A pertinent question to ask as we approach New Years’ Eve.  The answer is, as is so many things, “it’s complicated.”  According to the Straight Dope, the answer is “it depends.”

When an object falls, there are two main forces on it — gravity, and air resistance.  Air resistance depends on how fast something is moving, so the faster the bullet goes, the more air resistance.  So at a certain point, the forces of gravity and air resistance are in balance, and the bullet falls at a constant speed (since you need a net force in order to accelerate, or increase your speed).  That’s called terminal velocity.

So, is the terminal velocity of a bullet fast enough so it has enough energy to penetrate the skin?  Snopes reports that, back in the 1960’s, the army determined that the energy in a 0.30 caliber bullet falling from the air was about half that needed to produce a disabling wound.  Case closed? Not quite.  For one, different bullets have different terminal velocities AND different bullets require different speeds to penetrate the skin.

This is the only “Mythbusters” Myth to be rated both “plausible, confirmed, and busted” at once. If the bullet is fired straight up, it tumbles and falls at terminal velocity, they write.

But that’s not how bullets are fired, most are fired at a shallow angle.  That’s why, in most news reports of victims of falling bullets, the victim is pretty far from the shooter.  If the bullet is shot at a shallow angle, it doesn’t tumble, and can pack a hefty wallop — much faster than terminal velocity.

Conclusion:  If you’re stupid and shooting on New Year’s — at least use a protractor.  Fire at a 90 degree angle to the ground.  If you’re shooting at a shallow angle, you might kill someone.

Of course, we’d all be safe if people just stuck to the old tradition of firing shotguns with pellets.  Birdshot doesn’t have good ballistics — the bullets don’t spin cleanly, thus achieving the high velocities of bullets.  You’d just be showered with pellets, as if they were tossed in the air.

For the mathematically inclined, here is a detailed description by high school teacher Roy Mayeda:

As to the speed at which the bullet leaves the barrel (muzzle velocity), a REALLY SLOW bullet would do 500 mph (733 ft/s), like a light 38 Special target load or light 45 Auto.  The little old 22 Long Rifle high-velocity (“normal”) averages around 1200 ft/s (818 mph).  Typical 357 Magnum defense/police load leaves at about 1450 ft/s (988 mph), .30-06 Springfield at about 2900 ft/s (1977 mph), one of the new hot-rod varmint cartridges, the 204 Ruger has a factory load listed at 4225 ft/s (2880 mph).  As a rule, mainly older, lower-powered cartridges fire bullets with subsonic speeds, though the best competition-type 22 rimfire (22 Long Rifle) cartridges are also subsonic — avoids bullet experiencing turbulence that a supersonic projectile would encounter as it dropped to subsonic speed.  For maximum accuracy, it’s usually recommended to keep the projectile either supersonic or subsonic for the entire flight, rather than letting it drop through transition speeds.

One of the first photograps of the bullet in flight made by Peter Salcher with Ernest Mach in 1886

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Below I am reposting a rather long piece taken verbatim from the website of Steve Detweiler who just says that it’s an “amusing anecdote from a friend of mine.”  So, I’m not sure of the veracity of the story, and some claim that it’s an urban legend.  It may well be.  But it opened up some deep discussion on the PHYSLRNR email list, which I attempt to summarize below.

HEAVY BOOTS

About 6-7 years ago, I was in a philosophy class at the University of Wisconsin, Madison (good science/engineering school) and the teaching assistant was explaining Descartes.

He was trying to show how things don’t always happen the way we think they will and explained that, while a pen always falls when you drop it on Earth, it would just float away if you let go of it on the Moon. My jaw dropped a little. I blurted “What?!” Looking around the room, I saw that only my friend Mark and one other student looked confused by the TA’s statement. The other 17 people just looked at me like “What’s your problem?” “But a pen would fall if you dropped it on the Moon, just more slowly.” I protested.

“No it wouldn’t.” the TA explained calmly, “because you’re too far away from the Earth’s gravity.” Think. Think. Aha! “You saw the APOLLO astronauts walking around on the Moon, didn’t you?”

I countered, “why didn’t they float away?”

“Because they were wearing heavy boots.” he responded, as if this made perfect sense (remember, this is a Philosophy TA who’s had plenty of logic classes). By then I realized that we were each living in totally different worlds, and did not speak each others language, so I gave up.

As we left the room, my friend Mark was raging. “My God! How can all those people be so stupid?” I tried to be understanding. “Mark, they knew this stuff at one time, but it’s not part of their basic view of the world, so they’ve forgotten it. Most people could probably make the same mistake.”

To prove my point, we went back to our dorm room and began randomly selecting names from the campus phone book. We called about 30 people and asked each this question:

1. If you’re standing on the Moon holding a pen, and you let go, will it
a) float away,
b) float where it is,
or c) fall to the ground?

About 47 percent got this question correct. Of the ones who got it wrong, we asked the obvious follow-up question:

2. You’ve seen films of the APOLLO astronauts walking around on the Moon, why didn’t they fall off?

About 20 percent of the people changed their answer to the first question when they heard this one! But the most amazing part was that about half of them confidently answered, “Because they were wearing heavy boots.”

MORE ON THE BURNING QUESTION OF HEAVY BOOTS

I decided to settle this question once and for all. Therefore, I put two multiple choice questions on my Physics 111 test, after the study of elementary mechanics and gravity.

13. If you are standing on the Moon, and holding a rock, and you let it go, it will:
(a) float away
(b) float where it is
(c) move sideways
(d) fall to the ground
(e) none of the above

25. When the Apollo astronauts were on the Moon, they did not fall off because:
(a) the Earth’s gravity extends to the Moon
(b) the Moon has gravity
(c) they wore heavy boots
(d) they had safety ropes
(e) they had spiked shoes

The response showed some interesting patterns! The first question was generally of average difficulty, compared with the rest of the test: 57% got it right. The second question was easier: 73% got it right. So, we need more research to explain the people who got #25 right but did not get #13 right!

The second interesting point is that these questions proved to be excellent discriminators: that is, success on these two questions proved to be an extremely good predictor of overall success on the test. On the first question, 92% of those in the upper quarter of the test score got it right; only 20% of those in the bottom quarter did. They generally chose answers (a) or (b). On the second question, 97% in the upper quarter got it right and 33% in the lower quarter did. The big popular choice of this group was (c)…33% chose heavy boots, followed closely by safety ropes at 27%.

A telling comment on the issue of fairness in teaching elementary physics: Two students asked if I was going to continue asking them about things they had never studied in the class.

———————————

First off, here’s the physics.  Earth is not the only thing with gravity.  The moon exerts a gravitational force on things, but it just exerts less force, mostly because it’s just got less stuff.  Stuff attracts stuff, and so less stuff will attract other stuff less strongly.  If you drop a pen, it will fall slowly, because the acceleration due to gravity is weaker.  Earth is far away, but that doesn’t really matter — when you are on the surface of the moon, the gravitational attraction of the moon is stronger than that of the earth.  That’s why the astronauts could jump very high on the moon.  You would weigh less on the moon than you do on the earth.  If the astronauts jumped really really really hard, they could float away from the moon.  The same is true on the earth (but to jump that hard, you need rockets, and that’s what the space shuttle does).

So, here’s the discussion.  One instructor said that she had used similar questions in her class, and gotten similar results.  Many students thought the pen would float away.  One year, she asked them instead about a crescent wrench instead of the “apocryphal pen.”  They all answered that question correctly!  Another instructor, however, gave a similar set of questions to his class, and most of them answered correctly.  What’s he doing differently?

What’s the problem?  This question forces students to challenge a preconception that they had walking in the door — perhaps that “things float in space” or “heavy things get weighed down.”  Apparently the misconception that the moon has no gravitational attraction persists through most physics courses.  Even though they might be able to state that the moon has gravity (as evidenced by correct answering of the second question, as to why the astronauts stayed on the moon), they have trouble transferring that understanding to the “what happens when you drop a pen on the moon” question.  They are thinking, argued one instructor, in terms of the surface features of the problem (we’re on the moon!) rather than the underlying features (all chunks of matter have gravity).  Students transfer more when they’re interactively engaged in the material, says the research (e.g., Cognitive Development, 6, 449-468 (1991), Learning and Transfer: Instructional Conditions and Conceptual Change, Michelle Perry).

John Clement gave a few ideas for ways to address this misconception in class:

Given enough time you could propose a number of what-if questions which might help the TA understand what is going on.  Why did the rocket have to fire its engines to prevent a crash?  Why don’t rocks fly away from the moon?  What force pulled Apollo 13 around the Moon?  Whey when the astronaut dropped a feather and hammer did they both fall to the surface of the Moon? This last one has no heavy boots!!!

Another really important question is to ask why they think there is no gravitational attraction on the Moon.  A number of students will reply “because there is no air”.  The common misconception is think that “gravity” is due to the air pressing you down.  Or they may say because the Moon does not rotate, as this is another common misconception.  These are explained in the teachers manual for Minds on Physics, and students are asked questions
to bring out these misconceptions while building a coherent model of gravitational attraction.

So rather than attacking the “heavy boots” conception, the student has to internalize the model that there is (at least in classical mechanics) no threshold to the action of forces, and that unbalanced forces cause acceleration.  Then they have to apply it to a variety of cases, of course, along the way.  It helps to have them apply these conceptions to objects on other planets.  So blocks of wood in a water filled bowl all float at the same level on the Moon and the Earth, but springs supporting masses are stretched less on the Moon.

So the “heavy boots” is not the primary concern.  The concept of forces and acceleration are the primary concern.  Once the students have a firm model of forces, and of NTNs general gravitational law, the idea that things can float on the Moon will go away.

A few more comments that I liked:

Can the boots be heavy if the astronaut is not?  Are the boots heavier than the astronaut?  If not, do the boots weigh down the astronaut or does the astronaut way down the boots?  I think a few questions like this can make the logical inconsistency evident. (Jerry Touger)

But, countered Dave Van Domelen:

Actually, it goes along with ideas like blankets being intrinsically warm.  Qualities as properties of things, rather than the result of interactions.

And from John Clement, an idea I’d never heard before:

This comes from the concept that “gravity” exhibits a threshold effect.  You have to have enough of it to be pulled down, otherwise you float.

Which, pointed out a discussant, suggests that students are using buoyancy as an analogy — if you’re heavy enough you sink, if you’re light enough you float.  Or, perhaps, friction is the correct model — there is a threshold at which the force becomes effective.

Of course, trying to address these misconceptions as “problems” to be plucked out of the students minds won’t work.  They’re using these ideas because they fit with their experience of the world.  Trying to understand their underlying conceptions (without perjoratively labeling them as misconceptions) and working from there, will be most productive.  Dewey Dykstra has written quite a bit about this, and you can see my previous post on that.

Other resources:

• Minds on Physics (vol 4) has a good section on moon/earth comparisons
• An entire thesis was written on the “heavy boots” problem
• Chapter 4 (p 44-46) of another thesis also deals with this problem
• David Hammer, More Than Misconceptions” published in AJP in 1996.

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I came across a new science blog recently – A Schooner of Science — and really enjoyed Sarah’s fresh and funny writing style about all sorts of things that this blog doesn’t tend to cover — namely, biology and chemistry.  (I write about them when I can, but, well, it does all come down to physics, after all, right?  Right?).

Sarah cheerfully agreed to cross-post one of her recent articles in lemmings (yay lemmings!) and whether it’s really true that lemmings throw themselves off cliffs to reduce their population.  Enjoy, and check out a Schooner for more stuff (most recent posts — 5 most remarkable animal penises!).

——————————

Nothing like going retro and playing a game or fifty of LEMMINGS!!! I squandered many an evening in my youth curled over a computer screen, thinking of ways to keep those suicidal wanderers alive while they walked carelessly towards cliffs.

Some of the levels were so bizzarro I’d wonder “how did they get themselves INTO this mess? Do they WANT to die? Cute little green-haired munchkins!” For fun, here’s a real lemming – still cute but in a more hamsterish way:

At some point I was introduced to the idea that yes, lemmings DID want to die because of a brilliant population-control technique ingrained in their DNA. I’ve had many a teacher who have lamented the human desire to be fruitful and mulitply, crying that we were destroying the planet out of sheer numbers. Whether we would continue to survive would depend on whether we realised that space was running out and slowed down… otherwise we’d bang ourselves straight into extinction. Are we an r-selected species, or a K-selected species, is our growth rate exponential, or are we just midway up an S-curve? Whenever I read that the suicide rate was increasing, I would think to myself – maybe we ARE like the lemmings. Maybe this is all some genetic grand-scheme to thin out our numbers.
I don’t think that anymore, doesn’t it sound a bit ridiculous that you can have so many animals around that they disappear? It sounds a bit like Jimbo Kern from Southpark – “We’ve got to kill more animals so they won’t die!”
And the lemming thing – it’s all a LIE!!! Those lemmings didn’t kill themselves. The poor little rodents were chased off a cliff for a Disney movie. This Disney movie in fact, White Wilderness.

Hell, these lemmings weren’t even migrating and accidentally fell into the sea, or thought there was something worth swimming to on the other side of the ocean. Nope, they were pushed off the edge by film makers just to get the right shot. Lemmings aren’t even native to Alberta, Canada, where the footage was taken – they bought them from Inuit kids! So consider this myth debunked. It wouldn’t be the last time television pushed animals into killing themselves either – consider Happy Tree Friends.

If you want to play the game again and relive the lemmings good times, do it here Love the hair.

————————-

Originally posted on a Schooner of Science here.  See more of the Captain’s scientific musings over on her Schooner.

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Here’s today’s science classroom activity.  We’re surrounded by the crushing weight of layers of atmosphere above us, but we don’t feel it.  Why?  Our perception is tuned to differences, not absolutes.  If we were in a completely pink world, we would notice anything that wasn’t pink, but (I’m pretty sure) after a few minutes, we would become blind to pink itself, just like you don’t hear the noise of a fan in the room until it stops.

Similarly (though through different mechanisms), we’re not constantly aware of the intense pressure pushing in on our bodies.  (Would we really turn into mush in a vacuum?  No… read more about the effects of a vacuum on the human body here).  Thank goodness, because it’s quite startling.  I know, because I’ve felt it, in this wonderful science classroom experiment.

All you need is a big trash bag and an industrial strength vacuum cleaner, and a willing victim (er, “faithful subject of science.”) The victim (aka “subject) gets inside the bag, and once you suck all the air out of the bag with the vacuum cleaner, they’ll feel an intense pressure.  SAFETY FIRST!  Read this PDF writeup of the activity (from the Exploratorium’s Eric Muller) for all the ins-and-outs and safety factors in doing this with your kids.  (Words to the wise — don’t put your head inside the bag!)  It’s stunning — try it if you can.

Why do we feel this pressure?  Stop and think about it a moment.  What changed when we sucked the air out of the bag?  There’s the same atmospheric pressure outside the bag (14.7 PSI at sea level), that didn’t change, there’s still the weight of the atmosphere pressing down on you.  What changed is the pressure inside the bag.  What does that have to do with anything?

The high pressure outside the bag pushes the bag’s surface against your body, and the bag stretches against your skin.  We feel this stretching of the bag as it pushes on our skin and the little hairs on our body.  We don’t feel the pressure without the bag, because though the air pushes against our skin, it pushes the same in all directions.  The bag lets us feel what is already there — the weight of the air!

You can extend this activity a bit by measuring the pressure inside the bag (at the Exploratorium it was 1 PSI). Eric says:

Paul Doherty and I used a barometer watch to measure the pressure inside the bag when doing research on this activity. Some one that goes to the mountains a lot might be able to loan you one or you can buy one. You can also just get a barometer.  I found a bunch for sale on eBay. Lastly, you can make a home-made barometer.  If you do a Google search, there are a variety of easy to make barometer designs (but you still might need a good barometer to calibrate your homemade one)

Paul D. has a similar activity to let you feel the pressure in water with a plastic bag.  Stick your hand in a pail of water.  You don’t feel any pressure.  But stick your hand in a plastic bag and stick it in the water, and you’ll feel an intense pressure (that gets stronger with depth) as you put your hand in the bag.  Paul D. explains:

Why do you need the glove or the bag?
Human sensors detect differences or changes in a signal. When you stick your ungloved hand into the water the water exerts a uniform force on your hand. It flows around every hair and every wrinkle in your skin. Now a single hair is bent to the side. When this happens you cannot feel the pressure exerted by the water.

However when you wear the bag or the glove they will bend down the hairs on your hand, and the glove and the bag may have folds that exert uneven forces on your skin. So that you can “feel” the force exerted by the water.

Cocktail Party Physics has a great old post on the history of measuring pressure (fascinating stuff, really).  And Eric has more activities here.

A few relevant science toys from Arbor Scientific

One of Paul Hewitt’s favorite demos to show that suction cups stick because of the air pressure pushing (rather than a “sucking force”) is this atmospheric pressure mat.  You can lift a whole lab stool once you stick this down on it.  A similar but smaller version is these atmospheric pressure cups.

A vacuum chamber/pump will let you reduce the pressure on anything (they suggest marshmallows), explore gas laws, etc.    See the website for some example class activities. They also have a class set.

They’ve also got a durable hand-held vacuum pump for expelling air from any other kind of container.  (Good for the “coin and feather fall at same rate in vacuum” kind of demo).

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I’ve got a new podcast series out, and this one is my best yet.  I’ve been hired by the wonderful folks at the National Science Digital Library (they provide a central depot for great digital media for teaching science) to create a podcast for elementary educators on using polar science in their teaching.  We (me and my co-conspirator, the multi-talented Robert Payo) focus on misconceptions and myths about science and how to address them with science from the poles. This is part of the Beyond Penguins and Polar Bears webzine.

Visit the Beyond Penguins and Polar Bears Podcast!

So far we’ve done a show about the geology and volcanoes of the poles (and an Earthwalk with my old colleague Eric Muller of the Exploratorium), another one on ancient polar mammals, and the most recent (my favorite so far) on birds of the poles and how to use birdwatching to do real science.  These are all pretty short, 10-15 minutes. Here’s a direct link to the birds episode. I enjoyed this one in particular because I got to play so much with ambient sound and creating soundscapes, and my friend Tom has the *cutest* 10-year old neighbor who can deliver a script like nobody’s business.  Fun, fun storytelling.  And Jennifer Fee is awesome.  (And she’d read sciencegeekgirl before I called her up for the piece!)

To find out more and listen to the Birds episode, go here!

To go to the iTunes U site with all NSDL podcasts go here.

To see all my podcast series, go here.

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In keeping with my previous post on the International Year of Astronomy, this week’s 5-minute  Science Teaching Tips podcast is about our perception and the size of the moon.  What coin would just barely cover the full moon? You may be surprised. TI director (and recovering astrophysicist) Linda Shore explains how our brains distort the actual size of the moon. Listen to the full podcast — When the Moon Hits Your Eye.

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Satellite composite image of Western Hemisphere from NASA

Ooh ooh ooh, Bad Astronomy posted (a while ago) a fabulous list of Ten Things You Didn’t Know about the Earth. If you dig my science myths, check this one out. Such gems as “The earth is smoother than a billiard ball,” “Destroying the earth is hard,” and “Mt. Everest isn’t the biggest mountain.”

See also his earlier Ten Things You Didn’t Know about the Milky Way.

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Just got this from Bob Park’s What’s New column. Looks like Gingko has failed a double-blind study to see if it really improves memory. I’ve been taking it for a while, in hopes that it would defuzz my neuronal connections (I’m not that old, but my memory took a real hit ever since I was on crazy antimalarial drugs in Peace Corps 10 years ago).

This reminds me of when a friend told me that there’s no reason why Airborne would improve your immune system. I was really angry at him for telling me this. Airborne definitely seems to keep my colds from getting too severe. If that’s due to a placebo effect, then hearing scientific reasoning that it shouldn’t work will destroy my placebo effect. Especially since my belief structures are particularly sensitive to scientific evidence.

Here is what Bob Parks wrote. I’m a bit perturbed by what he writes at the end, that all these other remedies have failed double-blind tests. It sounds to me as if he expected this to happen, because herbal remedies are by their very nature “unscientific” or something. I don’t see why some of these “natural” remedies couldn’t have something to them. After all, we take Zinc to help our immune system. That’s just a mineral. What makes a mineral less “woo woo” than a plant (like Echinacea)?

3. GINKGO BILOBA: A TIP ON WHERE YOU CAN CUT EXPENSES.
Annual sales of the herbal remedy Ginkgo biloba in the US are at $249
million. It is alleged to prevent memory loss. It doesn’t. In its
first large trial, half of 3,069 volunteers 75 and older were given of
Ginkgo biloba daily, while the other half were given a placebo. They were
assessed for signs of dementia every six months for 6 years. Neither the
patients nor the doctors doing the assessment knew which group patients
were in. The group getting the placebo actually did slightly better,
although the difference was not statistically significant. France is
planning an even larger study. Ginkgo has a lot of company. One after
another, the most popular herbal supplements, ephedra, Echinacea, St.
John’s Wort, have failed in double-blind, placebo controlled studies.

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Molten glass - Mark Interrante - from NY Times

This is an addendum to my earlier (and popular) post about whether or not glass is a liquid.  If you haven’t read the previous post, the crux of the myth is that many of us are taught in science class that glass is a veeerry slow flowing liquid, and that’s why old windows are thicker on the bottom than the top.  News flash — actually you can find old windows that are thicker on the top than the bottom because it’s just an artifact of how glass windows were poured back in ye olde days.  But that doesn’t mean that glass is a simple thing.

A few months ago there was a great article in the New York Times Science section called “The Nature of Glass remains Anything But Clear“.  This very nice article about glass talks about how — even if it’s not a liquid — it’s a pretty complicated thing. “The arrangement of atoms and molecules in glass is actually indistinguishable from a liquid,” it says Solids tend to have atoms arranged in nice little tinker-toy stacks, whereas atoms in liquids aren’t so organized, more like someone threw the tinker toys across the floor in a rage, which is why they can flow.  The atoms in glass are more jumbled than organized.  So how come glass is so strikingly hard if its atoms don’t have a rigid order?

From the article:

“When cooled, a liquid either freezes, as water does into ice, or it does not freeze and forms a glass instead. In freezing… the molecules line up next to and on top of one another in a simple, neat crystal pattern. When a liquid solidifies into a glass, this organized stacking is nowhere to be found. Instead the molecules just move slower and slower and slower, until they are effectively not moving at all, trapped in a strange state between liquid and solid. .. This glass transition does not occur at a single, well-defined temperature; the slower the cooling, the lower the transition temperature. … By contrast, water, cooled quickly or cooled slowly, consistently crystalizes to the same ice structure at 32 degrees Fahrenheit.”

The reason glass forms is still a hot topic, with many competing theories.

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There have been several posts around the blogosphere of late regarding a report from journalist Steven Goddard that the arctic sea ice isn’t melting as quickly as we thought. In particular he was calling into question the validity of the data reported from the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado — I’ve included that graph below.

However, his analysis was not well-founded, and he’s since admitted his mistake. The Island of Doubt has posted a nice summary of what was wrong with his arguments. They write:

Goddard’s article is rife with scientific errors and evidence of his lack of familiarity with the science. His main argument, that the ice area up there is 30% larger than last year, not just 10%, is the product of the fact that Goddard based his story on his own analysis of images from the NSIDC and other sources. That analysis… consisted entirely of counting white pixels…. It turns out that Goddard got confused because he didn’t take into account map-projection distortion differences between competing images.

Once that little problem is dispensed with, it turns out that there is no discrepancy, the arctic is melting faster than normal, and may yet break last year’s record. Or not. Even if Goddard had been right, though, that says nothing about long-term trends. The point is, as Goddard proved, if you’re going to argue that an entire field of scientists got it wrong, you really should know something about the subject.

To Goddard’s credit, though, he admitted his mistake.

Sadly, the story has already started to make its way around the internet. So, just like myths like polar bear fur being a fiber optic (it’s not), or cats which grow wings (they don’t) it may be hard to get this one to go away. Why is it so much easier to spread rumors that something false is true than to fix the problem by telling people that something they think is true is actually false?  It’s made worse by the fact that some folks want to have fodder to fuel denialist claims, so they don’t have a lot of reason to correct erroneous information.

Deltoid also blogs about Goddard’s article here.

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