The sheer scale of the cosmos is hard to imagine, and even harder to put an accurate figure on. But thanks to some ingenious physics we now have a good idea of just how big it is.
Today we are fairly confident that the Milky Way is probably between 100,000 and 150,000 light years across. The observable Universe is, of course, much larger. According to current thinking it is about 93 billion light years in diameter. How can we be so sure? And how did we ever come up with such measurements from right here on Earth?