Question

why only one side of moon is visible to us

Answer 1

Only one side of the Moon is visiblefrom Earth because the Moon rotateson its axis at the same rate that theMoon orbits the Earth, a situation known as synchronous rotation or tidal locking. The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions cause the lunar phases.

Answer 2

The simple answer (and one that you've probably heard before) is thatwe only see one side of the moonbecause the moon rotates around the Earth at the exact same speed as it rotates around its own axis, so that the same side of the moon is constantlyfacing the surface of the earth.