Why can I see sometimes a horizontal half moon instead of a vertical one? Why do I sometimes see a horizontal half moon, instead of the more usual vertical one? I keep getting answers for crescent moons, but this is a half moon with its curve closer to the horizon, like a bowl of cereal.
I would like to know why it only appears sometimes that way.
It occurs because the lit half of the moon points towards the sun, along a great circle in the sky.  The half-moon will be 90 degrees away from the sun in the sky. So by (say) 8pm (at equinox for simplicity) the half-moon will be visible in the South west, but the sun will be below the horizon.  The great circle from the moon to the sun will go &quot;down&quot; relative to the horizon.
The angle that it points down will depend on where you are on Earth. Only near the equator could the moon be completely horizontal,