When you see a half moon, the sun, the Earth, and the moon form a right angle.

Here's why:

* Half Moon: A half moon occurs when the moon is illuminated by the sun at a 90-degree angle from our perspective on Earth.

* Right Angle: This 90-degree angle means the sun is positioned roughly 90 degrees away from the moon in the sky.

Think of it like this:

Imagine the Earth as the center of a clock.

* The moon is at the 3 o'clock position (half moon).

* The sun is at the 12 o'clock position (or the 6 o'clock position depending on whether it's a waxing or waning half moon).