I'd like to answer this yes or no question for a number of objects: "Is this star, at any point between these two times, going to be above the local horizon?". Say, I'm at the prime meridian at a latitude of 50 degrees, and I want to know whether, between the sidereal times of 11:00:00 and 16:00:00 (already calculated from GMT with appropriate corrections), a star of co-ordinates (R.A. 02:00:00, Dec. 05:00:00) would ever be above the horizon, how would I go about working it out? I'm assuming (correct me if I'm wrong) that if the R.A is within 6 hours of either time, it will be above the horizon at some point, so in this case it'd be between 05:00:00 and 22:00:00 (this star wouldn't be above the horizon). As for the Dec., does it just have to be between 90 and -40 degrees to be visible? Or is there some other criterion I'm missing? In addition to this, is it possible to determine, for each visible object, whether they'd be visible for at least one hour under 1.5 airmasses?