A toddler in the house means lots of book reading, singing, and play. Of course, the old classics come out. As I’m singing them to my child, I can’t help but to think about how they’ve aged and how they would be better expressed now for the twenty-first century. A current favorite is:
Twinkle, twinkle little star, how I wonder what you are
Actually, I don’t wonder that at all. Thanks to scientists studying you, I know you’re a huge ball of hydrogen, helium, and other gases with a fusion reaction going on at your core. No, what you are is a given. So, maybe the question should be:
Twinkle, twinkle little star, how I wonder where you are
After all, I know you’re up there, but how far away are you? I know you’re so far away that we measure distances in terms of how far light travels in a year. And that the nearest star is more than four light years away.¹ Most stars we see with the naked eye are hundreds or even thousands of light years away.
Of course, if your light is that old, then there are two more things I wonder. I wonder how far you’ve moved since the light I see came from you. But, since distance is measured in terms of time, the real question is:
Twinkle, twinkle little star, how I wonder when you are
This is how I sing the song now, with apologies to the future kindergarten teachers who will try to correct my son and who will undoubtedly have to hear the answer I give him when he asks the question: “how can you figure out when a star was?”²
 Proxima Centari, at 4.25 light years
 Triangulation using observations made 6 months apart taking advantage of the Earth’s orbit around the Sun works up to around 400 light years out. Further out, you have to use other tools collectively known as the cosmic distance ladder.