Wikipedia:Reference desk/Archives/Science/2015 February 14

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February 14

MMR Vaccine side effects

What are the potential adverse effects of the MMR, and do they differ in an adult who has received 1 dose previously, from someone who has never received 1 dose? — Preceding unsigned comment added by 198.255.253.60 (talk) 00:44, 14 February 2015 (UTC)

Assuming you've read MMR vaccine, talk to your healthcare provider, we cannot provide medical advice. μηδείς (talk) 00:48, 14 February 2015 (UTC)

What does N/A stands for?

I read about the PR interval in the table, and there is N/A in this squareץ What does it stands for?5.28.165.129 (talk) 02:09, 14 February 2015 (UTC)

It would be better to have a link to the page you are referring to, but it sounds like n/a. ―Mandruss ☎ 02:13, 14 February 2015 (UTC)

Typically "not applicable". ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:31, 14 February 2015 (UTC)

Please, look at the page here here is the original text (n/a)5.28.165.129 (talk) 03:35, 14 February 2015 (UTC)

• Edited the above to make it a link. --70.49.169.244 (talk) 10:32, 14 February 2015 (UTC)

Yes, that looks like n/a. <--- That is a link, click it. ―Mandruss ☎ 03:38, 14 February 2015 (UTC)

So, since the P wave was "absent or not related" in this idioventricular rhythm, there is no PR interval and therefore it is "not applicable". In another case, N/A might mean "not available". --70.49.169.244 (talk) 10:32, 14 February 2015 (UTC)

Thank you. 5.28.165.129 (talk) 16:59, 14 February 2015 (UTC)

What is the kink in a thermometer ?

What is the thing kink in the thermometer ? I have read this in one encyclopedia.I thought this one is the structure in mercury thermometer.Arvind asia (talk) 06:40, 14 February 2015 (UTC)

What are you talking about? Can you find an example on Google Images and link it here? ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:47, 14 February 2015 (UTC)

Here is a picture of a mercury thermometer. Where are you seeing a "kink" in it? ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:02, 14 February 2015 (UTC)

That isn't a medical thermometer. --Modocc (talk) 07:08, 14 February 2015 (UTC)

The OP didn't say anything about a medical thermometer. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:42, 14 February 2015 (UTC)

A Google search on kink in the thermometer gives results explaining that the "kink" is a constriction near the bulb that prevents the mercury from falling after its removal. --Modocc (talk) 07:08, 14 February 2015 (UTC)

That would make sense, if it's what the OP is asking about. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:42, 14 February 2015 (UTC)

I think it is. The medical thermometer article explains it (under "liquid-filled") but rather briefly. The point is that you want to measure the temperature at a particular place on a person's body (mouth, rectum, armpit, etc.) but it isn't convenient to read the thermometer while it's inserted there. And as soon as you pull it out, the temperature drops, so after inserting the thermometer you would have to read it right away to get a valid reading. Instead the thermometer is made with a constriction—a kink—between the bulb and the tube. Then when the temperature of the bulb drops, the mercury (or other liquid) will not return to the bulb, the reading stays the same, and you can read it at your convenience. After doing that, you swing the thermometer with the bulb outwards and this forces the mercury back down through the construction (this works particularly well with mercury since it's so dense). So the thermometer is ready to use again once it's been cleaned.

Meteorologists used to use the same kind of thermometer to determine the high temperature each day. I presume they now determine it based on records from some sort of digital thermometer. --70.49.169.244 (talk) 10:46, 14 February 2015 (UTC)

Yes. In the US, most weather stations went digital in the 80s. Though the maximum/minimum thermometers present immediately before that primarily used a different approach than the one you mention. They had two indicator pellets in the tube that were designed to stay at the maximum and minimum position respectively rather than having the column be constrained to prevent mercury from returning. Dragons flight (talk) 20:36, 15 February 2015 (UTC)

I'm not sure if it is relevant to the original poster, but a reversing thermometer also has a kind of "kink" in it. Dragons flight (talk) 20:36, 15 February 2015 (UTC)

There is more at Mercury-in-glass thermometer#Maximum thermometer including an image. Today we use a digital thermometer but we still have the mercury thermometers to make a weekly comparison. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 00:50, 16 February 2015 (UTC)

What is been the cause of local fighting in the East of the Republic Ukraine?

Not a Science Desk question. Robert McClenon (talk) 16:44, 14 February 2015 (UTC)
The following discussion has been closed. Please do not modify it.
1) Did the military local conflict in the East of Republic Ukraine had religious reasons, if as in Russia Imperia the Cathedra of Moscow Patriarchate in Russia had ensures the stability of the whole Orthodoxy, but because the split of the Cathedra of Moscow Patriarchate in Russia not had, and now could not had been at all, what is been the cause of local fighting in the East of the Republic Ukraine? 2) Did the local military conflict in the East of the Republic Ukraine been same similarity with the local military conflicts in the Republic of Moldova? — Preceding unsigned comment added by 83.237.221.6 (talk) 06:50, 14 February 2015 (UTC) --83.237.221.6 (talk) 06:50, 14 February 2015 (UTC) It's not about religion, it's about Russian expansionism. They already took Crimea, and they want as much of the rest of Ukraine as they can get. ←Baseball Bugs What's up, Doc? carrots→ 06:54, 14 February 2015 (UTC) I think that the West of the Republic Ukraine is been a passive participant in the local military conflict of the East of the Republic Ukraine.--83.237.193.210 (talk) 07:14, 14 February 2015 (UTC) Well, the western part of the country was instrumental in ousting the Putin buddy who was their president, and that's what triggered Russia's aggression into Ukraine. ←Baseball Bugs What's up, Doc? carrots→ 07:43, 14 February 2015 (UTC) WP:NOTAFORUM. There are plenty of sources which support your interpretation of events here, but at most you should be citing them, or discussing well-established facts, historical or otherwise. But this is not the appropriate place to be carrying on discussions concerning our personal speculation about the motives of the Russian government or the absolute proximal causes of the conflict. Snow talk 10:43, 14 February 2015 (UTC) Who are you talking to? And besides that, what is this doing on the Science page? ←Baseball Bugs What's up, Doc? carrots→ 10:47, 14 February 2015 (UTC) I suppose I was talking to both of you, but I can't expect the IP to have the experience to understand the distinction between what kind of discussions we are meant to be having here and how they differ from an open forum. But yes, you are correct -- clearly this the wrong desk for this question from a topical standpoint. Snow talk 10:59, 14 February 2015 (UTC) I seem that this page is not a public forum for a differents discussion, but the someway or anyway, the local military conflict is benign between of the East of the Republic Ukraine. Thanks!--83.237.240.49 (talk) 12:04, 14 February 2015 (UTC) Well, I don't understand what you're saying. Maybe someone will, and can help you better. ←Baseball Bugs What's up, Doc? carrots→ 13:00, 14 February 2015 (UTC) I recommend you go to List_of_wars_involving_Ukraine#Since_1991, which has links to the ongoing crises of the last year or so. ←Baseball Bugs What's up, Doc? carrots→ 13:03, 14 February 2015 (UTC) Not only is this not a public forum for discussion, it's also the science reference desk. The could be questions relating to the conflict which would fit here. But neither of the two above come close. Nil Einne (talk) 13:01, 14 February 2015 (UTC)

Friction force must be velocity dependent

I'm pretty sure that the friction force must vanish as the relative velocity of the two objects in contact approaches zero. But the equation $F=\mu F_{N}$ doesn't seem to be able to take this into account.

The naive $m{\frac {d^{2}x}{dt^{2}}}=-\mu F_{N}$ would only lead to position and velocity escaping without bound to negative infinity, so I obviously need a better equation of motion, e.g. for modeling when a shuffleboard piece comes to rest due to friction alone.--Jasper Deng (talk) 07:35, 14 February 2015 (UTC)

Your assumption is wrong. Gently push on a book on your desk. Does it move? Its velocity is 0 with respect to the desk, yet friction is non-zero, because the book doesn't move. --98.232.12.250 (talk) 08:06, 14 February 2015 (UTC)

That's not the point. This is assuming that there is no external force. An object in motion without any force but friction and the normal force still experiences a nonzero friction force, and will eventually come to a stop (think an ice hockey puck on a large sheet of flat (real) ice). If the friction force were then still nonzero, then the object would accelerate. Absent any motive force to move an object, the friction force must be zero at rest, because there cannot be a net force in any direction.--Jasper Deng (talk) 08:14, 14 February 2015 (UTC)

The coefficient of static friction is not always exactly the same as the coefficient of sliding friction, but for most materials they are very close. There is one error in your equation: The friction force does not always attain its maximum value, so the correct version of your first equation is $F$_max$=\mu F_{N}$, though the friction force will retain its maximum value whilst sliding continues.

You also seem to have a sign error not in your second equation but in your interpretation of it for sliding friction. The friction force always opposes relative motion. Dbfirs 08:21, 14 February 2015 (UTC)

If friction was proportional to relative speed, then objects would glide gently to rest like a well-driven car. If you observe carefully, they come to rest abruptly (as with harsh braking). Friction reduces to zero, of course, when there is no relative motion and no other force parallel to the surfaces. Dbfirs 08:25, 14 February 2015 (UTC)

Seems like the work-energy theorem is a more viable strategy for this kind of problem, since from what I can tell, there's a discontinuity between kinetic and static friction and I have no formula for the actual friction force (rather than its maximum).--Jasper Deng (talk) 08:31, 14 February 2015 (UTC)

Yes, work done by friction is a useful quantity to consider, but it uses the fact that sliding friction is constant at its maximum value. You can calculate static friction quite easily by considering other forces on the system in equilibrium, and remembering that friction cannot exceed its maximum value. Dbfirs 09:28, 14 February 2015 (UTC)

With a constant deceleration due to the friction, the finite distance traveled is v²/a. -Modocc (talk) 08:44, 14 February 2015 (UTC)

Actually it would be v²/(2a) using $v^{2}=v_{0}^{2}+2a(x-x_{0})$.--Jasper Deng (talk) 09:21, 14 February 2015 (UTC)

It's half as far (those 2s tend to sneak in there, yet it's still finite :-). I should have cracked open my forty year old physics book before I answered. --Modocc (talk) 09:37, 14 February 2015 (UTC)

Sorry, I believe that gravity is always determining the action of all the forces in nature of the planet Earth and also vectors of these forces, because in case of the absence of gravity the action of the forces is being disproportionate (mathematical dysfunction), that is, in this case the universal balance of the Law of conservation of energy did not observed in nature, besides that such physical-mathematical unit as mass never had inverse proportionate.--83.237.198.2 (talk) 08:58, 14 February 2015 (UTC)

Hi Alex, What on earth are you going on about? No, don't answer that question or people will really think you are a troll! Dbfirs 09:23, 14 February 2015 (UTC)

I’m sorry, the question being interesting, I’m still using translator, of course I’m not a troll.--83.237.198.2 (talk) 09:46, 14 February 2015 (UTC)

See [1] for a model of the recovery of static friction. It's behind a paywall, but libraries do have copies [2]. -Modocc (talk) 11:16, 14 February 2015 (UTC)

Maybe helpful: D'Alembert_Principle ? --217.84.69.159 (talk) 16:41, 15 February 2015 (UTC)

Friction is more complicated. For your desk example, you can place a book on a level desk and it will not move. You can incline the desk at 15 degrees and it may still not slide. All the forces changed and friction opposes motion. Both the force that wants the book to slide and the friction force are in balance. Friction and gravity re both non-zero but their net force (and motion) is zero. If you continue to incline the desk and overcome the static friction force, it's replaced with a dynamic force that can be quite different. Friction opposes motion but the force isn't necessarily zero when the object is at rest. --DHeyward (talk) 04:48, 16 February 2015 (UTC)

I should've clarified my assumption that friction is the only force except the normal force acting on the object in question.--Jasper Deng (talk) 05:01, 16 February 2015 (UTC)

Writing research reports

When writing research reports is it wise to write any of the discussion, evaluation or conclusions before you have a full set of results? I've heard you can write introduction and methodology but nothing after results section. 194.66.246.19 (talk) 11:54, 14 February 2015 (UTC)

A research report is means to document and communicate research findings. If there are organization- or publication-defined guidelines that you are required to follow, follow them. Otherwise, you can do whatever you want. Of course, some ways of structuring a report are better than others at helping the reader understand the report's content. Typically reports in general will have an abstract that explains what the report is about and a very brief description of its findings. It's there to help the reader quickly decide whether they want to read the actual report. There may also be an executive summary for those who need to get the gist of it but have no time or need for all the gory details. Concise statements of important facts, conclusions, and implications are appropriate there. To help the reader understand its content, you usually begin a report with an introduction that explains what problem the it addresses, why the investigation was done, the methodology used, the major findings and their significance. You may also want to provide some background information (relevant definitions, facts already known, etc) to help non-specialists understand the main content of the report. You then go into the methodology & design of the study, with enough details to enable the reader to evaluate the soundness of the methodology and to try to replicate the study if they wanted to. You then present the data collected and how they were processed to yield relevant statistics and interpretations. At this point the reader has been exposed to the details of the study and its findings. You can have a discussion of the findings and their implications if you want to. You structure your report to help your reader understand its content. Whether a particular document structure makes sense should be evaluated against that goal. If you're not constrained by rules that others require you to follow, just do what you think makes sense. --98.114.146.37 (talk) 14:44, 14 February 2015 (UTC)

Oops. After seeing Looie496's answer, I realized I misunderstood the question. --98.114.146.37 (talk) 18:51, 14 February 2015 (UTC)

• It isn't wise to write anything that depends on the results before the results are in, because there is a high probability that you will fail to spot some of the errors when revising the manuscript, causing incorrect information to get into the final product. Looie496 (talk) 17:28, 14 February 2015 (UTC)

I'd suggest that there may be arguments for writing a draft introduction (what you are trying to discover), and methodology (how you are trying to do it) before you do the research - this may help clarify your objectives. You will probably have to revise the drafts later, but they should provide a starting point. AndyTheGrump (talk) 20:06, 14 February 2015 (UTC)

The original question is somewhat unclear. You can write the paper in whatever order you want. Some people like to write the intro first so that they can get their thoughts together. Others write up the results as they build them, then write the intro so that it builds effectively toward the findings. When the paper is submitted everything has to be in the order specified by the journal (or your prof, or whoever).

There's really no right or wrong way. As always the three keys to writing are revision, revision, and revision. Short Brigade Harvester Boris (talk) 20:26, 14 February 2015 (UTC)

Testing a helmet by being driven over by a tank

Is that a meaningful method to test a helmet as Michael Schumacher did here Michael_Schumacher#Helmet? Tanks are neither too fast, and they distribute pretty well their weight. Add to this that the scenario is totally unrealistic. Noopolo (talk) 14:40, 14 February 2015 (UTC)

Which tank are we talking? They come in all sizes and weights. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 25 Shevat 5775 14:45, 14 February 2015 (UTC)

The article doesn't say, but any tank will distribute quite well its weight, this is the idea of having tracks.--Noopolo (talk) 15:43, 14 February 2015 (UTC)

I could see where a helmet might survive being slowly run over by a tank tread, but that doesn't prove it would survive a sudden, high-speed blow (nor would its wearer necessarily). ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:39, 14 February 2015 (UTC)

Ah, I see what he was talking about now. It's a racing helmet. I was imagining one of those WWII helmets. I think the tank most likely would have been some manner of Leopard 2 given that it's Germany, unless they dusted off an old Königstiger (the ultimate test). Sir William Matthew Flinders Petrie | ^{Say Shalom!} 25 Shevat 5775 20:38, 14 February 2015 (UTC)

This is the only image I could find of the tank vs helmet demonstration. I'm not clever enough to tell what sort of tank it is, but the bogies are quite different from those on an M1 Abrams, Leopard 2, Challenger 2 and Leclerc tank, which suggests either it's not a tank but another type of tracked vehicle, or a Russian design. Russian tanks tend to have wide tracks and a low track pressure so that they are effective in snow.

See also Pressure ballerina vs tank which demonstrates that an FV101 Scorpion armoured reconnaissance vehicle (tracked) or "light tank" has a lighter footprint than a ballet dancer. However, see also Tank Crushes Motorcycle Helmet from an admirable YouTube channel called tanks crushing things. Alansplodge (talk) 21:46, 14 February 2015 (UTC)

Tank! You want to see what a jet engine does to a helmet – unfortunately, in this clip, the guy was still wearing it at the time he got sucked into the engine. [3]. --Aspro (talk) 22:45, 14 February 2015 (UTC)

black body radiation 86.190.50.197 (talk) 15:39, 14 February 2015 (UTC)

when planck was developing his theory, how did the researchers measure the temperature of the source and the intensity of the radiation ?

You could be talking bolometers here for radiation. Maybe a thermometer would work for measuring the temperature?--31.51.19.57 (talk) 22:00, 14 February 2015 (UTC)

aurora borealis

I wondered why you left out the sound the Northern Lights makes. If conditions are right the Northern Lights can be heard by the naked ear. Sounds of the Northern Lights is now a science in its self. — Preceding unsigned comment added by 2601:6:E00:92C0:F0FD:CD00:80CD:824C (talk) 15:42, 14 February 2015 (UTC)

Wikipedia is an encyclopedia that anyone can contribute to. Please add your own references to this audio phenomena on that article page. This is how Wikipedia expands--Aspro (talk) 16:41, 14 February 2015 (UTC)

Could one of the planets of the Solar System previously be a space star?

Could one of the planets of the Solar System previously be a space star in the Solar System, if the distortions of planetary orbits of the space stars are always different from the distortions of the planetary orbits of the planets of the Solar System?--83.237.206.243 (talk) 16:16, 14 February 2015 (UTC)

In Holly Bible told us that the new space star was start shine when Jesus Christ was born, because astrology told us about born a new space constellation.--83.237.206.243 (talk) 16:31, 14 February 2015 (UTC)

No. Planets are not dead stars. And I doubt life on Earth would have survived a death of a star in the solar system. The Holy Bible is wrong. Noopolo (talk) 16:50, 14 February 2015 (UTC)

The Gospel of Matthew doesn't say anything about a new constellation, only a "star", and the concept of "star" to astrologers two thousand years ago was not what the same as it is today. Robert McClenon (talk) 17:07, 14 February 2015 (UTC)

Right. No one knows what the Star of Bethlehem actually was (assuming it actually existed). So the Holy Bible is not "wrong" in that sense. The Holly Bible would be the definitive work about techniques for raising shrubs with green pointy leaves and red berries. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:36, 14 February 2015 (UTC)

Doubt not the gospel of the red-suited one and his sleigh driving apostles! Sir William Matthew Flinders Petrie | ^{Say Shalom!} 26 Shevat 5775 03:39, 15 February 2015 (UTC)

There are many heavenly bodies outside our solar system. The number of stars is beyond count (but not estimation), especially if the multiverse is a thing. Constellations are man-made constructs, not anything of the universe like a nebula or a galaxy. I don't know anything about the history of the night sky, but if there was a bright light from space in the sky the night Christ was born, it was likely a very far-away supernova, which is a type of star death, whose light was just reaching us then. The actual star would have died thousands, millions of years ago depending on how far away it was. The sky is a constantly changing film of the past showing different heavenly bodies at different points in their lives throughout time. However, there's no dead stars in our solar system as Noopolo said. The different parts of the Bible were written over an almost millennium-long span when people had a more limited understanding of the universe—not that ours is much better, but we can explain some things through fancy equations, yay physics! Sir William Matthew Flinders Petrie | ^{Say Shalom!} 25 Shevat 5775 17:03, 14 February 2015 (UTC)

One problem with the "Jesus birth star" being real is that it couldn't guide anyone anywhere. Just like any other star, it would appear to be in a different direction depending on your location on Earth and the time of day, so it wouldn't "point" to a particular manger in Bethlehem. StuRat (talk) 03:32, 15 February 2015 (UTC)

Could be coincidence, or maybe all those miracles did happen in some form. We'll never know. The existence and nature of this particular Nazarene is on shaky ground as it is. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 26 Shevat 5775 03:39, 15 February 2015 (UTC)

Every space constellation always had its own shining star, why the case which described by me could be an exception?--83.237.241.134 (talk) 17:47, 14 February 2015 (UTC)

Constellations are typically patterns of multiple bright stars that people spot in the sky and then apply a mythical figure or shape. They vary from culture to culture. That particular star would have had to have been a very bright one and would suggest a supernova, though I don't remember how long a supernova is supposed to last. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 25 Shevat 5775 18:06, 14 February 2015 (UTC)

• A ty govorish po-Russki? μηδείς (talk) 18:39, 14 February 2015 (UTC)

For people who are not close friends or family, they'd use the 2nd person plural: "A vy govorite po-russki?". (And it's lower case r in russki.) But to whom was your question addressed? -- Jack of Oz ^{[pleasantries]} 03:39, 15 February 2015 (UTC)

Suelo tutear en la internet, y hablaba a http://whatismyipaddress.com/ip/83.237.206.243 μηδείς (talk) 04:18, 15 February 2015 (UTC)

Bitte, mein Luftkissenfahrzeug ist voller Aale. Short Brigade Harvester Boris (talk) 04:37, 15 February 2015 (UTC)

[[

File:Match-1.jpg|50px]]

Wenn es keine Zitteraale sind, wirf sie einfach wieder ins Wasser. ✦ _hugarheimur 14:04, 15 February 2015 (UTC)

"Suelo tutear" means "I am used to speaking on familiar terms"--I give the translation as I see it gives google translate fits. Here's my thanks for the eels!μηδείς (talk) 18:21, 15 February 2015 (UTC)

Medeis, I know we don't often agree, but funnily enough, the exact same question (albeit in much rustier Russian) occurred to me too. A familiar posting style? AlexTiefling (talk) 12:03, 15 February 2015 (UTC)

To summarize the above: No. The answer is no. Evan ^{(talk|contribs)} 02:21, 15 February 2015 (UTC)

• See Velikovsky's ludicrous Worlds in Collision. μηδείς (talk) 03:12, 15 February 2015 (UTC)

Velikovksy didn't say that Venus had been a "space star", whatever a space star is. He said that it had been a comet ejected from Jupiter. That doesn't change the fact that his claims were absurd. By the way, Babylonian records concerning Venus go back to 1700 BCE, before the time that Velikovsky said Venus had been ejected from Jupiter. Robert McClenon (talk) 04:08, 15 February 2015 (UTC)

Gravity being determinates all abilities (possibilities) of the Universe, as well as the all abilities (possibilities) of everything that existing in this Universe. Astrologers once upon was claimed that there was been a space constellation in the Solar System a luminate - luminary (shine) of which was been a new shine star. So be that’s the Sun is been a super-new shine star!--83.237.217.67 (talk) 08:49, 15 February 2015 (UTC)

The Universe is benign illuminates by light of constellations of galaxies which been nearest with the Solar System, while distant galaxies of the Universe are constantly been in the darkness, so it turns be out that the center of gravity of the Universe is been either the center of the Solar System, or it been in the nearest galaxies with the Solar System.--83.237.194.41 (talk) 10:07, 15 February 2015 (UTC)

Why did the work of magnetic energy – work of magnetism which is always been a gravity could be not ionize the atmosphere of planets and stars in the Universe?--83.237.241.114 (talk) 10:39, 15 February 2015 (UTC)

You remind me of Columbus on his voyage, except you're heading for the edge. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:48, 15 February 2015 (UTC)

Gravity is been a force and energy of the Universe!--83.237.245.203 (talk) 12:02, 15 February 2015 (UTC)

It's a force, no question about it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:25, 15 February 2015 (UTC)

Thanks, I did it!--83.237.245.203 (talk) 14:02, 15 February 2015 (UTC)

There is one more hypothesis fully-determining the value of gravity in the Universe, according to which the outside of the Universe there had been an excessive gravity, so that’s the Universe (galaxies of the Universe) been flaying in a different directions and soon will cease to existing.--83.237.245.203 (talk) 15:59, 15 February 2015 (UTC)

Near as I can tell, the Universe has not flayed anyone or anything as of late as it's a rather rude practice. As for how long it will last, the article, Future of an expanding universe is a good place to look. It's so far off though, that we don't really have to worry about it/will likely have wiped ourselves out by then. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 26 Shevat 5775 16:16, 15 February 2015 (UTC)

Excessive gravity been ripping (rip) the Universe into apart, so that could imagine how function the speed of light in the Universe.--83.237.246.150 (talk) 18:49, 15 February 2015 (UTC)

The speed of light should remain constant lest whoever's in charge of the Universe decides to increase it so as to make travel betwixt those much farther apart heavenly bodies easier, but that's getting into crystal ball territory. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 26 Shevat 5775 22:16, 15 February 2015 (UTC)

I thinking that, the absolute value of the all accelerations in nature is always been a value of gravity!--83.237.200.93 (talk) 09:20, 16 February 2015 (UTC)

In all cases it is always been ∆p=∆p, because impulse in nature is always been constantly (savedly).--83.237.200.93 (talk) 09:49, 16 February 2015 (UTC)

1) What mass had the Universe?--85.141.234.186 (talk) 06:35, 16 February 2015 (UTC)

2) The physical-mathematical value (unit) of mass of the Universe is been absolute or relative?--83.237.202.130 (talk) 07:08, 16 February 2015 (UTC)

Both your use of English and your understanding of physics are so poor that I'm not sure this is a conversation we can meaningfully have. AlexTiefling (talk) 09:32, 16 February 2015 (UTC)

Is it be c² = g = W (Joule’s)?--83.237.200.93 (talk) 11:54, 16 February 2015 (UTC)

As I’m know the physics told us that c ≈ a.--85.141.234.70 (talk) 15:08, 16 February 2015 (UTC)

Clearing the neighbourhood

Imagine that there are two Earths in the same orbit, each on the opposite side of the Sun, i.e. right now the other planet's at the spot we occupy in August. Would this, by definition, mean that neither one had cleared the neighbourhood? We don't see multiple bodies of comparable size in any single orbit, but I'm not clear if this is because (1) this just isn't the way things have turned out, or (2) in the very long term, this kind of situation isn't stable enough to persist indefinitely. Nyttend (talk) 17:44, 14 February 2015 (UTC)

Hm, I just discovered Co-orbital configuration and 3753 Cruithne. I'm not clear how these fit into my original question, since Cruithne definitely isn't in the same orbit, and I don't really understand the other terminology. Nyttend (talk) 17:49, 14 February 2015 (UTC)

I don't have a source, but that sort of orbit (opposite sides of the sun) is not a stable one. The "clearing its orbit" definition isn't scientifically defined in any case, it's an ad hoc qualifiction made by the IAU to the definition of a planet planet to exclude Pluto "Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves." Of course "clearing its orbit" also excludes Neptune, since Neptune hasn't cleared Pluto from its orbit. μηδείς (talk) 18:36, 14 February 2015 (UTC)

I don't know about that. Pluto doesn't actually cross Neptune's orbit due to the tilt in its orbit. Mingmingla (talk) 19:50, 14 February 2015 (UTC)

The obvious question, then, is "What crosses Pluto's orbit?" The whole issue is an embarrassing adhockery. μηδείς (talk) 02:07, 15 February 2015 (UTC)

The hypothetical planet opposite the Earth on the other side of the Sun would be at L3. As Lagrange point notes, that is an unstable equilibrium. (We know that there isn't a planet there because, if there were, its perturbations of Mars and Venus would have been observed.) Also, clearing the neighborhood can be said not to apply to Pluto, just as it doesn't apply to Triton and Nereid. Triton and Nereid are in stable orbits around Neptune, and Pluto is in a stable 3:2 resonance with Neptune. Robert McClenon (talk) 19:12, 14 February 2015 (UTC)

Thanks, Robert. I knew I had read that somewhere. μηδείς (talk) 02:05, 15 February 2015 (UTC)

Or even aside from that, our space probes would have seen it by now; we'd have two pale blue dots, for example. But why are Lagrange points relevant? If I understand that article rightly, the points are stable places for objects significantly smaller than the two big objects, e.g. minor moons or artificial satellites, not an object equal in size to one of the two. Nyttend (talk) 20:59, 14 February 2015 (UTC)

Lagrange points are relevant because they're the only configurations of three bodies where they maintain their geometrical relationship, and the Earth/Sun/Counter-Earth requires the three bodies to stay in a straight line. I think the wording about one body being smaller is only there because if it isn't then even the otherwise stable configurations may not be stable. --70.49.169.244 (talk) 01:44, 15 February 2015 (UTC)

Trivia note: One version of Krypton (comics) was that Superman's birthplace was in the same orbital plane and opposite from earth. Not scientifically sound, as noted, but it made sense for it to be called Krypton, that word coming from "hidden". ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:30, 14 February 2015 (UTC)

Yes, but the eventual conclusion was that Krypton had a red sun, and that on a planet with a yellow sun, its inhabitants would be supermen. Based on modern astronomy (not that of the golden age of comics), that would have to be a red dwarf, because we now know that a red giant is a late phase in stellar evolution in which the star, among other things, destroys its planets. Robert McClenon (talk) 21:20, 14 February 2015 (UTC)

Yes. The concept of Krypton (and of Superman himself) evolved substantially over the years. The original Superman comic (Action Comics #1 in 1938) as well as the early 1940s cartoons had him making huge leaps rather than flying. Making huge leaps makes more scientific sense, but flying is more dramatic. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:47, 14 February 2015 (UTC)

I heard somewhere-or-other that the flying was introduced in animations, because it saved work somehow. —Tamfang (talk) 08:09, 15 February 2015 (UTC)

Lagrangian point may be of interest. Maproom (talk) 21:32, 14 February 2015 (UTC)

My thanks to Robert for the revelant soyrce. μηδείς (talk) 21:34, 14 February 2015 (UTC)

Hint: On your keyboard you may find that just next to the letter Y and on the right, is a letter U. That's the one I think you want. 'revelant'... my L's sometimes wonder too. I put it down to too much retsina ;-) Actually. That would make a good ref desk question: Is it possible to quaff too much retsina?--Aspro (talk) 23:02, 14 February 2015 (UTC)

The problem is that if I took the toim to correct all me misspellings I'dv'd twoice as many edits as I already due. Sew I don't. And althow my username (and given) name's Greek, I myself am not a Hellene--not that their's anyfing wrong wivvat. μηδείς (talk) 02:54, 15 February 2015 (UTC)

It's natural for a Greek to confuse 'y' and 'u', especially after 'o'. —Tamfang (talk) 08:09, 15 February 2015 (UTC)

It's by no means a proper answer, but this may prove useful in visualizing the effects on the general stability of such a configuration, though unfortunately the "game" requires you to start with an arbitrarily-placed body and has a maximum time-frame of 500 years -- and getting two different planets on the exact opposite ends of an identical orbit could be tricky. Aaaaaand now I've just consumed a portion of the day for half a dozen ref-deskers. Which is ironic in that I recently came across that thing while scouring NASA archives for Wiki purposes. Snow talk 00:56, 15 February 2015 (UTC)

Thanks! It helped me to see that, even with a slightly off-placement of a same-size Earth, things went haywire within a few decades. Nyttend (talk) 03:01, 15 February 2015 (UTC)

You're welcome; I'm glad it proved useful! Though it's probably already obvious, it's worth noting that it would be technically possible for two bodies of exactly equal mass, traveling at equal velocity to co-exist in an identical orbit for a vast span of time, but the addition of even a single extra body that is close or massive enough to influence or be influenced by them to a significant degree, in terms of gravitation, is all but certain to make the overall system unstable (though it will not necessarily be the two bodies co-habitating in a orbit who will be dislodged from orbit first). Even then, it would be technically possible to have multiple sets of cohabitating planets provided that either A) their influence upon one-another with regard to gravitation was trivial, owing to their proximity to one-another and their masses relative to the star they all orbit, or B) if their orbits (and velocities within those orbits) were arranged in a highly regular fashion. But the chances against a star system developing such a regular arrangement are...well, I was about to say "astronomical", but given the circumstances, we can see even that term doesn't suffice.  :) Snow talk 08:45, 15 February 2015 (UTC)

Since Krypton has already been invoked, I might as well mention that in the Gorean Saga books by John Norman, the planet Gor is located in the Counter-Earth position and hence unknown to terrestrial scientists: Gor's resident Priest-kings (giant super-intelligent ants) use their advanced science to counteract all orbital instabilities and 3rd-body perturbations that would otherwise reveal it. (Don't take this as a personal recommendation of the series which, in addition to whatever literary merits it might (according to taste) exhibit, is based on a somewhat controversial morality.) {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 14:04, 16 February 2015 (UTC)

Planet retrograding motion

Can the 4 outer planets (Jupiter, Saturn, Uranus, and Neptune) go retrograde altogether? Is this even possible? -- (Angelos|Angelus (talk) 2:40 pm, Yesterday (UTC−5))

I would suspect that if they went retrograde they would be torn apart. See the last line of the linked section. Apparent retrograde motion is even less possible. Mingmingla (talk) 2:45 pm, Yesterday (UTC−5)

To put any planet, let alone a giant planet, into a retrograde orbit, that is, one orbiting the sun contrary to the direction of solar rotation, would require an extraordinary amount of angular momentum to be changed, requiring an extraordinary amount of energy. Can you explain your question further? Robert McClenon (talk) 4:24 pm, Yesterday (UTC−5)

See apparent retrograde motion. Each of the four planets are overtaken by the Earth and undergo apparent retrograde in 399, 378, 370 and 367 day cycles respectively. Thus they do so together at the same time when their retrograde cycles are overlapping. Mathematically, these are cycles that produce beat frequencies of which one is a reduction of frequency (see heterodyne) and it is a sum of their paired absolute frequency differences, giving us a recurrence every 1/abs(abs((1/399)-(1/378))-abs((1/370)-(1/367))) days or about every 23 years (~8536.5 days), which is too frequent to be notable. -Modocc (talk) 15:20, 15 February 2015 (UTC)

Hmm, we've got definition and discussions of Synodic period in articles but I don't think that's the question. My quick search has failed to find a good, explicit description of retrograde loop. Anyway, retrograde loops can indeed be simultaneous in a grand, approximate Syzygy (astronomy). Far as I see, the best recent chance that this might have happened was in March 1982; see The Jupiter Effect. Jim.henderson (talk) 15:42, 15 February 2015 (UTC)

This is a case where the first editors to answer a question didn't understand. The OP was obviously asking about apparent retrograde motion, in "retrospect". Since the orbits of the giant planets are large, and each of them is in apparent retrograde motion almost half of the time, it will occasionally happen. The complication is that Neptune and Pluto are in a 3:2 resonance, but I haven't figured out how that affects it. Robert McClenon (talk) 15:52, 15 February 2015 (UTC)

You're right. I did misunderstand. My apologies. My first answer is true, but not relevant, and my second answer was answering a question that wasn't being asked. Mingmingla (talk) 16:42, 15 February 2015 (UTC)

Pluto not a problem; OP must have read How I Killed Pluto and Why It Had It Coming. Jim.henderson (talk) 16:22, 15 February 2015 (UTC)

If you go to this link , there is a button in the image of the solar system frame, above right, (I don't really know the exact date and can't check because I'm running low on Kbs as I rinsed more than 500Mbs in less than ten days this time and I don't know how...) if you date back to around 1985, somewhere in between the year, the aforementioned planets regrograde. To understand the sign of retrograding planet, open the 'find' option of your browser, type the word glow. I'm surprised that all four planets are retrograding at the same time. Is this correct/possible? -- (Angelos|Angelus (talk) 19:06, 15 February 2015 (UTC))

Of course it is possible. Why do you think that it would not be possible? Since the orbits of the four giant planets are large compared to that of the Earth, each of them will be in apparent retrograde motion nearly half the time. This means that the probability of all four of them being in apparent retrograde motion is slightly less than 0.5**4, or slightly less than 0.0625. If the four giant planets are all in more or less the same direction from the Sun, the motion of the Earth will cause them to all appear to go retrograde at the same time. Robert McClenon (talk) 19:32, 15 February 2015 (UTC)

I was confused with the thought that, if it is true than people who were born around this time, were the 'special' or 'f***ed up'...! I'm just analysing astronomy with astrology. If it does occur once in a awhile, how and what would be the cause... You all made the matter clear... Thank you all. -- (Angelos|Angelus (talk) 07:11, 16 February 2015 (UTC))

Yes, that is a good ballpark figure and for a similar amount of time that I can calculate without probability: From my above calculation based on the coinciding centers of the cycles, there is a minimum of 121/8536.5 = 0.014 or 1.4% of the time. From the data in the chart, the probability is (121/399)(138/378)(151/370)(158/367) = 0.01945 or 1.945% of the time. The exact figure should be close to the latter figure, 1.945%, (I say close because the distribution of retrograde events, whenever they are, is only pseudorandom) and calculable (and verifiable with simulation). --Modocc (talk) 21:13, 15 February 2015 (UTC)

Mean interval? Jim.henderson (talk) 21:38, 15 February 2015 (UTC)

The 8536.5 days I calculated above is an arithmetic mean and it is a period that exists because the cycles and retrogrades are clearly deterministic such that every event repeats itself in that time frame with beats (assuming orbit stability). -Modocc (talk) 23:02, 15 February 2015 (UTC)

I understand, that it is true and possible... Thank you all. -- (Angelos|Angelus (talk) 07:11, 16 February 2015 (UTC))

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