Bad Astronomy
The entire universe in blog form

Jan. 4 2017 8:45 AM

The Snows of … Hawaii?

I’ve been fortunate enough to visit the big island of Hawaii a few times; it’s a fantastically beautiful place and an amazing science destination. There’s a seahorse farm, astronomical observatories, volcanoes, and an eclectic selection of environments, from forbidding lava flows to dry forests to lush tropics.

The edges of the island are at sea level (duh), but the highest point, the peak of Mauna Kea, stretches to 4,200 meters (2.6 miles) above. That brings a lot change in temperature, as you might expect. I watched the sun set from Mauna Kea while I shivered even when wearing a parka; I had gotten used to 35° C humid days.

Still, perhaps the last thing you might expect to see in Hawaii is snow.

Jan. 3 2017 8:45 AM

Seeing Through a Cloudy Lens

Going through old emails is like a treasure hunt sometimes. I found a gorgeous photo of a spiral galaxy recently, and then saw an email from photographer Brad Goldpaint about another type of object of my affection: lenticular clouds.

These are lens-shaped clouds sculpted by winds, usually found over mountains. The rising air can create standing waves, stationary up-and-down oscillations, downwind. The first crest, just downwind of the mountain, can form a cloud as the air rises and cools. The moisture in the cloud is swept downstream but is replaced by more air rising … and you get a stationary cloud, lens-shaped due to the flowing wind, that seems to hover near the mountain peak.

It’s no surprise people think these are UFOs sometimes. They’re honestly pretty amazing.

The photo above shows a magnificent example over Mount Shasta, which Goldpaint took in 2015. It shows the lenticular clouds lit by the Moon in the evening. As cool as that photo is, I like the one he took a bit earlier even better, when the clouds over Shasta were lit by the setting Sun.

Check out that laminar flow!

Brad Goldpaint, used by permission.

Holy wow! That smooth cloud over the mountain is an indication of laminar flow, smoothly moving air. I love how it follows the contour of the mountains.

Goldpaint also made a short time-lapse video showing how the clouds are stationary even as the air moves:

How cool is that?

I spot lenticular clouds pretty often, seeing as how I live near the foothills of the Rocky Mountains. I saw some just the other day, in fact:

Goats and lenticulars
They were more interested in me feeding them than turning around and admiring the sky. Typical.

Phil Plait

As I said when I posted this shot on Instagram, “Goats and lenticular clouds. Maybe the first time those words have ever been used together.”

If you like Goldpaint’s work, then check out his gallery. And if you want more lenticulars, I have links to some other spectacular shots below. Enjoy.

Jan. 2 2017 8:45 AM

Deep Field

The Universe is cycles.

Planets orbit their stars. Stars are born and die and seed the Universe with more gas and elements to create more stars. Galaxies spin. Closer to home, we see the Sun rising and setting, we awake and we sleep. Even the sounds we hear are cycles; musical chords are juxtaposed oscillations at specific frequencies, compression and rarefaction of air.

And here we are, at the beginning of a new, if somewhat arbitrary, cycle. With a fresh orbit around the Sun lying ahead of us, I want to show you something to think about in the coming year.

The image at the top of this article is called the Hubble Deep Field. It was the first of several subsequent projects using the venerable observatory to stare at a single patch of sky for days, weeks, building up an image of some the faintest objects ever seen. When it was first undertaken, we weren’t sure what we were going to find.

When the observations were processed and assembled, what we found was awe.

Nearly every object in that image is a galaxy, a vast, sprawling complex of gas, dust, and stars. Thousands were revealed in the image, despite the field of view being so tiny. If you held a grain of sand on your finger, your arm outstretched, it would occupy as much of the sky as the Deep Field does.

And yet, what lies inside those boundaries is the Universe itself.

This profundity inspires us. It did so to Eric Whitacre, a composer and conductor of classical music. He was moved to write an orchestral piece called “Deep Field,” and it’s a paean to this mesmerizing image of space. The piece is about 20 minutes long, and I urge you to sit, relax, and listen to the whole thing:

The piece mirrors the observation in many ways, metaphorically and literally.

It seems to be made of several themes, somewhat disjointed, that come together to form a greater whole, rising to a stirring climax. But note the image itself! In it, we see that the most distant galaxies—the ones we see when the Universe was young—are smaller, less well defined. We know that these galaxy fragments in the early Universe came together under their mutual gravity, merging to form the glorious, far larger and more well-organized galaxies in the modern Universe.

Hubble Extreme Deep Field
A section of the later Hubble Extreme Deep Field, released in 2012, shows glaxy fragments billions of light-years away.

NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Some of the music sections are dissonant, with conflicting chords, resolving themselves after a moment, or repeated with slightly different component notes to resolve them. I rather like that; it reminds me of seeing apparently overlapping galaxies in the image that become clear when zoomed in, or simply become easier to distinguish. And part of the point of the Hubble observation was to use its superior eyesight—what astronomers actually call resolution!—to distinguish all the objects in the field.

Two different meanings for the word, yet both come together where music and science meet.

Toward the end, the choral part starts, wordless voices in gorgeous, lush harmony with the orchestra. The tone of the whole work changes, but in a very organic, natural way, uplifting and hopeful yet retaining its pensive, even reverent attitude. It reminds me of the ending of Holst’s suite “The Planets,” where voices fade out at the end of “Neptune”, as if we are being borne away from the solar system, the Sun and its attendant planets receding and fading as we move away into the black.

If you want comments from the composer himself, Whitacre recorded a short video describing the piece (starting about one minute in):

The idea of using the audience phones is clever. Besides adding to the sound of the music, from the orchestra’s view they see dozens, hundreds of small lights scattered around them out in the black hall, mimicking the galaxies in the Deep Field itself. What a lovely idea.

I have listened to this piece of music many times now, and I must muse on its origin. These galaxies, these distant structures that dwarf us and crush our sense of space and time … they do not care about us. They are things, not alive, not sentient, incapable of feeling or purpose. And yet they are us. The galactic fragments and even full-fledged galaxies in the Hubble Deep Field are representative of the same cosmic constructs that formed the Milky Way, the first and second generation of stars in it, the supernovae and heavy elements that settled into swirling disks of material that birthed our Sun, our planet.

A galactic sunflower
M63, a nearby spiral galaxy and an example of unmitigated gorgeousity in our Universe.

ESA/Hubble & NASA

We evolved here, on this spinning planet, we grew from abiotic material to life, became complex, and eventually, after billions of years, became us. You. Me. Our sense of beauty and wonder and curiosity turned our gaze to the sky and allowed us to discover the pieces of Universe that were our origins, looking back across countless light years to how we came to be.

This in turn inspires art, prose and music, a unique outlook and perspective on nature that we can share and appreciate. The science created the art, and the art informs the science. Perhaps one could exist without the other. Perhaps. But together they are more than either individually or summed. Together they are the very basis of what it means to be human, the joy of understanding and expressing that humanity.

Which brings us back to cycles, and back to how this article started. Which is to say …

Happy new year. There will always be new years, but never quite this one again. Do what you can to make it a good one.

My thanks to astronomer, astronomy communicator, and friend Kim Kowal Arcand for the link to this wonderful piece of music.

Jan. 1 2017 8:30 AM

Happy New Tropical Earth Orbital Period! Kinda!

Another year, another repost: The article below is an updated version of one I try to post every year at this time — either because the topic is so much fun, or I'm lazy. Take your pick. But I love this kind of stuff; it's fun to research and to play with the numbers. If you like it too, read on. If you don't, read it anyway, because you might find out you do, and isn't that one reason we celebrate the new year? To try out and experience new things, or old things anew? You might also want to read about why we have leap years and even leap seconds. Science! I love this stuff.

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 if you're done kicking 2016 to the curb and trying your best to hope for 2017, sit back and let me tell you why we have a new year at all.

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 (unless you're a Geocentrist, in which case this stuff still all works, just the other way around). 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 (OK, ellipse, but they're pretty close in this case) 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?

Earth tilt animation
Measuring this turns out to be... interesting.

Wikipedia user Tfr000

Let’s say we used a stopwatch to measure the elapsed time. You'll find 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—better than a half a percent accuracy). That's an inconvenient number of seconds, though. I think we'd all prefer to use days instead. So 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, so 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.

synodic day
As the Earth orbits the Sun, it has to spin a little extra to catch up over the course of the day. Click to ensynodicate.

Illustration by Nick Strobel

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 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 March equinox (also northernhemispherictically sometimes called 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 that same equinoctal moment in the next. That’s called a tropical year (which is 31,556,941 seconds long). But why the heck would you want to use that? Ah, because of an interesting problem! Here’s a hint:

"Precess" is not the time before school break.

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.

OK, 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 March equinox always happens on or around March 21 every year.

Lend Me Your Year

Earth orbit
Perihelion is in early January and aphelion in July, but the exact date changes every year.

Phil Plait

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 (the farthest point is called aphelion). 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 an ideally 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 Ph.D. research because I was watching an object fade away over several years, and it made life a lot easier. Doctoral research is hard, shockingly, so you learn to take advantages where you can find them. 

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.

New Years Resolution
My New Years resolution is usually a few pixels. This joke NEVER GETS OLD.

Drew Saunders on Flickr

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 roughly every four years). In 2013 perihelion was on Jan. 2, but in 2017 it’s on Jan. 4. Same thing with the equinox: It can range from March 20 to March 21. 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—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. Every year (take your pick which kind) I say to myself I'll submit an updated article to Wikipedia listing all the different years and the number of seconds and days of each kind in them. 

Then every year I forget. But if you want to give it a shot, feel free. It would come in handy when I update this article every 365.26 days or so.

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.

And, finally (for real this time) 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.

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

Happy New Year! 

Dec. 30 2016 8:45 AM

2017: Wait Just a Second

If, like me, you’re counting the seconds until you can kick 2016 out the door, I have a tiny bit of bad news: You’ll have to count a little higher to do so. According to our clocks, 2016 will be exactly one second longer this year because scientists are adding a leap second to it.

Leap seconds are added to the calendar pretty often, actually, on average a little less than one every year. (Note: Leap seconds are wholly different than leap years, which come with their own set of mathematical funness.) This is done to keep our clocks in sync with the rotating Earth.

Basically, we have lots of ways of measuring time. One is to very carefully measure how long it takes the Earth to spin once on its axis. We call that a “day.” There are actually lots of different kinds of days—how long it takes the Earth to spin once relative to the Sun, or relative to the average position of the Sun in the sky, or relative to the stars. But all of these are based on the spinning Earth.

That was fine for pre-electronic technology, but nowadays we need something better. The Earth makes a pretty ratty clock when you focus in on very tiny timescales. Lots of forces affect our planet’s spin, including the tides from the Sun and Moon, continental drift, and even the way we dam up rivers. The overall change is pretty small: The length of our day now is only about two milliseconds longer than it was around 1820, which is when there were exactly 86,400 standard seconds in a day.

Earth's rotation graph
The length of Earth's day, from 1973 to 2015, measured relative to the 86,400 second day. It goes up and down by a few milliseconds (or a fraction thereof) on several timescales. Currently, the day is about two milliseconds longer than it was in 1820.

U. S. Naval Observatory

So yeah, our day is now 86,400.002 seconds. Give or take. I know that doesn’t sound like much, but it adds up. What it means is that the Earth spins a touch slower now and doesn’t keep time as well as the atomic clock does. So, every now and again, we have to correct for it.

Imagine you have two clocks. One thinks there are 86,400 seconds in a day, the other thinks that there are 86,401, so the second clock runs a tad bit slower than the first. Every day, it's one second behind, clicking over to midnight one second after the first clock does. Mind you, it keeps accurate time according to its own gears: every day has 86,401 seconds, so it's not slowing down.
However, to keep it synchronized with the other clock, we'd either have to subtract a second from the second clock (yikes, terminology is a bit confusing there!) or add one to the first clock every day. So we'd need a leap second every day, but not because the clock is slowing. It's only because it runs at a different (but constant) rate.

This is the part that confuses people. It’s not really that the Earth is spinning slower all the time, it’s just that right now it’s spinning slower than it did a while back. Even if the Earth’s rotation were perfectly constant now, we’d still have to add a leap second! I’ll note that some young-Earth creationists use this slowing to argue the Earth can’t be old, but, unsurprisingly, they’re wrong about this.

Anyway, we now use atomic clocks to keep standard time independent of our planet’s spin. They’re based on the frequency of an electron transition in cesium atoms—yes, seriously, and if you want details here they are. But this method is far more accurate, and can easily measure the time difference due to the slowpoke Earth.

When the difference between the atomic clocks and the Earth clocks differ by more than 0.9 seconds, a leap second is added to civil time (the time used by civilian authorities*). It could technically be subtracted if the Earth’s rotation were to speed up, but that’s never been the case since this method was adopted.

In fact we’ve been adjusting our clocks this way since 1972. In the 34 years since then, we’ve added 26 leap seconds to the calendar. The last one was in June 2015.

But this year’s end will see another next leap second. What does this mean?

On Saturday night, Dec. 31, 2016, at 23:59:59, one second will be added to our clocks. Instead of clicking over to Jan. 1, 2017, at 00:00:00, for one second the official time clock will instead read 23:59:60.

I know, right? Weird. But if we didn’t do this, all our computer clocks and everything else would get seriously messed up. Timekeeping is a serious business.

This probably won’t affect you in any way personally, unless you succumb to any existential dread of having this year be 0.000003169 percent longer. That’s been true of nearly every year for decades anyway.

And think of it this way: The motions of heavenly bodies affect us in ways you may not realize. I don’t mean astrologically, of course; that’s just hogwash. Obviously: Even the Sun and Moon can only change the Earth’s rotation by a wee bit over centuries, so their affect on you is even, um, weeer.

For me, this just reinforces my love of astronomy and science. After all, our human senses only perceive a tiny fraction of what’s going on around us, and our affairs are a minor thing compared with the Universe around us. I think that helps me keep a certain perspective; despite what happens on it, the Earth itself keeps on turning. Perhaps a scosh more slowly, but if that’s the price to pay to go along for the ride on this rotating ball of rock and water and metals careening through the cosmos, I think it’s an affordable one.

*Not to be confused with the time to be civil, which is nearly all the time.

Dec. 29 2016 8:45 AM

Amazing Illusion: Invisibility Cloak

My friend (and evil twin) Richard Wiseman is a staple here at BA HQ; he studies how our brains can be tricked into perceiving things that aren’t real … or missing things that really are there.

His latest entry into encephallusions* is masterful. Almost a century ago, the brilliant magician Robert Harbin came up with a way to seemingly cloak someone, as if part of them were to become invisible. It’s never been done in practice … until now. Richard—with a little help—made it real, and put together a wonderful video demonstrating it.

What do you think? How does this work? First, try to guess …

If you’re the kind of person who can’t stand not knowing—and good for you, that’s a big step in looking at the world scientifically!—then Richard made a second video explaining it:

Did you guess correctly? I did, kinda; I knew it had to do with reflections and angles, though I didn’t go to the trouble of doing the actual ray tracing (that is, figuring out the direction the light traveled). It’s very clever, and like a good showman Richard really sells it by moving his arm behind the ninja, making it look like she really is cloaked.

But there’s something he doesn’t explain in the video, and since it’s science, I will. Did you see him drop the bead in the bowl of water only to have it disappear? How does that work?

It has to do with refraction, the bending of light. When light travels from one medium (say, air) to another (say, water) at an angle it bends a little bit, changing the direction it travels. It’s actually changing its speed, and the amount of change depends on the material through which light is traveling. The amount by which the material can change the light’s speed (and the angle at which it bends) is called the index of refraction. It’s just a number that falls out of the equations, and, for example, water has an index of refraction of about 1.33.

bent spoon
Get bent.

Phil Plait

You’ve seen this lots of times; it’s why a spoon looks bent when you stick it in a glass of water (it also distorts the Sun, Moon, and stars when they set as their light passes through our atmosphere). If you put something transparent in water, though, things can get weirder. If that object has a different index, the light passing through the water bends a little when it passes through that object, distorting the view. Usually, this makes it a bit easier to see the object because you can see its edges better where the light is distorted.

But if the object has an index of refraction equal to that of water, the light passes right through it without distortion, and the object seems to disappear! The glass beads Richard used in the video have the same index as water, so when he dropped them in they disappeared from view.

This can be done with Pyrex and glycerine, for example, and many other pairs of transparent objects. It’s a very different effect than what Richard shows in the cloaking illusion, but in the end they both fall under the same category of “making stuff appear to disappear.”

Science! Ain’t it grand? And it’s useful: It can really help you understand when you’re being fooled, whether it’s just your brain not perceiving things the way they should be, or because someone is trying to fool you. But either way, it’s your best bet.

* Yes I just made that word up. I do that sometimes.

Dec. 28 2016 9:00 AM

Our Magnificent Moon

One really fine consequence of writing about astronomy is having astonishingly talented photographers send me images of the heavens. This burdens me with glorious purpose: Sharing these stunning views with you, and perhaps sneaking in an explanation or two.

While so many of these views display gas clouds, star clusters, galaxies, and more, sometimes you need go no further than our nearest celestial neighbor to find treasure.

Our Moon has been bombarded by asteroids and comets for eons, having its surface shaped and reshaped. Even a small telescope reveals this, and when you have someone who truly knows and cares how to process the images to bring out the details, what you get is simply wondrous.

The image above shows Gassendi, a huge 110 kilometer–wide impact crater, just visible in binoculars when the Moon is waning gibbous or near third quarter. This image was taken by master planetary astrophotographer Damian Peach, and is part of a set of high-resolution photos he took of the Moon in 2016.

Gassendi is interesting. Its floor is riddled with fractures, called rimae (singular: rima). It’s not known for sure what caused them, but a likely scenario is that the crater floor was filled with lava after the heat of the impact that formed it, and as this lava pool cooled it formed a crust. The crater bowl cooled and settled a bit and the lava crust above cracked, forming the rimae.

Gassendi is also the site of one of my favorite images from the Lunar Reconnaissance Orbiter, which shows a huge house-size rock that’s tumbled down a slope. Features like that thrill me, showing us a Moon that isn’t static and simple but instead has had a dynamic past that creates features both huge and subtle.

I strongly urge you to look at Peach’s collection of lunar images; make sure you have a glass of water handy to rehydrate your mouth after your jaw has hung open for a few minutes.

Another of my favorite photographers, Thierry Legault, has produced staggeringly high-resolution images of the Moon at a few different phases, including crescent and first quarter.

1st quarter Moon
The first quarter Moon ... seriously miniaturized to fit here. Click to enlunenate.

Thierry Legault, used by permission

I shrank the image to fit the blog here, but you really want to go look at the bigger versions. The full-size version of the one here is 51 Mb in size, and is 10,000 x 15,000 pixels! The maps are mosaics he made using a 35 cm telescope from the Alps in August. You really want to click through and see some of the frames he took that make up these images, because holy wow.

Here’s a crop (and somewhat shrunken) shot by Legault of the crater Plato, another favorite of mine:

Plato crater
Plato crater, a huge impact scar on the Moon.

Thierry Legault, used by permission

When it formed (or perhaps due to subsequent impacts or vulcanism), Plato's floor filled with lava and created a smooth surface ... at least until you look very closely, and find it filled with many smaller craterlets. Given its size—more than 100 km, nearly as big as Gassendi—it’s obvious in binoculars, with the dark basaltic rock of its floor and lack of large internal cratering giving it a striking appearance.

I have been observing the Moon my whole life, from the time I used a junky department store refractor, moving up to a 25 cm reflector, and even working on some Hubble images of it (that, sadly, did not pan out). Ironically sometimes the Moon irritates astronomers; it’s very bright, and when it gets near full it washes out the sky, making observing anything else difficult.

But even then, it’s an object of great beauty. I try to maintain perspective when the Moon is near full; observing it with binoculars or using my own ‘scope, I look for the rays of Tycho, highlighted by the steep sunlight angle, or peer near the day/night line (called the terminator) to see the sharp edges of shadows cast by crater rims and mountains, foreshortened by perspective. There is never a wrong time to cast your gaze toward the Moon, and despite its ancient and nearly unchanging physical face, it never seems to look the same way twice.

Dec. 27 2016 12:46 PM

A Fog Dome Rises in the Night

I love unusual meteorological phenomena. Iridescence, lenticular clouds, Kelvin-Helmholtz waves, and on and on. I’ve been fortunate enough to see a lot of these weird things for myself, but then my luck is leveraged heavily by my tendency to look up a lot and pay attention.

There are still quite a few events I’ve never seen, but I’ve heard of them. Asperatus undulatus clouds, crown flashes, more. I’m fascinated by all of these, and because I read a lot about them it’s rare for me to get wind (har har) of one completely new to me.

But then BA Bloggee Richard Jarvis sent me a tweet linking to a photo of a phenomenon I’d never heard of: a fog dome.

Dec. 27 2016 8:45 AM

Vera Rubin, Discoverer of Dark Matter, Has Died

Vera Rubin, a pioneering astronomer who discovered dark matter, has died. She was 88.

Her work in astronomy ushered in a revolution in how we saw the Universe. Dark matter is invisible, but it has mass, and affects the cosmos on a large scale. Rubin’s work was studying galaxy rotation curves, literally how spiral galaxies spin. When she plotted her data, she found that the graphs of rotation speed versus distance from the center could not be explained using the standard model for galactic structure. She realized that what her data showed is that there must be far more mass in the outskirts in galaxies than we can see. We now know this to be correct: Almost all galaxies are embedded in a vast cloud of invisible material we call dark matter. And these halos are truly massive; dark matter outmasses normal matter by a factor of more than 5 to 1.

Later work showed that entire clusters of galaxies had dark matter halos, and that this stuff actually helped the largest scale structures in the Universe form when the cosmos was very young. Without it, the Universe would look very, very different.

Do you see how important this is? Most of the Universe, Rubin discovered, is invisible to us, yet this material has had a profound effect on literally everything.

I didn’t know her personally, but everyone I know who did spoke highly of her. She was a mentor and role model to many, including many women. She was an advocate for women in astronomy, working for years for example to get more women into the National Academy of Sciences. Lots of people are sharing lovely stories online about meeting her and the effect she had on their lives, including Carolyn Collins Petersen and Dr. Chanda Prescod-Weinstein.

One more thing that must be said. As a woman, Rubin faced in uphill battle in much of her career. She deserved a Nobel Prize for her work but was overlooked year after year. I’ve written about this before; her work predates the discovery of dark energy by decades, yet the two teams of astronomers who made that discovery were awarded the Nobel in 2011. I do think the 2011 award was deserved, but why did the Nobel Committee skip over Rubin for so long? The last woman to win the prize for physics was Maria Goeppert-Mayer (for her work on atomic nuclear structure), and that was in 1963. The most recent woman before that was Marie Curie, in 1903. And that’s it. Just two women.

But the Nobel committee, by its own rules, does not give the award posthumously. So that’s that.

Except it isn’t. Her work lives on, and as she herself said when being admitted to the National Academy of Sciences:

Fame is fleeting. My numbers mean more to me than my name. If astronomers are still using my data years from now, that’s my greatest compliment.

How many women were inspired by her, how many structures in astronomical societies exist because of her? Even if the general public might not know her name, her positive influence will extend well beyond her own lifespan.

Dec. 26 2016 12:00 PM

Video Preview of Cassini’s Saturn Endgame

Not long ago I wrote that the mission of the fantastic space probe Cassini is coming to an end. On Sept. 15, 2017, after a series of quite daring maneuvers, Cassini will drop into the atmosphere of Saturn, returning a last few bits of data about the giant planet’s atmosphere even as it plunges to its death.

The wonderful podcast NASA 360 just released a pretty nifty short video showing this, with graphics that explain the end-of-mission so that you can picture for yourself how this will go:

This is bittersweet, to be sure. Of all the robotic probes humanity has sent into space, Cassini is one of my favorites. It’s been orbiting Saturn for more than 12 years; I remember watching the live feed of the orbital insertion burn with my daughter, then in grade school, while she asked me questions about it.

My daughter is now in college. That puts the longevity of Cassini into perspective.

But all good things … It costs money to maintain a spacecraft mission, and NASA has a limited amount of it. The longer the mission runs, the higher the chance something will go wrong, or it will run out of propellant—and Cassini’s tank runs low. NASA wants to make sure the probe cannot hit any of the moons; while it’s unlikely in the extreme, we don’t want our germs contaminating any places where there might be life. And this way we get extra data about Saturn’s atmosphere, too.

I always hate to see a mission end, but there is something noble and hopeful that our robotic proxy will send us information back even with its last breath. May all such missions do so well.