This topic is about looking at the tools of astronomy more
closely. Newton built on Galileo's work (and that of others) and
synthesised the laws of motion, understanding motion in terms of
forces. One of these forces is gravity, understood by
Newton to be universal: it explains (some) motions on Earth as well
as the celestial motions. Light from celestial objects tells us where
they are, but also tells us much more about them, if we know how to
interpret it.