In this modern age, we’re subjected to all kinds of outrageous and essentially unchallenged assertions: the earth is four and a half billion years old, the gross domestic product of Montenegro is $4.1 billion, it takes 364 licks to get to the Tootsie Roll center of a Tootsie Pop, and so on. Most of these just wash over us. You’d go crazy trying to challenge them all. But every now and then you see some number blithely mooted and say to yourself, how could we possibly know that?
It is approximately, we are told, 2.5 million light years, give or take, from the end of your nose to the Andromeda galaxy. So: if your bathroom mirror was hanging in Andromeda, you’d have to stare at it for 5 million years before you realized you had a little hair hanging out of your nose.
Think of all the ways we measure how far away things are. Now think how none of these things could work for something so very far away. It would take way too much measuring tape. You can’t drive there and back with an odometer. You can’t bounce radar off it and wait for the reflection. You can’t use trigonometry, because you don’t know how big it is.
Measuring such remote extragalactic distances makes use of something called the cosmic distance ladder. It’s a remarkable and complex set of measurements and algorithms, but this little video from the Greenwich Royal Observatory describes it beautifully. Watch it and you’ll feel a tiny bit more in control of this otherwise bewildering world.
(thanks to the cyclist for forwarding this)