Riddle

I linger high in the night sky

I'm only seen at night, nocturnal

I change size as the year goes by

Pale,blue,dark,sad

My blue luminescense illuminates the street at night

I watch over you, like a watchman, as you sleep

Sometimes a circle, sometimes a crescent

The sun illuminates me

When we align it creates something special

I cast a shadow on the world.