Science desk
< August 10	<< Jul \| August \| Sep >>	August 12 >

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.

August 11

Favored cigarette brands by ethnicity

Does anybody have this information? I'm doing research regarding the effects of racially-targeted advertisements on tobacco consumption. Kenjibeast (talk) 06:51, 11 August 2008 (UTC)[reply]

Hmmm. There could be some useful information here. Fribbler (talk) 10:21, 11 August 2008 (UTC)[reply]

Not about style of advertising but by volume: "2.6 times as many advertisements per person in African American areas as compared to Caucasian areas" [[1]]. Rmhermen (talk) 13:45, 11 August 2008 (UTC)[reply]

There have been a number of newspaper articles lately on the popularity of mentholated cigarettes amongst African-Americans, with brands like Kool and Newport targeting them specifically. The Newport article has a link to a study about brand preferences that looks useful, though it only breaks it down to "White", "Black", and "Hispanic." It's a few years old but it's a good start. --98.217.8.46 (talk) 17:01, 11 August 2008 (UTC)[reply]

What's this map projection?

I came across this unfamiliar projection. What is it? It looks like the Mercator, except with Greenland above North America. AlmostReadytoFly (talk) 13:59, 11 August 2008 (UTC)[reply]

It looks like it could be a take on the Goode homolosine projection. Fribbler (talk) 14:21, 11 August 2008 (UTC)[reply]

That does seem closest, but what the heck is Greenland doing? And Kamchatka? --jpgordon^∇∆∇∆ 00:23, 12 August 2008 (UTC)[reply]

sea anemones

how many years do sea anemones live ? thanks —Preceding unsigned comment added by 76.168.205.163 (talk) 17:40, 11 August 2008 (UTC)[reply]

From the BBC: "Sea anemones are anecdotally very long-lived, reaching 60-80 years and more. Like other Cnidarians, they do not age, meaning they have the potential to live indefinitely. Most fall foul to predators before a good age is reached. " Link:[2] Fribbler (talk) 17:56, 11 August 2008 (UTC)[reply]

(fascintating - does this mean anenomes are immortal?)87.102.45.156 (talk) 18:36, 11 August 2008 (UTC)[reply]

No, since they can still be killed. But they are "Biologically Immortal" in that their cells show no signs of ageing. Fribbler (talk) 18:47, 11 August 2008 (UTC)[reply]

(sorry to hijack this question) Do Sea anemones have any predators?87.102.45.156 (talk) 19:00, 11 August 2008 (UTC)[reply]

And are they the biggest/most complex 'immortals' ? 87.102.45.156 (talk) 19:02, 11 August 2008 (UTC)[reply]

(Outdent) Sea Anemones have few predators. Examples include the Butterflyfish and the Sea mouse. As for the largest Biologically Immortal animal, I'm not sure, but the Portuguese Man o' War must surely be up there. Fribbler (talk) 19:12, 11 August 2008 (UTC)[reply]

Thanks, I can't believe the 'sea mouse' has slipped me by all these 37 years. Well you're never too old...87.102.45.156 (talk) 19:32, 11 August 2008 (UTC)[reply]

Siphonophorae is interesting too - a colony of distinct individuals, unable to function separately, but not a multicellular organism as such. That's two things I've learnt today. (If you have any more please tell..). Thank you very much.87.102.45.156 (talk) 19:40, 11 August 2008 (UTC)[reply]

Hi. Please also see the articles turritopsis nutricula, an immortal jellyfish, and immortality. Thanks. ~A H 1^(TCU) 00:09, 12 August 2008 (UTC)[reply]

Inexpensive sheets of transparent elastic material

For a hobby project, I'm looking for an inexpensive material with the following properties:

elastic
transparent (if possible), or translucent (if it's almost clear)
available as, or can be made into, sheets of about 1 ft x 2 ft x 0.25 in

Basically I'm looking for something like the material used to make "gel" insoles, but I want it colorless and in a larger size. It would be extra desirable if the material can be easily molded into different shapes (it opens up more possibilities). Any suggestions? --71.185.228.156 (talk) 17:46, 11 August 2008 (UTC)[reply]

Check hobby shops for the covering for model airplanes. It is very lightweight, very strong, stretchable, and comes in clear or many colors. -- kainaw ™ 17:53, 11 August 2008 (UTC)[reply]

Thanks for the suggestion. When I said "elastic", I was thinking about elastic as in providing cushioning, not elastic as in stretchable. --71.185.228.156 (talk) 18:04, 11 August 2008 (UTC)[reply]

You wan't a 'gel' material (like jelly ?) - how strong does it have to be - water based gels are very common but weak.

I think what you want is a silicon rubber or another elastomer

seach for 'silicon rubber sheet' if that seems suitbale - it should be easily sourceable in the dimensions you specify.

The alternive is a foam rubber.87.102.45.156 (talk) 18:35, 11 August 2008 (UTC)[reply]

Not being familiar with water-based gels, I don't know if what I'm looking for is stronger than water-based gels are. If a material is strong enough to be used for insoles, it's strong enough. --71.185.228.156 (talk) 20:39, 11 August 2008 (UTC)[reply]

McMaster-Carr describes "Latex-Free TPE": This transparent thermoplastic elastomer has a gel-like consistency to absorb vibration and conform to irregular surfaces. It is super stretchy and has great tensile strength. It is nontoxic and nonallergenic. That sounds more or less like what you're looking for. You can enter "3430" in their search box for the appropriate catalog page, or enter "elastomer" and click around a bit. -- Coneslayer (talk) 18:53, 11 August 2008 (UTC)[reply]

or any other Thermoplastic elastomer qv 87.102.45.156 (talk) 18:57, 11 August 2008 (UTC)[reply]

(To Coneslayer:) The "Ultra-Elastic Clear Gel Rubber" sounds very much like what I was looking for. Thanks. --71.185.228.156 (talk) 20:39, 11 August 2008 (UTC)[reply]

FYI, if you only need a small amount the manufacturers will often be willing to send you a free sample. --Sean 13:45, 12 August 2008 (UTC)[reply]

You could also try folding up sheets of saran wrap. --Shaggorama (talk) 22:20, 12 August 2008 (UTC)[reply]

Legs on a whale

A while back I read somewhere that occasionally, when they haul a whale out, the fishermen notice that their catch sports vestigial hind legs. Simple question - anyone have pictures? I managed to find images of rudimentary leg bones found inside the body of the whale (which I gather is not particularly uncommon) but as I understand it, the legs in question here are *external*. Anyone able to satisfy my curiosity? Thanks. --Kurt Shaped Box (talk) 18:55, 11 August 2008 (UTC)[reply]

Given how tiny whale legbones are, I find this, err, unlikely. However there was a dolphin captured in 2006 with vestigial hind flippers, which sounds more par for the course. --98.217.8.46 (talk) 21:25, 11 August 2008 (UTC)[reply]

Have you seen the photo in the 1921 article “A remarkable case of external hind limbs in a humpback whale”? The photo from that article is available on the Internet (e.g. here). In general see the Wikipedia article Atavism and the New Scientist article “The ancestor within all creatures”, 2007-01-15, issue 2586. --Mathew5000 (talk) 00:45, 12 August 2008 (UTC)[reply]

Some more on whales and hind-legs at [3]. DuncanHill (talk) 00:49, 12 August 2008 (UTC)[reply]

Eeek! So many answers in so short a time. I was going to link Atavism for that link. --Kjoonlee 00:51, 12 August 2008 (UTC)[reply]

Thanks very much for the answers. That was exactly what I was looking for. :) --Kurt Shaped Box (talk) 18:11, 12 August 2008 (UTC)[reply]

Terrifying. Plasticup ^T/C 17:14, 13 August 2008 (UTC)[reply]

How fast will a fishhook rust

This weekend I was fishing in a pond and someone caught a large snapping turtle. When the person realized that the ~25 weight on his line was a turtle, not a fish, he said that he was going to cut the line leaving the hook in the turtle's mouth. Worried about the turtle's health, I argued that we should try to get ahold of the turtle and remove the hook. Everyone else said I shouldn't worry, the hook would just rust and fall out in a day or two leaving the turtle unharmed. Is this reasonable? Slideshow Bob (talk) 19:43, 11 August 2008 (UTC)[reply]

I doubt that any fishhook would rust away within a couple of days - unless it was made out of a very reactive metal (which would just be a silly thing to make a fishhook from). It might fall out on its own, or it might not. Depends where it's lodged, really. There is also the risk of the turtle swallowing or becoming entangled in the trailing line. Your friend should probably have reeled the turtle in and pulled the hook out. I occasionally see fishermen accidentally hooking gulls when walking on the seafront, at which point, I run over and implore them *not* to just cut the line and condemn the gull to a likely death. --Kurt Shaped Box (talk) 19:51, 11 August 2008 (UTC)[reply]

No, it's not "reasonable" to think a hook made of high-quality steel (the usual material in modern hook manufacture - see Fish hook) will dissolve or rust quickly. The belief that it will has been, to some extent, fostered by advice such as the following, which is copied from a government website. The topic is how to handle a fish that you have caught and are planning to release.

Use long-nose pliers to remove the hook quickly 
without tearing or injuring the fish. 
If the fish is hooked deeply, cut the line and 
leave the hook in when the fish is released.

If someone is faced with a choice between leaving a deeply-embedded hook in a fish and ripping it out, leaving it in is almost certainly the better choice from the point-of-view of the possible survival of the fish. However this does not mean the hook will dissolve or rust quickly. Wanderer57 (talk) 21:51, 11 August 2008 (UTC)[reply]

How about cutting off the barb with wire cutters? Preferably, the entire rounded part of the hook should be cut off. Or would the fishhook be too hard for that?

Andme2 Andme2 (talk) 22:05, 11 August 2008 (UTC)[reply]

If the hook is accessible, cutting through it is a definite option. A decent pair of cutters will cut through the hooks typically used in inland fishing (unless someone is fishing for very large fish.) Wanderer57 (talk) 22:18, 11 August 2008 (UTC)[reply]

One can buy a range of hook cutters at any good tackle shop. DuncanHill (talk) 22:21, 11 August 2008 (UTC)[reply]

I'm not sure I'd want my fingers close enough to an angry snapping turtle's mouth to remove a hook. Can anyone comment on whether this would really be a good idea? APL (talk) 02:27, 12 August 2008 (UTC)[reply]

Yes, I was definitely worried about how exactly we could remove the hook without the turtle taking someone's finger (we didn't have a hook cutter), but it never got to the point where we had to do something. The line broke before we had a chance to decide what to do, leaving the hook and an inch or so of line in the turtle. Slideshow Bob (talk) 12:06, 12 August 2008 (UTC)[reply]

This should not be taken in any way as advice - but if I had to do it, I'd probably let it bite down on the end of my boot (or a piece of wood, or whatever was handy), then use the opportunity to firmly grasp the head from behind and immobilize it (similarly to how I'd hold a gull or large parrot to avoid being bitten). --Kurt Shaped Box (talk) 18:11, 12 August 2008 (UTC)[reply]

Hi Slideshow Bob. This is the Science Desk after all, and it sounds like you have access to fishhooks and water ... :-) Maybe try two experiments, with and without salt. Report back! Saintrain (talk) 13:13, 12 August 2008 (UTC)[reply]

Zucchini plants with white lines

I planted several Zucchini plants a few weeks ago. I bought them when they were already grown. On the leaves, squiggly white lines have formed. Is this normal? 66.53.216.249 (talk) 22:09, 11 August 2008 (UTC)[reply]

Most likely it's Powdery mildew. I experience the same issues with my squash plants in the late summer and fall. Juliancolton ^Tropical ^Cyclone 22:26, 11 August 2008 (UTC)[reply]

These aren't spots, but very thin lines. 66.53.216.249 (talk) 22:50, 11 August 2008 (UTC)[reply]

Leaf miners? --Sean 13:57, 12 August 2008 (UTC)[reply]

Black holes in the solar system?

mooved from the misc desk Julia Rossi (talk) 22:51, 11 August 2008 (UTC)[reply]

If uranus was the size of Jupiter, would Jupiter be swallowed to create the largest black hole in the solar system. —Preceding unsigned comment added by 79.76.161.244 (talk) 21:52, 11 August 2008

No. My anus swallows planets with ease. — Twas Now ( talk • contribs • e-mail ) 22:16, 11 August 2008 (UTC)[reply]

What are you, 12? --98.217.8.46 (talk) 01:40, 12 August 2008 (UTC)[reply]

There are no black holes in the solar system, so if one formed, then it would be the largest. But it takes way, way, way, way, way, way, way, way more mass than is contained in two gas planets to create a black hole. --Masamage ♫ 22:38, 11 August 2008 (UTC)[reply]

Actually, there are theories that there are tiny tiny black holes floating all around. To answer your question though, black holes need to be very dense, and uranus is smaller than jupiter, so making it the size of jupiter would make it less dense. -mattbuck (Talk) 23:25, 11 August 2008 (UTC)[reply]

Hi. Also, if two gas planets collided, it would still create a massive explosion, but I don't know how massive. Neptune, by the way, is denser than Uranus, and perhaps more massive as well, but I don't remember. A non-mini black hole would have to be more than 2,000 times more massive than Jupiter and Uranus combined, if I calculated correctly. If the sun collided with another star, there might be a supernova, but I don't know if there will be any remnants, and if there are, it's still likely to not be massive enough to create a black hole. Now, the LHC is planning to potentially create mini-black holes on Earth, but those are likely to evaporate quickly (we hope). Hope this helps. Thanks. ~A H 1^(TCU) 00:01, 12 August 2008 (UTC)[reply]

Theory on tiny black holes heck, aren't we only days away from starting up our own black hole factory? Franamax (talk) 00:05, 12 August 2008 (UTC)[reply]

I'm guessing that the mention of black holes "evaporating" was meant in a general sense, but after reading that I'm wondering, if someone does create a black hole, how can they get rid of it?

If you made a micro black hole, you can control it by adding some positive charge, then orbiting some electrons to make a pseudo atom, and then doing a chemical reaction to fix it in place in some material. Perhaps to really elimenate them you would have to eject them from the solar system, but there may be no reason to do so if you can store it completely safely. The main danger might be a neutral black hole that could fall into the earth. But once it grabbed a proton, it whould be charged and then react with something. Electrons may be too hard to swallow in a micro black hole. Graeme Bartlett (talk) 03:17, 12 August 2008 (UTC)[reply]

To mix metaphors, a black hole is a bit of a white elephant. Wanderer57 (talk) 00:15, 12 August 2008 (UTC)[reply]

Wait a minute. If there is a possiblity of creating black holes, albeit small ones, in the LHC, then why is there not enough mass in Jupiter and Uranus combined for a black hole? There is certainly more mass in those two planets than is available to feed into the LHC. Wanderer57 (talk) 13:15, 12 August 2008 (UTC)[reply]

Presumably the poster meant they're not massive enough to make a black hole that wouldn't just disappear right away. --Sean 14:00, 12 August 2008 (UTC)[reply]

Or not enough mass to create a black hole through gravitational collapse (see Chandrasekhar limit and Tolman-Oppenheimer-Volkoff limit). If LHC creates micro black holes (big if) then it will do so by the alternative mechanism known as "hitting things together really hard". Gandalf61 (talk) 14:16, 12 August 2008 (UTC)[reply]

Okay, thank you. I didn't realize it was that easy to create black holes. So if I drop (say) a fifty pound lead weight from a height onto a big flat rock, I can create a black hole. How fast does the lead weight need to be going when it hits? Wanderer57 (talk) 14:46, 12 August 2008 (UTC)[reply]

99.999999% of the speed of light (or thereabouts). Gandalf61 (talk) 14:57, 12 August 2008 (UTC)[reply]

Oh. So we would really have to whip it in. Wanderer57 (talk) 17:54, 12 August 2008 (UTC)[reply]

Only violent events can form black holes because the forces involved must be enough to overpower electron degeneracy pressure and neutron degeneracy pressure. Collisions at any speed significantly below that of light are not energetic enough.

Even if Uranus had the mass of Jupiter and not just its volume, the gravity between the planets would not be enough for them to collide. From the articles Jupiter and Uranus, the former is at most 5.5 AU from the Sun and the latter is at least 18.4 AU, so the distance between them must be at least 12.9 AU. Using Newton's law of universal gravitation, if both planets had Jupiter's current mass, they would exert a force of 6.4 x 10^19 N on each other. The force between Jupiter and Saturn is 2.6 x 10^20 N, about three times larger, and yet you don't see them colliding at high speed. --Bowlhover (talk) 23:19, 12 August 2008 (UTC)[reply]

Deadliest lightning incident?

Hi. Wikipedia says it's either one of two plane crashes, one killing 81 and the other 91. However, I read a book a few months ago that contained a list of deadliest lightning incidents, and it lists one in an Egyptian army fuel depot in 1993 that killed 431 people, if I remember correctly. I don't know where they got the info from, and I couldn't find info on the Internet, but should this be included in an article even though I could only find one possible citation? Thanks. ~A H 1^(TCU) 23:51, 11 August 2008 (UTC)[reply]

Is it a reputable book? If it's not a comic book or a novel, yes, add the referenced information. We can always check it ourselves. Leave a diff link here, I'll try my local (city) library. Internet confirmation is not required. Franamax (talk) 00:00, 12 August 2008 (UTC)[reply]

And on review, you'll need the name and author of the book for the cite (and preferably page number and ISBN ref) - I might not have been clear on that. Franamax (talk) 01:47, 12 August 2008 (UTC)[reply]

According to this "Environmental Assessment for Permanent Aviation Fuel Facility" thing[4], that was Dronka, Egypt, in 1994. This is may be the book [5]? It's in some Oil & Gas journal too, but that costs money to see. --Haikon 01:55, 12 August 2008 (UTC)[reply]

Oil & Gas Journal costs money to see? Oh, pshaw. List the ref here, just in case one of the legions has a subscription. And remember there's always the WP:LIBRARY if you want to see it yourself. The second link you give looks like a picture-book, so I wouldn't give it huge credibility. The first one carries some definite weight. Adding in the O&G J would nail it down beyond doubt (if it pinned down lighting as the cause). Franamax (talk) 03:34, 12 August 2008 (UTC)[reply]

Hydrocarbon process safety: a text for students and professionals, Clifford Jones, PennWell Books, 2003, ISBN 1593700040 page 53 Ignition of fuel by lightnig
doi:10.1016/j.jhazmat.2005.05.053 581 deaths railway derailment in Dronka, Egypt--Stone (talk) 11:27, 12 August 2008 (UTC)[reply]

Hi. It's the exact same book mentioned here. I could perhaps go to the library to pick it up today. It's a book about extreme weather by H. Michael Mogil is all I remember. Thanks. ~A H 1^(TCU) 18:16, 12 August 2008 (UTC)[reply]

The Oil & Gas article ([6]) says "One report said the explosion was caused when flooding caused a bridge to collapse on pipelines. Another report said the depot was struck by lightning. About 15,000 metric tons of fuel were spilled," and that there were about 400 deaths. The J. Haz. Mat. article says only "The accident that caused most deaths (581) was a railway derailment in Dronka, Egypt, in November 1994." Hydrocarbon process safety says that aviation fuel and diesel leaked during a rainstorm and was ignited by lightning causing >410 deaths. -- BenRG (talk) 19:01, 12 August 2008 (UTC)[reply]

Hi. I have the source details:

title=Hurricanes, Tornadoes, Floods, Heat Waves, Snow Storms, Global Warming and Other Atmospheric Disturbances
author=H. Michael Mogil
ISBN 978-1-57912-743-5
pub date=2007, publisher=BlueRed Press Ltd., location=New York, NY
num of pages=304
info: Egypt; 430 fatalities; Army fuel depot; 1994
page/chapter which info is located: pg 135, chapter: "Lightning"

please try to include the info in an article, with this and other sources. Thanks. ~A H 1^(TCU) 22: 25, 12 August 2008 (UTC)

Does the book list the incident as the deadliest lightning strike? According to page 2 of this book, a lighting bolt that hit a church in Brescia, Italy ignited 100 tons of gunpowder and killed 3000 people. --Bowlhover (talk) 00:19, 13 August 2008 (UTC)[reply]

Hi. Actually, no, I think it says either since 1993 or 1990. Please add all this info to an article. Change the info in the articles, and GWR isn't considered a reliable source anyway. Thanks. ~A H 1^(TCU) 17:47, 13 August 2008 (UTC)[reply]

I have contradictory information. According to the "Environmental Assessment for Permanent Aviation Fuel Facility", "Blazing liquid fuel flowed into the village of Dronka, Egypt. The fuel came from a depot of eight tanks, each holding 5000te of aviation or diesel fuel."

According to Ben's source, "'the release occurred during a rainstorm and was said to have been caused by lightning. 420 fatalities reported One report said the explosion was caused when flooding caused a bridge to collapse on pipelines. Another report said the depot was struck by lightning. About 15,000 metric tons of fuel were spilled,' and that there were about 400 deaths."

What does your book say? I've so far added a brief description of the incident. --Bowlhover (talk) 08:19, 14 August 2008 (UTC)[reply]

Plate movement and longitude

Suppose you pick two monuments, one in Labrador and one in England. Because the North American plate and the Eurasian plate are moving away from each other, the east-west distance between the two monuments is increasing by about 34 millimetres a year, corresponding to about 1/600 of a second of longitude. My question is, assuming you could measure longitude (under WGS 84) with sufficient precision, after one year’s time would you find that (a) both longitudes had changed by about 1/1200 of a second each; (b) the longitude of the monument in England had stayed the same but the longitude of the monument in ~~Quebec~~Labrador had changed by about 1/600 of a second; or (c) something else? --Mathew5000 (talk) 23:51, 11 August 2008 (UTC)[reply]

Isn't longitude-zero measured from the Greenwich observatory? In that case, the longitude of the monument in England would have changed only to a negligible degree, while the farther one would have changed to reflect the continental drift. ~~Now as to the movement of the other monument from Labrador to Quebec, that would bear a little more investigation~~... ;) Franamax (talk) 23:57, 11 August 2008 (UTC)[reply]

WGS 84 longitude is relative to the IERS Reference Meridian (which is slightly east of Greenwich). I can' tell from our article whether or not it's fixed relative to the Eurasian plate or not. Algebraist 00:02, 12 August 2008 (UTC)[reply]

Yes. Reading in the IERS section, the reference meridian appears to be "on the pavement parallel to and northeast of Blackheath Avenue near the northwest corner of the fence surrounding the Pavilion Tea House". I'm not able to determine whether they move that fence every year to compensate for continental drift. :) I also see that "most countries have adopted a version of the IRM for their maps that is fixed to their own tectonic plate as it existed at the beginning of a specific year". So I'm all at sea here, the GPS article doesn't really help although it has some truly impressive mathematics. I know that my own GPS receiver has regional settings and map datums (i.e. WGS84), it also allows a date-time setting but I don't see how that correlates to continental drift rates. I know that I have to keep it on WGS84 when geocaching if I'm to have any hope of finding the cache - but I don't know if WGS84 is adjusted according to the regional settings!

So here's a call for help to any geographers or tectonicists out there: if we build a monument next to the Pavilion Tea House right at zero-degrees longitude, will it still be at zero next year? Will it matter if I'm measuring from it to Labrador, or to it from Labrador (i.e. will my GPS settings change anything)? Franamax (talk) 02:32, 12 August 2008 (UTC)[reply]

I don't quite read that part of the Prime Meridian article as saying that the reference meridian is fixed to that particular spot on the pavement, and if it does intend that, the citations don't support it. As for GPS, I believe that GPS is designed to use WGS84 as a "native" datum, and when your receiver is set to a different datum, the receiver is just using a certain conversion algorithm between WGS84 and the other datum. --Mathew5000 (talk) 18:07, 12 August 2008 (UTC)[reply]

I was asked to provide input here. I don't have the answer, but think for uses where measured accuracy is so important, I doubt latitude-longitude are used. In the U.S., there are state plane coordinate systems which cover specific areas and are relative to some point or monument marker in that region. I don't think GPS is designed to be that super accurate that 34 millimetres would make much difference. --Aude (talk) 20:02, 12 August 2008 (UTC)[reply]

After doing some searching, I found that WGS84 is not fixed relative to any continental plate. According to this webpage:

"A first step towards more consistency between the WGS84 and ITRS took place in 1993 when the complete WGS84 network was recomputed with respect to 8 GPS stations fixed to their ITRF91 position ([18]). This realization is known as WGS84 (G730), "G730" indicates its official implementation on GPS week 730 (Jan. 2, 1994) [...] Further improvements to the tracking station coordinates in 1996, led to WGS84 (G873). This realization consists of 13 sites (NIMA and Air Force sites); only one of them is located in Europe, namely in the UK"

"The main disadvantage of the ITRS and WGS84 lies in the fact that they are global systems. This means that, due to the plate tectonics, the coordinates in the different continents move with respect to each other. For example, expressed in the ITRF2000, the coordinates of Brussels change with about 2.5cm/year."

This suggests that the WGS84 is defined by GPS tracking stations with no compensation for continental drift. As the above users said, many countries/continents define their own datums that are fixed relative to their plates. Europe uses the European Terrestrial Reference System, North America uses NAD 83, while Australia implemented GDA94 (http://www.geoproject.com.au/gda.faq.html).

Incidentally, there is a datum that addresses tectonic movement (http://www.ngs.noaa.gov/CORS/Articles/Reference-Systems-Part-3.pdf). Those are the International Terrestrial Reference Frames defined by the strangely-named International Earth Rotation Service. The ITRF is not fixed relative to the European plate; rather, the combined angular momentum of all plates is assumed to be 0 and drift velocities of stations defining the ITRF are calculated with that assumption. --Bowlhover (talk) 22:47, 12 August 2008 (UTC)[reply]

Thanks; that four-part series by Snay and Soler [7] is very good reading, just what I was looking for. I will make some appropriate changes to longitude and other articles. --Mathew5000 (talk) 19:49, 13 August 2008 (UTC)[reply]

Glacial compensation by GPS

This question inspired me to ask about glaciers, which move much faster than continental drift. If any location on a glacier has a GPS coordinate assigned to it, this coord would seem to require frequent updates or some way to automatically account for the movement of the glacier. How do GPS units handle this ? (One option is to just exclude any location on a glacier, I suppose). StuRat (talk) 14:28, 13 August 2008 (UTC)[reply]

GPS units work just fine on glaciers. There is bedrock beneath glaciers, which does not move (except for continental drift), and in some spots the bedrock is exposed. The coordinates of locations on the bedrock really don't change. The glaciers do move, their coordinates do change. You can measure the movement of crevasses or other features on glaciers, by looking at the change in coordinates to determine how fast the glacier is moving. --Aude (talk) 16:33, 13 August 2008 (UTC)[reply]

You seem to be using the GPS to measure movement of the glacier, which is fine, but what about people who want to find features on the glacier, like crevasses. Unless those coords in the GPS unit are updated frequently and/or automatically, a GPS user may say "how far to the crevass ? Just a few more feeeeeeeeeeeeeeeee...splat !". StuRat (talk) 00:39, 14 August 2008 (UTC)[reply]

Your question is “how do GPS units handle this” but I think what you are really asking about goes beyond the basic function of a GPS receiver. The basic function is to use satellite signals to calculate latitude, longitude, altitude, time, and usually velocity. It carries on those functions without any “knowledge” of whether it is in a city, in the ocean, on a glacier, or wherever. If a particular GPS unit is able to tell you the name of the street you’re on (for example), that function is a frill: not part of the information supplied by the Navstar satellites but rather information that resides inside the GPS unit — sometimes you have to buy maps separately and load them in the unit either on a card or through a USB port, or the unit may have been sold with certain cartographic information already loaded in its memory. If your unit was designed for use in an automobile, or for use in a yacht, or whatever, then it probably won’t come loaded with detailed information on the features of individual glaciers. --Mathew5000 (talk) 06:44, 14 August 2008 (UTC)[reply]

While finding the position is certainly the core feature of GPS, a unit that doesn't put such info in context is of rather limited use. People on land want to know which geographic features are nearby, people in boats want to know which currents, shipping lanes, and ports they are near, and people in planes want to know which mountains, airpots, and national boundaries are in their proximity. You can, of course, take the GPS coords and then look them up on a series of maps and charts, but this can be time consuming, and possibly fatally so, if you are moving at a high velocity while attempting to determine your position relative to important features. StuRat (talk) 17:46, 15 August 2008 (UTC)[reply]

If you are working on a glaciers (such as a researcher), you would use recent satellite images as a "map". The satellite images would be referenced in relation to the bedrock. And the glacier is all relative to the bedrock below or whatever is sticking out. The exact precision of the GPS coordinates for this purpose of navigation isn't that important. One might use surveying techniques or something else to be more exact for scientific purposes. --Aude (talk) 10:10, 14 August 2008 (UTC)[reply]

I'm not thinking of scientists, but of explorers, adventurers, and mountain climbers, who would prefer to avoid falling down a crevass. Isn't there a GPS/navigation unit avilable for such people that has automatic updates provided for the revised positions of such features ? StuRat (talk) 17:31, 15 August 2008 (UTC)[reply]

Wikipedia:Reference desk/Archives/Science/2008 August 11

Contents