The softly glowing zodiac: lesson learned

By Phil Plait | July 12, 2012 6:57 am

Every day I post a short, pithy astronomy or space fact on Twitter and Google+. I call them BAFacts, and I have them all archived here on the blog. I try to make them as accurate as possible within the limitation of 140 characters. But I wrote one recently that, as it turns out, I had to retract for being incorrect. And I’m happy about it! Here’s why.

I recently was going through old posts and saw one that mentioned zodiacal light, a very faint glow in the sky that can only be seen on very dark nights. It’s a band of light that follows the path of the planets across the sky, which is technically called the ecliptic. It passes through the constellations of the zodiac, hence its name*.

This picture of the zodiacal light is by friend of the BABlog Brad Goldpaint [click to embiggen, and note this is a part of a larger shot that’s breathtaking]. The two bright "stars" are Venus and Jupiter, and you can see the glow from zodiacal light reaching up and to the left, following the ecliptic.

The origin of zodiacal light (when I learned about it, years ago) was thought to be dust from asteroid collisions. Asteroids out past Mars orbit pretty much in the same plane as the planets. When they smack into each other – and they do – they make dust. This reflects sunlight, so we’d see it as a faint band of light across the ecliptic. Case closed!

Of course, regular readers know me better than this. Read on!

So I wrote up a BAFact and tweeted it:

It says, "#BAFact: Zodiacal light is sunlight reflecting dimly on wreckage from asteroid collisions.", and the link goes to the article I wrote a while back mentioning zodiacal light.

Once it was up, I went on my merry way… for like three minutes. Luke Dones, who follows me on Twitter, tweeted this:

"@BadAstronomer: Dust in zodiacal cloud is now thought to be mostly from comets." The link he posted goes to this article in New Science magazine. That article also links to a professional astronomical journal paper, and sure enough, Luke was right. The researchers found that asteroid collisions cannot explain zodiacal light on their own. Not even close!

In fact, it looks like the majority of the reflected sunlight comes from particles created when comets disintegrate as they orbit the Sun (comets are mostly ice, rock, and dust; when the ice burns away from the Sun’s heat, they can disintegrate completely). These comets all would need relatively short orbital periods (less than about 20 years) and stick to the plane of the solar system. These are called Jupiter Family Comets, because their orbits will have been heavily influenced by that giant planet’s gravity over time.

Well! What’s a good scientist and blogger to do? Answer: correct the previous tweet and thank Luke. So I did:

… immediately followed by:

This is one of the many, many reasons I love science. I was wrong, because something I had learned a while ago had been updated by more recent research. When called out on it, I wound up learning something! And not just something small, but actually a fact about our solar system of which I wasn’t previously aware, a component that had an entirely different cause than I had once thought!

How awesome is that? It’s not often you get a whole new view opening up like that for you, to be able to see an old thing in a new light. Science is a wonderful tool for this, letting your knowledge get ever-closer to reality. It helps you see, and sometimes it lets the scales fall from your eyes. I don’t know of any better way to have that happen.

Image credit: Brad Goldpaint.

* This isn’t a coincidence. The zodiac is made up of constellations through which the Sun appears to move as the Earth goes around it. Since the planets orbit the Sun in more or less the same plane as the Earth, they follow roughly the same path the Sun does.

Related Posts:

When asteroids collide
Astrological sign of the times
Hubble captures picture of asteroid collision!
The Sun ate another comet


Comments (21)

  1. Daniel J. Andrews

    You’re supposed to double-down, not admit you were wrong. Don’t you know how the game is played? Your science training is overriding your cognitive biases, and flawed rationalizations. ūüėČ

    Last I heard it was asteroids too. Cool.

  2. drow

    nah, everyone knows its wreckage and debris from all the UFOs destroyed by USAF laser defense satellites.

  3. You know, the sad thing about this is that many people still think that finding out a mistake, or explaining something in a better way etc is a disadvantage of the scientific method and progress in general. Still the unreasonable belief that a “discovery” should remain “perfect” and 100% correct forever, prevails.

    Btw, I ve been to quite a few places with decent seeing and dark skies for the usual dim-enough DSO’s but I havent yet spotted the zodiac light.. *sigh* have to visit Atacama at some point ūüėÄ ūüėÄ *sigh*

  4. @Drow : Naaah, you ve got it wrong. These are startrails. The secret organisations responsible for the secret organizations behind the chemtrails, are now experimenting on ultra-mega-super-wow altitude spraying from the ISS in order to sunbjugate the whole human race and make us more receptive to well… make your choice :p

  5. “In science it often happens that scientists say, ‘You know that’s a really good argument; my position is mistaken,’ and then they actually change their minds and you never hear that old view from them again. They really do it. It doesn’t happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.” — Carl Sagan, 1987 CSICOP keynote address

  6. rob

    comets? asteroids? ufo debris? startrails? sounds like there is a lot of controversy.

    we need an internet poll to decide!

  7. Gary

    Here’s a take on why it’s so hard to admit and correct a mistaken position:

  8. Joesixpack

    “Those who have taken upon them to lay down the law of nature as a thing already searched out and understood, whether they have spoken in simple assurance or professional affectation, have therein done philosophy and the sciences great injury. For as they have been successful in inducing belief, so they have been effective in quenching and stopping inquiry; and have done more harm by spoiling and putting an end to other men’s efforts than good by their own.”
    Francis Bacon, “Novum Organum”

  9. Renee Marie Jones

    Headline: Bad Astronomer flip-flops on asteroid dust policy.

  10. Pascale

    HAHA, nice Renee.

  11. JimTKirk

    Drow – We shut those satellites down when I retired… We could only get 1.21 jigawatts outta them ūüėČ

    (Marty McFly – Doc, what’s a jigawatt?)

  12. Ok, now how is this going to kill us?

  13. Keith

    So this disproves evolution, right? If science *can* be wrong, then it is *always* wrong, ergo evolution is false. Whew! Glad we cleared that up.

  14. jcm

    For those interested, the paper is available here (free):

  15. Isaac

    Not just evolution, Keith, but global warming as well!

  16. Teacher Al

    Phil, you weren’t entirely wrong. It’s still particulate stuff in space. Not clouds, government experiments, or fairy dust.

  17. Ricardo

    @rob #6:
    Yeah, teach the controversy! I say the dust comes from the paint peeling off of Russell’s teapot and your science can’t prove me wrong! :-)

  18. mike burkhart

    well no one is perfect and even scientists can make mistakes. I’ve seen the zodiacal light in some books I’ve read it is also called the flase dawn because it lookes like the sky just before sunrise. In fact it appers before sunrise.

  19. Musical Lottie


    Also kudos* to you for putting in type the text in the images of the tweets so that people who use screen readers can also access the text :)

    *Or other laudatory statement of your choice.

  20. Salvatore De Dominicis

    This is not strictly related:

    “A Survey of Radial Velocities in the Zodiacal Dust Cloud” by B. May

    The fun thing is that the author is guitarist Brian May from “Queen” fame…
    Well, probably you already knew :-)

  21. Matt B.

    @11 For Marty McFly: A jigawatt is a gigawatt. You see, the first g is supposed to be soft because it’s followed by an i. This is a rule in anglicizing Greek. Unfortunately, computer people, who are notoriously bad at English, have managed to change common practice through their use of “gigabyte”.


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