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October 3

Centre of mass of the Earth

Does the center of mass of the Earth change its position with respect to the plane of the equator and its axis. —Preceding unsigned comment added by 113.199.208.88 (talk) 08:51, 3 October 2010 (UTC)[reply]

Yes. According to this site, "The Earth's centre of gravity constantly changes position within a three-centimetre cube in response to mass redistributions on the surface of the globe, caused by shifting masses within the ocean and by soil moisture, snow cover and groundwater."

I would also have expected some movement due to convection currents in the liquid portions of the mantle and core (see Structure of the Earth) moving around material of slightly different densities, but I haven't found a specific reference for that (yet). 87.81.230.195 (talk) 12:36, 3 October 2010 (UTC)[reply]

And what is this amazing centre of gravity constraining three-centimetre cube is made out of? Nickel-iron alloy, presumably. -- 111.84.196.147 (talk) 13:17, 3 October 2010 (UTC)[reply]

As I'm sure you know, the '3cm cube' is just a visualization trick. in fact, the actual center of gravity of the Earth is probably only definable in terms of a 3-dimensional confidence interval, which probably has a shape (at any given confidence level) like a flattened sphere or fat disk (on the assumption that most of the variation is in the equatorial plane, and less is in the axial plane) --Ludwigs2 19:56, 3 October 2010 (UTC)[reply]

We have an article on polar motion which seems to be what you're looking for. There is also an effect called nutation, which is due to the moon. Physchim62 (talk) 00:40, 4 October 2010 (UTC)[reply]

What about the tides from the moon? Do they cause any changes to radial velocity relative to the Sun, within the Earth-moon gravitational centre system? ~A H 1^(TCU) 14:34, 3 October 2010 (UTC)[reply]

The answer to the original question is no. Both the equator and the axis of rotation pass through the center of mass by definition. If the center of mass moves (in relation to something else, like the surface of a large part of the Earth), then the equator and/or the axis of rotation move with it. --Anonymous, 06:35 UTC, October 4, 2010.

The post just above by anonymous would apply if the Earth was a prefect sphere. However, in reality (as one is taught in school geography lessons) it is a slightly pear shaped oblate spheroid giving ellipsoid that is triaxial . Thus the 'geometric' and 'centre of mass' is not necessarily going to coincide. Satellite ranging has already shown that for Mars, the centre of mass and the geometric centres do not coincided exactly either. The World Geodetic System 84 gets round this problem by ascertaining the centre of gravity (mass) and generating a perfect theoretical gravimetric ellipsoid as a datum from which all other spheres can be compared.--Aspro (talk) 08:47, 4 October 2010 (UTC)[reply]

No, I was talking about the real world, not a perfect sphere, and the center of mass, not any sort of geometric center. --Anonymous, 16:02 UTC, October 4, 2010.

The OP has said nothing about ignoring the cartographers equator and axis and geometric shape, and so as it stands it is a reasonable question. If he was taking about your interpretation of equator and axis there would be no point in asking it. He is asking about the 'real' world which is the point of his question. Your second post also contradicts your first. To my mind some people do ask pointless questions here but you appear to be prejudging either his motives or his intelligence, either which is in my view despicable.--Aspro (talk) 18:44, 4 October 2010 (UTC)[reply]

The question specificially defined what it was asking about -- the equator and axis in relation to the center of gravity. I answered in those terms after other people answered in other ways. I don't see how it can be construed as inappropriately judgemental to answer the question that was asked, but I'll just say that I was not intending to malign anyone. --Anonymous, 03:17 UTC, October 5, 2010.

So it seems your just begging the question, Also, you haven't indicated what part you disagree with in the first explanation which which covers what earth scientists are discovering about the real world. Suppose you might as well just argue with yourself then. --Aspro (talk) 21:52, 7 October 2010 (UTC)[reply]

stelar cartography

I have a map of the galaxy as seen from what I will, for want of a better term, call above. However, this uses a system of coordinates based on angles from a line passing from the earth to the centre of the galaxy, rather different to that used to determine where stars are from earth. How, then, would I go about plotting the location of a particular star on this map using the coordinates given for its location within the sky?

148.197.121.205 (talk) 12:10, 3 October 2010 (UTC)[reply]

Does celestial coordinate system help? --81.153.109.200 (talk) 12:16, 3 October 2010 (UTC)[reply]

(EC) The article Celestial coordinate system has links to more detailed descriptions of these two systems - the Galactic and the Equatorial systems - (as well as other), and a section explaining how to convert between the Equatorial and Horizontal systems, but not explicitly the conversion you need. However you may be able to use the correct terminologies in these articles to help search for the appropriate conversion, which I'm sure is out there.

[Addendum] Doh! See the first external link in the Galactic coordinate system article! 87.81.230.195 (talk) 12:21, 3 October 2010 (UTC)[reply]

Bio engineering

Does bio engineering have good, strong prospects in the future? I know bio engineering is a broad field, but I have applied for undergraduates at the moment so a rough explanation would suffice. And also microbiology. Thanks.--119.155.118.204 (talk) 15:29, 3 October 2010 (UTC)[reply]

The article biological engineering explains the wide and robust areas of research. Is there a specific topic you wanted information about? -- kainaw ™ 16:02, 3 October 2010 (UTC)[reply]

The prospects for both fields are excellent -- bioengineering in particular is bound to grow vastly in the next couple of decades. The prospects for an individual going into a field also depend on the amount of competition, though, and I don't know anything about that. Looie496 (talk) 16:22, 3 October 2010 (UTC)[reply]

Also take a look at biochemical engineering and biogeochemistry. ~A H 1^(TCU) 18:17, 3 October 2010 (UTC)[reply]

And biomedical engineering. Regards, --—Cyclonenim | Chat 21:06, 3 October 2010 (UTC)[reply]

How does a rocket "steer"?

In the article on the Chinese Chang'e 1 lunar probe, there's a picture of the rocket carrying the probe blasting off. The rocket seems to have only one engine and no fins. How does the rocket adjust its direction of travel? —Preceding unsigned comment added by 96.227.60.253 (talk) 16:24, 3 October 2010 (UTC)[reply]

See gimbal#Rocket engines -- Finlay McWalter ☻ Talk 16:40, 3 October 2010 (UTC)[reply]

This page has some nice diagrams and animations illustrating the principle. I guess because they are NASA we could import them to Wikipedia? --Mr.98 (talk) 16:43, 3 October 2010 (UTC)[reply]

Once launched, spacecraft use a reaction control system to manoeuvre. For some operations, like moving from a Lunar orbit into a return-to-Earth trajectory, spacecraft orient themselves with the RCS (which tends to be a pretty low-powered affair) and then engage a larger rocket engine for the big-deltaV burn. -- Finlay McWalter ☻ Talk 16:45, 3 October 2010 (UTC)[reply]

I should mention that, while gimballing is very popular, it's not the only option. Gimballing requires either rotating the whole engine (which is pretty impractical for a serious rocket) or rotating the engine bell (which requires an articulation, which can be tricky to get right given the pressures and temperatures involved). Other options include vernier rockets to turn the rocket (they're a bit like the RCS, above) or putting robust vanes into the exhaust stream (that's how a V-2 rocket steered]]). Lastly you might consider movable fins (or fins with movable flaps) but they become ineffective in the higher reaches of the atmosphere, so they're mostly found on low-level devices like short-range missiles. http://exploration.grc.nasa.gov/education/rocket/rktcontrl.html has more. -- Finlay McWalter ☻ Talk 16:54, 3 October 2010 (UTC)[reply]

Skimming the lengthy users' manual for the Long March 3A rocket that launched Chang'e 1, it seems the first stage is steered by gimballing (and the wording suggests they do gimble the whole engine, despite my claim that this is impractical☻), and that the second stage is controlled by verniers. The first stage has little fins, but these appear to be fixed and so only for basic stabilisation. -- Finlay McWalter ☻ Talk 17:04, 3 October 2010 (UTC)[reply]

Fins would in any case I presume be only effective while the trocket was travelling through the atmosphere, which is a very small proportion of its journey. --rossb (talk) 17:55, 3 October 2010 (UTC)[reply]

Rockets spend a sufficient time at a low enough altitude that some designers have felt it beneficial to include aerofins; in some cases steerable ones. Saturn V had aerofins (although the article says that future flights might have removed them, as they "turned out to provide little benefit when compared to their weight". The LM3a has them, and the users' manual for the Soyuz-U says "Attitude control is carried out through two movable vernier thrusters and one aerofin [that's for each of the 4 boosters]. Three-axis flight control is made possible through these eight engines (two per booster) and four aerofins (one per booster)". N1 didn't and Ariane doesn't (as far as I can tell). Indeed it doesn't seem worth the bother, but who am I to argue with Von Braun and Korolyev :) -- Finlay McWalter ☻ Talk 18:38, 3 October 2010 (UTC)[reply]

I'm going to jump in with a follow-up question of my own: how do they control the rocket attitude just at launch? It seems to me that even a tiny imbalance in the thrust while it's sitting still or barely moving would tip the whole thing over. This seems much different than guiding the rocket in flight. Franamax (talk) 19:14, 3 October 2010 (UTC)[reply]

The engines gimbal even while it's still on the stand. If you watch a video of an Apollo or Shuttle launch, you see the main engine bells move around after they light, as the engines take over responsibility for keeping the vehicle righted from the stand. For a second or so the rocket hovers over the blast pit before it lifts. It's probably easier than in flight, because there's no dynamic pressure imbalance trying to pitch the rocket. This kind of dynamic hover behaviour is more evident in rockets like Delta Clipper and Pixel. -- Finlay McWalter ☻ Talk 20:32, 3 October 2010 (UTC)[reply]

They had computers that could handle that — reliably — in the 1960s? --Trovatore (talk) 20:54, 3 October 2010 (UTC)[reply]

Seems like a pretty basic feedback-based system: "gyroscopes in nose are reading tilts of (x,y) (or even better, are reading change in that direction), start moving rear thruster more in the direction of (x,y)". As long as rocket is tilted in that direction, the thruster will keep pushing more and more in that same direction until there's just enough torque around the center-of-gravity to start correcting the tilt. It works fine as long as you're dealing with "small" changes. If you tilt too far over and/or don't have limits on the rates of change, gets scarily easy to over-compensate and swing out of control...just like driving on ice. All sorts of inertial guidance systems even for following a complex route are at heart just a difference amplifier that reports "target-value minus current-value" and the result piped to whatever mechanism steers "away from off-course". I've seen the airplane analog ("adjust flaps/rudder to make the plane do exactly what the pilot says to do", which means need feedback-control to make "actual plane" match "pilot action" to negate wind effects) in intro-engineering courses. DMacks (talk) 21:28, 3 October 2010 (UTC)[reply]

Never mind 1960s, a V2 rocket had to do that in the 1940s, so clearly no computer. It's a damped feedback system running off a gyroscope (over damp it and it's underreactive and a breeze blows it into the stand; under damp it and it over-reacts and does a little somersault into a school). What's impressive is not that they could do this in the 1960s with transistors, but that they could do it in the 1940s with, well, I don't know. Baling wire and little pulleys, perhaps :) -- Finlay McWalter ☻ Talk 21:11, 3 October 2010 (UTC)[reply]

There is a film for the V1 rocket on youtube (watch?v=HQccOvNG_ZY) which gives the pneumatic steering mechanism for the V1. There is a gyroscope which is driven by pressured air in a tank. The whole "guiding system" is pneumatic. So you would need no computer or even electricity for the guidance. --Stone (talk) 06:51, 4 October 2010 (UTC)[reply]

That's why they call it rocket science! Actually, this is control theory, (arguably the most important subset of rocket science). For every time-step "t", what set of input signals are necessary (valve settings; mechanical actuators; and so on), are required to guarantee stability, both mechanically and thermodynamically, for the rocket? The difficulty is that certain outputs (like rocket attitude and real-time thermodynamics of the combustion chamber) are impossible to know exactly until the instant of ignition; so a sophisticated bit of engineering goes in to providing a feedback loop from sensors back to the control algorithm. Control theory was designed long before engineers had knowledge of computers; very sophisticated systems can be built using simple electronic amplifiers and even mechanical governors. Now that everything is digitized, we can control at higher frequencies and to higher degrees of precision; in the case of rocketry, though (especially the attitude-control of large rockets), many parameters are physically large - so the relevant frequencies are very low (maybe tens of hertz). For this low-frequency control, gigahertz-speed computers aren't needed. On the other hand, the thermodynamics inside the combustion chamber can occur very fast, so it is useful to monitor it at very high speeds. However, control of these parameters is limited - valves that are controllable at hundreds of hertz do not exist, so this limits the utility of a fast computer. Safety and "emergency shutdown" has improved since the 1950s, though, as unstable combustion can now be detected mere nanoseconds after the chemistry starts going haywire, and a computer can make a decision (for example) to shut down or detonate the rocket before an uncontrolled flight becomes catastrophic. Nimur (talk) 18:44, 4 October 2010 (UTC)[reply]

The Merlin rocket engines of SpaceX's Falcon 9 (next launch expected in November), while gimballed, also pivot their turbo-pump gas generator exhaust for roll control. -- ToE^T 00:24, 4 October 2010 (UTC)[reply]

You may also wish to read Pendulum rocket fallacy. Ariel. (talk) 01:09, 4 October 2010 (UTC)[reply]

Ensembl sucks as*!

Resolved

– Asked and answered. That the questioner doesn't want to read the answer is not a refdesk concern. DMacks (talk) 19:23, 3 October 2010 (UTC)[reply]

What are the alternatives? --178.98.78.190 (talk) 17:03, 3 October 2010 (UTC)[reply]

Uh. doesn't the article clearly say in the lead? Nil Einne (talk) 17:45, 3 October 2010 (UTC)[reply]

I don't read the articles; they're full of inaccuracies. But thanks. :) --178.98.78.190 (talk) 18:26, 3 October 2010 (UTC)[reply]

Nil Einne I just checked, the article lead doesn't say it sucks as* at all! Should we add that in? ;) Franamax (talk) 19:18, 3 October 2010 (UTC)[reply]

Wikipedia is not censored, so we should expand the wild-card to its actual value, which I expect is 'asphalt', or possibly 'asymptotes'. --Ludwigs2 19:59, 3 October 2010 (UTC) [reply]

More likely to be Asperger's syndrome... Physchim62 (talk) 20:17, 3 October 2010 (UTC)[reply]

If you can find a reliable source perhaps. On a related note, when asking for alternatives it's generally better to explain in more detail what your problems are, since it may enable people to give better answers. To give a simple example, even if we know Ubuntu 'sucks as', it doesn't help us know which of the hundreds or whatever Linux distros may suck less for you. Nil Einne (talk) 06:04, 4 October 2010 (UTC)[reply]

Global oxygen production of Algae

I am looking for reliable sources giving or commenting on estimates of the percentage of global atmospheric oxygen produced by Algae. I mean Algae, as they are defined in the article's lede: excluding the prokaryotic blue or blue-green "Algae". Thank you in advance, any help is appreciated (including references that say it is impossible to give a meaningful estimate).---Sluzzelin talk 17:05, 3 October 2010 (UTC)[reply]

This article states about 60%, might be a starting point for further searches. PЄTЄRS J VЄСRUМВА ►TALK 17:59, 3 October 2010 (UTC)[reply]

On a side note, the global measured population of plankton, which includes eukaryotes including plant algae has declined by 40% within the last six decades. ~A H 1^(TCU) 18:14, 3 October 2010 (UTC)[reply]

Thanks to both of you. ---Sluzzelin talk 08:19, 5 October 2010 (UTC)[reply]

Masturbation

Will stopping or declining of masturbation effect in increasing one's level of testosterone and, subsequently, more male-like behaviour? —Preceding unsigned comment added by 83.31.134.230 (talk) 19:27, 3 October 2010 (UTC)[reply]

Perhaps it will do the opposite. --178.98.78.190 (talk) 20:24, 3 October 2010 (UTC)[reply]

No it will not. I think it would probably do the opposite. It will, however, increase the amount of ejaculate next time you ejaculate, as it's stored up.--92.251.236.197 (talk) 20:28, 3 October 2010 (UTC)[reply]

Lack of orgasm might cause increased testosterone level and increased sexual drive. Edison (talk) 00:13, 4 October 2010 (UTC)[reply]

As our article on testosterone points out, variations in levels are due to a much wide range of causes than frequency of orgasm. Physchim62 (talk) 00:43, 4 October 2010 (UTC)[reply]

As an aside, there is some evidence that regular ejaculation reduces your chances of getting some types of prostate cancer. See Prostate_cancer#Ejaculation_frequency. There is no requirements as to the cause of this ejaculation, so whether it is done alone or in conjunction with another consenting adult, the benefits are the same. This effect has not been explained mechanistically, but it may have something to do with regulating testosterone levels in the body. --Jayron32 16:17, 4 October 2010 (UTC)[reply]