The moon and sun are at right angles during the first quarter and last quarter phases of the moon.

Here's why:

* Lunar Phases: The moon's phases are caused by the changing angles at which we see the sunlit portion of the moon from Earth as it orbits us.

* Right Angle: When the moon and sun are at right angles, the sun is illuminating half of the moon's surface, and we see this as either a crescent shape (first quarter) or a waning crescent shape (last quarter).

Let me know if you have any other questions about the moon and its phases!