I'm developing a Planetarium software and I have no idea about how what stars and planets are visible today (mm/dd/yyyy hh:mm) given a latitude and longitude.
Reading this tutorial, I have found how to use latitude to determinate north celestial pole position.
But I don't know how can I know which stars are visible given longitude in a particular date.
I have searched a lot on Internet but I haven't found anything (maybe because I haven't searched with the right words).
How can I calculate which star is at Zenith given a longitude and a time (i.e. now)?