Bad Astronomy
The entire universe in blog form

April 30 2016 9:15 AM

No, Saturn Won’t Be Huge in the Sky on May 1

Another day, another weird Facebook astronomy hoax.

The photo with the caption above have been spread around Facebook quite a bit the past few days; it shows Saturn looming huge in the sky over a city, with the annotation, “On 5/1/16 Saturn will be the closest it ever has been to Earth. It will look like this.”


I got a few notes from readers on Twitter about it. Unlike a lot of other wrong but possibly sincere memes, this one is clearly a hoax. It’s just silly and wrong on every level, so it couldn’t possibly have just been an honest mistake. It’s a joke.

What’s wrong with it? Well, two main things: Saturn isn’t the closest it’s ever been on May 1 (its closest approach to Earth this year is in June), and Saturn can’t get big enough to look like that, ever. Which is a good thing.

The closest Saturn ever gets to Earth is about 1.2 billion kilometers. That’s something of a hike. Saturn is big — 120,000 km in diameter, not including the rings which are 250,000 km across — but from that distance it’s not even big enough to appear as a disk to the eye. Through binoculars you can see the rings as a tiny ellipse, and through a smallish ‘scope you can more clearly see the planet as a disk and the rings circling it.

To be that big, Saturn would have to get as close to us as our Moon! In fact, that’s where the image comes from. It was easy enough to track down; a reverse image search on Google showed it’s from a Russian video intended to show what the sky would look like if the Moon were replaced with the other planets in our solar system:

It’s a fanciful idea, and as far as I can tell first done by my pal Ron Miller, a space artist, in 2012, and again by videographer yeti dynamics in 2013. In fact, yeti dynamics went ever farther and created a wonderful video showing what it would look like if Saturn were actually passing the Earth in the solar system:

As I wrote at the time, that would be the most amazing sight ever witnessed… and also the last thing you’d see. From that distance, Saturn’s gravity would inflict monstrous tidal forces on the Earth, causing apocalyptic floods, massive earthquakes, and generally dealing out death on a global scale.

As much as I love looking at Saturn, I’m pretty happy it’s over a billion klicks away.

So where is Saturn right now? It’s currently about 1.37 billion km away, where it always is. It rises around 10:30 p.m. local time, and shines at a magnitude of about 0.2, making it one of the brightest objects in the sky. In fact, it’s very much worth going out and taking a look around midnight; very close to it in the sky is the much brighter and much oranger Mars, and the bright red supergiant star Antares, the heart of Scorpius. Face south and look low to the horizon for the trio; your outstretched fist can cover all three.

Saturn by Damian Peach
Saturn, taken by master astrophotographer Damian Peach with a 35.6 cm telescope on Mar 18, 2016.

Damian Peach

And if you have a telescope, well then what are you waiting for? Saturn is one of the most magnificent objects in the sky. Patience pays off, too: in early June it will be at opposition, opposite the Sun in the sky, which means it rises at sunset and is up all night. As an added bonus, that’s when it truly is closest to the Earth, and appears as big as it can. But then, that means it’ll be 0.3 arcminutes across: Only 1/100th the width of the Moon in the sky.

But through a telescope, it’s glorious. So forget the dumb Facebook hoaxes, and go see the real Universe for yourself. It’s way, way better. 

April 29 2016 9:00 AM

The Warped Path to Understanding the Universe

The Universe is a bit bent.

You’d think that light would travel in straight lines, going directly from point A to point B. But it’s more complicated than that. Space is curved, warped, distorted by the matter lodged within it. It’s hard for our puny three-dimensionally adapted brains to wrap themselves around (so to speak), but what we think of as space is actually a framework that can be bent.


Mass does that bending, and we perceive that bending as gravity.

But we can see the effects of this bending, too. Like a car following a curve in the road, light follows the gravitationally induced curving of space. We poetically call this effect “gravitational lensing,” because a lens bends light as well. 

Sometimes the curve in space is profound, like when a black hole makes a warp so severe it’s almost a puncture in the fabric of reality. The path a beam of light takes can be whipped around quite viciously—we call this strong lensing—or even plunge down into the black hole, never to return. Clusters of galaxies can do this as well, warping light from even more distant galaxies into bizarre shapes.

Other times that curve is more gentle, when the mass is more spread out. Individual galaxies and some galaxy clusters are like that. Instead of an obvious distortion in the direction light takes, it’s subtler, only marginally changing the light’s path.

Although harder to detect, this weak lensing can tell us a lot about the mass doing the bending of space. And that’s why astronomers used Hubble to take that lovely image of the galaxy PGC 54493 seen above.

zoom in on galaxy
A closer look at PGC 54493. Lovely, isn't it?

ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

That galaxy dominates the view, our eyes drawn to magnificent spiral arms, the bar-shaped distribution of stars in its center, the blue patches where massive stars are being born by the thousands. But it’s not the most important thing in this image.

Look around it. See all the smaller, fainter galaxies? PGC 54493 is perhaps 500 million light-years or so away, but these other galaxies are much farther. The light we see from them was emitted when the Universe was far younger, and has traversed billions of light-years of space to reach us.

On that trip, this light passed near PGC 54493, traveling through the region of space subtly distorted by the galaxy’s mass. The shapes of these distant galaxies thus get distorted, too, the amount of change in their shapes determined by the amount of mass in the big spiral galaxy bending space. It’s difficult to determine that for any individual faint galaxy, but if you observe enough of them, you can show statistically how much they’re warped, and therefore where the mass of the intervening galaxy lies and how much there is.

That is critical. We know that normal matter—the kind of matter that makes up you, me, the computer you read this on, and essentially every visible object in the Universe—is in the minority. Dominating the amount of matter in the cosmos is a mysterious form of it that we cannot see, but which has mass, and therefore gravity. Dark matter can’t be observed directly, but its gravity warps space, and that distorts the light from those distant galaxies, and that we can indeed measure.

Astronomers have been mapping dark matter in this way for many years; perhaps most famously in the Pandora Cluster, which showed just how dark matter in clusters of galaxies can affect the light from more distant galaxies. They continue to observe other clusters to make even more maps of the distribution of dark matter in the Universe, and that’s why PGC 54493 was the target of these observations.

Although bright and beautiful, it’s the dark reach of that galaxy that concerns us, invisible matter making invisible bends in space, distorting more distant galaxies in almost, but not quite, invisible ways.

Sometimes the Universe is obvious, ostentatious, perhaps even gaudy. But other times it whispers, hints at us to follow the rules of math and science that shape it, and in so many cases it is following that path—distorted and subtle as it may be—that leads to seeing its underlying structures.

April 28 2016 9:15 AM

Climate Change Is Strangling Our Oceans

As our globe heats up, the effects of those rising temperatures are complex and varied.

Overall, we call these effects “climate change,” but that’s an umbrella phrase that covers a vast number of changes. Melting polar ice, glacial melt, changing weather patterns, more extreme weather, rising sea levels, ocean acidification … these are the topics we hear about quite a bit. But there are other changes going on, and these are every bit as much a cause for concern.


For example, global warming is strangling our oceans.

By that I mean that oxygen levels in the oceans are affected by global warming, too. This occurs in two general ways. One is that warmer water has a harder time holding on to dissolved gases—that’s a basic law of chemistry. So oxygen levels drop as water warms. The other is that as surface waters warm, they expand, and mix less with deeper water. Surface water gets most of its oxygen from phytoplankton that breathe it out. That water sinks and mixes with deeper water. As the water becomes more stratified due to global warming, less of that oxygenated water gets to lower depths.

The question then becomes, when will we start to see the effects of global warming on the oceans’ oxygen content? Like so many other aspects of climate change, we’re seeing that deoxygenation due to human-generated global warming occurring now. Not sometime in the distant future. Now.

That’s the result of a new study done by a team of scientists led by Matthew Long, an oceanographer at the National Center for Atmospheric Research in Boulder, Colorado. He and his fellow scientists used a sophisticated computer model that calculates the amount of oxygen dissolved in the oceans over the globe and with ocean depth. The model accounts for a great number of factors, including air temperatures, water temperatures, layers in the water, sea ice, and much more.

Global warming changes the air temperature, which affects the model. In the past, global warming was small, and natural variations in the environment changed the oxygen levels far more than human-made warming. But, as the world warms more and more, the effect of warming increases, and at some point becomes noticeable over the natural variations.

Think of it this way: Imagine being in a loud room, with many people talking, and someone nearby whispers at you. You can’t hear them at first over the ambient noise, but if they keep speaking more and more loudly, at some point you can distinguish their voice from the crowd.

The scientists running the model wanted to know when they could hear the voice of warming over the babble of the natural variations. They ran the model many times, varying the air temperature a small amount with each run. This gave them a range of outcomes that they could compare to physical measurements; that gave them a check to make sure they had real-world testable outcomes.

ocean oxygen level map
A map showing when the signal from human-caused global warming emerges from the noise of natural variability. Blues are where it's already being seen; by 2040 all colored regions are affected. Gray shows no predicted signal by the year 2100.

Matthew Long/NCAR

What they found is sobering. Deoxygenation due to human-made global warming is already detectable in the southern Indian Ocean, and in some regions in the eastern tropical Pacific and Atlantic. By 2030 to 2040—two decades from now—they expect to see more and more widespread deoxygenation over the globe. By the year 2100 (which is how far into the future they ran the models) a significant fraction of the global oceans will see some deoxygenation due to human activity.

This is, obviously, bad. The amount of deoxygenation may not be very much, just a drop of a few percent. But as we learned with increasing carbon dioxide levels in the atmosphere, and rising temperatures, it doesn’t take much change to destabilize a system.

The worst effects will come from areas already low in oxygen, called hypoxic zones, where the levels can be as much as 70–90 percent lower than average, and in suboxic zones, where it’s even lower. In those regions, a few percent drop can mean the difference between life barely holding on, and death.

Even when the change isn’t so dramatic, it can be devastating. You might think of the ocean as one big fish tank, but it’s actually incredibly diverse, depending on water temperatures, currents, pressure, and more. Changes in oxygen levels in the water reduce marine life habitats, stressing the inhabitants there. Changes in regional oxygen levels have caused migrations of fish, and even massive die-offs. Besides the effect on the life there, this has an impact on human activity including fishing, on which many countries depend.

Mind you, about half the oxygen we breathe comes from ocean phytoplankton. Messing with their habitat is like setting fire to your own house. Which is pretty much what we’re doing.

So you can add this to the list of deleterious effects of global warming to our planet. And don’t forget that the past six months have all had record-breaking high temperatures, with many scientists already expecting 2016 to be the hottest year on record globally, and carbon dioxide levels still rising.

Global warming is our future, but it’s also our present. It’s now. Once again, Americans, I implore you to consider this, what is perhaps the greatest long-term threat facing humanity, when you go to the voting booth in November. And for the rest of the world, know that most of us here in the U.S. are aware of the problem and doing our best to urge our leaders to take action as well. We can minimize the damage of global warming, but first we have to make sure our government is facing reality.

April 27 2016 8:30 AM

A Moon for Makemake

Well now, this is very cool: Astronomers have just announced that Makemake has a moon!

Makemake (pronounced MAH-kay-mah-kay; it’s named after a Rapa Nui god who created humanity) is a Kuiper Belt object (or KBO), a large icy world orbiting the Sun way out past Neptune. It’s what’s called a “classical” KBO, because its orbit is far enough from Neptune’s that it can orbit the Sun stably for billions of years (Pluto actually gets closer to the Sun than Neptune does, but their orbits are timed such that they never get very close together). At 1,430 kilometers across, it’s the biggest known classical KBO, and the third biggest such body known past Neptune. Only Pluto and Eris are bigger.


We know quite a bit about Makemake from previous observations. It takes about 7.77 hours to rotate once, and must have a very reflective surface. Spectra show it has a lot of frozen methane on it, too, more than any other object out past Neptune. Its orbit is elliptical, taking it as close to the Sun as 5.8 billion kilometers and as far as 7.9 billion kilometers out. Right now it’s almost at aphelion, the point in its orbit when it’s farthest from the Sun.

It’s been observed many times, but until now no moon had been found. The discovery was made using Hubble Space Telescope, which observed Makemake exactly one year ago, on April 27, 2015. The moon—which for now has the cumbersome designation S/2015 (136472) 1—was seen very close to the KBO, almost lost in its glare.

That may be why it hasn’t been seen before. For one thing, it’s much fainter than Makemake, making it hard to detect. The astronomers who discovered it also propose that we may be seeing its orbit edge-on, which means it spends much of its time too close to Makemake to easily detect from Earth. From its brightness it's probably very roughly 200 kilometers in diameter, so seeing it at all is remarkable, especially from more than 7 billion kilometers away!

discovery images
The raw, unprocessed discovery images showing Makemake and its moon (arrowed in the first panel). The streaks and dots are cosmic rays, different in every frame, but the moon persists.

Parker, et al.

The observations were split into two “visits”; one on April 27 and the next on April 29. The moon was seen in the first visit but not the second, meaning it was too close to Makemake to see in the second set. Each visit consisted of six long exposures, and the moon was seen in all six images on the first visit.

The problem is the time between the first and last images in the first visit was only a couple of hours, which was not enough to show any motion of the moon. That means the orbit isn’t well-known. The best the astronomers could do was say that the orbital distance is at least 21,000 kilometers, but may be as much as 300,000! For comparison, the Moon orbits the Earth at a distance of about 380,000 kilometers. The moon orbits Makemake with a period somewhere between 12 and 660 days.

Those are pretty wide ranges. And that’s too bad, because the orbit of a moon is really critical: It tells you the mass of the object it orbits. Gravity depends on mass of the primary object and the distance to the moon, so if the orbital period of S/2015 (136472) 1 had been measured, the mass of Makemake could have been found.

What makes this even more frustrating is that we know how big Makemake is, because in 2011 it passed directly in front of a star as seen from Earth; the length of time it blocked the star combined with its known orbital speed yielded the diameter of 1,430 kilometers. With that number, plus the mass, its density could be determined, and that gives a huge clue about what it’s made of. Rock is denser than ice, for example, so just knowing Makemake’s density would constrain its composition.

However, the good news is that now that we know the moon exists, more observations can be made. It may take Hubble to see it once again—Hubble’s really good at spotting faint things near bright things—but it can be done. And if carefully planned (which it will be), the moon’s orbital period and shape can be found.

There are some interesting implications of this, too. Earlier infrared observations of Makemake indicated the surface had at least two different materials on it, which had different thermal properties (darker material, for example, gets warmer than more reflective stuff). If that were the case, though, the brightness of Makemake should change enough to measure as it spins, but no such change was seen.

The moon may solve this problem! If it’s darker than Makemake, then that would throw off the infrared observations. Those telescopes couldn’t separate the moon and the KBO, so they merged together, confusing the interpretation.

Pluto and Charon
Pluto and its much darker moon Charon (relative sizes to scale, relative distances not to scale).


I find that very interesting indeed. As you may remember from last year, Charon, Pluto’s moon, is much darker than Pluto as well. If this new moon is indeed dark, that might indicate a similar origin story. Future observations with the James Webb Space Telescope, due to launch in 2018, may help figure this all out; it has the keen vision in the infrared to separate the two objects and determine how reflective the moon is.

Just the fact that the moon exists is interesting, too. Pluto has a moon, as does Eris and Haumea (another large trans-Neptunian object a bit smaller than Makemake). Are they ubiquitous? Two other such objects, Sedna and 2007 OR10*, have no known moons. Perhaps we just haven’t seen them because they’re too close to their primaries. Clearly, more and deeper observations are needed.

And let’s not lose sight of the more general nature of this: We’re still learning basic information about our solar system! Finding a moon like this gives precious insight into what the outskirts of our celestial neighborhood are like, a region that is terribly far away and dark, and very difficult to explore. Each discovery about it is a clue to how it came to be, how it changed over time, and why it looks the way it does now.

All of this tells us more about our existence, and informs us about our own world. A cold and distant moon orbiting a smallish ice ball may not seem like much, but it’s another piece of the puzzle, one worth investigating and understanding.

* Correction, April 27, 2016: I orginally wrote that object's name as 2010 OR10.

April 26 2016 9:00 AM

An Emerald Green Comet and a Billion Stars

I’ve said many times that there are no green stars, but that doesn’t mean there are no green objects in space.

Babak Tafreshi photo of a comet
The observatory, the comet, and the galaxy. Click to embiggen.

Babak Tafreshi


Oh my. Isn’t that gorgeous? In the foreground is one of the four smaller auxiliary telescopes that make up the Very Large Telescope Interferometer, a sophisticated piece of equipment that can take observations with far higher resolution even than Hubble. And by smaller, it’s a 1.8-meter wide mirror; that’s only small when compared to the 8.2 meter behemoths that make up the Very Large Telescope!

But that sky. To the right of the telescope blazes the billions of stars of Milky Way, our home galaxy, partially obscured by dark dust lanes and punctuated by the pink glow of hydrogen in the Lagoon Nebula.

But that green fuzzy glow above the telescope somehow manages to steal the show, perhaps because its color makes it stand out in stark contrast to the other objects in the sky. That emerald blob is the comet 252P/LINEAR, which passed the Earth in mid-March at a close distance of more than 5 million kilometers.

Many comets glow green; it’s due to the presence of a molecule made of two carbon atoms (C2), so that’s not a big surprise. But that’s about the only nonweird thing about 252P.

It turns out it was full of surprises. For one thing, it was followed closely by another comet, P/2016 BA 14, which had a very similar orbit, sparking hypotheses that they were once the same object that split. Both passed very close to Earth (the fourth and seventh closest passes of comets in recorded history).

This gave astronomers a chance to observe BA 14 with radar and lead to another surprise: BA 14 was way bigger than first thought. From its brightness it was at first estimated to be less than 200 meters across, and probably much less. But the radar observations showed it was more like a kilometer across! It turns out the solid nucleus of BA 14 is incredibly dark, reflecting less than 3 percent of the light that hits it. That makes it fainter than expected, which is why astronomers thought it was small.

radar observation of comet BA14
Radar observations of BA 14 show it rotating as it passed the Earth. It also allowed an accurate size to be determined.


Around the same time, 252P had a huge outburst, suddenly becoming 100 times brighter in a short period of time. It went from obscurity to becoming naked-eye visible, though unfortunately for us in the Northern Hemisphere it was too far south to see. But that was good news for Babak, who was able to capture it in the photo above while he was visiting the European Southern Observatory facilities.

It’s rare, but sometimes comets flare in brightness. This can happen if they clave (split off a piece), which can release a lot of ice that turns into gas; that reflects sunlight and makes the comet brighten. In 2007 Comet Holmes underwent a tremendous outburst that made it easily visible to the naked eye even though it was out past Mars; it’s brightness increased by a factor of a million! I was fortunate to be in a place where I could see it, and it showed a disk easily to the naked eye; the cloud was more than 1 million kilometers across. I’ve never seen anything like that before, nor since.

Which goes to show you, it always pays to keep your eyes on comets. Sometimes they fizzle, but when they perform, they can really perform, and all the time spent watching them can suddenly pay off.

April 25 2016 11:30 AM

Five Days in Paradise: Come to Science Luau 2016

Aloha! I’m pleased to announce registration is open for Science Luau 2016, a five-day vacation on the Big Island of Hawaii, with a special bonus: SCIENCE.

My wife and I run a company called Science Getaways, where we take people on vacations that are already really nice and add science to them. For this trip, we’re spending Sep. 19–23 on the Big Island, in the lovely and luxurious Mauna Lani hotel on the Kohala coast.

sea turtle
There's a pretty good chance you'll see more than one of these sea turtles swimming in the Mauna Lani waters.

Mauni Lani Hotel


We have several special extras planned. We’ll take a trip to the Kilauea volcano with an expert tour guide, ending in a stop at dusk at the summit crater, Halema'uma'u (ha-LAY-mah-oo-mah-oo), to see the lava pool glowing and illuminating the sulfur plume huffing and puffing out of it. We’ve chartered a boat to go to some coral reefs for snorkeling, and then we’ll go night diving with manta rays. We’ll also be taking a hike through a dry forest with another expert guide to look at the amazing native Hawaiian trees and wildlife. 

I took this shot of the Kilaeua crater on a recent visit to the volcano. Want to see this for yourself? Yes. Yes, you do.

Phil Plait

I know a thing or two about astronomy, so I’ll be bringing my solar telescope to show you what our active Sun is doing that week. On top of all that, there will be talks by experts about what we’ll be experiencing; I’ll be giving an astronomy talk as well.

We’ve been doing these Getaways for a few years now, and they have always been a lot of fun, with honestly wonderful people (many have made lifelong friends on these trips). We went to the Big Island last year and it was so popular we decided to do it again. 

I hope you’ll join us. All the information you need is on the Science Getaways page. Mahalo, and see you in paradise in September!

April 25 2016 9:15 AM

A Martian Crater Torn in Half

Sometimes geologists have it easy.

In astronomy, you can have two stars right next to each other, maybe even born in the same cluster, but you can’t be sure they’re the same age, or which one is older than the other. Differences can be really subtle, and have to be teased out of the data.

Geologists, on the other hand, get stuff like this:

WOW. That is a scene on Mars, taken by the wonderful HiRISE camera on the Mars Reconnaissance Orbiter. Its shows a small portion of Ganges Chasma, a huge, sprawling channel cut into the surface of Mars, probably due to catastrophic flooding a billion or more years ago. The violent rush of water cut through the landscape like God’s trowel, carving out the chasm.

April 24 2016 9:30 AM

A Twice-Lit Moon Kisses the Horizon

Sometimes you see a photograph that’s just so wonderful you can’t wait to show it to other people.


It was taken by Petr Horálek, a European Southern Observatory Ambassador—the ambassadors are a group of excellent photographers who shoot pictures of the ESO observatories for public outreach.

The photo was taken on April 6, just minutes before sunrise. Smack dab in the center is Venus, cruel twin of the Earth, covered in clouds so reflective they make the planet the third brightest natural object in our skies.

Below it is the crescent Moon, less than a day before its new phase. The crescent is so thin it’s almost an afterthought. Amazingly, the rest of the Moon’s surface was unlit by the Sun. So why can we see it? Earthshine! Light from the Sun hits the Earth, reflects off of it, illuminating the Moon, which then reflects it back into space, and to Earth. Our planet is very bright in the lunar skies, 50 times brighter than a full Moon. That’s enough to softly bathe the surface of our satellite in light.

Note too the Moon looks a little squished. That’s an effect of our atmosphere, which curves along with the Earth’s surface. Near the horizon, the light from the bottom of the Moon goes through a thicker layer of air than the top part of the Moon. The air acts like a lens, bending that light, making the Moon look flat.

Horálek timed this photo perfectly, getting the shot just as the Moon cleared the distant mountains. Had he waited much longer the Sun would have lit things up too much anyway. This was a time exposure, too: You can see the faint stars of Pisces surrounding the Moon and Venus.

And let’s not ignore the foreground! The silhouetted dome houses the 1.2 meter VLT Auxiliary Telescope, part of the Very Large Telescope array. See the two people crouched nearby? That’s my friend Babak Tafreshi on the left and Yuri Beletsky on the right, wrapping things up after a long night of photography in the incredibly rich and dark skies of this remote location in the high desert of Chile. I’ve featured both their works on this blog many times; click their names to fill your eyes and brain with delight.

I dream of capturing a photograph like this some day. But I just dabble in this; Horálek, Babak, and Beletsky are professionals. I might feel a pang of jealousy seeing shots like this, but it evaporates rapidly as I take in the sheer beauty. I’m glad there are so many people out their willing to collect the few photons the Universe graces us with, and share them with the world.

April 23 2016 9:30 AM

Valley of the Cosmos

Photographer Michael Shainblum is a wonder. His photographs are just stunning, one after another. I’ve been following his work for years, and every time he posts something it’s awe-inspiring.

Oh, you want an example? Then how about this for your eyeballs?

April 22 2016 9:15 AM

Noon on a Comet

A couple of weeks ago I posted a dramatic shot of the comet 67P/Churyumov-Gerasimenko backlit by the Sun, taken by the Rosetta spacecraft from a distance of a few hundred kilometers. I mentioned how the spacecraft was on a long looping path that would soon take it back to the comet, passing directly between it and the Sun at a distance of about 30 kilometers, which should make for some very pretty pictures.

Well, here you go: As promised, the image above shows that exact event! Rosetta was 29.9 kilometers above the surface of 67P when it took that shot on April 12, using the OSIRIS wide-angle camera. As you can see, shadows are nearly non-existent, and only appear where there’s a sudden change in the topography of the comet (cliffs, shelves, and the like) near the edge of the comet.


And that glowing spot left of center is real, too! There are lots of fancy names for it, including the zero phase effect and heiligenschein, but I think my favorite is “opposition surge.” Poetic and scientific, all at the same time.

There are two reasons for it. One is that from the spacecraft’s view, the Sun is directly behind it, so the Sun’s light is shining straight down on the comet. At that spot directly below the spacecraft you don’t see shadows, so the surface looks brighter. Not only that, but some types of terrain bounce sunlight straight back in the direction it came. If you’re on that line, you see that spot on the ground being brighter than the area around it.

The Sun is in the opposite side of the sky as that spot, hence the term “opposition,” and the surge is pretty obvious. Heiligenschein means “halo” in German, and if you’ve ever seen a faint glow around the shadow of your head as walk past a patch of dewy grass, you’ll understand that term, too.

As for zero phase, the technical term for the angle between the source of light, the observer, and the spot illuminated is the “phase angle.” When they’re all on a line that’s defined as zero. The term should actually be familiar to you in another guise: The angle between the Sun, you, and the Moon defines the Moon’s phase! Surprise! You know more technical stuff than you probably thought.

By the way, the Rosetta shot is looking up at the flattish “underside” of the comet, the broad region across the wider of the two lobes making up 67P’s rubber ducky shape. You can see part of the smaller lobe, the “head” on the left. The shape looks funny when shadows go away; it’s hard to tell what’s what. Shadows actually help us understand the terrain, and can be used to make 3-D maps of the surface. But this zero phase angle helps too; the surge can be used to understand better the surface on much smaller scales; different sized grains of ice or other materials change the brightness of the surge.

Fantastic! Science and geometry come together to form beauty. As they often do.