Here's why:

* Moon Phases: The moon's phases are caused by the changing angles of sunlight reflecting off its surface as it orbits the Earth.

* Vertical Orientation: A vertical moon simply means the moon is oriented with its long axis pointing up and down in the sky.

* Full Moon: The full moon is the only phase where the entire illuminated hemisphere faces the Earth, giving the appearance of a perfectly round, bright disk. This orientation naturally appears vertical.

While other phases, like the crescent moon or gibbous moon, can appear more or less vertical depending on their position in the sky, they don't have a distinct vertical orientation like the full moon.