Two faces of a moon

“Japetus is unique in the Solar System—you know this already, of course, but like all the astronomers of the last three hundred years, you’ve probably given it little thought. So let me remind you that Cassini—who discovered Japetus in 1671—also observed that it was six times brighter on one side of its orbit than the other.

“This is an extraordinary ratio, and there has never been a satisfactory explanation for it. Japetus is so small – about eight hundred miles in diameter—that even in the lunar telescopes its disk is barely visible. But there seems to be a brilliant, curiously symmetrical spot on one face, and this may be connected with TMA-1. I sometimes think that Japetus has been flashing at us like a cosmic heliograph for three hundred years, and we’ve been too stupid to understand its message.”

Arthur C. Clarke gave these words to his fictitious astronomer Heywood Floyd near the climax of his book 2001: A Space Odyssey. Like a lot of people, I read those words years ago and thought to myself: what is the dang deal with Japetus? Clarke wasn’t making up the part about the two-faced nature of Iapetus (as it is more commonly called), an oddball Saturnian moon.

The two-toned satellite is still mysterious, but now we have some quality snapshots from the visiting Cassini spacecraft. And, hoo boy! they do not disappoint. I find this APOD picture astounding: The Strange Trailing Side of Saturn’s Iapetus. Great Clarke! Look in the sky! It’s a cosmic snowball rolled in dirt… it’s a sugar-frosted chocolate space truffle… it’s… it’s a much better investment than the International Space Station. You know what we need? I’ll tell you what we need: more space robots with cameras and fewer accident-prone gold-plated Tang-sucking astro-cosmo-taikonauts.

All the pictures from the Hubble Telescope

Many people don’t fully realize that the appeal of amateur astronomy is cerebral rather than a visual. An expensive telescope can afford you some breathtaking views of the moon as well as a nifty view of Jupiter and its satellites. Saturn is a minor thrill, and a few of the larger nebulae also make for fun viewing. But at that point, as far as pure visual spectacle goes, you’re pretty much done. The moon will look the same the next time it’s full. It doesn’t change much. The Orion nebula doesn’t change at all. And it’s a pain in the ass to move that big telescope back into the yard. This is why, on a typical night, so many telescopes gather dust rather than starlight.

If the essential notion of it doesn’t thrill you, then observing variable stars is about the most boring thing imaginable. An enthusiast will get very exercised about the occultation of Regulus by the dark limb of the passing moon. But what are we really talking about here? The star is there, and then it winks out of sight. I can see both sides of this argument. On the one hand, what a majestic event! The inexorable interposition of massive heavenly bodies made plain to our tiny Earthbound eyebones. Oh, the grandeur! The hand of God is surely there. On the other hand: big freaking deal. I’m cold. Why did I stay up for this?

If, however, you had the Hubble Space Telescope at your command, it would be a different story. For a machine like that, the visual thrills never stop. So I was really happy to find a site that does a good job cataloguing all the magnificent pictures taken by Hubble over the years: HubbleSite.

I’ve always found the Hourglass Nebula MyCn18 to be particularly haunting. I’d be willing to bet that freely available pictures like this will do more to recruit more future astronomers than backyard telescopes ever did.

A year of full moons on Flickr

Full Moons over Flickr
Happy New Year! Not only is it a new year… a quick look out the window or at the Sky Clock reveals that there is a full moon in the sky tonight. Of course, you might not know that if you were locked in a windowless room. Suppose, for example, you were trapped in a room with nothing but a pen knife, a box of paper clips, and access to all the pictures on Flickr. Could you work out when the moon is full? If you are as clever as Jim Bumgardner you wouldn’t have any trouble at all. But since he’s the author of Flickr Hacks, I guess that’s only fair.

Bumgardner took pictures on Flickr that are tagged “full moon” and plotted them according to when they were taken (Flickr knows that because digital cameras encode it in the image). The result is this: A year of full moons. There’s another lovely image where he uses a similar approach to show the seasonal variation in sunset time: A year of sunsets.

This is an example of the informational residue that gets smeared absolutely everywhere on the web. You can learn the most fascinating things these days if you know how to scrape up the data slime. For instance, from the sunset picture mentioned above we can work out the average latitude of Flickr customers. Google Trends can also give you a sense of when the moon is full simply by watching what people search for. Not surprisingly, werewolf searches are somewhat correlated. [Bumgardner’s photos via the Kircher Society]

The earliest sunset

The earliest sunset
My part of the world was gray, dreary, dark, cold, and wet today. But it had one thing going for it, one very big thing: the sun set this afternoon a few seconds later than it did the day before. Ordinarily I wouldn’t bother splitting hairs over astronomical minutiae, but it helps get me through December to recall that, although the total number of daylight hours will keep shrinking until the 21st or so, the sunsets are now occurring later and later every day and so will continue until late June. Mmmmm… June. Why does the earliest sunset not match the shortest day? Here is a helpful diagram with ellipses and annotations to explain it.

Confusing ellipses

I hope that’s clear. If I work at it, I can understand how it all works for an hour or so, but then it fades.

Stellarium stargazing software

stellarium.jpg
I’ve been on a good run with free software lately. As part of some recent work I’ve been doing with my Sky Clock, I wanted to check my accuracy against a web site that showed the current sky. Was Saturn where I said it should be? As part of my Google search for such a web site planetarium, I came across a mention of Stellarium, a free open-source stargazing program for your computer. It looked pretty, and the fact that it was free made it a simple decision to install it and take a look at it. And lo and behold, it’s very good. Long ago I owned a copy of Red Shift from Maris, and it wasn’t a great experience. The interface was bulky and there was all kinds of weird content that I didn’t really need. Stellarium by contrast is very simple. Its interface feels a little Unix-y and text heavy (lots of single-letter accelerators that expert users can memorize), but it does exactly what I needed, it’s beautful, and it’s free.

As a professional software developer, this last part always gives me pause. I’ve lost count of the number of really useful really good programs I use that are completely free. Not ten minutes ago I downloaded the free Eclipse-based JavaScript/HTML editor Aptana along with a nice free font (Proggy Clean) to use with it. Last night I downloaded a free genealogy program called Personal Ancestral File. Again, it’s pretty basic, but it does exactly what I need. I don’t need and I don’t want to pay for all the fancy-pants features they cram into Family Tree Maker, another product I once spent money on.

Commercial vendors can charge for new features, but they must eventually run out of meaningful novelty. Their free competitors, who by definition can’t be run out of business, will ultimately swallow all features worth copying.
So I wonder: in the long run, will all software be free? I have become convinced that the answer is very nearly yes. In the long run, all software will be free, or hosted as a service, or both. Money will still pour through the system, but by a very different set of sluiceways.

The day the sun stood still

Happy solstice, that day on which the location of sunrise stops and reverses direction. I’m going to put another link to my Sky Clock here because I’ve added a few improvements to it. The yellow sun line can be seen crossing the blue line labeled “SS” (for summer solstice), thus signifiying the day. It must be summer! Also, the featured planets, stars, and constellations now sport tool tips and links. These links will take you off to the corresponding part of the sky on John Walker’s excellent Virtual Telescope site. Also, there’s a clock in the upper left and a moon phase display in the upper right. I’ll be adding proper horizon lines soon.

Note, by the way how close Mars and Saturn are in the western sky at sunset. It’s a sight worth going out of your way to see.

Early sunsets in December

sungraph.pngEvery winter I look forward to Earliest Sunset Day. Here in New England, the sunlight drains away with distressing speed in October and November, so I always feel a little warmer inside (even though there are currently 8 inches of snow on the ground) knowing that the sunsets will start getting later and later starting around now in December. In fact, when I went to check my trusty SunGraph program this year, it informed me that Earliest Sunset Day for my location was on December 9th. If you live near me, you too can celebrate the illusion that the days will now appear to be getting longer. Of course once you factor in the dawn, the days don’t actually lengthen until the honest-to-goodness solstice on the 21st.

It’s easy enough to see how ancient astronomers determined the solstice: they watched where the sun rose and set and noted when it stopped moving south and appeared to stand still (sol+stice derives from sun+still). But without accurate timepieces, I wonder how long it was before they realized that the earliest sunset did not coincide with the solstice. My guess is that some clever Greek had it all figured out a few thousand years ago.

Flickr’ing Saturn

I continue to be impressed with the community energy going on over at Flickr. I was struck by this lovely set assembled by Kokogiak (Alan Taylor) of pictures from the Cassini space probe at Saturn. What’s especially interesting here is that all the pictures come from NASA’s official Cassini site. Anybody could make a set like it, but no one else has (including NASA). The Mars rover page does a much better job, comparatively, in letting you get at interesting pictures. Taylor has done us all a wonderful service… his collection has the Wow factor that the NASA hadn’t yet supplied. Here’s how he describes it.

I’ve been fanatic about space exploration since I was a kid, and have been giddy the past year or so with the constant feed of imagery from Mars and Saturn. I’ve been trolling the 35,000+ images from Saturn ever since they started coming online. All of the images in this photoset come from NASA’s JPL website. They are the best of the lot, in my judgment. As self-appointed curator, I chose 101 images that spoke to me, and that I thought would speak to others. Especially the ones that made me say “whoa” to myself the first time I saw them.

While you’re at it, take a minute to look at his compact Amazon Light interface that makes a clutter free version of the Amazon site using far less real estate. Thanks Alan!

The earliest sunset

Living at 42 degrees north latitude (Boston, Massachusetts) I am jealous of the winter sunshine. I am sorry to see it depart and I am happy to see it return. If you’re like me and you live at a similar latitude, you’ll be glad to know that today is the earliest sunset. In Boston on December 9th, the sun sets at 4:11:32 PM. On December 10th, it sets exactly two seconds later, and on the 11th it sets a further four seconds later still.

But wait! Isn’t December 21st the shortest day? Yes it is, but because of some astronomical fudge factors, the latest sunrise, earliest sunset, and shortest day do not occur on the same day. The latest sunrise occurs on January 3rd. The fudge factor that throws things off comes from the fact the length of a single day is a surprisingly complicated notion. As Bertrand Russell said, “Everything is vague to a degree you do not realize until you have tried to make it precise.” The length of a day was originally defined as the time from one noon (when the sun reaches its zenith in the sky) to the next. But this definition gives a different length of day depending on where you are on the planet, what season it is, and where the earth is in its elliptical orbit around the sun. To keep our clocks regular and responsible, we have defined a uniform “mean tropical day” in which we imagine a fictional sun passing overhead everyday at exactly 12 PM. But the real sun isn’t typically overhead then. Sometimes it is ahead of and sometimes it’s behind this fictional sun. This variance in the occurrence of sundial noon and wristwatch noon is called the equation of time. If you’re curious about the astronomical details, be sure and check out Bob Urschel’s amazing Analemma pages. If, like me, you enjoy knowing when the sunsets will start getting later for you, download the SunGraph program from the same site.