I’ll be amazed if this has not been asked before, but it’s late and I’m tired (watched Serenity, History of Violence and Into the Blue all in one day) 
:o .
Anyway, a weird question popped into my head: If there was a planet, same size, on the exact opposite side of the sun from us on the same orbital path, would we ever know? Would there be any sign? And now I go to bed to spare you from more semi-random thoughts.
Yes, it would easily be detectable by its effect on the orbits of the other planets. And due to orbital eccentricity, it would be impossible for such a planet to remain always exactly on the opposite side of the sun so that we wouldn’t be able to see it.
Such a situation is unstable. Even if such an arrangement did occur, differing forces (the lack of a large Moon orbiting Opposite Earth, for instance) acting on each world at different times would cause one or the other to speed up or slow down relative to the other, so that eventually, the two would either collide or interact to eject one or the other or both into space.
Even if the two planets started out with precisely the same orbit, chaotic variations, in for example orbital eccentricity, would likely bring them near enough each other over the course of a few billion years to interact gravitationally. Then there’d either be a collision, or the two planets would get pulled into different orbits.
Curse your logic and your science! :mad:
Most likely, they’d just swap orbits, like Saturn’s moons Janus and Epimetheus, and then drift back apart until they came back on the other side of the circle and repeated the process.
Wasn’t one of the 19th century theories for the reason we couldn’t detect the orbit of Vulcan was because it’s orbit was opposite that of Earth?
No, not that Vulcan.
Just wanted to point out that Sam *Hell * has 666 posts
shiver
OMG, that’s not SamHell. That’s his Doppelganger !
No, not in any serious talks anyway. Vulcan was supposed to be found in an orbit between Sun and Mercury (the whole reason they proposed Vulcan’s existence was the small perturbation in Mercury’s orbit, which Einstein later explained), making its orbital period only a fraction of Earth’s. Therefore there was no way it could’ve stayed in the opposite side of the Sun to us all the time.
As has already been pointed out, it’s the kind of orbit that’s unstable. And therefore fun to simulate.
The most fun is of course had when the timing of the simulation is such that nothing seems to be happening, you go to do something else, and come back and find one planet (satellite, asteroid, whatever) has vanished.
The ‘Antichthon=opposite earth’ is actually where I got my screen name long ago (since cropped down to antich)
I actually remember an early-70s science fiction movie in which an astronaut ended up on just such an opposite Earth. Everything seemed exactly the same as on the original Earth from which he came - they even spoke English! - but he figured out what was up when he noticed two moons in the sky. That’ll do it!
Can’t remember who was in it or anything else about the plot, just that that was the shtick. Anyone else remember this movie?
I agree with everything that’s been said here so far, except for this:
I don’t think this is true. If you took the Earth’s orbit and rotated it exactly 180[sup]o[/sup] about an axis perpendicular to the ecliptic, that orbit is still a valid solution of Newton’s laws. Opposite Earth would then have its perihelion at the same time as our Earth, would have its aphelion at the same time, and so forth. In general, the Earth, the Sun, and Oppostie Earth would always be equally spaced along a straight line, although the spacing would change over the course of the year due to orbital eccentricity.
The stability argument still puts the kibosh on this idea, though.
Sounds like Journey to the Far Side of the Sun, AKA Doppelganger
, starring Roy Thinnes, fresh from the Invaders:
I hate this movie!
Once he gets there, everythin is the same as Earth, except that it’s backwards (the writing, etc.) Ultimately, he tries to get back, but fails because when he connects to a piece of counter-earth eqyuipment, it turns out the voltages are backwards.
Is this all stupid, or what? There’s no logical reason for any of it. Having set up this ludicrous situation, the only defence is to use it to make some moral or satirical point, but they don’t. They blow the entire concept on the polarity of the plug being reversed (essentially). This is the reason you got all these actors and moviemakers together, then went to the trouble of marketing and distributing this film? Plan Nine from Outer Space is more entertaining and innovative!
Plus, as noted above, the basic premise of a Counter-Earth, which has been used on and off for decades, isn’t sound in the first place.
Not sure I agree but I don’t have a cite to make a positive case. But this statement would be true only if the Sun were at the center of Earth’s elliptical orbit, and IIRC it is not. So when Earth is at perihelion and OE is at aphelion, they are on opposite sides of the Sun, and vice versa. But midway between those points, the three bodies would not form a straight line, and the two planets would have line-of-sight view of each other.
You misunderstand. Don’t put it in the exact same orbit. Rotate the whole orbit 180 degrees, so that when Earth is at perihelion, anti-Earth is also at perihelion, and so on. This would be an equilibrium situation, though as everyone in the thread has already noted, an unstable one.
I wish I still did .
The sun is centered at one of the foci of Earth’s elliptical orbit, but the center of the ellipse (and the other focus) is still within the radius of the Sun, IIRC.
I just wanted to note that, according to the “Alternate versions” section of the IMDB entry for this flick (cited above, by me), some prints actually have the “backwards” portions printed backwards, so that the words read correctly forwards. Evidently some technician saw the words printed backwards on the final product, didn’t realize they were supposed to be backwards, and fluipped everything over. (Kinda like the "color correction faux pas that happened with Susan Oliver in the pilot for Star Trek. No matter how green the makeup they applied, she kept coming out flesh-colored because the developing technician, thinking it was his fault, kept correcting the color).
As a result, the movie makes even less sense than usual, because Roy Thinnes’ character keeps referring to the text being backwards when it isn’t. Clever fans watching the film cleverly figured that they were watching the adventures of thinnes’ duplicate here on Earth. If only the filmmakers had actually been that clever. But alas, they weren’t.