertigo]

Yes and occasionally they do appear in the text of novels. But you are right in that I see this as a short story, I don't think the idea has enough in it for a novel. However I suspect a magazine would baulk at the idea. Not that I ever exepct to publish, it's just a fun idea at the moment.