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Using stars to tell if the world is round


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Hi,

Apologies if this is the wrong forum for such an enquiry - but I thought it would be worth a go.

I'm working on a novel set on a train that endlessly circles an alien planet. The train travels around the planet's equator through a featureless desert - from west to east, with the sun rising ahead of it each morning and setting behind it each evening. For reasons that eventually become apparent, the knowledge that the train is circling a planet has been lost by the inhabitants of the train (the journey has been underway for thousands of years). They believe that they are on a journey that will eventually reach its destination - with each new generation believing that it will be the one to finally arrive.

My question is: Could someone on such a train determine that the world is actually round based on observations of the stars? What feature of the stars above might cause someone to question the prevailing belief that they are travelling along an almost endless, flat plain?

Any ideas would be greatly appreciated!

Regards, 

Adam

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I the train was on one long straight journey, the stars and the constellations would change slowly, but would always appear in the same part of the sky and would only drastically change over hundreds of thousands of years. Circling the planet on a train would show the whole sky of stars and constellations move like we see them on earth; I.e. Orion would always appear in the winter time; Leo would appear more in the summer time etc, and the other constellations would move so that they would appear at certain times of the year as we travel around the sun.

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I would have thought that their sun rising ahead of them each morning and behind them each evening was a bit of a giveaway that they are not travelling on an endless flat plain. Unless they believe they see a different sun everyday, or have a myth a bit like the Egyptians who thought the Sun set in the west and was carried by boat around the edge of the world to rise again in the east.

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The speed of the train would be a factor if it were very fast. An earthly train travelling at 24000 mph round the equator counter to the earth's rotation would negate the rotation of the earth so the train would be in permanent day, night or twilight. If it were night an observer would observe the seasonal change in the constellations on view but no nightly rising or setting.

Olly

PS I think!!  :grin:

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If the lights were on in the train and assuming they were not red and that the occupants could not open the window or venture outside then perhaps the stars could not be seen at all.

Might solve the problem.

I think the fact that the stars change over time and that the sum moves front to back (overhead) would suggest that the sun also goes below them to reach its original starting point hence the world would need to be round.

Edited by beamer3.6m
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Of course the fact that Canopus was visible from Alexandria but not from Athens gave rise to plenty of ancient head-scratching :)

Yes, this sort of effect was one of the classical proofs that Earth is spherical (more generally, the effect of latitude on the height to which stars rise). The others were seeing things disappear over the horizon, and the circular shadow cast by the Earth during lunar eclipses.

But the OP imagines another planet of presumably endless desert (so things aren't seen disappearing over the horizon), with presumably no moon (so no eclipses), and with the train running round the planet's equator (so no change of latitude).

If I were on board I might wonder where the train gets its presumably limitless supplies of fuel, food for passengers, water etc. If the train makes stops to collect these, it provides points of reference that the passengers will eventually see recurring, thus proving their journey is a closed loop. And the refuelling stations presumably have to get their supplies from somewhere, so there might occasionally be roads seen to run off to the horizon. I also wonder how the train accommodates population growth. And lots of other things.

But personally, if a novel tells a good enough story then I'll go with it, no matter how illogical or impossible. And if it's perfectly logical but doesn't have interesting characters, events etc, I'll give up.

Edited by acey
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Hi,

Thanks for all the replies. It seems like there isn't really an easy way to prove this from stellar observations alone. Maybe I can use the idea of the sun moving overhead to trigger some doubts in the characters. The ultimate reveal comes from a different source, I'm just wanting ways to prod the characters in the right direction.

The points raised by acey are all valid and actually form the crux of the story. The train is massive and has fields, animals, recycling systems etc. on board - plus moisture scoops mounted on the outside to extract what little moisture there is the air for use as water. It never needs to stop and the view out of the windows is unchanging. The problem is that all the on-board systems are starting to break down and nobody understands how to repair them, plus the train itself is slowing as it begins to run out of fuel. The society that originally constructed it has devolved over time and now views the technology from a religious/spiritual perspective. The story examines how the society on board begins to break down, which isn't helped when they realise they are not actually going anywhere...

Thanks again for all the help!

Adam

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My question is: Could someone on such a train determine that the world is actually round based on observations of the stars? What feature of the stars above might cause someone to question the prevailing belief that they are travelling along an almost endless, flat plain?

 

Any ideas would be greatly appreciated!

 

Regards, 

Adam

If a passenger on the train decides to map the stars he might well notice a repeating pattern and realize that he is not travelling on an endless plane under endless stars (at he very least).

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Yep - anyone who noted the positions of the brighter stars would work out they were "in a sphere around the world", and hence the world must be spherical. On an "endless flat plain" the stars would not rise and set. However, you can fix this if you have the planet rotating at just the right speed that the same stars are almost overhead - unfortiunately that would mean no sunrise or sunset. I believe the solution is to make the nights cloudy (or the planet foggy) - that way the people on the train have fewer reference points.

(I hate to think, what powers this train? what if they run out of sandwiches/tins of beer? isn't the septic tank full? doesn't the driver/guard have a union? ..  .. what about delays. ..)

P

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post-39127-0-45323400-1446666459_thumb.j

Hi,

Apologies if this is the wrong forum for such an enquiry - but I thought it would be worth a go.

I'm working on a novel set on a train that endlessly circles an alien planet. The train travels around the planet's equator through a featureless desert - from west to east, with the sun rising ahead of it each morning and setting behind it each evening. For reasons that eventually become apparent, the knowledge that the train is circling a planet has been lost by the inhabitants of the train (the journey has been underway for thousands of years). They believe that they are on a journey that will eventually reach its destination - with each new generation believing that it will be the one to finally arrive.

My question is: Could someone on such a train determine that the world is actually round based on observations of the stars? What feature of the stars above might cause someone to question the prevailing belief that they are travelling along an almost endless, flat plain?

Any ideas would be greatly appreciated!

Regards, 

Adam

Objective: To find if an imaginary world is round when travelling on a moving train along it’s equator without prior knowledge that the world is a spherical planet, using the stars, and if necessary other means.  

Using the Earth as an example, the North Star (Polaris) is probably too low to be visible from the equator even with mean refraction at 34.5 minutes of arc at zero altitude. Instead, a line where the ‘pointers’ of the Great Bear cut the horizon viewed from the Equator would give the Northern axis zenith. For the Southern axis zenith, it’s where the long stem of the Southern Cross cuts the horizon.  Knowing this it is much easier to picture yourself on the equator of a spinning ball, although you could also imagine the heavens are rotating around you.

To calculate you are travelling around the equator of a ball the first step is to establish the speed of the train, timing an object thrown from the front, till when it passed the back, if no access to instruments. Then at night, position yourself at a known height of eye, say between 3 and 6 meters and have an assistant throw a lantern from the front of the train. As the lantern (now at rest) passes, start a stopwatch. When the lantern blinks out of sight stop the watch and calculate the distance at which the lantern dipped under the horizon. Using Pythagorean and Thales’ theorems with a 6 meter height of eye, and lantern dipping distance (Earth horizon distance nm = 2.08√HoE in m) of 9,427 m (5.09nm).

Using Pythagoras: A=90deg, b=9427m, c=6m giving C=0.0365deg

Refraction = √HoE 6m /7 = 0.35’ = 0.00583deg    C+refr = Cr = 0.04233deg 

Using Thales’ Theorem: Angle Cr multiplied by 2, giving CE=0.08466deg.

Using Pythagoras: A=90deg, C=0.08466deg, c=9427m, giving Earth radius 6,379,954m = 6,379.9Km. Actual mean Earth radius is 6371km.

Using circumference =2πr, 2π x6380km gives track length (Equator) 40,087km.

See diagram.

I wish to give credit to the new book called ‘To Explain The World’ by Steven Weinberg for inspiring the use of the Theorems.

Good luck with writing SF, it has many great attributes including introducing people to new ideas and proposing something too controversial for the scientific method to be applied. Also it is my preferred novel type.

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you might want to read (re-read?) Christopher Priest's novel, Inverted World. It has some similarities to your concept.

Alastair Reynolds' Absolution Gap also has similar ideas. In this story, a train makes a never ending journey around a planet in order to maintain a constant vigil on that planet's moon. The authorities are always on hand to crush any inquisitive thoughts.

Edited by Roy Challen
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