What evidence shows the Milky Way is a spiral galaxy and where we are in it?
A viewer asks what evidence is used to claim the Milky Way is a spiral galaxy and to locate our position within it.
Answer
They do not really know; they are making educated guesses, because there are no cameras out there taking pictures of the Milky Way, and we cannot even get a probe reliably past Pluto. We assume it is a spiral because on a clear night you can see a thin band of stars across the sky, which is usually a sign of a spiral galaxy, and because our nearest galaxy, Andromeda, which we can photograph from the outside, is clearly a spiral. Seeing that band, and that there are stars behind us as well, they estimate we are about two-thirds out from the center on one of the arms. By the way, these galaxies are spinning about ten times too fast for our present physical laws, which is a problem modern science does not like to discuss.