Lookback time – my new article in Sky & Telescope
November 2, 2016
I’ve been writing for Sky & Telescope, the American astronomy magazine, for a year now. My first feature article was published last December (details here), my second came out this April (ditto), and my latest is in the current (December 2016) issue, which should be hitting newsstands this week. I’ve also been writing the “Binocular Highlight” column since June.
My latest feature article, “Twelve Steps to Infinity”, is my favorite thing I’ve ever written about astronomy, and maybe my favorite thing I’ve ever written, period.* I’m posting about it here because the concept should be interesting to all students of the past: the speed of light is finite, so when we look out into space, we are also looking back in time. We see the moon as it was 1.28 seconds ago, the sun as it was 8.3 minutes ago, Jupiter anywhere from 33 minutes to over an hour ago, depending on whether we’re on the same side of the sun or not, and Neptune after four hours – at that distance, our 16-light-minute swing around the sun hardly makes a difference. Most of the stars visible to the naked eye are within 2000 light years, which is 2% or less of the diameter of our Milky Way galaxy. With binoculars or a small telescope you can track down numerous external galaxies and see them as they appeared tens of millions of years ago. One of my favorite observations is seeing the light of the quasar 3C 273, which started traveling 2.4 billion years ago, when our single-celled ancestors were gearing up for the Great Oxygenation Event. (If you’d like to replicate that feat yourself, you can get a very capable, “lifetime” telescope for a little over a hundred bucks. I recommend the Orion SkyScanner 100 – see this and this for more information.)
My new Sky & Tel article doesn’t go nearly that far back – in fact, I don’t even make it out of the Cenozoic. But the concept scales all the way out, so if a particular event in Phanerozoic history is near to your heart, there is probably a star, nebula, cluster, or galaxy whose light left at the right time, which you could observe with binoculars or a small telescope (although the distribution is gappy between half a million and 30 million light years, where there just aren’t that many nearby galaxies). The Messier and Caldwell catalogs are good places to start, and there are hordes of online resources (many funded by your tax dollars by way of NASA) you can use to find a match. If I get really motivated I might post a table of easily-observed celestial objects and their lookback times. In the meantime, if you have a date in mind, leave it in a comment and I’ll find something temporally close for you to go look at.
Lots of people provided assistance and inspiration. Steve Sittig, who runs the Hefner Observatory at the Webb Schools here in Claremont, helped me refine the idea through numerous conversations, and did a trial observing run with me last autumn. Fellow paleontologists Alan Shabel and Thierra Nalley guided me on hominid history (needless to say, any remaining errors are mine). My editor at Sky & Telescope, S.N. Johnson-Roehr, made numerous small improvements, and the S&T art department made the article even more beautiful than I had hoped. Finally, the little plesiadapiforms at the end of the piece are there thanks to Pat Holroyd, who introduced me to them when I was at Berkeley. Many thanks, folks!