A program has been developed for the Hewlett-Packard Computer 9845 to represent in a graphical form the period of observability, for individual nights, of periodic phenomena of stars with respect to the phase. One version of the program (Fig. 1) gives the observability with respect to the phase for the whole year. The other version (Fig. 2) gives the beginning and end of the suitable observing interval for several observatories around the world thus facilitating preparation of a program for 24-hour continuous observation.