Measuring distances is an always active part of astronomy, as I understand it, but it starts with parallax. See this page for more info: https://en.wikipedia.o

Measuring distances is an always active part of astronomy, as I understand it, but it starts with parallax. See this page for more info: https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder