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.)


Our place in the Milky Way, from a talk I put together on the same subject.

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!

* Other contenders: my favorite paleo thing is the RLN paper, and my favorite thing I’ve written about myself is this essay. And that’s quite enough navel-gazing for one post!


11 Responses to “Lookback time – my new article in Sky & Telescope

  1. […] late and my computer is almost dead, so I’m just going to link to the longer announcement/acknowledgment post on my paleontology blog. […]

  2. Mike Taylor Says:

    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 think it’s going to need more than a $100 telescope to replicate the great oxygenation event.


    Seriously, though, this is great stuff.

    If I get really motivated I might post a table of easily-observed celestial objects and their lookback times.

    You should totally do this — not as a blog-post, but as your next S&T article. And of course add a third column, the corresponding event in the history of Earth. (Unless the big table would overlap too much with the present article, which is hard to tell without being able to read it, which seems to be the case.)

  3. Jim Ruebush Says:

    Thanks for your article. I’ve been a long time reader of S&T.

    I published a series of blog posts about the Cosmic Distance Ladder and how we see farther into space. I’d be happy to share the links if you’d like them.

  4. Dale McInnes Says:

    Hey Matt … As an side here, Time is amazing. One could also look back some 1500 light years @ a star and peer 2000 years into the future. I’m thinking about that star with the 20% shading (and growing). I forgot the designation tag for it. If that is a so called Dyson Sphere in the making and we can evesdrop on it then, we would truly be looking back in Time to go forward into our future. Did your Astro partners ever mention that star in conversation ? That alone would make for a very exciting ongoing blog don’t you think ?

  5. Matt Wedel Says:

    Thanks for the kind words. Jim, I’d love to see your posts on the Cosmic Distance Ladder – please post the links at your convenience.

    Dale, I appreciate the enthusiasm about KIC 8462852 (the media-hyped Dyson Sphere star), but I am deeply skeptical that it’s actually a growing Dyson Sphere, for the following reasons:

    1. It’s not just 22% occulted and growing – it has dips of 22% in its light curve, which then rise back to normal. So there’s no evidence yet that anything is changing there, vs being a stable situation.

    2. I suspect that we have about as good a handle on Dyson Sphere dynamics as the ancient Greeks had on flying to the moon, and that whatever solutions advanced civilizations actually come up with for harnessing all of a star’s energy* will resemble the current conception of a Dyson Sphere about as much as a Saturn V or space elevator resembles a winged chariot or a throne tied to a bunch of birds.

    * If that’s even useful – probably simpler at that point of technological evolution to build or capture a small black hole, fire matter into its accretion disk, and harness the resulting x-rays. That would put stellar-level energy output to shame (see quasars).

    3. What little we do understand about Dyson Spheres and Matrioshka swarms suggests they’d be built up evenly around the host star as a series of satellite swarms or rings, not just a big eggshell that grows from one off-center point. The former would produce a gradual constant dimming, which is not what we see. The big light dips we actually do observe would maaaaaybe be consistent with one big shell growing from a single point, but given what we know about orbital mechanics that seems like the dumbest and most energy-intensive way to do it, since it would be wildly unstable and would probably gravitationally destabilize the rest of the system.

    4. We are constantly being surprised about natural phenomena. Just in the past year we’ve been slapped in the face with the fact that no, Pluto is apparently not too distant, cold, and small to be tectonically active. We already have a bunch of reasonable, non-technological hypotheses that could account for the light curve of KIC 8462852 – running past them to clutch at the Dyson Sphere hypothesis seems deeply unwise. Especially given our track record of confusing natural phenomena with technological ones – people entertained the idea that pulsars might be extraterrestrial signals for a while, too. This isn’t the argument from incredulity, it’s an inductive argument from sampling – so far, all of the proposed ET signals have been something else. And this isn’t even a very good ET signal, for the reasons explained above.

    So I don’t think KIC 8462852 would make a good subject for an ongoing blog, because it doesn’t seem to be evolving, at least not any timescale we can detect or make use of (if the dimming is from a protoplanetary disk, it may have a system of planets in 100,000 years, but it won’t be burning up the presses in the meantime), and all of the hype over it ignores some fundamental facts and reasonable inferences in favor of wildly unlikely speculation. I can get that from the Amphicoelias fragillimus fanboys.

    Sorry to be a downer. I’m just more excited about these deep-dimming stars for what they’ll tell us about the natural evolution of stellar systems, than about any possible alien megastructures.

  6. Dale McInnes Says:

    Thanks for the input Matt. No such thing as a downer in science. It’s all speculative until the evidence proves otherwise. Every effort must be made to isolate a possible new/ old natural phenomenon before taking the next step. I thought that star was too old to still contain a proto-planetary disc. Harnessing a tiny black hole makes more sense from our early 21st C perspective. Problem with any aliens is that we wouldn’t really know what the hell they were up to. Any new laws of physics that they might be using, probably, wouldn’t make any sense to us anyway. Time will certainly tell.

  7. Matt Wedel Says:

    My guess is that we’re going to find that there are ways for a disk to persist on billion-year timescales, possibly by being perturbed by large invisible or barely visible masses like super-Jupiters and brown dwarfs, in sort of the same way that Jupiter keeps our own asteroid belt from coalescing into a planet. In fact, “large invisible mass” is a pretty good description for whatever is dimming KIC 8462852, and that cover a lot of potential ground. I wonder if it could possibly be a failed binary system, with a whole star/disk system of junk orbiting the primary star. Like you said, time will tell! Anyway, thanks for taking my long comment in the constructive sense in which it was intended.

  8. Mike Taylor Says:

    … wildly unlikely speculation. I can get that from the Amphicoelias fragillimus fanboys.


  9. Dale mcinnes Says:

    Double ouch on that last comment Matt. That barosaur cervical makes one stand in awe. Leave a little mystery for us.

  10. Matt Wedel Says:

    Ha, no worries! I wasn’t talking about anyone here – I was throwing some drive-by shade regarding our periodic infestations of Carnivoraforum fanboys. I’m equally on board with Supersaurus/Big Baro being just as awesome as we think – not just hope – it was. As for Amphicoelias, the Morrison is a big place. Staggeringly immense, in fact. So there’s still plenty of room for mystery.

  11. […] It’s about a third of the way to the Orion Nebula. (I have these distances loaded in RAM for a reason.) It’s a little less than half of the 1,100-light-year thickness of the ‘thin […]

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