Why does the illumination of the moons surface appear to change when viewed from earth even through the sun continuously illuminates half of the moon?

The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. This is caused by the different angles from which we see the bright part of the Moon's surface. These are called "phases" of the Moon.

Explanation:

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.