Final answer:
The phases of the moon are determined by its position relative to Earth and the Sun, which is influenced by the gravitational interactions between these bodies as per Newton's universal law of gravitation.
Step-by-step explanation:
The different phases of the moon are caused by the alignments of the Sun, Earth, and Moon, which result from the gravitational forces as described by Newton's law of universal gravitation. As these celestial bodies move in their orbits, we see varying portions of the moon's illuminated part, contributing to the phases ranging from new moon to full moon. The gravitational pull of the Sun and Earth on the Moon creates a dynamic interaction that, along with the relative positions of these three bodies, defines the lunar phases we observe from Earth.