Stellarium
Stellarium is a free and open source (FOSS) software, written in C, C++ and GLSL among others, that renders realistic skies in real-time with OpenGL. It is available on multiple different platforms such as web, desktop app, play store and app store.
It is recommended to download the application on your phone as it is much easier to carry and reference from as compared to from a laptop or a desktop.
The Stellarium user guide in its entirety can be found here.