The universe's size turns out to depend on two things: how long it has been expanding and how fast. This film traces that measurement from the ancient Greeks, who first treated the sky as a subject for investigation rather than the work of gods, to Edwin Hubble and his assistant Milton Humason at the 100-inch Hooker Telescope on Mount Wilson in the 1920s, who showed that fuzzy "nebulae" were actually distant galaxies rushing away from us. That discovery, now Hubble's law, points back to a single moment of origin, the Big Bang, dated by precision measurements to 13.7 billion years ago and giving the universe a light-travel radius of about 1.3 quadrillion kilometers. But space itself has been stretching the whole time, so the Hubble Space Telescope's 1995 survey found galaxies now nearly 46 billion light years away in every direction, and up to 92 billion light years apart from each other. The film closes on a newer idea still: that this entire observable universe might be a small patch within something far larger.