Here's why:

* Phases of the Moon: The different phases of the moon (new moon, crescent, half moon, gibbous, full moon) are caused by the changing angle of sunlight reflecting off the Moon's surface as it orbits Earth.

* Half Moon: A half moon occurs when half of the Moon's visible surface is illuminated by the Sun. This happens when the Sun, Earth, and Moon form a right angle.

Imagine a triangle with the Sun at one corner, the Earth at another, and the Moon at the third. When the angle between the Earth and Moon is 90 degrees (a right angle), we see a half moon.