• The Winter Solstice


    Record rainfall continues in southern California, so my solstice is coming and going today without one of my favorite ritual of marking the furthest south sunset of the year. But here is a remembrance of solstice past.

    If you had walked out into my backyard around 4:40 the last few afternoons you would have been greeted with the orange ball of the sun setting with a final low glare over the tops of the buildings that I can see low on the horizon out across the Los Angeles basin. At this time each late afternoon I like to get out the binoculars that I keep next to the back door, and I step outside to watch the last seconds of the sun setting and to find the spot where the last glimmer of light for the day appears. Every night that glimmer has moved a little further to the south. Just a few weeks ago the last glint vanished just behind the cupola of the Pasadena city hall. By just the next day, the cupola was clear, but the sun disappeared behind the building to the left of city hall. Last night it set 4 or 5 office buildings further to the left, still, behind an anonymous office tower that I can't recognize, but through the binoculars appears impressive with the sun directly framing it and the occasional stray bit of light going through a window on the far side, rattling around on the inside, and emerging as the last bit of bit of light before a long winter night.

    Tonight I watched again, and the sun set behind exactly the same anonymous tower. It hadn't moved at all. Today, therefore, must be the solstice.


  • Lunar Eclipse

    (images via Wikipedia)

    For the past two weeks, as we've followed the moon nearly halfway around the sky, I've talked about the relative positions of the earth and the moon and the sun using a simple system clock-based system to point to everything. As we've been doing it, we're looking down from above the north pole of the earth, which you can draw as a clock face.

    The sun is off in the distance in the 6 o'clock position. Almost two weeks ago, when we first saw the little sliver of a moon and the remainder of the disk full of earthshine, the moon was around the 5 o'clock position. When it flew past Jupiter almost a week ago it had moved up to 3 o'clock, and now, as it is coming around to full, it is nearly to 12 o'clock.

    Often when you present this way of looking at positions of the sun and the earth and the moon to kids for the first time (and it work even better if you use a lamp for the sun, your head for the earth, and a Styrofoam ball for the moon), you will be asked "but wait, how come we can ever see the full moon?"

    It's true that in this simple view every time the moon moves to the 12 o'clock position it should fall into the earth's shadow and disappear. We should never see a full moon. The only reason we do is because the orbit of the moon is tilted by just a little bit, so most of the time when the moon reaches the 12 o'clock position it is a little above or a little below the long shadow of the earth.

    That's the same reason, of course, that we don't get eclipses of the sun every time the moon is at the 6 o'clock position. It's too bad, really, because if eclipses of the sun actually happened every month maybe I would finally get a chance to see one. Which I still haven't. Which makes me bitter.


  • Yelping at Saints


    (Photograph of "Halo Moon" contributed to the Boing Boing Flickr Pool by BB reader Scott Wililams.)

    If your December evening skies have been clear recently you probably can't help but have noticed the slow growing of the moon as it has risen from being for a twilight sliver almost two weeks ago, to a half-illuminated disk passing Jupiter to an almost-full orb rising only an hour or two before the sun sets. There's nothing new here. It does essentially the same thing every 28 days, but it is still a show worth watching.

    On Tuesday, as the moon finally goes from just-barely-not-full to finally-completely-full, the moon will finally brighten its last incremental amount and it will be its brightest of the month, though only a little brighter than it was the night before.

    This gentle brightening to a muted peak sounds prosaic and reasonable. But it is not true.


  • You Betcha: Speculation, space, and markets for alien life

    (photo by Katrina Tuliao, via Wikipedia)

    This morning, on the web site of Science magazine, the news and research journal of the American Association for the Advancement of Science, the title and summary of an article came up that was so intriguing that I had to quickly log in to my computer at work, get behind the pay wall, and read to find out the news. I mean, who would not want to know the details of how "Alien Planets Hit the Commodities Market"?
    Having been behind the pay wall, I am here to inform you that it is a perfectly pleasant article about the continued discovery of planets around other stars. But the commodities part just came from a clever headline writer.


    I was really hoping that the article was going to be about someone setting up a commodities market where you could buy shares in the date of when we first make alien contact, much like how in "election stock markets" you can buy shares in your prediction for who is going to win presidential elections. It has been shown repeatedly that these election stock markets are excellent predictors of who will win the race.

    Markets are quite amazing things.


  • Due Date

    (Image via Wikipedia: Views of a Foetus in the Womb, c. 1510 – 1512, a drawing by Leonardo da Vinci.)

    I will admit to occasional single-minded ranting. You might think that, as an astronomer who studies the outer part of the solar system, my rants are restricted to issues like classification of planets, bad weather at telescopes, and the possible effects of secular perturbation on the perihelion evolution of detached Kuiper belt objects. But my other main job, being a parent to a now-5-year-old daughter, provides me a plethora of new things to rant about, also.

    My daughter provided me the very first opportunity before she was even born. Back then, she was code-named Petunia, and all I really wanted was some way to understand what Petunia's July 11th due date actually meant. The ranting really didn't begin until sometime in the third trimester. Here is an excerpt from How I Killed Pluto and How It Had It Coming from the moment when simmering frustration turns into full-scale rant.


  • How Many Dwarfs Are There?


    (Image courtesy Wikipedia)

    Most people know by now that Pluto has been downgraded. Astronomers have decided that, conceptually, we should reserve the word "planet" for the small number of dominant bodies in the solar system. Pluto doesn't come close to making the cut. But it didn't just get shoved into the corner as "insignificant object," it got to be part of a brand new class of objects never before defined, the "dwarf planets."

    Now, before you complain that, clearly, by virtue of the power of the English language, a "dwarf planet" must certainly be a planet first, a dwarf second, I would just like to mention two things. First all adjective noun combination in the English language are not noun first, adjective second. A matchbox car is, in fact, not a real car. It's OK if a dwarf planet is not a real planet. Second, though, I will acknowledge that the language is unfortunate and misleading. I preferred the term "planetoid" myself, rather than the (intentionally?) misleading "dwarf planet."

    Still, forgetting the vagaries of language, we are left with dwarf planets which are not planets. How many are out there besides Pluto? And what is a dwarf planet?
    The International Astronomical Union (the group responsible for all astronomical nomenclature) has officially declared there to be five dwarf planet (in order of mass: Eris, Pluto, Makemake, Haumea, Ceres), and we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.


  • Santa Claus is coming to your sky


    (image via Wikimedia Commons: Space Station over Sedona.)

    For the first couple of Christmas eve's of my daughter's life, I would take her over to the computer and show her Santa's current position on the sweet and dopey Santa NORAD tracking site. Even by the time she turned four years old, though, she was beginning to get suspicious that this cartoon Santa had little to do with the real one.

    I needed a better shtick. And last year I found the best one ever. I will definitely be pulling this one again this year.

    Just around her bed time I warned her that Santa was coming soon and she definitely needed to be asleep if he was going to stop. We put out milk and cookies and carrots as I kept checking the time on my watch.

    At precisely 7:34 I said "I think he's about to fly overhead right about now."


    "Yeah, let's go outside and look," I whispered.


  • Star log for Monday, December 13, 2010


    (photo: "Starry Twilight," contributed to the Boing Boing Flickr Pool by Mike Geiger of Ottawa, Canada.)

    I'm assigning no new astronomical homework for tonight. It's your chance to catch up in case your schedule or your clouds or your sleep have prevented you from going out and seeing the sights yet.

    Mercury is still visible just as the sky gets dark after sunset. I saw it last night, but the skies were thoroughly clear and the horizon was thoroughly flat if you miss it this time around, fear not there will be better chances coming soon, so keep yourself aware by following something like the fabulous nightly Earth & Sky site.

    If you were ever unsure which object in the sky is Jupiter tonight is definitely your night. First, find the moon, the find the really really bright thing next to it. Bingo. Even with the moon so bright you can still get out those binoculars and check out where the moons of Jupiter are this time.

    The moon has climbed now all the way to first quarter. It rises around noon; see if you can impress your friends by pointing it out in the afternoon!

    And, the best treat of all, tonight should be the peak of the Geminid meteor shower, which is well worth waking up in the middle of the night for.

  • The Geminids: A Fiery Death


    (Photo, via Wikipedia: "Leonid Meteor Storm, as seen over North America in the night of November 12./13., 1833. Source, E. Weiß: "Bilderatlas der Sternenwelt"; Published 1888)

    The Earth is hurtling through mostly empty space at nearly 70,000 mph. But space is only mostly empty. Throughout the solar system is debris left behind by comets, colliding asteroids, and even dust from interstellar space. When the Earth hits these things – at 70,000 mph – it puts on a nice show.

    You can see the show any time of the year. Just find a dark moonless sky and stare up for a while. You'll eventually see the quick streak of a shooting star. That "shooting star" was what happens when one of these tiny dust-sized pieces of debris gets in the way of the earth. It burns high in the atmosphere. Very very occasionally the earth will run into an even larger piece of debris.

    The best I ever saw in my life was when I was sitting on a beach in Hawaii and noticed a bright light behind me, turned around, and saw a huge bolide split multiple times in the air before disappearing behind a volcano behind me.

    These days they are often caught by security cameras or by whoever happens to have a video camera in their hand at the time.

    Some parts of the solar system are dirtier than others, and right now the Earth is plowing through one of the dirtier ones. We are plowing right through the orbit of a former comet, and that orbit is full of dust and small rocks sputtered out by the comet over centuries. The comet has lost so much debris, in fact, that by now it appears as a barely distinguishable rocky asteroid quietly orbiting the sun.

    If you want to see shooting stars and see a lot of them, now is a good time to get outside and do it.

    When? Where?


  • Look Up!


    Did anybody catch Mercury for the first time last night? I had just enough hazy cloud on my western horizon last night that Mercury was lost in the much. If you missed it, keep trying. And if you still can't find it, don't fret: your assignment for tonight is much, much easier.

    The planets all travel around the sun in flat disk. Since we sit inside this disk too, when we go outside and look for planets they will all lie along one giant circle around us. Planets move slowly, so waiting for one of them to trace out the giant circle can take a while, but the Moon takes only a month to circle around us, so we can use it to trace the paths of the planets in the sky.

    If you've been watching the moon the last few days, you have seen it climbing in the evening sky still growing towards its first quarter (which comes up on Monday – so quickly! Wasn't it a tiny sliver just days ago?).

    The earthshine is fading away, as the view of the Earth from the Moon is also moving from full to third quarter. (more…)

  • The Seven Planets


    Back in the good old days everyone knew how many planets there were, then scientists came along and screwed everything up. How could something that was always a planet suddenly not be one? It made no sense. Chaos ensued, people protested, and scientists were thrown in prison.

    I'm not making up that prison part, either.

    It was dangerous being one of the first scientists to go against the traditional view of what was and was not a planet. But, regardless of the danger, 467 years ago, Copernicus stood firm. "The Sun and the Moon are not planets", he declared. Two of the seven known planets gone like that.

    The other five known planets were doing fine though, it's just that they now orbited the Sun instead of spinning around the fixed Earth. Those five — Mercury, Venus, Mars, Jupiter, and Saturn – remained. But in perhaps the biggest change to the word "planet" in human history, the Earth under our feet was declared to be on par with those other five.
    Galileo – defending these ideas with observations of the actual night skies through his newly acquired telescope – was imprisoned by the church.

    Those seven initial planets – the "wanderers" from which the original Greek word came – are still out there and still part of our every day experience. The names of the seven days of the week come from the original seven planets. Sunday is obvious. Monday pretty easy. Satur[n]day? Check. The others are a bit obscure from old English and Latin, but they're all there.

    Before artificial lights, I suspect that most people had seen most of the seven planets. These days? Almost no one.


  • I think there's another planet past Pluto

    (Palomar Observatory. Photo: Mike Brown)

    Last night's sliver of a moon grows a bit bigger and sets a bit later in its trek across the skies this month. Can you notice the earthshine getting a little fainter?

    While the view of the setting moon from my backyard last night was among the more spectacular I have ever seen, December is not always the best time of the year for sky watching, even here in southern California.

    In solidarity with those who have nothing but a blanket of clouds, I bring you an excerpt from my book How I Killed Pluto and How It Had It Coming, published just yesterday, of a December night 11 years ago, when my skies were bringing nothing but misery.


  • It's Only a Sliver Moon (Daily Astronomy Log with Mike Brown)


    (photo via Wikipedia)

    This past Sunday began the last lunar cycle of the year, which, for the first time in almost 20 years, includes a full moon on the winter solstice itself. I'm going to spend some of the time between now and then using the moon as a guide for a little tour of the night sky.

    Tonight, if your clouds part enough in the evening to reveal the just-set sun – very nearly as far south as it will ever get – you should also get to see a tiny sliver of a new moon hanging like an ornament above it in the not-yet-dark skies. This sliver moon is, to my mind, one of the most impressive sights to periodically grace our skies. To me, the ethereal part is not the sliver itself, looking like a razor sharp sickle glowing in the sky, but the ghostly outline of the rest of the moon that can be faintly seen.

    What is that ghostly outline? If you pay close attention you might even noticed that it disappears after a few days. By the time the moon is up to first quarter all you see is that bright sunlit half of the orb. It's hard to tell precisely what is happening, because as the moon waxes towards full it gets brighter and brighter and you might just think that you're having a harder time seeing that ghostly outline in the presence of that brighter moon. But, no, the outline is indeed getting fainter.

    What's going on? With a little thinking about what is illuminating the moon we can figure it out pretty easily and even make sense of the little details of when it is brighter and when fainter.


  • Die, Pluto, Die!


    I've spent a lot of time over the last few years trying to understand the astonishment that people feel over the demotion of Pluto. I mean: it's really really really really tiny, compared to most of the real planets in the solar system. So what's the big deal? Some people have blamed the outsized affection for the undersized ice ball on Walt Disney, others on American nationalism, and still more on the general tendency to root for the underdog. Whatever the reason, for many people, a solar system without Pluto as one of the nine major members feels wrong. But why?

    Recently, I think I finally had the flash of insight that allowed me to understand how the rest of the world feels. The solar system that many people have in their minds looks something like this picture (above, at top of post) I took of a placemat that my daughter uses daily.

    How could you possibly say that Pluto deserves the boot? It's about the same size as Mercury, and, really, not that much smaller than Earth or Mars or the rest. Astronomers, clearly, are just mean.