Innokenti asks

 Why are observatories dome-shaped? They house a telescope… but why the dome? The things can swivel around and stuff without being encased in a dome. So what purpose does it serve?

 Short answer: they don’t have to be. Behold the Very Large Telescope array at the Paranal observatory, which is decidedly un-domey. The dome shape is just a convenient one that happens to get used a lot.

 Long answer: The dome superstructure exists to protect the telescope from the elements. Observatory telescopes contain incredibly sensitive optics, but they also tend to be build in rather remote, inhospitable places (the middle of the desert, on top of a mountain, on top of a mountain in the middle of the desert etc.) which can suffer somewhat inclement weather from time to time. Some kind of superstructure is necessary if you actually want your telescope to survive. However, the superstructure also has to be built in such a way that it allows the telescope to rotate through a full 360 degrees and to elevate to an altitude of up to 90 degrees from the horizontal in order to observe as much of the sky as possible, while also retaining the ability to open and close and protect the telescope when it is not in use.

 Now, obviously you don’t have to use a dome to achieve all this, but unless your observatory has some sort of special purpose that requires other factors to be taken into consideration a dome-shaped superstructure is rather attractive because it’s the most lightweight one that will do the job. The heavier your dome is the more difficult it is to shift it round to look at different points in the night sky, and so a dome shape was very popular before we mechanised the whole shebang. These days it’s more a matter of tradition than anything else; there’s no real reason to use domes any more, but there’s no real reason not to either, and observatories are now inextricably linked with the dome shape in the public psyche. PR is more important to large-scale scientific endeavours than you might think, which is why having an observatory that looks just like people think it should do is quite handy. And of course for low-budget observatories where cost and weight are still an issue the dome shape is still the best available: easy to source, easy to install, easy to use.

Baroness Josh asks

http://www.bbc.co.uk/news/science-environment-18803212

How is there a moon we hadn’t noticed yet? How do we expect to find habitable extrasolar planets if we can’t even lock down our own back yard? Just how much thumb-sucking goes on in astrophysics anyway?

 This is where I start wishing I’d figured out a way to write that post on the Kuiper Belt that didn’t make me sound like a lunatic. Well, more of a lunatic, anyway.

 Right, first off, calling this thing a moon is a little bit generous and the only reason we’re doing so is because we lack the proper astronomical terminology to draw distinctions between the Hubble Space Telescope and the actual Moon. According to the technical definition they’re both moons of Earth, even though there’s a fairly considerable size difference between the two. This new moon they’ve found around Pluto is “between 10 and 25 km in diameter” which is tiny when compared to the vast majority of solar system moons and more directly comparable to Phobos and Deimos, the two asteroid-like moons which were captured by Martian gravity at some point in the distant past. Considering this thing is about 27 times further away than those are – as well as likely having roughly the same reflectivity as a piece of road asphalt – I’m amazed the Hubble picked it up at all.

 Second, if you’ll recall my post on exoplanets, the vast majority of exoplanets aren’t found by direct observation but rather through techniques involving indirect measurements of the parent star such as radial velocity and the star’s light curve. Why didn’t we use those techniques to pick this new moon up? It’s really small, meaning any gravitational force it exerts on Pluto will also be really small, and Pluto is now part of a five-body system to boot which will ensure any gravitational tugs it does manage to exert will be lost in the noise created by the other three moons, which are much larger. The two situations aren’t remotely comparable.

 Third, don’t expect things to stop there. Extensive computer modelling and laboratory experiments (if you really want references on this ask in the comments and I will, after some grumbling, go and track down the relevant papers) has shown that Pluto and the other objects within the Kuiper belt are subject to something called collisional evolution. What this means is that the Kuiper belt could be densely populated enough that collisions between Kuiper belt objects are relatively common occurances, and so depending on how fast the impacting body is going a collision will either result in the two bodies “sticking” together and agglomerating into a single new body, or else blasting both bodies apart into a collection of smaller objects. That Pluto has so many “moons” bound into what appear to be stable orbits with it indicates that it was a victim of the latter as it’s highly unlikely it would be able to capture all of these things so neatly on its own. At some point in Pluto’s past, then, it was probably struck by something that blasted off a chunk of Pluto’s mass. Some of it probably escaped. Some of it probably fell back onto Pluto. And some of it formed the moons we see today. It wouldn’t surprise me if, when the New Horizons probe gets there in 2015, it manages to find a couple more moons like this one orbiting Pluto. As far as I’m concerned this further cements Pluto’s position as something separate from a planet or a moon; it’s in a class of its own, and “dwarf planet” is a hilariously inadequate term for describing what that class is.

The post The Quickfire Round. appeared first on The Scientific Gamer.

RRRrrrrrssssssssssnnnnnncccccccceeee.

I’m sorry, I was doing an impression of a resonating object there, mainly because I feel that starting with a description of what resonance is would be pointless. Everyone knows what “to resonate” means, and as far as physical principles goes all schoolchildren get taught about Galileo Galilei and oscillating pendulums when they’re doing their GCSEs. Basically if you have something undergoing a persistent, periodic motion, then it will transfer and store energy with more efficiency at certain specific frequencies resulting in a greater amplitude of motion. Most people understand this in the context of pendulums, as above – where, if you release a pendulum and let it settle for a few seconds, it will eventually swing with a period 

where l is the length of the pendulum and g is the pendulum’s acceleration due to gravity.

In a constant gravitational field – say, at the surface of the Earth where human beings tend to congregate – g will be a constant 9.81 ms^-1, making the period T entirely dependent on the length of the pendulum l. The frequency is just the number of oscillations per second, and so when the pendulum swings at a period T we can find its frequency 1/T. This is the pendulum’s resonant frequency; the frequency of oscillation at which it will produce the largest oscillation for the smallest energy outlay. Since it loses only a tiny amount of energy with each swing (and that only because the point where the pendulum is attached to whatever it’s swinging from will not be frictionless, and if it’s swinging in an atmosphere you’ve also got the question of drag to consider) the pendulum will swing with that period for a long, long time – hence why pendulums “prefer” to swing at this period T determined by their length: it’s the most efficient way they can swing.

So resonance is motion where an object oscillates/vibrates in a way which ensures the maximum possible motion for the minimum possible energy outlay. Objects can have many different resonant frequencies (which are an integer multiple of the fundamental frequency) and resonance is crucial to the real world operation of many things – opera singers breaking wine glasses by vibrating their vocal chords at the glass’s resonant frequency; tuning forks which, when struck, will quickly settle down to resonating at a certain set pitch governed by their resonant frequency; and string instruments like pianos which rely on hammers hitting steel strings of different lengths held under tension which then vibrate at their resonant frequency, producing a single tone depending on the length of the particular string. It can also lead to some amusing mishaps when applied to large-scale engineering projects, like this one:

Now, there are differing views on whether Tacoma Narrows really was an example of resonance. My undergraduate physics textbook would have me believe that it was, while Font Of All Knowledge Wikipedia says that it was not. However, since there are recorded examples of bridges which have collapsed due to resonance from, e.g., army formations marching over them in lockstep, and that as a result of this resonance is one of the things engineers have to account for when building bridges these days, it’s as good an example as any to illustrate that minor, repetitive perturbations of an object can have a very powerful effect on that object if they happen to match one of its resonant frequencies thanks to the efficient transfer and storage of the object’s energy. This holds true even when those minor, repetitive perturbations are one planet gravitationally interacting with another planet as it orbits around the sun.

Here is an example of orbital resonance:

This is the 4:2:1 resonance between Ganymede, Europa and Io. (You can think of those numbers as integer numbers of some fundamental frequency that governs orbits, as with mechanical resonance.)  As you can see, Europa orbits Jupiter twice for every orbit Ganymede makes, while Io orbits four times. This is called a mean motion orbital resonance because the ratios of the orbits of the moons are simple integers. Every time two of the moons are on the same side of the planet their mutual gravitational pull is greatly increased since they are closer together, with potentially catastrophic consequences.

Now, you may be thinking that it is something of a massive coincidence that Ganymede, Europa and Io just happen to have orbits that interlock so perfectly with one another, but it’s the same sort of massive coincidence that has led to you sitting here today reading the words that I have written. In other words, the Galilean moons did not just pop into existence in those positions and with those orbital resonances; they instead evolved over time until they reached their current stable, self-correcting orbits. We think of the Solar System as being immutable and unchanging, but that’s only over human timescales. Over million- and billion-year timescales the orbits of the planets and moons themselves change; this is either because they were not stable in the first place, or else because they are exchanging angular momentum with other, smaller bodies such as asteroids and comets (this will be important next week).  For example, there’s a laser reflector on the Moon that the Apollo astronauts left behind that scientists use to very, very accurately measure how far the Moon is from the Earth. What they found was that the Moon is gradually slipping away from the Earth at the rate of 4 cm per year – this may not sound like much, but over millions of years it all adds up¹. The orbits of the planets have similarly evolved and continue to evolve; as the Sun converts mass into radiative energy its gravitational hold on the planets will slacken and diminish, leading to them migrating further outwards in their orbits². And the same sort of thing is true of Io, Ganymede and Europa, which started closer in to Jupiter and migrated outwards until they reached their current arrangement, at which point their positions became self-perpetuating thanks to the orbital resonance.

The 4:2:1 Galilean resonance is an example of a stable resonance that resists any change in the orbits of its component moons, but not all orbital resonances are like this. Many orbital resonances are destabilising resonances, where the repeated gravitational perturbation of having two bodies in close proximity during their orbits moves one of them out of its stable orbit and ejects it from the Solar System. The best way to think about this is by imagining the gravitational force between the two bodies as an impossibly long rope strung between them. Usually the rope is just taut and under tension, but whenever the two bodies pass close by to each other the big one gives an almighty tug on the rope, yanking the smaller one towards it. Over thousands of orbits these repeated tugs on the gravitational rope can drag the smaller body off of its orbital path and from that point on it’s bye-bye, Solar System.

You’ve probably seen the effects of resonance, even though you might not have known it at the time. The most famous example is the rings of Saturn, the structure of which is a product of both stabilising and destabilising resonances. The gaps in the rings are caused by a number of different things, and one of these things is a destabilising resonance with certain of Saturn’s moons that are orbiting in phase with that portion of the rings³. On the other hand some of the rings exist in orbital regions where they should have dissipated long ago, and this is thanks to stabilising resonances with – again — large moons such as Titan. Other not so obvious examples of resonance in the Solar System include the Kirkwood gaps in the asteroid belt, which are governed by mean motion resonances with Jupiter. And finally there’s Pluto, which exists in a 2:3 resonance with Neptune.

Now, normally Pluto could never exist where it does. It has an orbital path with a high eccentricity which crosses that of Neptune (again, this should have been a big hint that Pluto Isn’t A Planet). If it’s going that close to the orbital path of one of the gas giants it should have been booted out of the Solar System a long time ago. However, Pluto survives thanks to its 2:3 resonance; any time it gets close enough to Neptune that it starts to become a hazard, Neptune gives it a gravitational kick that speeds it up and shifts it to a wider orbit further away from Neptune. Conversely, any time Pluto looks like it might be getting far enough away from Neptune that it will escape this resonance, Neptune gives it another gravitational kick from the opposite direction that slows it down and corrects its orbit. So while the orbit of Pluto is constantly shifting over time thanks to interactions with Neptune, these interactions and orbital shifts cancel each other out leading to Pluto existing in this stable 2:3 resonance.

(This is illustrated with a pretty animated gif on this website if you’re having trouble visualising exactly what is going on. The orbital paths are plotted with respect to Neptune which is stationary from the gif’s point of view, so Neptune is the blue dot which never moves. The orbit of Pluto is in pink, with the Neptune-crossing part of its orbit represented as the loop, or “twist”. As you can see, any time this dangerous part of its orbit looks like it might get too close to Neptune – which would be fatal for Pluto – the gravitational interactions of the 2:3 resonance “corrects” Pluto’s orbit and shunts it back in the opposite direction.)

And that’s orbital resonance. Nearly all the examples of resonance we see in the Solar System today are stabilising resonances; destabilising resonances are generally indicated by the absence of stuff where stuff should be as per the Kirkwood gaps. With Jupiter and the other gas giants being as large as they are their orbital resonances have had a very significant effect on the way the Solar System has formed and evolved over time, and particularly so in the case of Pluto and the other TNOs that form the Kuiper Belt, the Scattered Disc and the Oort Cloud. Which was kind of the point in me explaining resonance in the first place: I cannot adequately describe those areas without explaining why they are structured the way they are, and I cannot do that without talking about resonance. HOORAY.

1. The reason for this is tides, but not in the way you might think. Every time the Moon causes a high tide it creates a slight bulge in the ocean. This bulge is situated a little bit ahead of the Earth-Moon axis thanks to the rotation of the Earth, causing an uneven gravitational force that drags the Moon forward very, very slightly. The upshot of all this is that the Earth is losing rotational momentum to the Moon, which is converting it to orbital momentum and moving further away from the Earth. Wikipedia tells me that this interaction would in theory go on for another fifty billion years until the Earth and Moon become spin-locked in – what else – a resonance, but that the Moon will never get that far because the Sun is due to increase in size and luminosity and vaporise the Earth’s oceans 2.1 billion years from now. So that’s nice, I guess.

2. This, incidentally, is why nobody has any goddamn idea what is going to happen to the Earth when the Sun gets big and strong and turns into a red giant.

3. Other gaps are caused by moons orbiting inside the rings themselves, while the remainder are unexplained at this point in time.

The post Resonating Resonances! appeared first on The Scientific Gamer.

The K.B. stands for Kuiper Belt. SCIENTIST HUMOUR.

I picked up a compendium of H.P. Lovecraft’s short stories a couple of weeks ago – mainly so that I could amuse myself by playing Lovecraft Bingo (counting how many times he uses words like ‘accursed’, ‘pnakotic’ and ‘squamous’ ) – and ended up being rather amused when I came across the story “The Whisperer in Darkness”. This consists of about sixty pages of the usual confused rambling while the reader waits for him to get to the damn point already, but ultimately turns out to be about advanced aliens from Pluto who have set up a mining colony in the furthest reaches of the American hills that they’re trying to keep secret. This brought a wry smile to my lips for two reasons:

1. Lovecraft’s shameless attempt to piggyback off of something that was very much part of the public zeitgeist at the time, what with him starting this story the same month that Pluto was discovered. It’s the literary equivalent of somebody sticking dark matter into their awful sci-fi novel because they think it sounds exotic and dangerous.
2.  That there was ever a time when people thought Pluto was an important place that might support life of any kind.

 The whole Pluto “controversy” a few years ago stems from a variety of roots: a bureaucracy which ran a sloppy vote, scientists with vested interests and pet theories on both sides of the argument, and –  bizarrely – a great deal of public sentiment for what is essentially a lump of ice and rock out in the middle of nowhere. If you want to trace it back to the beginning, though, you have to start with the discovery of Pluto in 1930. Are you sitting comfortably? Time to Listen with Mother.

The best picture of Pluto currently available, which combines HST imaging data with a surface map provided by studying the light changes caused by Charon as it passed in front of Pluto.

Once upon a time there was a man called Alexis Bouvard.  Bouvard was engaged in the tedious task of compiling astronomical tables predicting and describing the orbits of the outer gas giants according to Newtonian mechanics, but he hit a snag when he came to Uranus: it wasn’t orbiting the way Newtonian mechanics said it should. There were only two possible explanations for this: either Newtonian mechanics were wrong (unthinkable at the time, although this eventually turned out to be the case) or else there was some unknown eighth planet orbiting beyond Uranus exerting a gravitational force that was interfering with its orbit. The matter then passed to Urbain le Verrier, a specialist in celestial mechanics, who cranked through some feverish calulations based on nothing more than the minor perturbations in Uranus’s orbit and came up with a predicted position for the hypothetical eighth planet. He mailed his prediction to the Berlin observatory, who duly pointed their telescopes at the appropriate piece of sky and wound up discovering Neptune in 1846.

 This was an astonishing achievement for le Verrier, but all was not hunky dory in astronomy land. After some examination of Neptune it was determined that the new planet was, on its own, not enough to account for the irregularities in the orbit of Uranus. This led astronomers to posit the existence of a ninth planet, which they eventually dubbed Planet X.

Fifty years went by. Percival Lowell spent a decade looking for Planet X, a search which ended with his death in 1916. The search stalled for another thirteen years while a legal battle over Lowell’s bequest to his observatory was wrung out, before being handed off to the 23-year old Clyde Tombaugh in 1929. Tombaugh spent the next year examining pairs of photographic plates; each pair of plates was a picture of the same portion of the night sky taken several days apart, and Tombaugh’s job was to pretty much to see if he could spot the difference. If he saw something on the second plate which had moved from its position on the first plate, he had a candidate for Planet X. This was tedious work involving the painstaking examination of hundreds and hundreds of plates, but it eventually paid off and he discovered his candidate body in January 1930.

This is probably pretty hard to spot when you've been looking at similar plates for sixteen hours straight.

Everything up till now has been Good Science, but this is where the Bad Science starts. Telescope technology in 1930 wasn’t that great. The new “planet” — named Pluto by an English schoolgirl – appeared as a point of light to astronomers rather than a distinct disc like the other planets. This could mean any or all of a number of things:

a)      It wasn’t very big.

b)      It was a very very long way away.

c)      It wasn’t reflecting much sunlight (it had a low surface albedo).

However, because astronomers thought Pluto must be responsible for the additional perturbations in the orbit of Uranus it followed that it had to have enough mass to exert the level of gravitational force required to do so. This led them to discount point a) and estimate the size of Pluto as being about 8,000 km in diameter – or just a bit larger than Mars. In order for Pluto to be this big it would need a very, very low albedo to account for such a large object reflecting such a tiny amount of sunlight, and so astronomers decided it was about as reflective as a lump of road asphalt. Despite a vocal minority proclaiming that these astronomers were full of shit and that Pluto’s orbital eccentricity and brightness meant it had far more in common with comets than it did with planets, the size estimate and the planet title stuck.

Time continued to pass. Pluto’s size was revised further and further downwards as telescope technology improved and more accurate observations were made. Eventually Pluto’s “moon”, Charon, was discovered in 1978. This allowed astronomers to measure the total mass of the Pluto-Charon system based on their mutual gravitational effect, which turned out to be about five hundred times smaller than that of the Earth. After Charon serendipitously made a series of planar transits across Pluto from 1985-1990 their respective sizes were worked out as well*. Pluto has a volume less than one two-hundredth that of the Earth. It is tiny.

So Pluto was now known to be unlike any other planet in the Solar System. Fortunately (for Pluto) nothing else had yet been found that remotely resembled Pluto, so Pluto remained a planet simply through dint of having been called one so long that nobody could think of a good reason not to. Nothing lasts forever, though, and Pluto should have started looking over its shoulder in 1992 when the catchily-named (15760) 1992 QB1 was discovered. This was the first confirmed occupant of the long-posited region of space called the Kuiper Belt, a population of icy bodies outside the orbit of Neptune which are left over from the formation of the Solar System. I’ll be posting more on that later, but after the Kuiper Belt was discovered and more people started combing it for interesting objects it was only a matter of time before somebody found something in it that was decidedly Pluto-like. Quaoar, Haumea, Ixion and Varuna were all discovered in the period between 2000 and 2005, and all of them had sizes comparable to – but not exceeding – that of Pluto (between 500 km and 1,200 km diameter; Pluto has a diameter of 2,000 km). The straw that broke the camel’s back was the discovery of Eris in 2005; this had a diameter of 2,400 km, making it some 25% larger than Pluto.

A mock-up of large TNOs shamelessly thieved from Wikipedia.

Eris really brought things to a head. After the size downgrades certain segments of the astronomical community had been complaining for years that Pluto wasn’t a planet at all, but Eris presented the IAU with an outright logical conundrum. Scientists love consistency. To call Pluto a planet would also mean calling Eris a planet. But if they called Eris a planet there’d be no good reason not to call the other four planets as well – they were all orbiting in the same area of the solar system and they were all of comparable size, so why not? And – even worse – it was almost certain that astronomers would go on finding these big Pluto-like objects as more people looked with better instruments. Do they call all of them planets? If they did, it’d be entirely possible that in a hundred years time we’d end up with a list of planets that was 100-200 items long.

 And so the IAU conference in 2006 to decide what exactly constituted a planet and what didn’t. Exactly what happened at that conference I do not know and there are some scientists (notably Alan Stern, the principal investigator behind the New Horizons probe that’ll be arriving at Pluto in 2015) who are still very bitter over its result, which was that the largest of the Pluto-like bodies (the ones with the most accurate size measurements) were to be placed in a new category of objects called “dwarf planets”. Further, Pluto itself was to be demoted from planet status and put in the category of dwarf planet.

 Now, you can argue all you want about how good the new definitions for dwarf planets and planets are is. There’s definitely an argument to be had since the two defining criteria of a planet are now that it is “is in hydrostatic equilibrium” and “has cleared the immediate area around its orbit”, but nobody really knows what the hell the second one means given that a strict reading of it leads to us discounting Earth as a planet given the existence of 3753 Cruithne and other similar bodies, not to mention the Jupiter Trojans. However, what is clear to me after studying the Kuiper belt for most of the last five years is that Pluto is rather obviously a member of this discrete population of objects and not a planet in its own right. It formed with the belt, it evolved with the belt and it’ll die with the belt. Pluto might or might not be a dwarf planet, but what it definitely isn’t is a planet.

These people are being ironic. At least I hope they're being ironic.

 Which is why the outrage over Pluto’s demotion eluded — and continues to elude – me. Even I get a little bit unsettled when I run into schoolkids who have been taught from day one that Pluto is a dwarf planet and nothing else, but the scientific argument for it no longer being a planet is irrefutable. When respected scientists like Alan Stern come out with something like

“Firstly, it is impossible and contrived to put a dividing line between dwarf planets and planets. It’s as if we declared people not people for some arbitrary reason, like ‘they tend to live in groups’.”

My response is an entirely predictable collection of four-letter words. It is not in the least bit contrived to draw a distinction between planets and dwarf planets, just as it is not contrived to draw a distinction between adults and children, or between gas giants and terrestrial planets, We called Ceres an asteroid for two hundred years simply because we had nothing else to call it, even though it was demonstrably different from everything else in the asteroid belt, and we didn’t start calling it a planet when we figured out it was round. Pluto is demonstrably different from a comet, or a small Kuiper Belt Object (they’re pretty much the same thing), but that doesn’t mean it automatically qualifies as a planet since it’s demonstrably different from those as well. So if you ask me, the classification of dwarf planet works quite well even if the definition does not.

But then the media and the public came up with far more spurious reasons as to why Pluto shouldn’t be demoted and should instead be “grandfathered” in as a planet, as if science worked that way. It’s been a planet for seventy-five years! It’ll upset schoolkids who will have to find a new mnemonic to remember the planet names! We’ll have to rewrite all the textbooks! Cry me a river, guys; nobody discounted tectonic plate theory just because it would have been a pain in the ass to change our scientific paradigm of the interior of the earth. There’s always cultural resistance to a new idea but it still surprised me in this case because they were kicking up a fuss over basically nothing. Pluto is the same object discovered by Clyde Tombaugh eighty-two years ago. We just call it something different now, that’s all.

Oh, and I nearly forgot to mention: those peturbations in the orbit of Uranus that started this whole mess? When the Voyager 2 probe flew by Neptune in 1989 astronomers were able to use its orbital trajectory to get a more accurate measurement of Neptune’s mass, which turned out to be 0.5% less than previously thought. This meant that Neptune on its own was now enough to account for the variations in the orbit of Uranus, and that ultimately the entire Pluto = planet debacle has been the result of science running down this massive blind alley.

 (Of course nobody ever thinks about the real victims of the IAU’s decision: the astrologers.)

 P.S – I’ll post later on in the month in more detail about some of the science behind why Pluto is so different and the general mechanics & formation of the Kuiper belt/scattered disc. I’m by no means done on this subject.

 *In detail: sometimes a planet or a satellite will move in front of some light-emitting or –reflecting object, blocking some of the light we see from that object. If the planet/satellite is close enough to us to block out all the light from the object, this is called occultation**. Solar eclipses are the most obvious example of occultation, but stars are small enough to be occluded all the time by various Solar System objects. Charon occluded a star in 1980, allowing an estimate of its diameter to be made – astronomers knew how fast Charon was moving and they knew how long it took to move from one side of the star to the other, which gave them the size.

 Now, if the blocking object appears smaller to us than the light-emitting object, all it will do is block a portion of the object’s light and so reduce the intensity of the light we receive from that object. This is called a transit, and this is what Charon did to Pluto a number of times between 1985 and 1990 – it passed between Pluto and the Earth, which was remarkably lucky given the precession of Charon’s orbit means that happens only once every hundred years or so. Astronomers measured the decrease in light intensity as Charon moved across Pluto, and since they already knew how big Charon was from the 1980 occultation they were able to use the transit time to work out the size of Pluto.

 **Not to be confused with occlusion, which is a word dentists use to describe how the top and bottom teeth fit together when your mouth is shut.

The post Love Don’t Live Here Anymore. appeared first on The Scientific Gamer.