Science desk
< February 1	<< Jan \| February \| Mar >>	February 3 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

February 2[edit]

Ob. XKCD question.[edit]

This looks plausible (forget the last few panels):

   http://xkcd.com/620/

...it looks like it would be a heck of a lot of fun too. I wonder why there aren't people doing it for real? You'd think there would be people like base-jumpers doing it...maybe even have these rigs in theme parks. Any ideas? SteveBaker (talk) 05:07, 2 February 2010 (UTC)[reply]

There was some dude who did this, or something very like it, off of a bridge in the Tampa Bay area. I am researching it now. --Jayron32 05:19, 2 February 2010 (UTC)[reply]

See Sunshine Skyway Bridge, under "Bridge Suicides". A group of daredevils tried a pendulum swing from the bridge, and failed to take into account the additional stresses on the cable when it was actually swinging. The cable broke and several people were injured. --Jayron32 05:24, 2 February 2010 (UTC)[reply]

Doing it old-style usually doesn't work. Astronaut (talk) 05:35, 2 February 2010 (UTC)[reply]

It would be an engineering nightmare. You'd need some sort of automatic belaying system that could apply a constant tension to someone who's squirming around and flapping wings. It would need to be accurately tuned to each different person who went on it. It would quickly be tiring for the person on the cable; even if if most of the weight is taken off by the cable, you still have 60 kg of mass to pull around. You'd basically be stuck going up or down, because any deviation to the side and the cable would act as a pendulum and apply a restoring force to the middle. And I wouldn't have any clue how to flap my wings to apply force in a particular direction, and would just end up flailing around. Maybe an effective flapping technique could be developed with practice (and trial and error), but human arms aren't exactly built for flapping. In any case, the experience needed would rule out any sort of carnival ride. I invite someone to try building something like this; I'd like to see how it turns out (videos would be appreciated) And of course, neither me nor Wikipedia assume any liability for anyone ~~crazy~~ brave enough to try it. Buddy431 (talk) 06:01, 2 February 2010 (UTC)[reply]

Certainly the cable would act as a restoring force to keep you vertically below the attachment point - but if the cable is long enough, the deflection angle could be kept fairly small - perhaps enough to allow a modest amount of lateral flight. Having some restoring force to keep the flier within some reasonable boundaries would be a useful thing. Seems like you could build some kind of computerized winch to keep the tension in the rope constant - and also perhaps to limit the rate of descent if the person looks to be in danger of smacking into the ground at high speed. We can weigh the participant in advance to understand the amount of tension that would be needed. But it seems like the ability to experience what it would be like to literally fly like a bird would be worth the effort. SteveBaker (talk) 14:17, 2 February 2010 (UTC)[reply]

With some tracking cameras and some clever software, you could put the winch on a pair of tracks, and keep it directly over the user at all times. Compensating for the pendulum effect. APL (talk) 15:33, 2 February 2010 (UTC)[reply]

Peter Pan does it (video). Cuddlyable3 (talk) 11:37, 2 February 2010 (UTC)[reply]

Although I have not seen peeople doing this with a cable attached, there is a Base Jumper version of this already. In fact, we have an article on Wingsuit flying, which describes the practice. --Avic enna sis 17:47, 2 February 2010 (UTC)[reply]

Wingsuits are really only gliders - what I'm thinking of is something where you actually flap to generate thrust...powered flight. Think "Bat", not "Flying Squirrel". SteveBaker (talk) 20:38, 2 February 2010 (UTC)[reply]

Flapping-wing suits in low gravity are a time honored trope in written science fiction. I'd love to know how much upward thrust a normal human could reasonably generate with a well designed pair of wings without exhausting himself. APL (talk) 02:01, 3 February 2010 (UTC)[reply]

This trope is sometimes portrayed as happening in a hyperbaric flying stadium at 1/6 g on the moon, implying that the authors felt that thicker air was needed to generate sufficient lift even in such reduced gravity. 124.157.247.221 (talk) 02:23, 3 February 2010 (UTC)[reply]

If the XKCD cartoon is to be believed (and that guy is pretty good at getting science facts right), then it requires the 0.09g gravity of titan and 50% denser atmosphere...so at 1/6th g, the air density would have to be pretty high. Of course you can easily "fly" at 1.0g in a fluid as dense as water...it's just that we call it "swimming". That's mostly because the bouyancy of the water cancels the gravitational force on a typical human body so you don't need much thrust in order to change altitude. SteveBaker (talk) 16:29, 3 February 2010 (UTC)[reply]

Death[edit]

How long after brain death occurs does it take for absolutely all nerve impulses in the body to stop? —Preceding unsigned comment added by 114.75.61.49 (talk) 12:45, 2 February 2010 (UTC)[reply]

This article at HowStuffWorks confirms that even after brain death the heart may still beat without assistance and some spinal reflexes may be present. So, if a patient is on a ventilator and is receiving fluid and nutrients, then I think the interval between brain death and cessation of absolutely all nerve impulses in the body could be a very long time indeed. Gandalf61 (talk) 13:01, 2 February 2010 (UTC)[reply]

I was thinking more about without any sort of life support, but thanks for the article. —Preceding unsigned comment added by 114.75.61.49 (talk) 13:50, 2 February 2010 (UTC)[reply]

Mike the Headless Chicken lived 18 months without a brain. Cuddlyable3 (talk) 20:37, 2 February 2010 (UTC)[reply]

He still had most of his brain stem, which is what controls the heart and lungs. Buddy431 (talk) 22:53, 2 February 2010 (UTC)[reply]

GLUT transporters[edit]

Hi guys, I'm currently reading about carbohydrate metabolism and I came across GLUT in the different organs. It is stated that GLUT2 transporters in eg pancreatic beta-cells are low-affinity transporters while GLUT3 transporters in brain are high affinity transporters. Can anyone explain clearly what low affinity vs high affinity transporter means and the significance of it, with respect to the organ which the transporter it is found in?

Thanks a million!! :) —Preceding unsigned comment added by 121.6.60.209 (talk) 14:16, 2 February 2010 (UTC)[reply]

Basically a transporter has to grab hold of a molecule before it can transport it, and then hang on to it during the transport process. The affinity is basically the "grip strength" -- a high affinity transporter is more likely to get hold of a molecule that is passing by, and tends to keep hold of it longer once it has gripped it. All else being equal, a high-affinity transporter can be expected to be more efficient. Looie496 (talk) 15:40, 2 February 2010 (UTC)[reply]

Hi, just to clarify, do you mean that since a high-affinity transporter has a low michaelis constant, so it's more efficient as Vmax is reached at a lower concentration of glucose? Thanks in advance :) —Preceding unsigned comment added by 121.6.60.209 (talk) 15:55, 2 February 2010 (UTC)[reply]

Yes, low K_M = high affinity. --Mark PEA (talk) 20:22, 2 February 2010 (UTC)[reply]

ohms law and current[edit]

In ohms law current is designated as I why not A for amps or C for current —Preceding unsigned comment added by 69.129.88.122 (talk) 14:22, 2 February 2010 (UTC)[reply]

This [1] suggests that it might be I for Integral (of current density), but I don't know whether that was the original reason for choosing the letter I. Dbfirs 15:17, 2 February 2010 (UTC)[reply]

While I does happen to be equal to the (surface) integral of the current density J, I would be rather surprised if that were the origin of the notation. It should be noted that the link above simply provides the formulae and their definitions; it doesn't explicitly address the question of how the symbols were chosen. TenOfAllTrades(talk) 15:24, 2 February 2010 (UTC)[reply]

(ec) There are at least two competing stories out there (both referenced in this discussion). One is that the I came from the French intensité (literally, intensity), a synonym for current. Another version of the story is that the I came from impetus, yet another historical synonym.

Some sources note that if C were used for 'current' it could lead to confusion with the also-important-in-physics constant c, the speed of light. While such confusion would indeed be problematic, the widespread use of c for the speed of light didn't really occur until around the beginning of the twentieth century: [2]. (Einstein himself didn't make the switch from V to c until 1907.) A (for amps or ampere) wasn't used because a lot of the equations involved were developed by Ampère himself — and he wasn't so boastful that he was going to name an important variable after himself. TenOfAllTrades(talk) 15:24, 2 February 2010 (UTC)[reply]

I agree that the integral is just coincidental. TenOfAllTrades beat me to the real reason(s), but all I could find was opinion. Dbfirs 15:41, 2 February 2010 (UTC)[reply]

Back in 1860 a scientific handbook(p281) discussing Ohm's Law might have used E for the electromotive force, R for the sum of resistances, and F for the "force of the current." Then see Ferguson "Electricity" from 1873 which says "The quantity of the electricity passing in a current, or the strength of the current (Fr. Intensité, Ger. Stromstärke), is estimated by the power of the current to deflect the magnetic needle, by the chemical decomposition it effects, or by the temperature to which it raises a wire of a given thickness and material." An 1872 journal calls it "intensite de courant." An 1875 journal notes the standardization of previously vague and conflicting terms for electrical quantities, in which "intensity" sometimes meant current strength and sometimes meant electromotive potential. It notes that "intensité" was universally used by French writers for current. Some writers in 1875 still used "i" for "intensity of the electrostatic field" or "electrical force at a point" and "C" for current strength (p 65)." The "Ampere" was apparently not adopted as the international unit of current until 1881. Edison (talk) 16:24, 2 February 2010 (UTC)[reply]

My raw nuts are green - have they gone to seed?[edit]

Sorry for the bad pun. Normally I eat roasted mixed nuts, but because I heard that raw/unroasted nuts are healthier, I bought some. When I opened the package this morning, though, some of the nuts had a green colour...is this mould, or just their natural, uncooked colour? I'd heard somewhere that peanuts can often have fungus, and even if the mould isn't harmful, I don't find the idea of eating it very appetizing.... Thanks in advance! Quietmarc (talk) 15:19, 2 February 2010 (UTC)[reply]

What kind of nuts exactly are in the package? Photo please? Comet Tuttle (talk) 15:31, 2 February 2010 (UTC)[reply]

Stupid me, left the package at home, and I'm at work. I'll take and upload a picture as soon as I'm home. A similar (but not identical) product from the same store has cashews, almond, pecans, filberts, and brazil nuts listed as ingredients, but there may be some variation in the product I actually got. I've texted my roommate to see if he can take a picture sooner....Quietmarc (talk) 15:54, 2 February 2010 (UTC)[reply]

Pistachios are often naturally green when eaten, so it may be likely, if your mix contained pistachios, that it would be perfectly normal. Some distributors used to dye their pistachios a bright red color because the green bothered some people, but I see less and less of this than I used to. --Jayron32 17:39, 2 February 2010 (UTC)[reply]

Funny, my first thought at the idea of red nuts was betelnut. Nyttend (talk) 13:55, 3 February 2010 (UTC)[reply]

Okay, false alarm. I double-checked and it looks like it was just green pistachios. I swear when I looked at them yesterdcay morning (at 5am, far too early for me to be up) that several kinds of nuts had the greenish colour, but yesterday it was clearly just them. Thanks guys.Quietmarc (talk) 14:20, 3 February 2010 (UTC)[reply]

counterfeit stamps[edit]

There are a great number of security devices on US banknotes to prevent things like photocopies and such, do stamps have similar security features? Googlemeister (talk) 17:00, 2 February 2010 (UTC)[reply]

Per this 2007 news story, "the investigation into counterfeit stamps was triggered after postal inspectors discovered that hundreds of letters were being rejected for delivery because the stamps lacked the required phosphor tagging." So yes, there are security devices. — Lomn 17:05, 2 February 2010 (UTC)[reply]

Here is another article on the security features. (Google "postage stamp security"). Unsurprisingly, the higher value stamps have more intricate features. --Mr.98 (talk) 17:31, 2 February 2010 (UTC)[reply]

In the UK, a phosphor stripe is used by the machinery to distinguish between first and second class letters. I'm surprised phosphor tagging is not mentioned in the postage stamp design article, but I'm not sure how widespread it is.--Shantavira|^{feed me} 09:21, 3 February 2010 (UTC)[reply]

Not sure about third-world countries, but it's common in the developed world to use tagging. Virtually all US stamps have been tagged since the 1970s (or 1980s? I can't remember for sure), with untagged stamps being either (1) the occasional error, or (2) older ones. There's not really a ton of need for security in stamps; unlike in the nineteenth century, people don't produce postal forgeries (conterfeits to deceive the post office) very often, as it doesn't pay very well. Ironically, most forgeries today are made to deceive collectors, and the stamps typically counterfeited are nineteenth-century stamps (such as the one pictured) that were made with security devices similar to traditional US banknotes. Nyttend (talk) 14:01, 3 February 2010 (UTC)[reply]

Hmm, now that I read the article from Linn's, I should note — while bank tellers are trained to know counterfeit money (they handle enough of the real stuff, after all), most stamps aren't handled by many people. After all, if you produce postal forgeries and use them, you're going to be the only one handling most of them; unless you get caught, the large majority will stay on the envelope all the way into the trash can. While some of the features mentioned in the article have been used on US stamps in the past — especially the watermarking, which is sometimes obscure and makes it hard for those of us who collect early twentieth-century US stamps — they're not as common, and ideas such as the sycopated perforations and the holograms (unless I'm remembering badly) have never been used on US postage. Nyttend (talk) 14:10, 3 February 2010 (UTC)[reply]

Interestingly, the U.S. government experimented with blurring the line between stamps and currency during the civil war: postal currency. TenOfAllTrades(talk) 15:40, 3 February 2010 (UTC)[reply]

Recipe for production of hydrogen by electrolysis[edit]

I would appreciate a basic recipe for efficient production of a liter of hydrogen ( and a half liter of oxygen) by electrolysis of water. Our article on Electrolysis of water does not go into specifics, since Wikipedia avoids being a "how-to" guide. I expect to use a DC power supply set at whatever voltage is optimal, with carbon electrodes (from old drycell batteries) submerged in water with some acid added. I want to do it on the cheap with materials available in my workshop, including sulfuric acid (battery acid, specific gravity 1.265) added to tapwater (would added to distilled be better?). I could use hydrochloric acid, salt, or baking soda if there were some advantage). I would collect the gas by displacement of water in a 1 liter bottle over the electrodes. It has been many years since I took chemistry, but I am aware of basic safety precautions such as adding acid to water rather than the converse. Is carbon (old battery carbons) inert enough for the process, since I don't have platinum electrodes? Would stainless steel be better in some way? How much of the sulfuric acid per liter of water is optimal? What is the ideal voltage (like 1.5 volts? 6 volts?)? Is there some problem or issue of diminishing returns with more than the minimum voltage? Is there some problem of diminishing returns with more than a minimal acid concentration? What volume of gas would then be expected per ampere hour? Is use of dilute sulfuric acid better in quantity or purity of hydrogen and oxygen produced some way than dilute hydrochloric acid or sodium chloride solution? Thanks. Edison (talk) 17:52, 2 February 2010 (UTC)[reply]

You don't want to use a chloride ion, because you'll get a significant amount of chlorine gas instead of oxygen. I think sulfuric acid is best, though of course it's nasty stuff and you need to be careful. Here's one setup: it's probably a bit smaller scale than you're looking for, but it's a good starting point. They use 0.1 M sulfuric acid, and say carbon electrodes are fine. They also use a variable voltage power supply, and use it to control the current. That seems like a good idea to me, rather than use a battery at a fixed voltage. Buddy431 (talk) 22:48, 2 February 2010 (UTC)[reply]

If the formula weight of sulfuric acid is about 98, the 1.265 specific gravity of my stock solution would indicate 2.70 molar, wouldn't it? (I never did chemistry for a living, many years since I took a course). Would a 27 parts water to 1 part stock solution volume dilution be about right to get from my stock solution of sulfuric acid down to the 0.1 molar solution (not sure if the volume would be greater or lesser than the sum of the parts)? Edison (talk) 06:00, 4 February 2010 (UTC)[reply]

This says that acid with a specific gravity of 1.2609 (close to yours) is 34.63% acid (by weight, I assume). That's a molarity of about 4.45 mole/L. To dilute to 0.1 M, add about one part acid to 45 parts water. The volume might change a little, but not enough to worry about. Buddy431 (talk) 06:35, 4 February 2010 (UTC)[reply]

Percent loss[edit]

If you want to find percent loss of water,

(mass of water lost / initial total mass of water) x 100?

I keep getting this is wrong. What's the real formula? —Preceding unsigned comment added by 142.58.156.63 (talk) 18:15, 2 February 2010 (UTC)[reply]

That formula looks correct. -- Flyguy649 ^talk 18:29, 2 February 2010 (UTC)[reply]

Yes, percentage loss and gain are always expressed as percentages of the original amount unless there are special circumstances that dictate otherwise (such as retail profit where profit was traditionally expressed as a percentage of takings which were a known quantity). Dbfirs 19:17, 2 February 2010 (UTC)[reply]

I would disagree with the "x 100", since 0.7 (say) actually is 70%. I'd replace it with "x 100%", but this is a particularly minor detail. Grandiose (me, talk, contribs) 19:41, 2 February 2010 (UTC)[reply]

Dental Cavities[edit]

I scanned the article on dental caries already, but I found no information about why dental caries cause a color change in tooth enamel. What happens chemically to cause the whitish enamel to cause a dark stain when there is a cavity?130.127.52.67 (talk) 19:33, 2 February 2010 (UTC)[reply]

The destruction section of the Tooth enamel article seems to give a lot of details. Beach drifter (talk) 20:51, 2 February 2010 (UTC)[reply]

Indeed it does, but none of them refer to colour changes. Richard Avery (talk) 07:11, 3 February 2010 (UTC)[reply]

Color change in enamel and dentin occurs because bacterial products and debris become stuck in the hollow areas at which sound tooth structure used to be present. DRosenbach ^{(Talk | Contribs)} 04:52, 4 February 2010 (UTC)[reply]

vision acuity between 20/80 and 20/100 or 20/200[edit]

If the vision is between 20/100 and 20/200 is value identify? Do 20/140 or 20/170 exist? Is 20/300 or 20/90 exist? I never seen any doctors calling [3] 20/90 or 20/130 or 20/250. I seem to hear either black or whites on 20/x00s I only hear is either 20/100 or 20/200 or 20/400 nothing between--209.129.85.4 (talk) 21:19, 2 February 2010 (UTC)[reply]

Yes. See our Visual acuity article; the article is pretty heavy going, but if you scroll down halfway there's a chart and an accompanying discussion about what is meant by measurements like 20/30. Comet Tuttle (talk) 21:27, 2 February 2010 (UTC)[reply]

This system is easy to understand. What it (roughly) means is that what this person can distinguish at the first distance, a 'normal' person would be able to distinguish at the second distance. So 20/20 vision means that this person can see at 20 feet what normal people can see at 20 feet - in other words, normal vision. 20/100 means that what this person can see at 20 feet - normal people could see at 100 feet. 20/10 would mean that this person can see at 20 feet what most people would have to get 10 feet away from to see...that's pretty good eyesight! So, sure, someone's vision could be 20/140 or 20/170 or 20/300 or 20/90. Probably the reason they round to a set of standard numbers is that with a simple eye chart, they only have letters printed at particular sizes - and it's not such an exact science since you're relying on what the viewer is telling the doctor - which could be inexact. But in principle, your vision could be 20/123.456789 if someone could measure it that accurately. The use of 20 as the first number is simply because that's as far back as you can get from the eyechart in most doctor's offices. SteveBaker (talk) 02:08, 3 February 2010 (UTC)[reply]

hydrogen ion concentration[edit]

I think I've managed to determine the hydrogen ion concentration and the pH when 2.16 g of hydrobromic acid is dissolved in water; if the molar mass is 80.9 mol^-1, then 2.16 g divided by 80.9 mol^-1 equals 0.027, and -log(0.027) = 1.6. A pH of 1.6 seems reasonable for hydrobromic acid, though a little on the high side, I was expecting negative pH. First, I guess the appropriate question is, have I got that part of it right? But that's assuming /mol dm³, and I want to know what the hydrogen ion concentration and pH will be if the final solution is 0.05 dm³. Suffice to say I don't really understand this, but my first guesses are getting me to take 0.027, multiply by 0.05 to get 1.35 x 10^-3, and then -log(1.35 x 10^-3) gives me 2.87. I don't think that answer is right but I can't see what I'm doing wrong. Sorry for the formatting I don't know how to get math stuff here. Differentially (talk) 21:26, 2 February 2010 (UTC)[reply]

Your mistake is that you do not say how much water the acid is disolved in. In your first set of calculations, you take the -log of the number of moles of HBr which is dissolved. This is where you went wrong, since the pH is based on the concentration which is measured as the number of moles per liter. So you would need to divide the .027 by the volume of the solution in liters, and then take the -log of THAT number. --Jayron32 21:59, 2 February 2010 (UTC)[reply]

Hmm, I kind of tried that; my volume of final solution is 0.05 dm³, and 0.027 divided by 0.05 gives me 0.54; but the -log of 0.54 is 0.268. Correct me if I'm wrong, but does a pH of 0.27 make any sense at all? That's why I had discarded it as one of my options. Differentially (talk) 22:11, 2 February 2010 (UTC)[reply]

That's right, I think. A pH of 0.27 is certainly pretty acidic, but you have a lot of acid in only 50 mL of water. Buddy431 (talk) 22:32, 2 February 2010 (UTC)[reply]

I think he may be close to surpassing the solubility of HBr water there. You can only assume complete dissociation before there is too little water to solvate all the HBr and the HBr starts boiling out of solution. John Riemann Soong (talk) 01:42, 3 February 2010 (UTC)[reply]

As with hydrogen chloride (which dissolves to form hydrochloric acid), hydrogen bromide is extremely soluble in water. Our article gives a figure of 193(!) grams HBr per 100 mL of water. Though that sounds high, that's about the same solubility on a mole-for-mole basis as HCl. TenOfAllTrades(talk) 02:05, 3 February 2010 (UTC)[reply]

Yup, HBr as a 48% aqueous solution is a standard commercial chemical. On the other hand, "pH == -log (H⁺ concentration) and HBr fully dissociates" are each not really correct at extremely high concentrations. The so-called strong acids are not infinitely dissociative, only "really high, essentially 100% at moderate concentrations"--the pKa is merely measurably a little negative. And the activity coefficient can also alter the actual pH even if the proton concentration is known...diverges more as concentration gets large. On the other-other hand, all of that is not usually considered for simple school-work calculations. DMacks (talk) 03:25, 3 February 2010 (UTC)[reply]

To answer the OP's question on whether a pH of 0.27 is OK. Its perfectly fine. pH is somewhat eroneously assumed to run from 1-14 because the "median value", 7, falls nicely in the middle. However, a pH of seven isn't really a median value. Its merely a result of the autoionization of water, and pH has no hypothetical upper or lower bounds. pH can be a negative number, and it can be higher than 14. A pH of 0.27 is very reasonable for a strong, highly soluble acid like HBr. --Jayron32 03:35, 3 February 2010 (UTC)[reply]

Differentially says he expected a negative value, so why he thinks 0.27 "doesn't make sense" is a bit hard to reconstruct. --Trovatore (talk) 03:41, 3 February 2010 (UTC)[reply]

Heh, it had been a very long day. I'd first got 0.27 (and discarded it) before I'd researched HBr and determined that a negative pH was likely. Differentially (talk) 09:10, 3 February 2010 (UTC)[reply]

Thank you for the help. Differentially (talk) 09:11, 3 February 2010 (UTC)[reply]

cuts on penis[edit]

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis, prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page.

-- Flyguy649 ^talk 17:40, 3 February 2010 (UTC)[reply]

Most recent common ancestor for humans and dogs, or How closely related am I to my two dogs?[edit]

Given that all life on Earth evolved from a single common ancestor, I'm curious as to how closely related I am to my two dogs. Has last common ancestor between humans and dogs been identified or know long ago it lived? A Quest For Knowledge (talk) 23:52, 2 February 2010 (UTC)[reply]

This tree of life suggests about 60 million years ago, while this indicates closer to 80 mya. You can also check out Evolution of mammals, which indicates that there's some disagreement about when and where certain types of mammals developed, but it's sort of ambiguous when the branching is though to have occured. As both dogs and humans are placental mammals, an upper bound is about 125 mya, as this is when the first placentals emerged. Buddy431 (talk) 00:27, 3 February 2010 (UTC)[reply]

Interesting -- the divergence happened before the Jurassic period? John Riemann Soong (talk) 01:39, 3 February 2010 (UTC)[reply]

The Jurassic period ranged from 200 mya to 145 mya. The Cretaceous period ranged from 145 mya to 65 mya. 124.157.247.221 (talk) 02:15, 3 February 2010 (UTC)[reply]

I take it "mya" is "million years ago"? Would that be 200 million years before today, or 200 million years before 1 January 1950, as in Before Present? --Trovatore (talk) 03:38, 3 February 2010 (UTC)[reply]

I suppose that's a joke (you can use one of these :) guys to indicate that it is - it's hard to pick up tone from a short post like this). If it's not, consider that when we throw out a nice round number like 200 mya or 145 mya (both clearly rounded to the nearest five million years), it doesn't really matter if there are a few dozon, or even a few dozen thousand years of ambiguity. Even so, it would be nice to have a standard. We have a small mya (unit) article, but it doesn't clear things up. Buddy431 (talk) 04:04, 3 February 2010 (UTC)[reply]

Man visiting museum: "How old is that dinosaur?" Guard:"65 million and 7 years old" Man:"How can you be so accurate?" Guard:"Well when I started here they told me it was 65 million years old, and that was 7 years ago." --Frumpo (talk) 11:55, 3 February 2010 (UTC)[reply]

To avoid confusion, I put my jokes in small text, a smiley face, and an edit summary saying that it's a joke. Since I started doing that, no one's taken my jokes seriously. A Quest For Knowledge (talk) 04:39, 3 February 2010 (UTC)[reply]

Thanks, AQFK, a useful cautionary tale. --Trovatore (talk) 04:42, 3 February 2010 (UTC)[reply]

Indeed, a difference of 0.000060 million years isn't relevant to the ages of the Jurassic, which Wikipedia gives as 199.6± 0.6 mya to 145.5± 4 mya, but it would be interesting to know if the formal definition resolves it. (Precise definition may even vary by usage.) Before Present#Usage of "BP" addresses some dispute over whether the term "BP" should be reserved for radiocarbon estimations. I would assume that a common baseline would be useful where stratification and radiocarbon dating methods overlap, but it would be good to hear from a geologist. 124.157.247.221 (talk) 04:42, 3 February 2010 (UTC)[reply]

Mya isn't really a formally defined, and tends to be discouraged in the professional literature in favor of BP and annum (though you still see mya a lot in popular science writing). Everyone professional, in my experience, bases BP at 1950 regardless of whether they mean radiocarbon years or calendar years. I've also seen the less common B2K (before 2000) crop up occasionally in recent years. I know the BP article talks about whether "BP" should always mean radiocarbon, but I've yet to meet a geologist who is actually in favor of that. Only people who live and die by radiocarbon seem to like that idea. Dragons flight (talk) 18:48, 3 February 2010 (UTC)[reply]