# A mirror in the sky to look at past Earth

Earth reflects light from Sun into Space; are there any "mirror like" objects which reflect back to Earth this light? Has anyone been able to use this method to recover some information on how was the Earth in the past (how it looked, what was it temperature, etc)?

• How long ago in the past are we talking? A couple of seconds? I believe that light from Earth's atmosphere reflected by the Moon has been used for the analysis of Earth's spectrum. The experiment was done to calibrate exoplanet measurement models, I believe... but I would have to look for it. Other than that... probably not. – CuriousOne Sep 6 '14 at 20:28
• If I may restate your question, you want to know if there are any extremely large reflective surfaces that are, perhaps, hundreds to thousands of lightyears away such that we can see how Earth looked in the past? – Jim Sep 6 '14 at 20:29
• @Jim It's a bit broader, but includes what you say. Also for example: would I be able to watch a broadcasted TV show from the '50s? – Brainstorming Sep 6 '14 at 20:33
• One could put one's hope on a well-polished Russell's Teapot... – Johannes Sep 6 '14 at 20:35
• Anything far enough away to give us a look into history that isn't already recorded would also be far enough away that Earth's "image" in this mirror would be unresolvable at any frequency, especially against the background of the Sun. We might be able to determine Earth's mass and the presence of an atmosphere off such a reflection, but nothing we don't already know – Jim Sep 6 '14 at 21:12

Detecting reflected light from a "past Earth" is an interesting thought. Even a spectrum might tell you something about the composition, temperature of the atmosphere etc, even if as been correctly said in other answers/comments, the spatial resolution to actually image the Earth would need to be several orders of magnitude better than the Hubble Space Telescope can achieve. [To be exact, an image of the Earth seen at 2000 light years would subtend an angle of $1.4\times10^{-7}$ arcseconds, which is 400,000 times better than can be achieved with HST.]
In this related question you can see how I calculated that the apparent magnitude of the Earth, reflected from a large plane mirror at a distance of 1000 light years, would be $m \sim 36$ (a non-specular reflection might be as faint as $m\sim 85$) and hence far too dim (by $>2$ orders of magnitude) to be even detected by current technology - the limiting magnitude for HST ultra deep field observations is $m \sim 30$. (Leaving aside the question of why or how there was a plane mirror out there!)
Given that we already know a lot about the Earth over the last 1000 years, you would actually want the reflector to be a lot further way than this to see further into the past and the received signal will decrease as $d^{-2}$.