Yay! Tonight at midnight it’s New Year!

But what does that mean, exactly?

The year, of course, is the time it takes for the Earth to orbit the Sun, right? Well, not exactly. It depends on what you mean by "year", and how you measure it. This takes a wee bit of explaining, so put down the champagne, take the lampshade off your head, and hang on.

First, I will ignore a few things. For example, time zones. These were invented by a sadistic watchmaker, who only wanted to keep people in thrall of his devious plans. So for now, let’s just ignore them, and assume that for these purposes you spend a whole year (whatever length of time that turns out to be) planted in one spot (though I’ll note that as I write this, it’s already 2007 in Australia and other points west of the international date line).

However, I will not ignore the rotation of the Earth. That turns (haha) out to be important.

Let’s take a look at the Earth from a distance. From our imaginary point in space, we look down and see the Earth and the Sun. The Earth is moving, orbiting the Sun. Of course it is, you think to yourself. But how do you measure that? For something to be moving, it has to be moving relative to something else. What can we use as a yardstick against which to measure the Earth’s motion?

Well, we might notice as we float in space that we are surrounded by zillions of pretty stars. We can use them! So we mark the position of the Earth and Sun using the stars as benchmarks, and then watch and wait. Some time later, the Earth has moved in a big circle and is back to where it started in reference to those stars. That’s called a "sidereal year" (sidus is the Latin word for star). How long did that take?

Let’s say we used a stopwatch to measure the elapsed time. We’ll see that it took the Earth 31,558,149 seconds (some people like to approximate that as pi x 10 million (31,415,926) seconds, which is an easy way to be pretty close). But how many days is that?

Well, that’s a second complication. A "day" is how long it takes the Earth to rotate once, but we’re back to that measurement problem again. But hey, we used the stars once, let’s do it again! You stand on the Earth, and define a day as the time it takes for a star to go from directly overhead to directly overhead again: a sidereal day. That takes 23 hours 56 minutes 4 seconds = 86,164 seconds. But wait a second (a sidereal second?) — why isn’t that exactly equal to 24 hours?

I was afraid you’d ask that — but this turns out to be important.

It’s because the 24 hour day is based on the motion of the Sun in the sky, and not the stars. During the course of that almost-but-not-quite 24 hours, the Earth was busily orbiting the Sun, so it moved a little bit of the way around its orbit (about a degree). If you measure the time it takes the Sun to go around the sky once — a solar day — that takes 24 hours, or 86,400 seconds. It’s longer than a sidereal day because the Earth has moved a bit around the Sun during that day, and it takes a few extra minutes for the Earth to spin a little bit more to "catch up" to the Sun’s position in the sky.

Here is a diagram from Nick Strobel’s fine site Astronomy Notes that will help explain this:

See how the Earth has to spin a little bit longer to get the Sun in the same part of the sky? That extra 4 minutes (really 3 m 56 s) is the difference between a solar and sidereal day.

OK, so we have a year of 31,558,149 seconds. If we divide that by 86,164 seconds/day we get 366.256 days per year.

Wait, that doesn’t sound right. You’ve always read it’s 365.25 days per year, right? But that first number, 366.256, is a year in sidereal days. In solar days, you divide the seconds in a year by 86,400 to get 365.256 days.

Phew! That number sounds right. But really, both numbers are right. It just depends on what unit you use. It’s like saying something is 1 inch long, and it’s also 2.54 centimeters long. Both are correct.

Having said all that, I have to admit that the 365.25 number this is not really correct. It’s a cheat. That’s really using a mean or average solar day. The Sun is not a point source, it’s a disk, so you have to measure a solar day using the center of the Sun, correcting for the differences in Earth’s motion as it orbits the Sun (because it’s not really a circle, it’s an ellipse) and and and. In the end, the solar day is really just an average version of the day, because the actual length of the day changes every, um, day.

Confused yet? Yeah, me too. It’s hard to keep all this straight. But back to the year: that year we measured was a sidereal year. It turns out that’s not the only way to measure a year.

You could, for example, measure it from the exact moment of the vernal equinox in one year to the next. That’s called a tropical year. But why the heck would you want to use that? Ah, because of an interesting problem! Here’s a hint:

The Earth precesses! That means as it spins, it wobbles very slightly, like a top does as it slows down. The Earth’s wobble means the direction the Earth’s axis points in the sky changes over time. It makes a big circle, taking over 20,000 years to complete one wobble. Right now, the Earth’s axis points pretty close to the star Polaris, but in a few hundred years it’ll be noticeably off from Polaris.

Remember too, that our seasons depend on the Earth’s tilt. Because of this slow wobble, the tropical year (from season to season) does not precisely match the sidereal year (using stars). The tropical year is a wee bit shorter, 21 minutes or so. If we don’t account for this, then every year the seasons come 21 minutes earlier. Eventually we’ll have winter in August, and summer in December! That’s fine if you’re in Australia, but in the northern hemisphere this would cause, panic, rioting, bloggers blaming each other, etc.

So how do you account for it? Easy: you adopt the tropical year as your standard year. Done! You have to pick some way to measure a year, so why not the one that keeps the seasons more or less where they are now? This means that the apparent times of the rising and setting of stars changes over time, but really, astronomers are the only ones who care about that, and they’re a smart bunch. They know how to compensate.

Okay, so where were we? Oh yeah– our standard year (also called a Gregorian year) is the tropical year, and it’s made up of 365.24 mean solar days, each of which is 86,400 seconds long, pretty much just as you’ve always been taught. And this way, the vernal equinox always happens on or around March 21 every year.

But there are other "years", too. The Earth orbits the Sun in an ellipse, remember. When it’s closest to the Sun we call that perihelion. If you measure the year from perihelion to perihelion (an anomalistic year) you get yet a different number! That’s because the orientation of the Earth’s orbital ellipse changes due to the tugs of gravity from the other planets. It takes about 100,000 years for the ellipse to rotate once relative to the stars! Also, it’s not a smooth effect, since the positions of the planets change, sometimes tugging on us harder, sometimes not as hard. The average length of the anomalistic year is 346.6 solar days, or 29,947,974 seconds 365.26 days, or 31,558,432 seconds. What is that in sidereal days, you may ask? The answer is: I don’t really care. Do the math yourself.

Let’s see, what else? Well, there’s a pile of years based on the Moon, too, and the Sun’s position relative to it. There are ideal years, using pure math with simplified inputs (like a massless planet with no other planets in the solar system prodding it). There’s also the Julian year, which is a defined year of 365.25 days (those would be the 86,400 seconds-long solar days). Astronomers actually use this because it makes it easier to calculate the times between two events separated by many years. I used them in my PhD research because I was watching an object fade away over several years, and it made life a lot easier.

So there you go. As usual, astronomers have taken a simple concept like "years" and turned it into a horrifying nightmare of nerdy details. But really, it’s not like we made all this stuff up. The fault literally lies in the stars, and not ourselves.

Now if you’re still curious about all this even after reading my lengthy oratory, and you want to know more about some of these less well-known years, then check out Wikipedia. They have lots of info, but curiously I found it rather incomplete. I may submit something to them as an update (like how many seconds are in each kind of year; they only list how many days, which is useful but could be better).

I have to add one more bit of geekiness. While researching this entry, I learned a new word! It’s nychthemeron, which is the complete cycle of day and night. You and I, in general, would call this a "day". Personally, if someone dropped that word into casual conversation, I’d beat them with my orrery and astrolabe.

Incidentally, after all this talk of durations and lengths, you might be curious to know just when the Earth reaches perihelion, or when the exact moment of the vernal equinox occurs. If you do, check out the U.S. Naval Observatory website. They have tons of gory details about this stuff.

Hmmmm, anything else? (counting on fingers) Years, days, seconds, yeah, got those. Nychthemeron, yeah, Gregorian, tropical, anomalistic… oh wait! I know something I forgot to say!

Happy New Year.

There have been some server issues the past few hours, and while doing some repairs there have been some other flaky behaviors. For example, right now commenting is all screwed up. I’m working on it! Please be patient while I most likely make things worse.

William Burroughs Burrows is a longtime space writer, journalist, and author. He’s written several books on space policy, and is a faculty member at NYU. Needless to say, his vast experience in writing and space makes him something of an expert on it.

So when he makes a list of the Top Five Space Books, you should read it and pay attention.

And, of course, when he lists Bad Astronomy as his number 5 book, why, you should run out and buy a copy!

Seriously, I was surprised to see my own book on his list, especially when he also lists Andrew Chaikin’s "A Man on the Moon" and (yikes!) "Lost Moon" by Jim Lovell (from Apollo 8 and 13, yes, that Jim Lovell) and Jeffrey Kluger!

Here is what he said:

5. “Bad Astronomy” by Philip Plait (Wiley, 2002).

Philip Plait is a California astronomer who evidently became so exasperated with the contemporary warping of science by ideology or just plain ignorance that he wrote “Bad Astronomy” as an antidote. This primer on basic astronomy explains, among much else, why the moon sometimes hits your eye like a big pizza pie (it happens when the moon reaches the perigee of its elliptical orbit and is closest to us). But Plait’s astronomical discussions also take on creationism. My favorite part of the book: when he goes after the crowd that claims the Apollo moon landings were a hoax. Years ago, Buzz Aldrin showed one way to deal with this bizarre belief when someone shoved a Bible at him and demanded that he swear he actually landed on the moon; Aldrin decked the guy. Plait achieves the equivalent with words.

That last part is pretty cool; I like the imagery of verbally punching Bart Sibrel. I have to say, though, that the reason the Moon looks big on the horizon is not due to it being at perigee; it’s a combination of two illusions that tricks your brain into thinking it’s bigger when it isn’t (in reality, it’s slightly smaller on the horizon than when it’s overhead).

Still, I’m deeply honored by this. Writing a book is a big process, taking months and years of effort, and of course if it sells well you know it’s been worth it. But it helps a lot to be recognized by people who understand the field, and this just made my weekend. It’s a great way to end the year!

Tip o’ the space helmet to the many folks who emailed me about this!

Are you still wondering if there is a First Amendment in these United States? I am.

According to this press release from PEER (Public Employees for Environmental Responsibility), Bush White House appointees are suppressing real science in order to promote creationism. Specifically, at the Grand Canyon National Park, a book is on sale that says the canyon was formed in Noah’s flood. Also, guides at the park are not allowed to answer questions about how old the canyon is, despite scientists’ incredibly detailed and intricate knowledge of the formation mechanism, scheme, and history of the canyon (hint: some of the oldest rocks in the canyon are two billion years old).

How angry does this make me? Well, how hot is a supernova? That’s about the same level.

It’s not bad enough that this White House is stepping on the throat of science everywhere it can, but now this Administration (through proxy appointees) is actively engaging in promoting a belief system which is demonstrably false… oh, and it happens to be unconstitutional, too. The very first Amendment to the U.S. Constitution really could not be any more clear on this.

“In order to avoid offending religious fundamentalists, our National Park Service is under orders to suspend its belief in geology,” stated PEER Executive Director Jeff Ruch. “It is disconcerting that the official position of a national park as to the geologic age of the Grand Canyon is ‘no comment.’”

The book, called "Grand Canyon: A Different View", talks about the idea that the Grand Canyon is very young. This is standard creationist rhetoric, and is grossly wrong. I might even call it a lie… no, let me rephrase this. I would definitely call it a lie. To see for yourself, try reading Red State Rabble’s Grand Canyon essays. There are four of them, slamming the creationist dribble. You can also read the wonderful National Center for Science Education’s rebuttal to this book. The NCSE rocks, by the way. They have tours of the Grand Canyon where you’ll get the real story, and not some made up nonsense.

What I find interesting is that this book has been around for years. When it first came out, people started to complain, and the National Park Service Chief of Communications David Barna leapt into action said something that turned out, um, not to be true:

Despite promising a prompt review of its approval for a book claiming the Grand Canyon was created by Noah’s flood rather than by geologic forces, more than three years later no review has ever been done and the book remains on sale at the park…

… NPS Chief of Communications David Barna told reporters and members of Congress that there would be a high-level policy review of the issue.

According to a recent NPS response to a Freedom of Information Act request filed by PEER, no such review was ever requested, let alone conducted or completed.

Emphasis mine.

Three years! How long does it take to read this book? Of course, having a scientist write down just why creationist arguments are wrong might take about that long. There are a lot of wrong arguments. But still, that’s 0.05% of the age of the Earth!

I’ll note that at the very least, the book should have been taken off the shelves until further investigation had been done. Yet there it sits, lying to the millions of visitors to the park every year.

If I were a Grand Canyon park geologist, I would be screaming bloody murder. But maybe I should leave the talking to them:

As one park geologist said, this is equivalent of Yellowstone National Park selling a book entitled Geysers of Old Faithful: Nostrils of Satan…

I would not be at all surprised to see that title coming soon to a government book store near you.

There is some good news, though: the Grand Canyon website does indeed talk about the canyon forming over millions of years. I wonder how long it will be before the White House changes that link to go to Answers in Genesis?

I’m still working on my "Runners Up" post about the (almost) best images of 2006, but in the meantime, feast your eyes on a page containing dozens of incredible cloud images. I have seen a lot of these kinds of clouds myself (lenticular clouds are awesome, and weird). How many have you seen?

This past year has been — like most are — up and down for science in general and astronomy in particular. We’ve had stunning successes and heartbreaking setbacks, all of which seem huge when dealing with them at the time. But while the science of astronomy is many things, one of the more subtle yet deeply profound aspects of it is its ability to provide a sense of perspective.

The year’s end is a traditional time to look back and try to gain perspective on the events that occurred during this last circuit of the Sun. Astronomers have been hammering away at the sky, taking images that have profoundly pushed forward our understanding of the Universe. But there’s been more than that, too. They’ve slapped down some bad science — and deleting a negative is indeed a positive. They have also looked at old things in a new way, or new things in a new way, and even some new things in an old way.

With all this in mind, I decided to create my list of Best Astronomy Pictures of 2006. I went through hundreds of images (maybe thousands), checking NASA, APOD, the ESA, BAUT, and a few dozen amateur and professional sites featuring pictures as well. The criteria I kept in mind were beauty, of course, but also scientific value. But both of these could be trumped by the coolness factor. All three are subjective, but what the heck. It’s my blog. So here is what I came up with.

All the images below are hosted at Flickr, and they link to the original sites with higher resolution images (many of the pictures are suitable as wallpapers).

Remember, it’s my list. If you disagree, or you agree but don’t like my ordering, then post a comment! Let’s see what you think should have been here. Maybe I’ll post my runners-up list.

Number 10: The Comet and the Ring

A comet almost had to make this list, since they’re so darn pretty. But there was one this past year that, to me, was extra cool.

Periodic comet 73P/Schwassmann-Wachmann (say that three times fast) was discovered in 1930, and in 1995 it was seen to have broken up into many pieces, most likely due to heating from the Sun. The fragments passed close by the Earth once again in May 2006, where many were easily visible through binoculars (I saw them myself, both through my binocs and my ’scope).

The comet was big, bright, and passed by many astronomical showpieces… including the famous Ring Nebula, a cloud of gas a light year across ejected by a dying star. When the two were close together, astronomers Paul Martinez & Philip Brents took this spectacular shot:

The Ring is the disk-shaped object in the upper left. Here’s a close-up of it:

A lot of images were taken of this pairing, many showing more detail and with the comet closer to the Ring, but this image speaks to me (in fact, it’s my desktop image at work). The scale is big– a lot of sky is in this shot, and it shows better the contrast in apparent size between the two objects. The comet looks bigger only because it’s so much closer: the Ring is actually about 10 trillion times bigger than the comet! But it’s a tad bit farther away.

Number 9: Painting the Eclipse

Lunar eclipses are fairly common: the Moon passes into the Earth’s shadow roughly once or twice per year on average. Since the Moon is bright and easy to photograph, there are zillions of lunar eclipse pictures available to view.

I thought I’d seen ‘em all, but then I saw this one and it floored me:

How cool and wonderful is that? The photographer, Laurent Laveder, set this image up very carefully, making sure that the model was placed just so when the Moon was just starting to be eaten by the circular edge of the Earth’s shadow. The result was this very clever tongue-in-cheek photo. I love it! He has many more images on his site worth checking out, too.

I also wrote about this image in September 2006, and have some more comments there.

Number 8: The Tarantula Writ Large

Our Milky Way Galaxy is a giant spiral collection of stars, gas, and dust. It has many smaller satellite galaxies, and one of them is the Large Magellanic Cloud, or LMC. The LMC is a fuzzy cloud-like object easily visible to the naked eye if you happen to be far enough south of the Equator (I saw it with my own eyes from Canberra, Australia in 2004). Through a telescope, though, the LMC is dominated by a cloud of gas called the Tarantula Nebula, perhaps the most active stellar nursery known.

You’ve probably seen images of the Orion Nebula, right? At 1500 light years away, it’s one of the brightest nebulae in the Milky Way, and is easily visible to the unaided eye. It’s about 30 light years across.

The Tarantula, however, is 160,000 light years away, and yet is still about as bright to the eye as the Orion Nebula. That’s because it’s frakking huge: it’s something like 1700 light years across, fifty times Orion’s size! If the Tarantula were placed at the distance of the Orion nebula, it would fill half the sky.

That’s big.

And so is this next image. The good folks at the European Southern Observatory stitched together several images of the Tarantula to make a mosaic of it that has 256 million pixels. Let’s see your store-bought camera do that!

This is a very, very compressed image of the big one. You could download an insanely monstrous 211 Mb 9000 x 8000 pixel image, but I recommend you go to their zoomable image of it instead, and tour around it. See if you can spot Supernova 1987A, a star which blew up and eventually led to me getting my PhD.

But try not to get lost. It’s a big place.

Number 7: The Face Defaced

Ah, the "Face" on Mars. Where would Richard Hoagland be without it? Shilling some other snake oil, I would guess.

But that’s a dream; people promoting antiscientific garbage always find some way to offload their claptrap. Still, it’s always nice to see them slapped in the face — or the Face — by reality.

This next dose of reality comes courtesy of the European Space Agency, whose Mars Express orbiter took some great high-resolution images of the Cydonia plain on Mars where the face is located. By taking images from different angles and with varying solar illumination, they were able to create a three-dimensional image of the "Face". Perhaps when this image was released Hoagland waited with bated breath to see his ravings confirmed, but that’ll be a long, long wait:

Wow, it’s uncanny, isn’t it? It looks just like a face… if that face was hit repeatedly with high speed projectiles and then covered with lumpy mashed potatoes.

That’s no face… it’s a butte! Yes, I know it’s actually a mesa and not a butte, but let a guy have a joke at someone else’s expense once in a while, OK?

Anyway, the ESA also put together a nifty 3D rotating animation of this, and there is another image taken from a different angle as well. That last one is marginally more face-like, but you have to kinda squint really hard to see it. There is also a cool suite of "Face" images online as well.

Number 6: Robots on Mars

Speaking of Mars, we humans have been sending our machines there for a long time. We still lose the odd one or two (getting to Mars is pretty hard in reality), but in general we’ve been getting better at it, and better at building them as well.

In 2006, the Mars Reconnaissance Orbiter switched on its HiRISE camera, a phenomenal device capable of taking images of the Red Planet’s surface with half a meter resolution. The pictures returned have been devastatingly amazing, with an incredible scientific return, but sometimes the best pictures have little scientific value, but are still, well, cool!

This is a very small piece of a HiRISE image of Victoria crater on Mars, showing just one part of the rim. And sitting right there on the edge of the crater is a little metal robot named Opportunity. Launched in July 2003, that rover was designed to work for only 90 days, yet it just celebrated its 1000th day on Mars! You might have a hard time seeing it in that image, so here’s a close-up:

There it is! You can see the shadow of the camera mast, and even the tracks of the rover.

Incredible.

Of course, I know in my brain that Opportunity is sitting on Mars, and I have seen all sorts of pictures it’s taken of itself. But somehow, seeing that image makes it really real.

We have robots on Mars! Humans are so smart.

Number 5: The Shuttle, the ISS, and the Sun

I still think it’s funny that most people are unaware that they can see man-made satellites easily with the naked eye. There are even websites that can tell you when a given satellite will pass near you!

The Shuttle, when it’s up, is a pretty bright object, as is the International Space Station. So if you do your homework and plan your observation extremely carefully, you just might make my Top Ten list.

Thierry Legault did just that. Not only did he get a picture of the Shuttle and the ISS, he nailed them while, from his viewing point, they were passing directly in front of the Sun.

This shot is simply stunning, and shows a tremendous effort in planning, timing, and execution. The picture was taken on September 17, 2006, less than an hour after Atlantis had undocked from the ISS. By capturing them in silhouette against the Sun, he could take such a short exposure that any atmospheric distortion was frozen out. This means he got incredible detail in his picture. Take a look at the zoomed image:

You can see different structures on the ISS, and even the vertical tail on the Shuttle! Given that the spacecraft were hundreds of kilometers away from Legault, this picture is truly an amazing feat.

Number 4: Direct Evidence of Dark Matter

This next picture takes a moment to set up, so please forgive me. Plus, I like to lecture sometimes.

As I was perusing images, I realized I didn’t have many that had strong scientific value, which was ironic. But that happens: most scientific images aren’t published because they’re pretty, and pretty pictures sometimes only get in the news because they’re pretty. But there was one image this year that has both beauty and a far deeper scientific significance.

It’s been known for decades that there is a lot of dark stuff out in space, between galaxies. We see its effects on the way galaxies rotate, and the way they behave when they live in clusters (like a city of galaxies). We know that this dark matter is ten times as common as regular matter (like the stuff we are made of: atoms of hydrogen, carbon, oxygen and so forth), but it was undetectable, so it was somehow different than normal matter. But how, exactly? No one was sure.

One theory was that dark matter was made of weird particles that could interact with normal matter or other dark matter through gravity, but that was it. In other words, two colliding clouds of dark matter could pass right though each other like ghosts.

But how do you detect something like that? One way is through gravitational lensing. Matter has gravity, and gravity bends light. So if matter, even dark matter, gets between you and some distant object, it can act like a lens, distorting the light from the more distant object. By mapping out those distortions you can "see" dark matter.

So you know what those clever astronomers did? They looked at two colliding clusters of galaxies, which together are called the Bullet Cluster. Galaxy clusters have lots of gas pervading them, like fog in a city. When the clusters collided, the gas from each cluster smacked into the other head-on, grinding them to a halt. But if the dark matter is really this ethereal stuff it would keep on going, undeterred. If this were the case, you’d see the normal matter from the cluster closer to the center, with the dark matter on the outside.

And behind door number two…

Voila! The pinkish light is coming from the normal gas in the cluster. The dark matter reveals itself through its gravitational distortion of more distant objects, which is colored blue here. And look! The dark matter is on the outside, and the normal matter on the inside, just as predicted!

To an astronomer, this is completely convincing evidence that dark matter is real, and that the majority of the Universe is made up of stuff we simply don’t understand. What is it? Beats me, and it beats a lot of other scientists, too.

I love mysteries! That means there’s more to learn.

Number 3: Solar Shock Wave

The next image on my list makes me a little sore. When it was released I was busy and figured it wouldn’t be that interesting. I was completely wrong.

The Sun has a powerful magnetic field. The surface of the Sun is so hot that the atoms of gas have their electrons stripped off (the atoms are ionized), and this in turn makes them susceptible to those magnetic fields. In fact, the field is coupled with the matter: the gas follows the shape of the magnetic field lines, and as the gas moves the field lines also follows the gas. As the gas churns and boils on the surface, the field lines get all tangled up. A lot of energy gets squeezed in a tight space, and when that happens the magnetic field lines can suddenly and catastrophically reconnect, releasing vast amounts of energy in a solar flare.

On December 6, 2006, a big flare detonated on the Sun. The release of energy screamed outward over the surface, expanding in a circle. Astronomers at the National Solar Observatory caught this expanding shock wave in the act:

The fuzzy white ring is the expanding wave. The scale here is numbing: the Sun is 1.7 million kilometers across, so this ring was hundreds of thousands of kilometers in diameter. Heck, just the width of the ring is far larger than the Earth!

So why is this one of my top picks for 2006? After all, the still image doesn’t look like much! Ah, but the astronomers at NSO strung together a series of images into a dynamite animation. It’s totally cool. I couldn’t find the total energy released in this flare, but a typical big flare might blow off 10²⁵ Joules… which is 10% of the total energy emitted by the Sun every second.

Still not sure how much that is? Think of it this way: a big hydrogen bomb might have a yield of about 10¹⁶ Joules, so this flare was the equivalent of one billion hydrogen bombs.

Yikes.

See why I picked this as number 3?

Number 2: Evidence of Water on Mars

Mars figured prominently in the headlines this year, and so it does here as well. This third image of the fourth rock from the Sun is my number two pick because it shows direct evidence for what might be the biggest discovery on Mars yet: the presence of recent water activity on the surface!

We know that there’s water on Mars, but it’s frozen. The polar caps have water ice, for example. And we see lots of evidence that water flowed billions of years ago on the surface too: there are gullies, river beds, and flow patterns of erosion. But if there’s any water near the surface, it must be frozen beneath it, like a permafrost.

But we’re humans, and we like our water to be liquid. Could there be any on the surface?

Recent pictures from the Mars Global Surveyor indicate that there is. It doesn’t last long, and there’s maybe not a huge amount of it, but it’s there.

The image on the laft was taken in August 1999, and the one on the right in September 2006. The difference is obvious: the later image shows a gully filled with a lighter-toned material that was not in the earlier one (in fact, it was later seen on images from Febraury 2004). Something seeped out from below the surface and deposited that material during the intervening time. The evidence that it was water is indirect, but very compelling. Dry dust flows are generally darker, and the flow shape indicates the medium was a liquid like water.

If it was water, then there was only enough in that flow to fill a few Olympic sized swimming pools. Not enough to keep a colony drinking, but then again I wouldn’t have wanted to be standing downstream when it erupted from the ground. It’s a fine start.

Again, while this isn’t rock-solid proof of recent water activity on the surface of Mars, it’s the simplest explanation, and that is extremely exciting.

What else could it possibly have been?

This image has it all. It’s of a familiar object, seen in an unfamiliar way: back-lit by the Sun, a view impossible from Earth. It shows the whole planet, a rarity from space missions. The image shows very faint details and has very high resolution, a must.

But there is sheer artistry at work here. The colors, the lighting… I love the sun splash in the lower left limb of the planet, and the fans of ethereal mistiness shooting out from the rings. The shading on the planet itself is lovely, while the rings provide a geometric symmetry that is very appealing to the eye.

All this is necessary for the image to be the best, and together they may even be sufficient. But like all true winners, it has that extra addition, the over-the-top detail that pushes it into "all-time" status:

That dot in the center of the image is the Earth. It’s us. Cassini was nearly one billion miles from us when it took this image, orbiting a giant ball of gas as exotic and alien as any place we can imagine. From such a terribly removed location, the entire Earth is reduced to a single point of light, just one among an anonymous many as seen from our robotic proxy as our generation, for the first time in all of history, seeks out our neighborhood and takes a really good look.

That’s why this is the best astronomy image of 2006. And it’s one of the best of all time.

Still and all, a year is a long time. In 2007 we’ll see more astronomy missions launched into orbit, more telescopes built, more people than ever perusing the images from Mars, from Saturn, and from the depths of space. What portraits of the Universe will make the list next year?

I’m not sure I agree with his gross generalization of YouTube commenters, and I’m sure not all Hoax Believers are as dumb as he says (but I have seen precisely this level of deductive reasoning by some HBs myself), but I feel I must link to this XKCD comic because if I don’t, I’ll get emails about it anyway.

Click to see the rest of it. Some NSFW language!