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One of the STEREO satellites has been at the earth-sun L4 point, so we have "looked behind the sun", and verified that there is no counter-earth there. That was not the purpose of their mission though.


According to the Wikipedia article Counter-Earth, this idea has been floated around for millennia. However, modern day earth-bound techniques would easily detect a 'Counter-Earth' via the following (but not limited to) these methods: Gravitational perturbations would have been observed to have occurred on the other planets (terrestrial) As Earth's orbit ...


It would be detectable due to its gravity effect on other planets, comets and the Sun. Check these sites for further info: http://en.wikipedia.org/wiki/Counter-Earth#Scientific_analysis http://en.wikipedia.org/wiki/Lagrangian_point#L3


There is a very nice web-based utility called Staralt that I frequently use before observing runs. This has lots of observatories pre-programmed or you can put in your own latitude and longitude AND altitude (can you do that in Stellarium?). To quote the website - "you can plot altitude against time for a particular night (Staralt), or plot the path of ...

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