Earth orbits the Sun, which is situated in the Orion Arm, one of the Milky Way’s four spiral arms. (Even though the spiral arms of the Milky Way or any other galaxy are not solid structures, the size scale of the galaxy is so large that the density wave will last for millions of years; it is therefore appropriate to say we are “in” the arm at this period in cosmic history.) Earth and the Sun are about twenty-five thousand light-years away from the galactic center.