Bad Astronomy
The entire universe in blog form

Sept. 23 2015 10:00 AM

Watch the Lunar Eclipse on Sept. 27–28!

On Sept. 27 and 28, the Moon will enter Earth’s shadow, creating a total lunar eclipse. These only happen a couple of times a year, and are a great event to watch.

First, the quick stuff you need: The Moon begins to enter the dark part of Earth’s shadow starting at 01:07 UTC on Sunday night/Monday morning. To be clear, for folks in the U.S. that’s Sunday night, starting at 9:07 p.m. Eastern time; for most of the country the Moon will be low to the eastern horizon. At that time you’ll start to see a dark “bite” taken out of the Moon on the part of it nearest the horizon (the lower left, again for Americans).

It’ll take just over an hour for the Moon to pass fully into the shadow, and the last sliver of it will slip into darkness at 02:11 UTC (10:11 Eastern). It’ll stay dark for more than an hour, and then start to be illuminated once again at 03:23 UTC (11:23 Eastern). “Last contact,” when it is out of Earth’s shadow, occurs at 04:27 UTC (00:27 Eastern, after midnight).

People in the western U.S. (west of Colorado) will see the Moon already in eclipse when it rises. Here’s a map to show if you’ll be able to view the eclipse:

map of eclipse viewability
Map of the eclipse viewability.

Drawing by NASA/Fred Espenak

“U1” is when the Moon begins to enter the dark part of the shadow (the umbra), so everyone east of that spot in the U.S. will see the whole eclipse. Everyone west of the line marked “U4” (when the last part of the Moon leaves the shadow) will miss the event. Sorry. We live on an opaque spinning planet, so someone always misses out. Anyway, the diagram below shows the Moon's path through the Earth's shadow. 

lunar eclipse diagram
The Earth casts two shadows in the sky: the (outer, fainter) penumbra and (inner, darker) umbra. This map shows the Moon's path through them during the September eclipse.

Drawing by NASA/Fred Espenak

You don’t need any special equipment to see this; just go outside and look at the Moon. (This is different than a solar eclipse, where you need eye protection from the bright Sun.) Having said that, I’ve always found binoculars to be best aid to viewing. The Moon can take on an odd three-dimensional appearance when you use binoculars during an eclipse, and it’s pretty cool to see. A telescope is great, too, if you have access to one. If there’s a local astronomy club or observatory near you, see if they’re running a star party for it.

I’ve written before about exactly how and why we get lunar eclipses; the dance of the Moon, Earth, and Sun has to play out just right for the Moon to move through Earth’s shadow in the sky. Go there to get details, but there’s one thing I want to emphasize.

When the Moon is fully eclipsed it usually turns red, though sometimes the effect is more subtle than other times. This is because from the Moon’s point of view the Earth is blocking the Sun, and sunlight gets filtered through the thin layer of Earth’s atmosphere, reddening it. If you were standing on the Moon, it’s like you’re seeing every sunrise and sunset on Earth all at once!

How about that?

You can read more about the eclipse at the Time and Date site, EarthSky (which separately lists the timing for the eclipse in different U.S. local time zones), and Wikipedia. Also, the Virtual Observatory will be running a live viewing of the event online, in case your weather isn’t cooperative. So will my friend Adam Block at the Mt. Lemmon SkyCenter.

Also, I just so happen to have done an entire episode of Crash Course Astronomy on eclipses. Watch!

I’ll note that this eclipse happens very close to when the Moon is at perigee, the part of its elliptical orbit when it’s closest to Earth. Its average distance from Earth (center to center) is about 384,000 kilometers, and during the eclipse it’ll be only 356,900 km distant, 7 percent closer than average. That means it’ll look 7 percent wider than average … but I doubt you’ll notice. If you took a picture when it’s at apogee (farthest from Earth) on Sept. 14—when it’s 406,500 km away—and then compared it with a picture taken on Sept. 28 you’d definitely see the difference! But just going out and looking during the eclipse you’re unlikely to be able to tell.

Of course, the Moon will be rising for many people in the U.S. at the start of the eclipse, so the Moon may look huge due to the well-known Moon Illusion. But don’t be fooled! A lot of websites will no doubt be hyping up the “Supermoon” (a full Moon at or near perigee), but don’t be taken in by them. The difference in size isn’t all that much.

Not to pooh-pooh any of this! Lunar eclipses are fun; they play out relatively slowly, so you can take a look, go inside for a few minutes, then go back outside to see more of the Moon gone. It’s a great opportunity to try your photography skills, and because you don’t need any equipment, it’s nice to get friends and family together outside to take a peek.

I strongly recommend marking your calendar. The next total lunar eclipse won’t happen until January 2018! So watch this one if you can. 

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Sept. 22 2015 9:45 AM

No, That’s NOT What the Earth Would Look Like Without Water

A weird animated graphic depicting a distorted, lumpy Earth has gone viral over the ‘Net in the past few days, claiming that this is what the Earth looks like “without water.”

There’s only one problem with it: nope. Nopity nope nope nope.

That’s not at all what it shows. What it actually depicts is the Earth’s geoid: a way of describing Earth’s gravitational field. The original graphic is a product of the MATLAB package described by Ales Bezdek (credits at that link). Here it is in all its knobby goodness:

Earth’s gravity isn’t smooth at the surface but is stronger in some places than others. That’s because the Earth isn’t a perfectly homogeneous sphere (that is, the exact same density throughout its interior) but has some places where it’s more dense and places where it’s less dense. That affects the surface gravity.

When you stand on the surface of the Earth, it feels like gravity is pulling you down to the center. But if you stand next to a denser region, its gravity pulls you a little bit to the side, away from the center. The geoid in the viral graphic shows this; in that map gravity always pulls you perpendicular to the surface depicted.

I know that sounds weird, but basically it’s saying that if you’re on the side of a “hill” shown in the geoid graphic, a plumb bob (a heavy weight tied to a string) will not point toward the center of the Earth, but perpendicular to the surface where you’re standing. The actual graphic is hugely exaggerated on purpose, making it easier to see the Earth’s lumpy gravity field.

I have to laugh (if somewhat ruefully). One thing I find whenever some wrong science factoid goes viral is that it’s usually exactly wrong; it states the opposite of what’s actually going on. That’s true here! How?

Another way to describe the geoid is that it’s the shape of an object if it’s perfectly fluid; if the surface is allowed to flow freely.

For a perfectly homogeneous object (say a big nonrotating drop of water in space) the geoid would be a sphere. For the Earth, well, it’s what’s shown in the graphic. In other words, that graphic doesn’t show the Earth without water, it shows what the shape of the Earth’s surface would look like if the surface were entirely covered in water.

See? Exactly wrong.

It’s easy, given the caption, to think that this is what the Earth’s solid surface under the oceans looks like. But look at the scale bar in the graphic; it goes from about +80 meters to -80 meters. That’s a teeny tiny fraction of Earth’s size. In physical reality, even if the Earth were covered in water it wouldn’t be anywhere near as lumpy as depicted. Again, it’s exaggerated for clarity.

Think about this, too: The deepest part of the Earth’s ocean (the Mariana Trench) is about 10 kilometers deep. The Earth is nearly 13,000 kilometers across! Take away all the water from the Earth’s surface and you’d hardly notice; the elevation difference between the highest mountain and lowest point in the ocean is less than 20 kilometers, about a tenth of a percent of the Earth’s diameter.

Earth without water
This is what the Earth looks like with all the water gathered up. The big drop is all the water; the next biggest is fresh water in the ground, lakes, swamps, and rivers; the smallest is just fresh water from lakes and rivers.

Graphic by Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution; Adam Nieman.

And yes, that drop is the size of the sphere you’d get if you extracted all the Earth’s ocean water (as well as atmospheric water vapor, lakes, ice caps, and so on). It’s not much compared with the whole planet, is it?

The lesson here? Beware of factoids without evidence to back them up. Also beware of science factoids presented by nonscience sites. Heck, beware of them even from science sites; we make mistakes sometimes.

But be super-duper skeptical of stuff on the ‘Net given without attribution, too. That usually means it’s been through at least one layer added by someone who doesn’t necessarily understand what they’re writing. It could even be something just made up out of thin air.

And that usually means … it doesn’t hold water.

Note: I think this was originally posted to Twitter by 9GAGGifs, a site where people can upload images without any attribution, practically guaranteeing stuff that abuses science can go viral with virtually no fact-checking. (I’ll note I don’t have anything against such sites in theory, but in practice a lot of stuff is posted without attribution, which, not to put too fine a point on it, sucks.) It was then picked up by DesignTimes, a Twitter feed that, upon my inspection, also appears to commonly post stuff without attribution (or, in this case, fact-checking).

Sept. 21 2015 10:00 AM

A Companion’s Glittering Beehive

We live in the Milky Way, a gigantic galactic disk of stars, gas, and dust. Our galaxy also has several smaller companion galaxies, mostly elliptical and irregular dwarf galaxies. One of them is called the Large Magellanic Cloud, about 160,000 light-years away. It’s fairly irregular, though recent studies have shown it may have used to be a small barred spiral galaxy.

I have studied a bit of this galaxy (it was the host of a star that blew up in 1987 which was the subject of my Ph.D. work), but it still manages to surprise me. I didn’t know, for example, that is was the host of a few globular clusters. These are dense balls of stars, sometimes contains up to a million, all orbiting each other in a spherical knot.

One of these globulars near the LMC (as those of us in the know call it) is NGC 1783, a spectacular example of such a beast. How spectacular?

This spectacular.

Sept. 20 2015 10:00 AM


A microburst might sound like some astronomical minicatastrophe, but it’s actually a much more terrestrial event: A sudden accelerating downdraft of wind from a cloud that can, at times, be quite violent.

Some are dry, with just air descending at high speed, while some are wet, loaded with water. Photographer Bryan Snider caught the latter when he was taking time-lapse footage of a thunderstorm over Tucson, Arizona, on Aug. 8:

You can see the rain falling, then at about 00:11 you can see the microburst develop and drop. A second one happens just moments later, too. It’s fascinating to see it hit the ground and blow outwards in a circle. The position of the Sun was perfect to illuminate just the bottom of the event. (Note: After I wrote this, but before it went live, I found that a webcam at Kitt Peak National Observatory caught the same microburst.)

This one didn’t look too violent, despite the scale. But microbursts can be; one that let loose in Montana in 1998 knocked down trees like they were twigs. Even less powerful ones can be a danger to aircraft, and they’re difficult to predict.

I’ve never seen an actual microburst, though I did once see a series of downdrafts from clouds forming beautiful fingers as they descended, one after another in a row. It was mesmerizing, and interestingly enough I asked some meteorologists to help me identify them, but we came up empty.

It’s not unusual for astronomers to be interested in meteorology, and meteorologists to be interested in astronomy. After all, we both look up for a living, and there’s always something amazing waiting to be seen in the sky.

Tip o' the umbrella to Henry NL.

Sept. 19 2015 10:00 AM


Sometimes, I just wanna post a cool picture. Like now.

Whoa. That photo was taken in June 2013, when a lightning storm raged over Cerro Paranal in Chile. See those four bumps on the top of that hill? Those are the enclosures of the four sub-units of the Very Large Telescope, each an 8-meter behemoth. A person standing there would be too small to see in this photo.

But what I really love about this—besides the obvious drama—is over on the left. That patch of clear sky reveals a star barely visible over the mountains’s flank: Procyon, the 8th brightest star in the sky. It’s a relatively old star, a couple of billion years of age, and is actually a binary: a massive, hot star orbited by a small white dwarf, the exhausted core of a star long dead. They’re separated by a couple of billion kilometers, but from our distance of more than 100 trillion kilometers, they appear as a single star by eye.

It’s a reminder to me that there can be much sound and fury, but it’s amplified by being nearby; from a great distance much larger fury can be subdued into a flickering dot.

Sept. 18 2015 10:00 AM

Crash Course Astronomy: Neutron Stars, Pulsars, and Magnetars

Despite my fascination with asteroid impacts, black holes, supernovae, and other über-violent astronomical catastrophes, they don’t scare me.

I mean, sure, they’re scary, but they don’t frighten me personally, viscerally. The wonder and awe I feel from them is real, but I don’t get that holy-yikes-these-things-can-kill-us vibe. They’re just too far away.

Then there are magnetars.

These are freshly minted neutron stars, the collapsed cores of high-mass that have exploded, leaving behind the ridiculously dense, rapidly spinning, fiercely magnetized ball of neutrons.

In 2004, one of these beasts halfway across the galaxy decided to throw a tantrum, and its effects were physically felt on Earth. From 50,000 light years away.


So they give me the heebie-jeebies. Just a little.

So how do magnetars get their power? How does a neutron star form in the first place? And how do pulsars play into this?

Here’s a thought: Watch this week’s Crash Course Astronomy!

It’s funny how quantum mechanics — the physics of the very very small — plays such a huge role in astronomy, which generally deals with things that are very very big. The Universe doesn’t discriminate against size, and in fact size itself can play a massive role in how objects form and how the behave.

Like with black holes… but I get ahead of myself.

Sept. 17 2015 10:00 AM

Painting the Wings of the Butterfly Nebula

As I’ve mentioned a bazillion times before, I’m fascinated by planetary nebulae. These are winds of gas blown off by dying stars, illuminated and set to glow by the hot stars in their centers, and generally sculpted into fantastic shapes that can be anything from thin soap bubbles to highly elongated cylindrical barrels.

One of the most spectacular is M2-9, a doubled-lobed beastie very roughly 2000 light years away. The image above was recently released, using older images from Hubble combined with a new one taken using STIS, a Hubble camera I actually worked on for many years. The colors in this image are a little different than usual; the blue is from tenuous oxygen in the nebula that’s usually colored green (it appears about the same color as a green laser pointer to our eyes), and red —usually indicative of hydrogen — is instead from sulfur.

Some people call this the Butterfly Nebula, but I’ve never thought it looks very lepidopterate. It looks more like two squids kissing.

The extreme elongation of this nebula is rare, and a bit puzzling. Usually, this sort of symmetry comes from the central star being a binary, two stars orbiting each other. Gas blown off by the dying star of the pair gets thrown out along the equator of the system, forming a thickish disk. When the star loses enough of its outer layers, the hot core is exposed. A faster wind picks up, but finds its path blocked by the disk. It blows up and out instead, forming the bipolar shape.

But there are some subtle issues with this scenario when you look closely at the nebula. For example, there are nested shells; you can see an inner reddish one and an outer green one. The red one appears to be brighter on the bottom, and in both lobes. That’s weird. You sometimes get bright spots in both lobes, but they’re on opposite sides. That’s usually caused by jets of material blasting out from the star, and if it’s tipped a bit it hits the lobes on opposite sides (hold a pencil between your thumb and forefinger, then tip it a little; one end points to the left and the other to the right. Same thing; and we call that a precessing jet, where the spin axis wobbles around).

diagram of M2-9
A schematic of the interacting winds blow by the two stars in the central binary of the nebula.

Drawing by Livio and Soker

I found a paper by my friends and colleagues Mario Livio and Noam Soker, and they came up with a clever explanation. They propose that the two stars are a dying red giant and a hot white dwarf. The red giant is blowing a slow, dense wind. The white dwarf is drawing that material in, which forms a disk around it. This collimates (focuses) the wind from the white dwarf, which is much faster. But this fast wind gets bent to the side when it hits the slower wind.

In the diagram above it bends to the left. But this only shows the upper half of the picture; there’s also a lower half that’s a mirror reflection. In that, the white dwarf wind blowing down is also bent to the left, and that’s why we see brightening on the same sides of the lobes.

The two stars orbit each other every 120 years or so. As they do, the white dwarf wind moves around, slamming into the sides of the lobes, making a different part of the nebula glow. It’s like standing in the middle of a room with a flashlight and spinning around; you illuminate different parts of the walls with time.

Animation by R. Corradi, M. Santander-Garcia, Bruce Balick

This animation was put together from actual observations of the nebula; note how the glow moves around over time, illuminating the same sides of both lobes:

That’s exactly what Mario and Noam would predict it to do. Their hypothesis isn’t proven by this, but it’s very compelling to me. It makes a lot more sense than a precessing jet.

I mentioned Noam not long ago, in my Crash Course Astronomy episode about planetary nebulae. I also mention that for many nebulae, the binary stars would have to be very close together to make these shapes, “improbably close”. I may need to reassess that idea. Under some circumstances they don’t have to be all that close to generate these strange and fantastic shapes.

But then, that’s what science is all about, right? Reassessing your ideas, making sure you’re not holding on to ones that aren’t correct. I spent a while studying these objects, and I’ll be honest: It’s pretty cool to find out that sometimes even a few of my basic ideas about them need to be tweaked.

Sept. 16 2015 10:00 AM

Plumes of Magellan

Every now and again, I see a picture of an astronomical object that has me scratching my head.

The image above is from Hubble Space Telescope, and shows a section of a nebula in the Large Magellanic Cloud, a dwarf galaxy companion of our Milky Way. Lovely, isn’t it? Delicate streamers wafting across a field of stars …

But what is it, exactly? I was thrown when I saw it. Usually, blue wisps like this are dust—microscopic grains of silicates or long carbon-based molecules. Dust is very good at scattering blue light, in the same way our atmosphere scatters blue light from the Sun, making the sky blue.

But that didn’t make sense to me. Dust scatters blue, so stars behind the dust look red: The blue light from them is scattered away, and we don’t see it. But the stars behind this material don’t look any redder than the other stars in the frame.

That’s when I checked the filters used for the image. Aha! Normally, images like this use a filter that lets through light emitted by warm hydrogen, called H-alpha filter. This strongly emphasizes gas in the image.

In this case though, the filters chosen were different. What you see as red is actually from the infrared. What you see as green is actually coming from a filter that lets through orange light; what’s displayed in the picture as blue is actually a mix of blue-violet and ultraviolet light.

So those wisps are gas, not dust—in fact, this is showing the outskirts of the Tarantula Nebula, a vast star-forming gas cloud in the LMC, with which I’m fairly familiar. The color threw me off!

Tarantula nebula
A more "normal" (and much wider) view of the Tarantula Nebula.

Photo by M. Schirmer, T. Erben, M. Lombardi (IAEF Bonn), European Southern Observatory

Elemental gas—say, hydrogen—emits a variety of colors. The strongest in visible light is in the red, that H-alpha I mentioned earlier. But it also gives off blue and green. Oxygen is very strong in the green, but also emits some blue. So the gases we’re seeing in this image are the usual ones in nebulae, but displayed differently.

It’s a little disconcerting to see that; I’m used to colors being displayed a certain way in images like this. But sometimes mixing things up can help. Structures you might miss pop out, for example. Also, some regions of nebulae are at different temperatures, which means oxygen or helium might dominate the emission over hydrogen. Changing the colors can make this easier to spot.

So while at first this image knocked me off my game, as I looked at it more I got to appreciate what it was showing me. It’s easy to get too comfortable doing things the same way all the time. Sometimes it helps to shake things up a bit.

Sept. 15 2015 9:45 AM

Cloud Streets Bering Down

I love odd and unusual cloud structures, especially when they form interesting patterns. There are lots of different kinds, but when you’re looking down from orbit, I think cloud streets are the coolest.

“Cloud streets?” you ask. “What are those?”

cloud streets
The sky is paved with cloud streets!

Photo by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC

That is the view from NASA’s Aqua satellite, designed to watch our planet’s water. It’s part of the critical (and politically threatened) Earth Observing System, birds that keep track of the Earth’s water, air, land, and climate.

That shot (go grab the much embiggened version, which is astonishing) was taken over the Bering Sea, and is centered about halfway between Russia on the left and Alaska on the right. The little squiggly island just above center is St. Matthew Island, and you can see part of the Aleutian Islands arc to the lower right.

The cloud streets are the long, linear fingers of clouds flowing down and to the left (roughly southwest). I’ve described them before; they form when cold air from over the land blows offshore over warmer water. The air over the water is rising, but the cold air acts like a lid. The warm air hits it, gets spread out to the sides, then starts rotating in long cylinders. Here’s a diagram:

cloud street diagram
Rising air slams into cooler air above it and starts to rotate in oppositely spinning cylinders.

Diagram by Daniel Tyndall, Departmant of Meteorology, University of Utah

Each “street” spins in an opposite direction, and the rotation can be stable for hundreds of kilometers. They occur anywhere cold land blows air onto warm waters, so they appear over Greenland’s coasts, the Great Lakes, and obviously Alaska. I don’t think I’ve ever seen this for myself; my maritime experience hasn’t included steaming off of a wintry location. Although the scale is pretty big, it must be neat to be under these. They probably look like mackerel clouds but on a much larger scale.

I didn’t know these existed until I became a satellite photo nerd just a few years ago. It’s amazing what nature can dream up, and what we can see when we make the effort to look.

Sept. 14 2015 10:00 AM

Pardon My Flocculence

Spiral galaxies are just so danged pretty.

Seriously, look at that! It’s an image of the nearby galaxy M63, aka the Sunflower Galaxy, taken using the Hubble Space Telescope. M63 is relatively close to us, a mere 37 million light-years, easily visible in small telescopes. I’ve seen it a few times myself through my own ’scope—I vaguely remember finding it by accident when I was in high school, looking for the more famous M51 Whirlpool Galaxy. They’re not far from each other in the sky, and in fact they aren’t far from each other in space; both are part of a small group of galaxies.

M63 is what we call a flocculent galaxy—the arms are patchy, like tufts of cotton. Roughly a third of all spirals are flocculent, and apparently it’s still not well understood what causes it. One idea is that local gas clouds get stretched and sheared by the galaxy’s differential rotation (that is, stuff closer to the center makes an orbit around the galactic center in less time than stuff farther out).

Another is that star formation is localized, starting randomly in spots in the disk of the galaxy, triggering more star formation around it, creating that patchwork. In galaxies like the Milky Way, the spiral arms themselves trigger star formation, so you get more of a sweeping, grand design to the spiral arms.

M63 Spitzer
Dusty, too.

Photo by NASA/JPL-Caltech

Spitzer detects infrared light emitted by warm objects, so what you’re seeing here is mostly dust gently heated by nearby stars. The arms look a little more organized here, but you can still see they’re patchy. 

I love big, sweeping spiral galaxies, but there’s something to be said for flocculent ones as well (like NGC 2841, NGC 3521, and NGC 1398). And besides being just pretty, they also show us that there’s more than one way to make a spiral arm, or to spawn millions of stars.

Nature’s fairly creative. And it has a broad canvas upon which to work.