On January 4, 2012, I posted my first BAFact: a short astronomy fact that was brief enough to put on Twitter but informative enough to be interesting. I posted the first one on perihelion – the point in Earth’s orbit when it’s closest to the Sun – and the last one will be a year later.
Because 2012 is a leap year with 366 days, July 5th was the 184th day: the first day of the second half of the year. That means I’m more than halfway done!* Appropriately enough, here’s the July 5 BAFact:
I post the BAFacts on Twitter, Google+ (where I can flesh them out a bit more – and add pictures – since there’s no character limit), and have a complete archive of them on the blog as well. With 180+ already in the bag, reading those should keep you busy for a while!
I generally link them to previous blog posts dealing with the topic in question, but not always. I’ve actually been surprised at how difficult it can be to reduce a topic to 100 or so characters (leaving room for the leading "#BAFact: " and shortened link, plus room for retweets), and how that limits some topics. I have also been surprised to find out I haven’t written about some topics! For example, I was thinking recently of making a BAFact about the nearest known black hole, Cygnus X-1, and discovered I had literally never even mentioned it in a blog post! That’s weird… but by coincidence that got fixed just this last weekend.
So this exercise in brevity has given me new things to write about. I’ll note that there have been arguments over the accuracy of some of the BAFacts, too. Sometimes that’s just due to having to be so brief that the description might be misleading if you don’t click the link; I struggle with those but usually make them as clear as possible, and hope people actually read the post to clarify. And once I really did just make a mistake; as I recently mentioned I didn’t know that recent research had found that zodiacal light is mostly from comet dust and not asteroid collisions, and had to post an immediate correction! But that’s OK; I love learning new things, too.
So as we enter the second half of these, I hope you keep up with them and enjoy them. And if you have a beef with them, find a mistake, have something to add, or know of a good picture or story relating to them, follow it up with a tweet of your own! The whole point here is to have fun and learn things. Which, when it comes to science, are exactly the same.
* Well, kinda. Perihelion is actually on January 2, 2013, roughly a day earlier than usual because we have an extra calendar day this year. The Earth orbits the Sun not caring at all for our calendrical contrivances, so when the time comes I’ll decide whether to post the last BAFact based on the Earth’s orbit our roughly-hewn measurement of it.
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!
Regular readers know the phenomenal work of Stéphane Guisard: he takes astrophotos showing stunning, deep views of the sky (see Related Posts at the bottom of this entry). And he’s done it once again: using a fish-eye (very wide angle) lens, he captured stunning video of the entire sky from Chile. You can see the whole thing on that link, or he’s uploaded the video to YouTube:
[I strongly urge you to set the resolution to its highest (1080p) and make this full-screen. Seriously.]
OK, this needs a wee bit o’ explaining…
First of all this was taken on December 5, 2010, at the European Southern Observatory’s Paranal Observatory in Chile. You can see the telescopes nearby. On Stéphane’s page (and on YouTube), you can see the usual view where the sky appears as a circle, and the horizon wraps around. But what he did here is to "unwrap" the sky so it appears rectangular. It starts in the east on the left, goes through south, then west in the middle, then through north and back to east on the right. So you can see stars rising on the extreme right and left sides of the frame, moving toward the middle, and then down to the west. It takes a little getting used to!
The amazingly talented astrophotographer Stéphane Guisard has done it again! Check out this amazing 360° view of the entire sky:
[Click the picture to 2 π steradianate and get access to a zoomable, panable image.]
There’s so much going on here it’s hard to know where to start. Basically, Stéphane wanted to get the darkest sky possible for this shot. So he went to the Atacama desert in Chile, not far from the Paranal observatory. At that latitude, and at that time of year, the Milky Way — usually seen as a band of light across the sky — circles the horizon! That glow you see around the picture is not from cities or anything else like that, it’s from the galaxy itself.
Since the Milky Way is so low in the sky, its soft light is minimized. He also took this picture at new Moon, so there was no light from that, either. Zodiacal light is sunlight reflected back to Earth from dust in the plane of the solar system, and he chose the time of the picture to minimize that as well.
The glow you can see in the picture is called gegenschein, and is sunlight reflected back to Earth from particles in the solar system, but this light is more concentrated in the area of the sky directly opposite the Sun’s position. That’s why it appears so bright in that oval, and fades away to the sides.
What Stéphane has essentially achieved here is a picture with the darkest sky possible.