I've wondered for a while about how far away we could see the earth if we were using our current technology and we had our current light output. At what distance could we see the artificial light the earth produced?
This paper Detection Technique for Artiﬁcially-Illuminated Objects in the Outer Solar System and Beyond provides some sort of evidence on the question
Existing optical telescopes and surveys can detect artiﬁcially illuminated objects comparable in total brightness to a major terrestrial city at the outskirts of the Solar System
For this signature to be detectable, the night side needs to have an artiﬁcial brightness comparable to the natural illumination of the day side. Clearly, the corresponding extraterrestrial civilization would need to employ much brighter and more extensive artiﬁcial lighting than we do currently since the global contrast between the day and night sides is a factor ∼ 6 × 10^5 for the present-day Earth
Lets ignore that our telescopes will get better (about 2.5% a year?) so how quickly at current rates will the earth take to be this bright?
“over the last three centuries, and even now, the world spends about 0.72% of its GDP on light. This was the case in the UK in 1700 (UK 1700), is the case in the undeveloped world not on grid electricity in modern times, and is the case for the developed world in modern times using the most advanced lighting technologies.”
But what is that in terms of extra light produced? "In 1700 a typical Briton consumed 580 lumen-hours in the course of a year, from candles, wood and oil. Today, burning electric lights, he uses about 46 megalumen-hours—almost 100,000 times as much."
So that means if we grew at the same rate we have for the last 300 in about 300 years we will be producing enough light that if it was all shone out into space the earth would be as bright at night to an outside observer as it is in the day. I dont think you can make projections this far and it still leaves the question of how long after that ambient external light would be that bright. Still I think its interesting that even with current detection technology as civilisation the 300 years advanced from us would be nearly visible to us. I would guess this prediction will reduce but 300 years is my first estimate to when we could see ourselves.