Why does the photograph of mercury always half?

Answer 1

Because the sun can only shine on one half at a time. This is true enough, but not a complete reason. Mercury appears in half phase because its orbit is located between the Earth and the Sun. It is so close to the Sun as viewed from Earth that it is most easily seen and photographed when it is in it's most lateral position to the Sun; the position in which it is illuminated in half phase. As it moves around the orbital arc, the angular separation from the Sun's disk quickly becomes so small that viewing any other phases is difficult or impossible without very specialized equipment. The reflected light from Mercury is overwhelmed by the brightness of the Sun.