Galaxies look stationary, so why do scientists say that they rotate?
Answers
It's because galaxies are unimaginably huge. An object traveling at high speed across a very long distance appears to be moving slowly when viewed from far away.
Planets and galaxies do not look stationary; to the contrary, they can be visibly observed to rotate. Our earliest observations through telescopes showed that the surfaces of Mars and Jupiter changed over time, implying rotation. The obvious exception to earthbound observation is our own moon, which is tidally locked to earth’s rotation and therefore only shows one face to us.
Galaxies obviously rotate given the spiral shape so many of them assume. The long, curved arms of galaxies are the result of the effects of gravity determining the differing speeds of stars around the galactic center.