On March 14 (Pi day) together with Clear Sky Blog we will host a "Flashmob for Science" event in Park am Gleisdreieck. You can register (and let your friends know about it) on Facebook, and you can read more about the event (in German) on this Clear Sky Blog post or from Wissenschaft im Dialog.
The idea behind our Flashmob for Science events is to understand how much variation there is between observations from different users. Everyone has slightly different eyesight, different phone displays, and a different threshold for what counts as "seeing" a star. On top of that, the order of the stars we suggest and accidental mis-identifications can affect the result (which is why viewing more than the minimum 8 stars is a good idea!). These differences aren't a problem for the science, as long as we understand how large the variation due to them is.
Our first-ever Flashmob for Science in 2012 was a relatively small affair. This time we're aiming for a much larger event. The more participants we have, the better our understanding of these variations will be! So please come and join us in Berlin on March 14!
Note: When deciding whether a star is "visible" or not, the best technique is to say it's not visible unless you can always or nearly always see it with direct vision. If it only occasionally twinkles into view, say it's invisible or that you can only see it with averted vision.