Tomorrow (Saturday December 10) the Moon will pass into the Earth’s shadow, causing it to plunge into ruddy darkness, an event called a total lunar eclipse. These happen roughly twice a year somewhere on Earth, but this is the last one visible in North America for more than two years, so even though it’s in the morning it might be worth a look for you.
You can get all the info you need on watching the eclipse from my pal Alan Boyle over at the Cosmic Log, including timing, locations, and where to watch live online, too. NASA has a page with more detailed information as well. This one favors US folks farther west; the Moon will have set when the eclipse really starts for East Coast folks.
But the fun begins when the Moon starts to enter the deepest part of the Earth’s shadow at 12:45 UT (04:45 Pacific US time), and the last bit passes into shadow at 14:06 UT (06:06 Pacific). Deepest eclipse is about 25 minutes after that. Interestingly, for people in the western US, that’s around the same time as sunrise. For me, the Sun rises at 07:12 (Mountain time) Saturday, and the Moon sets at 07:14 — when it’s still partially eclipsed! Unfortunately, the mountains to the west will block my view of the setting Moon.
But for those of you with a clear horizon to the east and west, you may get an extraordinary opportunity to very briefly see the Sun and eclipsed Moon at the same time! Normally this isn’t possible; by definition the Moon and Sun have to be directly opposite each in the sky to get an eclipse at all.
But due to a quirk of geometry and atmospheric physics, it is possible. Read More
Last week, I was checking my feed reader, catching up on all my favorite web comics. One of them is sci-ence, a comic you really should be reading. It’s drawn (in part) by artist and science afficianado Maki Naro, and (like xkcd and Saturday Morning Breakfast Cereal) it’s both funny and scilicious.
I got a snicker out of the comic he had just posted, dealing with my pal Neil Tyson and the Moon. Go read it!
Back yet? OK.
Now, I know that just last night I was praising Neil, and today I have no cause to bury him. But I will nitpick a wee bit…
First, of course, who hasn’t wanted to chase Neil Tyson down the street while yelling incoherently at him? But that aside, I must point out that this explanation of the Moon Illusion, while very common, is not actually correct.
The Moon Illusion is when the rising (or setting) Moon looks huge and fat, squatting on the horizon, but appears far smaller when up high in the sky. But it’s not because you’re comparing it with foreground objects! I’ve seen this illusion when out in the open plain, with nothing between me and the horizon but Kansas farmland, which is like a geometric plane, except flatter.
The European Southern Observatory just posted this lovely picture of the moon setting behind the Very Large Telescope observatory in Chile:
[Click to embiggen.]
Photographer Gordon Gillet was 14 km (8.5 miles) away from the observatory when he caught the full Moon behind it. The sky is pink because behind him, the Sun was rising — as it must be when the full Moon is setting.
But I had to chuckle when I read the description:
If you’ve ever seen the Moon rising over the horizon, looking so fat and looming that you felt like you could fall right into it, then you’ve been a victim of the famous Moon Illusion. And it is an illusion, a pervasive and persuasive one.
So, how does this thing work? Ah, step right up.
One of my favorite brain-benders is the Ponzo Illusion. You’ve seen it: the simplest case is with two short horizontal lines, one above the other, between two slanting but near-vertical lines. The upper line looks longer than the lower line, even though they’re the same length.
The illusion works because our brains are a bit wonky. The slanted lines make us think that anything near the top is farther away; the lines force our brain to think those lines are parallel but receding in the distance (like railroad tracks). The two horizontal lines are physically the same length, but our brain thinks the upper one is farther away. If it’s farther away, then duh, our brain says to itself, it must be bigger than the lower one. So we perceive it that way.
While procrastinating on reddit, (you do look at reddit, don’t you, especially the science section?) I found this beautiful example of Ponzo:
Heehee! You’d swear up and down* that the red vertical line on the right is much longer than the one on the left, wouldn’t you? It looks almost twice as long to me. It’s a very powerful perception.