3 ms·How do they know the photons were from billions of years ago?by yeukhon 11y agoHow do they know the photons were from billions of years ago?cheez 11y agoIf an object is measured to be a billion light years away, that means that light from that object takes a billion years to get here.yeukhon 11y agoRight... but how do you know it takes a billion year to get here?cheez 11y agoMeasuring 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_ladderyeukhon 11y agoThank you!