Fellow Posted January 9, 2004 Share Posted January 9, 2004 Let's imagine a planet which has sun and moon whose apparent size is same, just like in Earth. If the lengths of the year, month and day on the planet were same, then the planet would always turn same side to the Sun. Moreover, if the moon was all the time between sun and the planet, then in the center of the light side there would be eternal solar eclipse. The climate of this area would be cold, so it would come some kind of third pole of the planet. Is borning of that kind of planet (same year, month and day) in practice possible. Another question: what kind of combination of year, month and day there should be, that in some area of the planet there would be eternal solar eclipse in the day, but also normal night so that sun would rise and set moon all the time in front of it. Link to comment Share on other sites More sharing options...
Sayonara Posted January 9, 2004 Share Posted January 9, 2004 You can ignore the month as far as this problem goes. I'm not sure a moon can stay between planet and star. It pretty much has to orbit the planet, not just hang nearby. Link to comment Share on other sites More sharing options...
JaKiri Posted January 9, 2004 Share Posted January 9, 2004 It wouldn't be just 'hanging' nearby, it would be doing an orbit, just one with exactly the same period as the planet. Even if something like that could happen (can't be bothered to work it out) tidal forces would change the moon's orbit, so it wouldn't be sustainable. Link to comment Share on other sites More sharing options...
Sayonara Posted January 9, 2004 Share Posted January 9, 2004 How can the moon be orbiting the planet if it maintains the same position between the planet and the sun? To MSpaint with you. Link to comment Share on other sites More sharing options...
JaKiri Posted January 9, 2004 Share Posted January 9, 2004 Sayonara³ said in post #4 :How can the moon be orbiting the planet if it maintains the same position between the planet and the sun? To MSpaint with you. Can't be bothered with paint. Assume the planet isn't rotating, and points in the same direction all the time. This means that theres a constantly changing face of the planet, well, facing towards the sun, and, hence, the moon. If the face of the planet towards the moon is changing without the planet rotating, the moon must be in orbit around it. Link to comment Share on other sites More sharing options...
multi Posted January 9, 2004 Share Posted January 9, 2004 it would not be a moon of the planet if it was not orbiting the planet. It would be a smaller planet or moon orbiting the sun with the same peroid as the planet it was eclipsing. Link to comment Share on other sites More sharing options...
Sayonara Posted January 9, 2004 Share Posted January 9, 2004 MrL_JaKiri said in post #5 : Can't be bothered with paint. Assume the planet isn't rotating, and points in the same direction all the time. This means that theres a constantly changing face of the planet, well, facing towards the sun, and, hence, the moon. If the face of the planet towards the moon is changing without the planet rotating, the moon must be in orbit around it. Oh I get it. All rest frames are equally valid. What is the likelihood of this scenario? Link to comment Share on other sites More sharing options...
multi Posted January 9, 2004 Share Posted January 9, 2004 In an infinate universe the probability is very high that it does occur as in an infinate universe all probabilities are likely. ...If of coarse it is possible which I have no idea on ... Link to comment Share on other sites More sharing options...
Sayonara Posted January 9, 2004 Share Posted January 9, 2004 The amount of matter and energy in this universe is finite though. Therefore the number of star systems must be too. Link to comment Share on other sites More sharing options...
multi Posted January 9, 2004 Share Posted January 9, 2004 sorry can't answer on the probability or likly hood not my field i'm afraid... Link to comment Share on other sites More sharing options...
JaKiri Posted January 9, 2004 Share Posted January 9, 2004 multi said in post #8 :In an infinate universe the probability is very high that it does occur as in an infinate universe all probabilities are likely. ...If of coarse it is possible which I have no idea on ... The universe isn't infinite in that sense though. It's infinite in that you'll never reach the end if you travel for ever, but in size it is always finite. Link to comment Share on other sites More sharing options...
Sayonara Posted January 9, 2004 Share Posted January 9, 2004 multi said in post #10 :sorry can't answer on the probability or likly hood not my field i'm afraid... Don't sweat it, I was asking MrL Link to comment Share on other sites More sharing options...
JaKiri Posted January 9, 2004 Share Posted January 9, 2004 Sayonara³ said in post #7 : Oh I get it. All rest frames are equally valid. What is the likelihood of this scenario? Given the precise nature of it, the chance (as for any individual event) is nigh on nil. [edit] I'll just get a simplified version of the mathematics done... [edit2] Cancel that, I can't find pen or paper. Link to comment Share on other sites More sharing options...
multi Posted January 10, 2004 Share Posted January 10, 2004 MrL_JaKiri said in post #11 : The universe isn't infinite in that sense though. It's infinite in that you'll never reach the end if you travel for ever, but in size it is always finite. Hense the reason I did not say it was a certanty... Link to comment Share on other sites More sharing options...
multi Posted January 10, 2004 Share Posted January 10, 2004 question: In an infinite universe every possibiltiy would occur an infinite number of times right? Link to comment Share on other sites More sharing options...
Radical Edward Posted January 10, 2004 Share Posted January 10, 2004 MrL_JaKiri said in post #11 : The universe isn't infinite in that sense though. It's infinite in that you'll never reach the end if you travel for ever, but in size it is always finite. well it could be infinite in extent, but the problem arises of course that you can only see as far as the light has been travelling as a result of the finite age of the universe. i.e. right now we can only see 13.7 billion light years maximum. Link to comment Share on other sites More sharing options...
JaKiri Posted January 10, 2004 Share Posted January 10, 2004 Radical Edward said in post #16 : well it could be infinite in extent, but the problem arises of course that you can only see as far as the light has been travelling as a result of the finite age of the universe. i.e. right now we can only see 13.7 billion light years maximum. It could have no end, yet be finite (like the surface of the earth). Indeed, if you accept that at some point the size of the universe was finite (big bang), then it must always be finite. Link to comment Share on other sites More sharing options...
VendingMenace Posted January 10, 2004 Share Posted January 10, 2004 In an infinite universe every possibiltiy would occur an infinite number of times right? Not nessesarily. It would depend on the size of the universe's infinity and the number of possible things. If the infinity that is the number of possible things is larger than the infinity of the universe (i belive it is, but i forget the reasoning) then it is false to claim that all things must happen. Link to comment Share on other sites More sharing options...
JaKiri Posted January 10, 2004 Share Posted January 10, 2004 VendingMenace said in post #18 : Not nessesarily. It would depend on the size of the universe's infinity and the number of possible things. If the infinity that is the number of possible things is larger than the infinity of the universe (i belive it is, but i forget the reasoning) then it is false to claim that all things must happen. .... Link to comment Share on other sites More sharing options...
Radical Edward Posted January 10, 2004 Share Posted January 10, 2004 MrL_JaKiri said in post #17 : It could have no end, yet be finite (like the surface of the earth). Indeed, if you accept that at some point the size of the universe was finite (big bang), then it must always be finite. a closed surface is one option of course, but so is a surface of infinite extent. you can say that at the big bang singularity, the distance between all points was zero. this does not limit the number of points later on, i.e. the number of points one metre apart from one another, to be a finite quantity. Link to comment Share on other sites More sharing options...
Radical Edward Posted January 10, 2004 Share Posted January 10, 2004 sorry, I can't edit posts from this machine because it is in chinese. I meant to add that there are a finite number of points that are one metre apart on a closed surface like a sphere. Link to comment Share on other sites More sharing options...
Radical Edward Posted January 10, 2004 Share Posted January 10, 2004 back on topic though, what you need is a moon with an orbit of a year, and then you need to check that this orbit is actually a stable one (i.e. it won't get pulled off by the sun or any other bodies in the solar system. assuming that this is possible, and assuming it is done with our earth sun and moon, then I suspect the size of the shadow would be miniscule, if not microscopic. you might have better luck positioning the centre of this moon at a lagrangian. Link to comment Share on other sites More sharing options...
JaKiri Posted January 10, 2004 Share Posted January 10, 2004 Radical Edward said in post #20 : a closed surface is one option of course, but so is a surface of infinite extent. you can say that at the big bang singularity, the distance between all points was zero. this does not limit the number of points later on, i.e. the number of points one metre apart from one another, to be a finite quantity. That's a logical fallacy; there can be infinite number of points in a finite space therefore it's possible for the space to be infinite isn't possible without a non-finite rate of expansion, which I don't believe is under consideration and I've just seen your edit. Back on topic, as I said before Redward, it is entirely possible for the system to be in existance, just not stablely (nasty nasty word); this can be because of tidal effects, either in the planet or the sun, influence of other bodies (be it a gravitational effect from another planet, or a collision with a comet or the like), but whatever it is, the system will be a positive feedback loop, so it's going to end sometime. Link to comment Share on other sites More sharing options...
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