To clarify, this refers to two spacecraft that are parked at the L4 and L5 Lagrange points. As you can see in the link, from those points you can clearly be able to see anything orbiting in opposition to Earth (approximately, but not exactly, at L3).

In fact, there are no objects at L3, according to... top... men.... [Always cite your sources, gang!]

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Yes, the STEREO mission (Solar Terrestrial Relations Observatory) was a pair of satellites launched into solar orbit in 2006, one going slightly faster than the Earth, one slightly slower, so that over years they move to the opposite side of the Earth's orbit from us and are able to see the "back side" of the Sun. STEREO-B (Behind) died in 2014, STEREO-A (Ahead) is still operational.

By a funny coincidence, as you're asking this question now, STEREO-A has almost completed its first "lap", and is coming back past the Earth this summer. So it can't currently see what's going on on the other side of the Sun because it's got almost the same view we do.

https://stereo.gsfc.nasa.gov/ https://stereo-ssc.nascom.nasa.gov/where.shtml

There's also the Parker Solar Probe and the ESA Solar Orbiter, which are currently making tight elliptical orbits close to the sun, which bring them to the "back side" once every few months. Right now the Parker probe is pretty close to the far side of the Sun from us, and the Solar Orbiter is between us and the Sun.

http://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Where-Is-PSP https://www.esa.int/Science_Exploration/Space_Science/Solar_Orbiter/Where_is_Solar_Orbiter_Track_ESA_s_Sun_explorer

I believe OP is trying to describe seeing what is on the other side of the sun, at any given time, from the vantage point of earth.

To essentially see “through” the sun to observe what an earth bound viewer can never see.

I assume that. Our own solar system has planets that differ A LOT in size, speed, orbit, material composition, etc. I think it’s more plausible to find aliens in mars than another planet hidden on the other side of the sun.

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Wouldn’t the rotation of the earth bring the water to the equator? Bulge and all that…

Obviously he is talking about sneaky aliens, duh.

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[Edit: It seems like lots of commenters are interpreting this question quite differently. I interpreted OP as imagining an initial condition of an effectively 2-dimensional single-molecule-thick free surface of the Earth. I think I did this because otherwise I wasn't sure how the "smooth sphere" assumption would have been particularly necessary.]

Water drifts toward the equator. If there is already stuff there, like the surface has a bunch of water, or it's smooth and in moving the water has to climb up to move over from its initial position (since the Earth begins as perfectly smooth), then you have a complication where once the water gains altitude it is moving "too slowly" compared to the linear speed at that larger distance from the Earth's center. Thus the water appears to drift backwards (West) until friction with the surface below brings it to the "correct" speed that is locked with the rotation of Earth. The Westward drift from gaining altitude is of course the coriolis effect. If it's just a single drop of water that has to climb up once over the hard surface, then that's all there is to it, and it can continue onward [Edit: I'm dumb -- as the drop moves toward the equator, its distance from the axis of rotation of the Earth of course increases (i.e. that is the "horizontal" component of the radius), so the drop continually finds itself moving too slow for the Earth underneath -- thus still appearing to drift Westward.] -- unless the surface is frictionless I suppose?

If in this scenario the Earth is an entire (2D) ocean of water, then it gets especially interesting, as the water will try to bunch up at the equator and reach an equilibrium height with a bell-shaped curve outward. The formation process will keep the water moving East-West currents parallel to the equator until it reaches equilibrium, at which point it all has to come to rest.

If you add our Moon in there, with tides, then you'll definitely have continuous currents from the coriolis effect, since the water will be moving both vertically and in whatever off-axis (or even on-axis) direction the Moon is orbiting in this scenario.

The earth is spinning and the mass wants to move to the outer edge of the spin, which is the equator, which is why the earth bulges a little there. For the same reason, the water would want to move to the equator.

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I understand that now, but we wouldn't need a satellite(even if we have many now). We'd know by the way other objects/planets interact with it

If you draw a circle, the smaller you draw it, the more it seems to look like a dot. The larger you draw the circle, the more the curve is so spread out, that when you focus on one part of that huge circle, it begins to look like a long line with a slight curve. Keep making it larger and the curve seems to be less and less noticeable.

You can also see this effect by holding a ball and moving it closer or further away from your eyes while focusing on the curve at the edge.

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He’s going on about the twin earth theory where there’s a second earth on the exact opposite side of the sun in lockstep with our earth so in theory we would never know what’s there unless we just leave a satellite hanging around somewhere that isn’t in a orbit around earth like mars or something

> This was the plot of the movie Melancholia. There was another Earth on the same orbit as ours, but we had no idea because it was always on the exact opposite side of the Sun ftom us. Until something happens and the two worlds are set to collide.

That's... not it at all.

Melancholia was a rogue (planet not in orbit of our star or any other star, but flying freely through the galaxy) gas giant (not another "Earth") that entered the solar system without being noticed at first because it entered on the other side of the Sun.

The collision happens because Melancholia enters into Earth's orbital path as it's coming through the solar system - not because it was "always on the same path."

I live on a hill in a hilly town. If this surface is so flat why did my car roll away when I forgot to put the parking brake on?

???? What are you talking about? Why something would be exactly at the opposite side of the sun, relative to earth? Nothing has the same speed, mass, gravity, etc to make this event possible. Are you contemplating the possibility that there’s something hidden behind the sun?

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I think I understand what you're trying to say, that water would sit at the poles since there will be less inertia there? But that would be missing the entire point of inertia, think about the practical applications that would often be labeled as centrifugal force. The fastest point on a spinning sphere is the equator.

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We used to call the notional "opposite planet" by the name Nemesis. Pretty sure it's not there.

This was the plot of the movie Melancholia. There was another Earth on the same orbit as ours, but we had no idea because it was always on the exact opposite side of the Sun ftom us. Until something happens and the two worlds are set to collide.

I don't have an answer to your question though, sorry.

I would THINK even if we didn't have visual confirmation, we might be able to detect an object in other ways. Asteroids making weird turns because of the gravity or something.