Moon phases: In the course of one month the Moon passes through all four phases. The Moon itself does not generate light; it is lit up by the Sun. As the Moon orbits the Earth, the portion of illuminated Moon that we see changes – giving rise to the phases of the Moon.
Moon phases are determined by the relative positions of the Moon, Earth, and Sun. We see the Moon go through a changing cycle of phases each month due to its orbital motion around Earth and the changing geometry with which we view it.
Changes in the Moon's position as it revolves around the Earth results in more or less of the sunlight reflected from the Moon being visible when observing the Moon from the Earth. This causes the Moon to appear to change shape from Earth.
The moon's phases are caused by the changing angles of the earth's shadows and reflected sunlight as the moon revolves around the Earth over the course of about 1 month (28 days). An imaginary line where the Earth is tilted. The earth completes one revolution around the sun every 365 days.
Astronomers have broken down this cycle into four primary Moon phases: New Moon, First Quarter, Full Moon, and Last Quarter. There are also four secondary phases: Waxing Crescent, Waxing Gibbous, Waning Gibbous, and Waning Crescent.
As the moon revolves around the planet Earth, the half portion of the moon that faces the sun will be lit up. The moon will be lit up because of the sun's light. The different shapes of the lit part of the moon that is visible from our planet i.e. Earth are known as the moon phases.
It is so because sun is a luminous body which emits light from every angle ,whereas moon is a non-luminous body which does not emit light , it only reflects the light of the sun.
Phases of the Moon: As this celestial body revolves around the earth we can see different amounts of the region illuminated by the sun. Thus, it appears to change its shape.