Transcript: It took a long time for people to figure out the true sizes and distances of celestial objects.  Part of the reason is psychological. It was always difficult for humans to imagine that there might be things so much larger then them or so much larger than the Earth.  The other reason is that when you look at something in the sky all you can really see is its angular extent, and its angular extent tells you nothing about its true distance or true physical size.  For example, when people talk about observing things in the sky, they might say, "It looked as big as a dinner plate," when in fact the object might be very far away and very much larger than a plate.  Without a measure of distance, it is impossible to guess, or estimate, the true physical size of an object in space.