Bad Astronomy

One of my favorite events of the year is the Mecca of nerdery, San Diego Comic Con. According to some estimates, eleventy billion geeks (actually, closer to 150,000) converge on the town every July to talk and hear from people involved in creating their favorite TV shows, movies, books, and comics. It's a seriously huge event.

At the 2012 SDCC, I was very excited to have my friends Chris Hardwick (from Nerdist) and astronomer Neil Tyson ask me to be on a live video version of Neil’s popular radio program “Star Talk”. Also appearing was Gary "Baba Booey" Dell'Abate (from the Howard Stern show), who is a huge gadget nerd. The four of us sat in a booth on the cavernous exhibit floor of the San Diego Convention Center and talked about gizmos—the tech, the science, the silliness. It was a lot of fun, and Hardwick just put the video up:

The video is about 20 minutes long, but we actually talked for over an hour. I hope more of that goes up at some point; Chris had to leave early to go to another event (I think he was meeting up with, seriously, the cast of Doctor Who!) and Gary also had to run, so Neil and I talked astronomy and space tech for a while.

After the interview, Neil’s producer mentioned that some of the cast of “Star Trek: The Next Generation” were signing autographs at a nearby booth and wanted to meet him. Neil asked me to tag along, so we went over and met Brent Spiner, Marina Sirtis, and LeVar Burton. That’s when this happened:

Oh my.

SDCC is always amazing, and last year I had the great pleasure to moderate a panel on the Science of Science Fiction, which is becoming something of a tradition. I have every plan on doing it again this year, and when I do I’ll be sure to let everyone know here on the blog. I hope to meet a lot of BABloggees there!

The Milky Way galaxy—our home galaxy—is erupting. Two monumental geysers are blasting out of its heart in opposite directions, and astronomers recently got the clearest view of them ever seen.

The image above shows the Milky Way in visible light, as we see it on a very dark night—stars, gas, and dust strewn across the sky. Superposed on that is the radio emission from those vast winds of material blasting outward (which is invisible to the eye; it's colored blue so you can see it). Those radio waves were detected by the Parkes radio telescope in Australia. These winds been seen before using both radio telescopes and Fermi, an orbiting observatory that detects gamma rays (the highest energy form of light), but only at low resolution. Until now they haven't been mapped so clearly and in such detail.

The scale of this image is difficult to grasp. I’ve cropped it here to let you see the structures, but if you look at the original image it shows the whole sky…and you can see these eruptions of matter are so vast they stretch across two-thirds of the entire sky!

I’m actually rather stunned at this. If you had radio-vision, and you could see these streams of matter, you’d have to physically turn around to see the whole thing end-to-end. In real numbers, the material is about 50,000 light years long—half the length of the galaxy itself—and is rushing away from the center of the galaxy at a mind-numbing 1000 kilometers per second!

When I read that, the hair on the back of my neck stood up. My first thought was, “What the frak could power something that vast?”

And then I found out: the geysers contain the energy equivalent of a million exploding stars!

At that point I may have blacked out for a moment or two. If you want to know what humbles an astronomer, then this is pretty much your go-to scenario.

It’s hard to express the colossal nature of this. Think of it this way: Take all the energy the Sun emits every second (enough to power the entire Earth’s needs for nearly a million years). Now multiply that by 31 million, the number of seconds in a year. Now multiply that by 10 billion, the numbers of years the Sun will be around. It’s a huge number, staggering, and that’s still only about 1% of the energy output of a single supernova. That means these geysers contain a hundred million times the Sun’s entire lifetime supply of energy.

See? That’s why I was overwhelmed.

I’ll note that these geysers present no danger at all to us here on Earth. If they did, we’d have been zapped a long time ago; this structure is pretty old, millions of years old at least. But we’re a long way from the action; the core of the galaxy, where the geysers are generated, is 26,000 light years away, and the material itself is not headed anywhere near us. We’re safe.

So still, what could possibly generate this much energy? For a long time it was thought to be the supermassive black hole we know exists in our galaxy’s center; matter falling in can be ejected away at high velocity. Another competing idea was that vigorous massive star formation over millions of years would generate huge winds of material, boosted even more when those stars died as supernovae. We know this happens on a smaller scale in the galaxy; bubbles of gas and dust erupting outwards have been seen before, like in this image from the Herschel space telescope:

At the lower left is a small herniated region (colored blue in this false-color far-infrared image) caused by supernovae and the winds from new stars blowing material out of the galaxy. Even though this is pretty big on a human scale (many light years across), it’s peanuts compared to the Parkes observation. Still, the idea is the same.

And the new Parkes observations finally do resolve this. As the material blows out from the galaxy it carries with it a magnetic field. Careful analysis of the affect of that field on the material using the Parkes data shows the energy source to be star formation, and not winds from the black hole. The shape and structure of the geysers indicate there must have been several different episodes of star formation, in fact, and not just one long, continuous event.

I’ll note I’ve been reading about these competing ideas for a long time, and the debate has been pretty strong. Until now, it wasn’t clear which was right, so it’s nice to see this resolved.

And it’s amazing, too: It’s incredible to think that something with so much power could have been hiding from us for so long; it’s only because it’s spread out over so much sky that we missed it.

Such an incredible image, on such a scale! It’s wonderful to know that we can learn so much about our own home. Even better, it reminds us that we still have so much more left to learn.

That’s yet another reason I love science so much. It’s a magnificent puzzle that never ends. There's always another piece of it waiting to be found around the corner, and there's always more to learn.

[Update: I got a note from my friend Carolyn Porco—imaging team leader for the Cassini Saturn mission—saying that technically, the term "geyser" is only used to describe eruptions of water. She should know, since she studies the geysers at the south pole of Saturn's moon Enceladus! I'll note I use the term here as an analog, a metaphor if you like. There are many terms for such events—jets, bubbles, cavities, and so on—but none seemed to fit like "geyser". This is a common problem in communicating science; being technically accurate while still being able to actually describe things to people who may be unfamiliar with the science. I apologize for using any misleading terms, and I'm happy to hear of any better ones!]

At 04:37 UTC today (11:37 p.m. Eastern US time last night), the Earth reached perihelion, the point in its orbit when it’s closest to the Sun for the year. At that time, the center of the Earth was 147,098,161 kilometers from the center of the Sun.

You probably didn’t notice. There were no fireworks, no ball dropping from a spire, no breathless celebrities celebrating it. But it happened just the same, as it did last year, and the year before that, and all the years stretching back to the dawn of our planet 4.5 billion years ago.

The Earth orbits the Sun in an ellipse, not a circle. This fact wasn’t discovered until the early 1600s, when astronomer Johannes Kepler published the first two of his three laws of planetary motion. Until that time, for thousands of years previously, everyone thought the planets orbited the Sun moved along perfectly circular paths.

But orbit in an ellipse we do, which means sometimes we’re closer to the Sun, and sometimes farther.  For the Earth, the difference isn’t all that much: about 5 million km (3 million miles) separate closest and farthest distance from the Sun—very roughly 147 - 152 million km (91 - 94 million miles). It’s a change of only about 3%, which to the eye would make it look pretty much like a perfect circle. In fact, I used Wolfram Alpha to draw a circle and an ellipse with the same shape as Earth’s orbit. Can you see the difference?

The one on the right is the ellipse. Hard to tell, isn’t it?

Some people think that the distance to the Sun is the reason we have seasons. But as you can see, this difference is so small it hardly has any affect at all. There’s some, but it’s small. We’re a little bit cooler on average when we’re farther away, but the tilt of the Earth’s axis is a far bigger influence on temperature than our distance from the Sun. Note that we reach perihelion in January, in the dead of winter for the Northern Hemisphere! That’s the opposite of what you would expect if distance to the Sun alone were the cause of the seasons.

Calculating the exact time of perihelion turns out to be a bit complicated; it’s different every year (last year it was on January 5). I was surprised initially to find out the biggest effect changing the time of perihelion is from the Moon! Upon reflection it makes sense, though. As the Moon orbits the Earth in a big circle 770,000 km (475,000 miles) across, the Earth makes a smaller circle too. Picture it this way: Imagine two children holding hands and spinning each other around. One kid is bigger than the other, so the smaller child makes a big circle, and the bigger kid makes a small circle as they swing around, with both of them revolving around their center of mass (what astronomers call their barycenter). The Moon and Earth are locked together by gravity, but the principle is the same.

Because the Earth makes that smaller circle once per month, the shape of its path around the Sun is a bit wobbly. This can change the time of perihelion by several hours! Astronomers need to account for that when they calculate the time of perihelion.

Not only that, but the distance is calculated between the centers of the Earth and Sun; that’s because the equations used involve gravity, and that is done assuming the objects are actually point sources, literally infinitely small points that have mass and gravity. Given the huge distance between the Earth and Sun compared to their sizes, that math works out pretty accurately. When the Earth was at perihelion, its center was 147,098,161 km (91,402,560 miles). Your exact distance from the Sun at that moment depended on where you were on Earth, your latitude and longitude. But the Earth’s radius is about 6371 km, and the Sun’s is 696,340 km, so their surfaces were about 146,395,450 km (90,965,914 miles) apart.

Roughly.

So what does all this mean? Well, if you very carefully measured the size of the Sun, it would appear a bit bigger today than any other day of the year. The difference is pretty small, as this picture from astrophotographer Anthony Ayiomamitis shows:

That’s the Sun at aphelion (left) perihelion (right), and clearly it’s not a huge deal. You’d never notice.

But it also means that every day, from here on out until we reach aphelion, we’ll be a little bit farther from the Sun, heading upward in our orbit. Then, on July 5, 2013, we’ll reach the top of our orbit, aphelion, and start to head downhill once again.

A lot of people like to make New Years resolutions, some way to improve their lives starting with the beginning of the year. Have you? If you did, aphelion makes a good benchmark for a checkup. Between now and then the Earth will have moved 468 million km (290 million miles) around the Sun, and will be 5 million km further from it. What will you accomplish in that same time?

[Update: I mistakenly wrote that the last perihelion was on January 2, 2012, when it was in fact on January 5, 2012. It has been corrected in the text above. Also, I wrote that it had been thought planets orbited the Sun in circles, but I should've said that simply that planets moved in circles—the idea they orbited the Sun came along with Copernicus, though he too thought they moved in circles. This has been corrected in the text as well.]

Yesterday, on the last day of 2012 when the world was preparing to celebrate the arrival of a new year, the Sun blew off a few fireworks of its own:

This event is called a prominence, and it happens when huge amounts of energy stored in the Sun’s magnetic field is eruptively released. Loops of magnetic field lines pierce through the Sun’s surface, their bases forming sunspots. Each spot is like a magnet’s north or south pole, and the loop connecting them can contain a lot of energy. I mean a lot: big ones can explode with the equivalent of billions of nuclear weapons. If the lines get tangled or the spots get too close together, it’s like shorting out an electric circuit. The lines snap, the energy is released, and mayhem occurs.

In this case, it was a relatively mild event. Only a few million tons of hydrogen were blasted a hundred thousand kilometers up from the Sun’s surface against its fierce gravity, and the explosion was probably only the same as a paltry few million megaton-class bombs. No biggie.

The images to make the animation were taken by NASA’s Solar Dynamics Observatory, which stares at the Sun 24 hours a day. It can see well out into the far ultraviolet part of the spectrum, where activity like this is more obvious (in this case, the images were at a wavelength of 30.4 nanometers).

The Sun is approaching the peak of its 11-year magnetic cycle, which should mean more sunspots and more activity. However, it’s not easy to predict what will actually happen; it may be a smaller peak than usual, or it may be bigger. We need our eyes on the Sun like SDO to let us know what’s happening! Big explosions create waves of subatomic particles that can scream across space; when they reach Earth they can damage our satellites and trigger blackouts here on Earth, causing billions of dollars in damage.

Literally, space exploration and our knowledge of astronomy and physics can help save money, property, and our modern technical civilization. There’s a thought to start your new year off right!

Tip o' the lens cap to Little SDO on Facebook.

Yay! It’s a new year!

But what does that mean, exactly?

The year, of course, is the time it takes for the Earth to go around 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 while the antacid is dissolving in your stomach to remedy last night’s excesses, sit back and let me tell you the tale of the year.

Round and Round She Goes

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 billions 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 dang 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?)—shouldn’t that be exactly equal to 24 hours? What happened to those 3 minutes and 56 seconds?

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.

A diagram from Nick Strobel’s fine site Astronomy Notes (shown here; click to embiggen) helps 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 3 minutes and 56 seconds 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.

The Sun Rose by Any Other Name

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—a specific time of the year when the Sun crosses directly over the Earth’s equator in March—in one year to the vernal equinox in 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, by 21 minutes or so. If we didn’t account for this, then every year the seasons would 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, people leaving comments in all caps, and so on.

So how do you account for this difference and not let the time of the seasons wander all over the calendar? 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, not to self-aggrandize too much, 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 365.25 mean solar days (most of the time, actually), 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.

Lend Me Your 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 (which in 2013 is tomorrow, Jan. 2, and I’ll have a post about that as well, never fear). If you measure the year from perihelion to perihelion (called an anomalistic year, an old term used to describe the shape of an orbit) 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, taking 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 31,558,432 seconds, or 365.26 days. 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.

Where To Start?

One more thing. We have all these different years and decided to adopt the tropical year for our calendars, which is all well and good. But here’s an issue: Where do we start it?

After all, the Earth’s orbit is an ellipse with no start or finish. It just keeps on keeping on. But there are some points in the orbit that are special, and we could use them. For example, as I mentioned above, we could use perihelion, when the Earth is closest to the Sun, or the vernal equinox. Those are actual physical events that have a well-defined meaning and time.

The problem, though, is that the calendar year doesn’t line up with them well. The date of perihelion changes year to year due to several factors (including, of all things, the Moon, and the fact that we have to add a leap day every four years like we did this year). In 2012 perihelion was on Jan. 5, but in 2013 it’s on Jan. 2. Same thing with the equinox: It can range from March 20 to the 21st. That makes using orbital markers a tough standard.

Various countries used different dates for the beginning of the year. Some had already used Jan. 1 by the time the Gregorian (tropical) calendar was first decreed in 1582, but it took time for others to move to that date. England didn’t until 1752 when it passed the Calendar Act. Not surprisingly, there was a lot of religious influence on when to start the new year; for a long time a lot of countries used March 25 as the start of the new year, calling it Lady Day, based on the assumed date when the archangel Gabriel told Mary she would be the mother of God. Given that a lot of ancient Christian holidays are actually based on older, Pagan holiday dates, and the fact that this was on March 25^th—very close to the equinox—makes this date at the very least suspicious.

Still, in the end, the date to start the new year is an arbitrary choice, and Jan. 1 is as good a day as any. And as a happy side effect it does help establish the Knuckle Rule.

Resolving the New Year

So there you go. As usual, astronomers have taken a simple concept like “years” and turned it into a horrifying nightmare of nerdery and math. 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. It has 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 list only how many days, which is useful but could be better).

I have to add one more bit of geekiness. While originally researching all this, 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 challenge them to a duel with orreries at dawn.

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. It has tons of gory details about this stuff.

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

Happy New Year!

[Note: This article is a modified, updated version of one that has run on the Bad Astronomy blog in the past.]

Today is the last day of the year, traditionally a time of reflection on the past and hope for the future.

We all see time through the lenses of our own minds, our own lives. What matters to us individually differs from person to person, of course, but there can be a lot of overlap. Most of us worry about the same things: family, finances, conflict, peace, equality. Some of these concerns are personal, some grand, some specific, some enveloping the whole world.

I am no exception. My thoughts range from the close to the distant: My family, my daughter prepping for college, friends, writing, government, matters overseas, and the existence of hatred and small-mindedness when even a small amount of compassion and tolerance can overwhelm them.

It’s easy to get inundated when being reflective, easy to be overwhelmed yourself. When that happens to me—and it does, every day—I literally think to myself, “Look up.”

And I do. I look to the sky, and I see great beauty there, filled with amazing things, from tiny asteroids to vast sprawling galaxies. Now, for the first time in history, we’re just starting to venture out into it. The groundwork laid for centuries is now paying off in our ability to leave the ground, perhaps forever.

We’re still figuring out how to do it, and we make lots of wrong steps as well as right ones, but the path, I think, still leads upward. I was thinking about this very thing today when I happened to see this picture:

This photo was taken by Kevin Ford, an astronaut on board the International Space Station. On Dec. 21, looking out a window, he took this picture of an approaching Russian Soyuz capsule containing three more astronauts: the crew of Expedition 35, which will take over command of the ISS for the next few months.

You can see the capsule almost swallowed by the canvas of black around it. Below, seemingly close in the picture but still hundreds of kilometers away, is the gentle blue curve of our planet’s horizon. And above, more distant by far, the half-lit face of the Moon. In this short exposure no stars can be seen; it’s just our planet, its one natural satellite, and one of many human-created satellites. The Moon above has no one on it; just artifacts left by a short visit decades ago and an uncrewed handful since. The Earth below is teeming with people, all living under that narrow blue arc of air. And in between, three more humans guided by the hand of Newton’s laws, headed for an outpost in space.

If there’s a better metaphor for the end of a year and the beginning of another, I’m not sure I know it. Humans are explorers. We’re a curious bunch, and we love to stick our heads into places unknown, moving from one thing to the next, learning about everything around us.

There’s a lot of everything to know. And we cannot possibly understand it with our eyes closed, our minds narrowed, our heads tilted down.

So look up! Because when we do, even for a moment, our view increases from here to infinity.

If you do one thing this upcoming year, just look up.

—

My thanks to Expedition 35 Commander Chris Hadfield for putting this photo on Twitter, and for reminding me why I love doing what I get to do.

I love clever filmmakers. I love science. So of course, I love it when clever filmmakers makes clever science videos. John Boswell is one of the creators of Symphony of Science, a fantastic series of videos featuring scientists and science communicators auto-tuned to original music. He also makes other videos about science under the name MelodySheep on YouTube … which is how I found a fantastic short piece called “Our Story in One Minute”. It shows the entire 4.5 billion year history of the Earth compressed down into one minute. Well, the highlights at least, with a stress on humanity’s involvement. It’s fascinating and wonderful. Watch:

Pretty cool. And is that my friend Brian Cox standing on the mountain at the end? I think it is.

If you liked this video, you might also like “Timeline: The Age of the Universe”, which has the 13.73 billion year history of the entire Universe compressed to 13.73 minutes; a billion years per minute. It was created by Craig Hall, who wrote a piece of music specifically for the video. He also posted helpful comments in the video itself to tell you what was happening in the Universe and when.

Both of these videos should be shown in science classrooms! I think they’d help inspire kids to learn more about the cosmos we live in.

... and maybe they should be shown to Congress, too.

I’m fascinated by the Sun. As an astronomer, I love the idea of being able to study a star up close, where we can watch it in high-resolution and on both short and long time scales (also, I appreciate being able to observe it during the day rather than having to stay up all night).

As a human, I know it’s the source of light and heat for our planet, and is a key to life itself.

But it’s a star, with all that implies: It’s massive, energetic, and prone to throwing the occasional hissy fit. And when it does, the scale is awe-inspiring.

All of this is why I’m excited to let you know that I’m hosting the Discovery Channel TV show “Solar Storm” for their series “Curiosity”. The show airs tonight, Sunday, December 30, at 9:00 p.m. Eastern time (check your local listings, etc. etc.), and is all about how the Sun makes its tremendous energy, and how that sometimes comes in the form of epic solar tantrums flung out into space.

Here’s a promo clip:

A second clip is also online, too. A couple of reviews are up as well; one on Evil Scientist and another on TV By The Numbers.

Solar storms are a concern of mine; while they can’t hurt us directly on Earth (sorry, folks, you won’t turn into the Incredible Hulk after a big flare), they can damage satellites and, more worrisomely, cause massive power outages. The details are complicated, but essentially a big solar storm can launch billions of tons of subatomic particles toward Earth. These interact with our planet’s magnetic field, which in turn can create massive currents of electricity that surge into our power grid. If the effect is large enough it can overpower the grid, causing transformers to blow and creating widespread power outages. This happened in March 1989 in Quebec, and a big storm could do more damage.

While this threat isn't necessarily immediate, it’s something our governments and power companies should be taking very seriously. I took it seriously enough to write about it in my book Death from the Skies! (note: affiliate link). And I know a lot of solar astronomers who fret about it as well. When I wrote the Sun chapter, I took pains to create a good balance between talking about the dangers without being overly alarming. I think “Solar Storms” also strikes this balance (which can be harder for a TV show, since the addition of high-res CGI visuals makes it more visceral).

Solar storms are a real problem, and my hope is that bringing attention to it will raise awareness and make it an issue people take seriously. There are ways to avoid the worst of the damage; for example, by adding more capacity to our power grid, which right now is carrying about as much current as it can (which is why induced currents are bad; they overload the wires). Another is to for power companies to invest in things like building more transformers to replace those blown by an overload, and also to investigate other technologies that can reduce the problem of induced currents induced by solar storms.

Solar storms and asteroid impacts are the only two realistic threats that come from space that have any real chance of doing damage to humanity and our civilization. But the good news is that they are also the two we can actually mitigate. We just have to choose to do so. It will take time and cost a lot of money, of course, but if we wait until after a big storm, the cost could be much higher.

All of this and more will be discussed on “Solar Storms”. I hope you watch it, and I hope you enjoy it!

As the Moon orbits the Earth, we see it sliding across the sky, making a complete circle once every month or so. Its motion is slow, almost invisible to the naked eye, though if you’re patient you can see it move very slowly relative to the stars in the sky. Sometimes, it actually passes directly in front of a bright star, an event called an occultation. Less frequently, things line up just so, and it occults something brighter, like a planet.

Last week, from North America, Jupiter and the Moon were very close to together; from my house in Boulder, Colorado they were less than a degree apart (roughly twice the width of the Moon on the sky). But from South America the angle was slightly different, and on Dec. 25, 2012, viewers there saw the Moon directly occult Jupiter.

Via UniverseToday.com I saw this video from astronomer Rafael Defavari, who was able to take video of the event through his Celestron 20 cm. telescope in Brazil!

That’s pretty cool. The video is sped up by a factor of five to show the process. At the end he also got a shot of Jupiter reappearing, with its moon Europa Io popping up just ahead of the planet. You can see the shadow of Europa Io on Jupiter’s cloud tops, too. Bear in mind, the Moon was about 400,000 km (240,000 miles) away at the time, but Jupiter was over 600 million kilometers (360 million miles) farther out! It looks small here, but in reality Jupiter is over 40 times wider than our Moon. [Update: My apologies; I originally misidentified the moon Io as Europa.]

I’ve seen a couple of occultations like this in the past. Unfortunately, there won’t be any good ones visible from the United States this coming year, but if you live in South America or Africa you’ll get lots of chances: the US Naval Observatory has a list of them coming up. Jupiter and the bright star Spica in Virgo get blocked by the Moon several times over the next few months, and you don’t need anything but your eyeballs to see them. If you have a decent-sized telescope, Pluto gets occulted by the Moon a few times as well. Those happen over the Northern Hemisphere, so check the list if you want to try for it—but you’ll need a good ‘scope and camera to detect faint Pluto, so it’s probably only an event for people who have some experience looking at such things.

You may be wondering why the Moon doesn’t block Jupiter (and every other planet) every time it orbits the Earth. It’s because the Moon’s orbit is tilted with respect to the plane of the solar system. The solar system is flat, so to us the planets appear to move around the Sun in a line on the sky called the ecliptic. But the Moon’s path is tilted, so it can only occult a planet when it crosses the ecliptic and a planet happens to be at that point. It’s a bit like safely crossing the street; you’re crossing the path that cars travel on, but as long as no cars are actually there when you cross you’re OK.

Most of the time that’s true for the Moon as well, but sometimes a planet is at that spot, and the Moon glides in front of it, blocking it. Because the Moon is big, and the planets move slowly, in general if you get an occultation one month, you get one or two more over the next couple of months. That's just what we're seeing here.

Unfortunately, from here in the States we don’t get to see these Jupiter occulations, but again, if you live south of the Equator, check the USNO site and see if you can watch one!

Today, Dec. 28, 2012, marks a somewhat infrequent and cool event: the 13^th full Moon of the year.

Technically, the Moon was most full at 10:21 UTC (04:21 Eastern US time this morning), but will appear full to the eye all day and night. It rises at sunset no matter where you are, and stays up pretty much all night.

So what’s the big deal? Well, to be honest, it’s more of a mathematical curiosity than anything else. First, if you want to know why we have phases of the Moon in the first place, it’s not due to the shadow of the Earth on the Moon! That’s actually a common misconception.

In reality, it has to do with the angle between the Sun, Moon, and Earth. Basically, the Moon is a sphere out there in space, with half being lit by the Sun. As the Moon moves around the Earth once per lunar orbit (and the angle between the Moon and Sun changes), we see different amounts of the Moon being lit. Sometimes we see the half that’s dark (when the Moon is new), sometimes the half that’s fully lit (the full Moon), and sometimes in between. There are approximately eleventy billion web sites about this; here’s a video that explains it pretty well in five minutes.

The time it takes from the Moon to go all the way through its phases—called the Moon’s synodic period—is 29.531 days, or 29 days 12 hours 44 minutes. That’s just about a month, and in fact is where the word “month” comes from (think “moonth”).

So you might expect to get 12 full Moons every year, and you’d be close. A year is about 365.25 days, so there should be 12.37 full cycles of lunar phases a year (dividing 365.25 days per year by 29.531 days per lunar cycle), which means 12 full cycles—so 12 full Moons—plus a little bit, about a 1/3 of a cycle more.

But that extra bit is important. That means the cycle of phases and the length of the year don’t line up. At the end of one year you have about 1/3 of a cycle left over. By the end of the second year, you have twice that much, or 2/3. That means the third year has a full cycle left over, so you get a 13^th set of Moon phases for free.

Here’s another way to think of it: If we had exactly 12 cycles per year, we’d have 36 cycles every three years. But really we have 3 x 12.37 = 37.1, so we get an extra cycle every three years or so.

And here we are! 2012 is the year for that. Tonight’s full Moon is the 13^th of the year.

There are a couple of more things this means, too. For one, if we have 13 full Moons in one year, there must have been a month that had two full Moons in it! This year, that happened in August, when there was a full Moon on both Aug. 2 and Aug. 31. The second full Moon is commonly called a Blue Moon. That’s just a nickname; it’s not really blue, and using it to mean the second full Moon in a month is a relatively modern custom.

Here’s something else that’s kinda nifty: If the full Moon occurs on or before January 10, then there will be 13 full Moons in a year. Like I said, the cycle of Moon phases is 29.531 days. That means you get 12 full Moons in 354.37 days (12 x 29.531), which is about 10 days less than the length of a year. So if the full Moon happens between Jan. 1 and Jan. 10, there’s enough time left in the year to get a 13^th full Moon. That also means it has to happen after Dec. 21 or so.

By the way, the last time we had 13 full Moons in a year was 2009, when the 13^th full Moon happened on—get this!—Dec. 31. Since, like I said, the 13^th one has to happen between Dec. 21 and the 31^st, a New Year’s Eve full Moon does sometimes happen. In fact, it happens once every 19 years, when the lunar cycle matches up with the length of the year—19 years is almost exactly 235 lunar cycles (19 x 365.25 is very close to 235 x 29.531)—which is called a Metonic cycle. The next New Year’s full Moon will be in 2028…which, amazingly, will also coincide with a total lunar eclipse.

Incidentally, if the full Moon happens on Dec. 31, then it must also happen on Dec. 1 or 2, making a New Year’s Eve full Moon the second full Moon of the month, so it’s also a Blue Moon.

Phew! I could go on, but that's probably enough. And now you have a head full of  the cool nerdy math that goes into this. I love this stuff; the whole Universe obeys a set of rules that makes a lot of it predictable and understandable. Once you see that, it’s hard not to be amazed and awed by it. The more you know, the more amazing it gets.

And that feeling never goes away. I’ve been in love with all this for years. So trust me: It’s not just a phase.

[If you want to know the phase of the Moon for any hour in 2013, NASA has a web page that is relevant to your interests. I have a blog post explaining it all, too.]