Imagine we have a shade launched in space that can be turned opaque and transparent. We can use it to communicate in binary, and mathematically sound extraterrestrials can pick up the pattern. Of course the shade will be small enough and far enough from the sun, and will block a tiny angle of the sunlight, thus "shading" the message to different star systems as it orbits the sun.
Is this possible, and has it been purposed?
We detect planets around other stars by this method - if they pass in front of their star (ie between earth and their star), we detect the drop in brightness.
If the structure was big enough it could therefore certainly cause fluctuations in brightness visible to extraterrestrials - that's not too far from the proposed explanations for the brightness variations in Tabby's Star; the so-called "alien megastructure" star.
How big is big enough? The Wikipedia article I linked to above has a chart that shows masses and radii of planets that have been detected by this method. Planets with a tenth of the radius of earth have been detected successfully.
(Edited to add better information)
Welcome aboard, jhgjmhgvjhg. That is a truly marvelous idea indeed!
What we do here is to try and help people build an environment/world according to the requirements and restrictions of the situation.
Your idea looks very suitable for short distance (up to Jupiter or at most, Saturn) communication. However, I am afraid that the idea would fail for long distance communication.
The reason is that sun is a very, very, very big object. In fact if you were standing on a planet of our nearest star (Proxima Centauri) and looked toward our sun while Jupiter was passing in front of the sun, you would not easily see Jupiter. That is because the sun is so large and Jupiter (at such far distance from the sun) so small that you would not see any difference whether Jupiter was passing in front of the sun or not. You could see Jupiter if and only if you had some really, really powerful telescope with you.
Now think how much difference would a little piece of glass make and how difficult it would be to notice the shadow cast by it. No, it would not work at all even if the shade glass was the size of a continent!
