Bad Astronomy

[This post is a modified, updated version of an article I wrote a few years back. Since today is the anniversary of the event, I thought it could use an update.]

Eight years ago today—on Dec. 27, 2004—the Earth was rocked by a cosmic blast so epic its scale is nearly impossible to exaggerate.

The flood of gamma and X-rays that washed over the Earth was detected by several satellites designed to observe the high-energy skies. RHESSI, which observes the Sun, saw this blast. INTEGRAL, used to look for gamma rays from monster black holes, saw this blast. The newly-launched Swift satellite, which was designed and built to detect bursts of gamma-ray from across the Universe, not only saw this blast but was so flooded with energy its detectors completely saturated—think of it as trying to fill a drinking glass with a fire hose. Even more amazingly, Swift wasn’t even pointed anywhere near the direction of the burst: In other words, this flood of energy passed right through the body of the spacecraft itself and was still so strong it totally overwhelmed the cameras.

It gets worse. This enormous wave of fierce energy was so powerful it actually partially ionized the Earth’s upper atmosphere, and it made the Earth’s magnetic field ring like a bell. Several satellites were actually blinded by the event. Whatever this event was, it came from deep space and still was able to physically affect the Earth itself!

So what was this thing? What could do this kind of damage?

Astronomers discovered quickly just what this was, though when they figured it out they could scarcely believe it. On that day, eight years ago, the wrath of the magnetar SGR 1806-20 was visited upon the Earth.

Magnetars are neutron stars, the incredibly dense remnants of a supernovae explosions. They can have masses up to twice that of the Sun, but are so compact they may be less than 20 kilometers (12 miles) across. A single cubic centimeter of neutron star material would have a mass of 10¹⁴ grams: 100 million tons. That’s very roughly the combined mass of every single car on the United States, squeezed down into the size of a sugar cube. The surface gravity of a neutron star is therefore unimaginably strong, tens or even hundreds of billion times that of the Earth.

Yikes.

There’s more. What makes a neutron star a magnetar is its magnetic field: it may be a quadrillion (a 1 followed by 15 zeros: 1,000,000,000,000,000) times stronger than that of the Earth! That makes the magnetic field of a magnetar as big a player as the gravity. In a magnetar, the magnetic field and the crust of the star are coupled together so strongly that a change in one affects the other drastically. What happened that fateful day on SGR 1806-20 was most likely a star quake, a crack in the crust. This shook the magnetic field of the star violently, and caused an eruption of energy.

The sheer amount energy generated is difficult to comprehend. Although the crust probably shifted by only a centimeter, the incredible density and gravity made that a violent event far beyond anything we mere humans have experienced. The quake itself would have registered as 23 on the Richter scale—mind you, the largest earthquake ever recorded was about 9 on that scale, and it’s a logarithmic scale. The blast of energy surged away from the magnetar, out into the galaxy. In just 200 milliseconds—a fifth of a second, literally the blink of an eye—the eruption gave off as much energy as the Sun does in a quarter of a million years.

A fireball of matter erupted out of the star at nearly a third the speed of light, and the energy from the explosion moved—of course—at the speed of light itself. This hellish wave of energy expanded, eventually sweeping over the Earth and causing all the events described above.

Oh, and did I mention this magnetar is 50,000 light years away? No? That’s 500 quadrillion kilometers (300 quadrillion miles) away, about halfway across the freaking Milky Way galaxy itself!

And yet, even at that mind-crushing distance, it fried satellites and physically affected the Earth. It was so bright some satellites actually saw it reflected off the surface of the Moon! I’ll note that a supernova, the explosion of an entire star, has a hard time producing any physical effect on the Earth if it’s farther away than, say, 100 light years. Even a gamma-ray burst—an event so horrific it makes the hair on the back of my neck stand up just thinking about it—can only do any damage if it’s closer than 8000 light years or so. GRBs may not even be possible in our galaxy (they were common when the Universe was young, but not so much any more), which means that, for my money, magnetars may be the most dangerous beasties in the galaxy (though still unlikely to really put the hurt on us; see below).

Here’s what Swift detected at the moment of the burst:

As Swift scientist David Palmer describes:

This is the light curve that [Swift's Burst Alert Telescope] saw, showing how many gamma rays it counted in each sixteenth of a second during six minutes of observation. I didn’t draw the main spike because it was 10,000 times as bright as the tail emission, and you would need a monitor a thousand feet tall to look at it.

The blast was so strong Swift saturated, counting 2.5 million photons per second slamming into it, well off the top of that graph (and the actual blast was far brighter yet, as other satellites were able to determine).

See the pulsations in the plot? After the initial burst, which lasted only a fraction of a second, pulses of energy were seen from the magnetar for minutes afterward. The pulses occurred every 7.56 seconds, and that’s understood to be the rotation period of the neutron star. The crack in the crust got infernally hot, and we saw a pulse of light from it every time it spun into view. This same pulsing was seen by other satellites as well.

So here's a recap, in case your brain still remains uncrushed: this was an explosion by an object with the mass of the Sun, squeezed into a ball a few kilometers across, with gravity billions of times stronger than Earth's, a magnetic field quadrillions of times stronger, all of of which is spinning every 7.5 seconds.

Still, even given all that, the damage from the explosion was actually rather minimal here on Earth. But that’s because SGR 1806-20 is so very far away; had it been one-tenth that distance, the effects would have been 100 times stronger. We’d have lost satellites at least, and it would have caused billions of dollars in damage in NASA hardware alone. Of the dozen or so known magnetars, none is that close (though a couple are about 7000 light years away). Magnetars aren’t easy to hide, but it’s possible there are some within 5000 light years. It’s unlikely, though, and I’m not personally all that concerned.

The tantrum from SGR 1806-20 is one of the best studied events of its kind, and is certainly the most powerful ever detected in the modern era. Astronomers will be studying the magnetar, and others like it, very carefully to see what can be learned from them. If you want to read more, then I suggest the NASA page about the event, as well as the Sky and Telescope magazine page on it, too.

And if another blast like that one comes from SGR 1806, or any other magnetar, don’t worry: I’ll report it right here. Unless it fries my computer. Or just my brain, thinking about it.

On Dec. 24, 2012, the private space exploration company SpaceX gave its prototype Vertical Takeoff/Vertical Landing (VTVL) rocket called Grasshopper a chance to make a small hop: 40 meters straight up. During the test Grasshopper went up, hovered, and landed back on its tail successfully. Watch the test—and turn your speakers up for maximum effect!

That is so cool! It stayed aloft for a total of 29 seconds.

Grasshopper is actually the rocket tank from the first stage of a SpaceX Falcon 9 equipped with a single engine and landing legs. The idea is to be able to reuse rockets instead of dropping the used parts into the ocean when they’re done boosting the payload. The Space Shuttle did this with the solid rocket boosters, which fell into the Atlantic where they were recovered by boat. In the case of the Shuttle the financial savings of reusing the SRBs versus just building new ones was debatable. However, rocket design has improved since the SRBs were first built, and it’s possible reuse can save money as well as time.

This test of the Grasshopper was important because it used what’s called “closed loop thrust vector and throttle control”, which means the rocket engine was automatically pointed and thrust throttled as needed to compensate for things like changes in wind and fuel remaining (as the rocket burns fuel it gets lighter, so less thrust is needed to hover).

If you look carefully on the right hand side of the rocket in the picture above, you may notice what looks like a person riding on the landing leg platform. That's actually a cowboy mannequin the folks at SpaceX placed there as a lark. Elon Musk, SpaceX CEO, tweeted about it during the test flight. I suspect Wil Wheaton had something to do with this.

On Nov. 1, 2012, a test of Grasshopper had it going up less than 10 meters, so this test was pushing it harder. SpaceX plans on ramping up tests to the point where Grasshopper is traveling supersonically, to better mimic actual flight conditions of an orbital launch.

Other rocket companies have done similar tests, such as Armadillo Aerospace and Blue Origin. The idea is not new—the Apollo Lunar Modules were VTVL, after all. With improvements in onboard computers and mechanical techniques, though, the technology has come a long way. I’ll be very interested to see where this goes.

And a note: I am still hearing people grumbling that crewed space flight in the United States is dead with the Shuttle. That is simply ridiculous; they’re dead wrong. In fact, I’m excited for the next few years of putting humans into space. NASA is working on their Space Launch System (which just passed a major technology review), and there are several private companies working on taking people into space, into orbit, and beyond.

The future of humans in space looks pretty good to me right now. And I have a hope—a realistic one—that we’ll see a human walking on the Moon in the next fifteen to twenty years. Possibly sooner. We’ll see, but the idea is no longer a fantasy. It’ll be reality sooner or later. I’m hoping sooner.

Photographer Randy Halverson is good. Very, very good. So much so that I picked one of his shots to go in my gallery of the year’s best astrophotos.

But had I known about this picture, I might’ve picked it instead:

Click that to embiggen it! When you do, you’ll see why I like it. The Milky Way glows over the landscape, the combined might of billions of stars, their vast distance reducing their luminosity to a mere whisper. In the foreground a storm rages, lightning illuminating the clouds to a red and purple glow.

And above it, apparently attempting to connect the two, the brief flash of a shooting star: A bit of cosmic debris no bigger than a grain of sand, heated to luminescence by its tremendous speed as it rams through our air.

As an added bonus, on the right is the ruddy glow of Antares, a red supergiant star that marks the heart of Scorpius, the scorpion. One day that star will explode, and from its distance of only about 500 light years it will shine about as bright as the full Moon! But that may not happen for hundreds of thousands of years.

I love the sense of scale in this shot: The clouds a few kilometers away, the meteor a few hundred, Antares a few thousand trillion, and the merged stars of our galaxy deep in the background, a hundred thousand trillion kilometers further yet.

If there’s one thing I love about astronomy—and there are far more, I assure you—it’s the sheer size of it. From here to eternity, indeed.

I was wondering what to post for today; a short philosophical piece, a longer discussion of holidays and family, a pretty picture, or something quick and funny.

Then the Universe delivered unto me a picture perfect present: Behold!

Trust me, you want to see this bigger! This is a Spitzer Space Telescope view of the star Zeta Ophiuchus (or Zeta Oph to its friends), a massive star plowing through the gas and dust floating in space. Zeta Oph is a bruiser, with 20 times the Sun’s mass. It’s an incredibly luminous star, blasting out light at a rate 80,000 times higher than the Sun! Even at its distance of 400 light years or so, it should be one of the brightest stars in the sky … yet it actually appears relatively dim to the eye.

That’s because it’s sitting in a dust cloud, dense opaque material that absorbs the light from the mighty star and diminishes it. However, infrared light can penetrate the murk, allowing us to peer into the cloud and see what’s going on.

Zeta Oph is blasting out a fierce wind of subatomic particles (think of it as a super-solar wind) that expands around the star. Not only that, but the star itself is moving rapidly through the dust at a speed of about 25 kilometers per second (15 miles per second), so it’s violently compressing the material ahead of it. This creates that wave structure, which is similar to the wave off the bow of a boat, though more like the supersonic shock wave generated as a fighter jet screams through the air.

That curving wave is roughly four light years long: That’s 40 trillion kilometers! The colors we see here represent different wavelengths of infrared light, well outside what the human eye can see, but are very clear to the Spitzer telescope, designed to see this flavor of light.

I love this shot; if it had been released just a week earlier, I would’ve included it in my list of the Best Pictures of the Year! In fact, an image of it from WISE, another infrared ‘scope, did make my Top Pictures list in 2011.

This is one of my favorite objects of all time, so it was easy to decide to post it today. And c’mon: It’s red and green, and is literally wrapped in bow. How could I resist?

The Universe is simply amazing, and is the gift that keeps on giving. Enjoy your holidays, folks.

One of the questions astronomers always get from people is, “What movie depicts science the most accurately?” Most of us usually say, 2001: A Space Odyssey, which is true enough. There are some mistakes in the film, but you have to be an über-dork to see them (the phases and locations of the Earth and Moon are shown incorrectly a couple of times—which, to be fair, is important to the plot, but still pretty dang nerdy to spot).

Another movie that holds its own is Contact, based on the novel by Carl Sagan. It goes out of its way for accuracy, adding lines of dialogue that are wholly incomprehensible to the layman but which get astronomers nodding sagely to themselves. For example, when Jodie Foster says the signal’s frequency is “pi times hydrogen,” she is referring to the characteristic radio signal emitted by hydrogen atoms in space (about 1.4 GHz) but multiplied by the number pi, which is a sign of the signal’s intelligent origin.

I like Contact quite a bit, and in fact just recently watched it with my daughter, who saw it for the first time (she loved it too). So I was pretty psyched when my pal Veronica Belmont asked me to be on her web show “Fact or Fictional” (part of Revision3's Tech Feed channel on YouTube) to talk about the science of the movie. It’s a short interview, but fun:

And if you haven’t seen Contact, go! Watch it! And then go and read the book by Sagan. As good as the movie is, the book is far better, and the ending—literally, the last page—still gives me a chill just thinking about it.

André van der Hoeven is a Dutch astrophotographer whose work has graced my blog many times in the past. He sent me a note recently: He’s been taking archived images of astronomical objects from telescopes like Hubble and Subaru and combining them. This way he can create an image that has both the fine detail due to Hubble’s excellent eyesight as well as the wide field and faint detail that comes from a bigger telescope like Subaru (an 8-meter giant in Hawaii).

The reason he sent me the note was because he had finished working on a picture of the Ring Nebula, one of the finest examples of a planetary nebula—a dying star shedding winds of gas—in the sky. His final product is nothing short of amazing:

Gorgeous, isn’t it? I’ve observed the Ring Nebula literally hundreds of times; it’s big, bright, and very easy to find in the summer and autumn sky. Even through a small telescope you can make out that inner ring, which looks like a hazy smoke ring floating in the eyepiece. It looks almost like the solid disk of a planet, which is why these objects are called planetary nebulae.

But what’s truly amazing is all that outer material roiling around it, completely invisible in nearly every picture I’ve seen. That stuff is ethereally thin and incredibly faint, so it only shows up in deep images taken with big telescopes.

All of this gas was expelled by the star in the very center of the nebula, which was once very much like the Sun (though probably about twice our star’s mass). After billions of years of converting hydrogen to helium in its core and generating fierce amounts of energy, it started to run out of fuel. The star expanded into a red giant, blowing a wind of subatomic particles into space; that’s what makes up the shells of gas you see in the deep image.

Eventually, the star started to contract, heating up and blowing a faster wind that caught up with and slammed into the older material. That’s what forms the brighter inner ring. Interestingly, it’s not actually shaped like a ring: Studies have shown it’s actually barrel-shaped and oriented so that we’re looking down the barrel. It only appears to be shaped like a ring due to our viewing angle.

All of that is cool just by itself. But wait! There’s more!

While assembling the images together, van der Hoeven noticed something odd. A foreground star that happened to lie between us and the inner ring appeared to move between images. Curious, he found older images of the nebula, one from 1959 and another from as far back as 1922! Adding them together, you can clearly see this star moving across the frame:

How cool is that? The triangle connects three fixed stars, and the moving star—called 2MASS 18533272+3301234—is the red one in the middle of the triangle. Its motion is real: Like planets orbiting the Sun, stars orbit the center of the Milky Way galaxy. That motion can be hundreds of kilometers per second, but appears diminished to invisibility due to their vast distances. Over time, though, it adds up, and can be detected. Astronomers call this proper motion. This star seems to be moving about 0.1 arcseconds per year, a very small amount: The Moon is 1800 arcseconds across, so it would take this star nearly 20 millennia to move across a parcel of the sky the size of the Moon.

However, with telescopes, and the patience to wait 90 years, that motion is still detectable. Van der Hoeven also did some rough measurements and found the star is likely to be about 800 light years away, about a third of the distance to the Ring nebula itself.  

He has more information about this on his website, including more images of the star’s motion.

From what I can tell, this star’s movement has never been noticed before, so congrats to van der Hoeven for his keen eye! Perhaps we should name the star after him. “Van der Hoeven’s star” has … wait for it, wait for it … a nice ring to it.

Oh, my, but do I have a treat for you today. Feast your eyes on the spectacularness of NGC 5189, a dying star seen by the Hubble Space Telescope:

Holy Haleakala! You absolutely must grab the embiggened version, since I shrank it considerably to fit the width of the blog here. There’s also a 7Mb version at 4,000 x 4200 pixels that will have your jaw hitting the floor. Honestly, this is one of the most spectacular Hubble images I’ve ever seen, and I’ve seen a lot.

[If you like this picture, the folks at Hubble Heritage are hosting a contest about it, asking if it reminds you of some other object like a dragon. I see a dog's face looking obliquely left. See it?]

Now that your eyes have had their fill, it’s time to feast your brain. What are you seeing here?

This object is an expanding cloud of gas rushing away from a dying star. Right in the very center you can see the star itself, a tiny blue dot whose appearance belies its power. Once a star like the Sun, the central star of NGC 5189 is now a dense, extremely hot cinder called a white dwarf. It’s probably only the size of the Earth but is 100,000 times denser than our planet.

A few thousand years ago this star was dying. It had swollen into a red giant, a huge, bloated thing that was expelling a strong, thick wind of gas into space. Over time the star shrank and heated up, turning bluish and starting to blow a thinner but much faster wind. The fast wind caught up with and slammed into the older, slower, thicker wind, carving out a cavity in it. We call these kinds of clouds planetary nebulae, becasue through small telescopes some of them look round and green, like planets.

So what’s with the huge backwards S-shape?

To be honest, it’s not completely understood. However, the most likely cause is a jet of material beaming away from the poles of the central star, powered by the intense heat and magnetic fields of the fierce white dwarf. These types of jets are common in situations like this.

If the star were just sitting there, the jets would plunge up and down, slamming into the material, punching through it along the star’s axis of rotation. However, something is causing the white dwarf to wobble. Perhaps there is a second star we can’t see tugging on it, or there could be planets in the doomed system whose gravity is affecting the white dwarf. But instead of simply rotating, the star is precessing, that is, wobbling like a top that’s spinning down. This changes the direction of the jets over time, sweeping out a huge circle above and below the star.

The S is actually where the jet has rammed into the material surrounding the star, expelled in its earlier winds. You can see streamers of material blowing away, outward, where the jet has hit denser stuff clumped around the star. In fact, this tells us the orientation of the entire structure: Since the nebula is expanding, material moving toward us must be on the near side, and stuff moving away must be on the far side. The picture above uses this information to map the nebula: Parts colored red are moving away from us and therefore on the far side; the blue material is heading toward us and therefore closer.

As you can see, the top of the nebula is tipped away from us. Although I can’t be sure, the current orientation of the jets of material blasting from the star is probably from the upper left to the lower right, where the S shape suddenly stops. As the jet sweeps around, the ends of the S will continue on in those directions.

But it’s actually pretty difficult to tell. It’s not at all clear what’s going on here—and I’ll note I spent a few years studying these types of objects. Their structure is complex and tangled, making interpretation a nightmare. In the near side/far side map, you can see there is also a ring of material around the star like the waist of an hourglass. This type of feature is again common in such nebulae, but the arcs of the S overlap it so it’s fiendishly hard to figure out what’s what here.

So it’s a puzzle! And the only way to solve it is to take more detailed observations, use more careful analysis, and apply the math and physics we’ve developed over the past couple of centuries. It’s only this way that we can properly learn about what we’re seeing here.

That’s one of my favorite things about astronomy, in fact. It really is a treat for both our eyes and our brain. Such devastating beauty is supported, magnified, amplified by the awe we feel when we truly understand it.

Speaking of asteroid impacts…

In September 2011, I gave a talk at TEDxBoulder about asteroid impacts, why they are such a big threat, and what we can do about them.

I’m pleased to note the talk became very popular, and the video of it eventually made it to the main TED site; as I write this it has over 800,000 views! It’s gratifying to know so many people have heard this message that I think is critically important.

In even better news, TED has partnered with the Huffington Post to highlight one talk every Friday in what they’re calling TEDWeekends, and they chose my presentation for this week! The video of the talk is available there, and they also posted a new article I wrote about the talk to update it with more current information about this threat from the skies.

The talk is also on Youtube, and why not, here it is just to make it easy on you:

I encourage you to go to the HuffPo site, though, so you can read my article as well. 

I must note that I made a couple of errors in the talk. Two are minor, and outlined in a blog post I wrote at the time. There is one other place I blew it, and it’s a bit embarrassing: I greatly overstated the explosion of the K/T impact, saying it was a million times the entire planet’s arsenal of nuclear weapons. It was actually more like 20,000 times as large (what I meant to say was that it's about a million times the largest bomb ever detonated), which is still a mind-numbing blast capable of global extinctions. As we know that one was.

The good news is that people are beginning to take action about asteroid impacts, and I think that if we continue to take this threat seriously we’ll have a formidable defense against impacts set up in the next couple of decades. Given the statistics of impact frequency, that’s likely to be soon enough to prevent one from turning us into the dinosaurs.

And as a note: If someone ever asks you what the value of space exploration is, just show them a picture of a dinosaur skeleton collecting dust in a museum somewhere. Then ask them how much they want to see our our own bones in such an exhibit.

Space exploration can save the human race. Literally.

Yay! Asteroid 2011 AG5 will not hit us in 2040! Up until now, I couldn’t say that with perfect certainty.

Here’s the scoop: Last year, a 140-meter rock was discovered on an orbit that takes it very near the Earth. Projecting the orbit forward in time showed that it had a small but non-zero chance of impacting the Earth in the year 2040. That would be bad: It would explode upon impact with a yield of more than 100 megatons, far larger than even the biggest nuclear weapon ever detonated on Earth. While that wouldn’t cause a worldwide extinction event—this is no dinosaur-killer—an explosion equal to blowing up 100 million tons of TNT is something to be avoided.

The odds of impact were only about 1 in 500, but that’s still too high to rest easy. The problem is that initially, the asteroid’s orbit was difficult to determine well enough to predict where it would be more than a few years in the future. The analogy I like is pretending you’re an outfielder in a baseball game, and as soon as the batter hits the ball, you have to close your eyes. How do you know where the ball will be when it comes down? If you can only get a second to look at it after it’s hit, you only have a vague idea where it will land. But the longer you can track it, the more accurately you can see where it’s headed. Track it long enough and you can catch it.

It’s the same with asteroids. Earlier in 2012 only a few observations of AG5 could be made before it got too close to the Sun to see. Those allowed the crude estimate of where it would be in 2040, and that big fuzzy volume of space included the Earth.

However, new observations taken with the monster Gemini telescope in Hawaii allowed a far better orbit to be calculated. The path of the asteroid in 2040 was found, and now clearly does not include the Earth. It will be a clean miss, by about 900,000 kilometers (550,000 miles). This is more than twice the distance to the Moon, if that helps.

The diagram above shows this: The upper picture shows the uncertainty in the region of space the asteroid would pass in 2040, and Earth is in that region. The lower picture shows the new calculations, with the asteroid missing us entirely.

I reported on this asteroid back in March 2012. At the time, with the impact risk of 0.2 percent, some people were concerned about AG5. One of those was Apollo 9 astronaut Rusty Schweickart, who is now with the B612 Foundation, a group dedicated to understanding and preventing asteroid impacts. He urged NASA to work on a plan to do something about the asteroid, should further observations make the odds for an impact higher.

Part of the problem at the time was that the asteroid was too close to the Sun to observe, and while there was a short window to observe in October 2012, it wouldn’t be until September 2013 that AG5 would be clear enough from the Sun to easily observe it. Schweickart didn’t want to lose a whole year should the asteroid prove dangerous.

Eventually NASA did perform an initial investigation into this, but now, happily, it won’t be needed. The October window worked out, the orbit refined, and now we know it’ll miss.

But this exercise was hardly a waste of time. The lessons learned were important. There are more asteroids out there, and given enough time the odds climb to certainty that we will be hit if we do nothing. The good news is we are looking for them, and hopefully, should we find one with our name on it, we’ll have enough time to be able to do something about it.

As for what we can do, stay tuned. I’ll have more about that shortly.

Tip o’ the Whipple shield to AsteroidWatch on Twitter.

Happy solstice!

I’ve been saying that today’s date is meaningless when it comes to doomsdays, which is true. But it does have astronomical significance, and for Northern Hemisphereans it’s a happy one: Today, at 11:12 UTC (06:12 Eastern time) it was officially the winter solstice. That means the nights are getting shorter, the days longer, and that half of winter is behind us.

There are a lot of different ways to describe this. One is to observe the Sun over the course of the year. In the summer it’s higher overhead at noon, and in the winter it’s lower. If you keep careful track of the arc the Sun makes in the sky every day, you’ll find it’s highest around June 21 every year, and lowest around Dec. 21. That’s a rough guide to the time of the solstices.

Another way is to measure the position of the Sun against the background stars. Astronomers use a coordinate system for the sky that’s much like using latitude and longitude on the Earth, but (for historical reasons) we call them Right Ascension and declination. Just like latitude measures your position north or south of the Earth’s equator, declination measures a star’s position north or south of the celestial equator (which is really just the Earth’s equator projected on the sky). If you measure the Sun’s position on the sky, you’ll find that every year around June 21 it reaches its northernmost declination, and around Dec. 21 its southernmost. What astronomers define as the winter solstice is the exact moment the center of the Sun’s disk reaches its southernmost declination. Today, that was at 11:12 UTC.

So what causes this? The Earth’s tilt! We orbit the Sun once per year in a path that is very close to a circle. The Earth also spins once per day, of course. The axis of that spin, though, is not exactly perpendicular to the plane of its orbit. Instead, it’s tilted by roughly 23 degrees. That’s why every school globe you’ve ever seen is tilted, in fact!

As the Earth orbits the Sun, the axis stays pointed in one direction on the sky—the north pole more or less points toward the north star Polaris. So sometimes the Northern Hemisphere is tilted toward the Sun, sometimes away. When we’re tipped toward the Sun, the Sun gets higher in the sky, heating the ground more directly, warming us up. It also means the Sun stays up in the sky longer (days get longer) so there’s more time to warm up every day. It’s summer!

The opposite is true in the winter: We’re tipped away from the Sun; the Sun is lower in the sky; it heats the ground less efficiently; and days are shorter. It gets cold: Welcome to winter.

Today, the Northern Hemisphere was tipped as far away from the Sun as it gets: the winter solstice.

But that’s a good thing! Every day for the next six months, we’ll slowly round the Sun and have our axis point more toward it. The Sun will get higher, the days longer and warmer.

That’s why ancient civilizations celebrated the solstice. It meant the return of the Sun and warmer days ahead. While in the United States we tend to call this the first day of winter, I think it’s more like the halfway mark. After all, the past six weeks the Sun has been getting lower in the sky, and for the next six it gets higher. The solstice is the midway point between those two, so for me it makes more sense to call this midwinter’s day; the midpoint of winter.

And I can’t help but mention this: The end-of-the-world crowd really screwed this one up. For ancient peoples this wasn’t a day for doom and gloom! It was generally a day to be happy, to celebrate. And I suppose it still is. We have a far greater understanding of astronomy now, and how the Earth interacts with the Universe around it. And we still get to enjoy the idea that warmer days are afoot.

It’s the best of both—of all—worlds.