Wikipedia:Reference desk/Archives/Science/2017 June 26

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June 26

A guy where I work contends that CSPAN is actually harming democracy and functioning of usa legislature.

He says because of cspan, politicians can't make deals, and can't have personal friendships across party lines. Is this true? Also, are there political scientists and journalists who have discussed this claim, either pro or con?65.103.249.243 (talk) 00:00, 26 June 2017 (UTC)

In the wake of the shooting of a Republican congressman, I heard some members of both parties talk about how they all get along personally, they just differ over policies. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:33, 26 June 2017 (UTC)

Question transferred to the Humanities Desk, whose topics include politics—here. --76.71.5.114 (talk) 00:33, 26 June 2017 (UTC)

Engine knocking

According to the engine knocking article, the phenomenon is distinct from, but routinely follows, instances of pre-ignition. Lower in the article, I read that "pre-ignition can destroy an engine in just a few strokes of the piston". Presumably you wouldn't have knocking if the engine were destroyed, so how does this work? Is it simply that pre-ignition sometimes doesn't destroy the engine in such a short period of time, so the knocking has a little time to manifest before the whole thing becomes useless? Nyttend (talk) 00:04, 26 June 2017 (UTC)

I think you are reading too much into it. I know of one engine that runs in preignition and knock for 10s of hours on the dyno, as a durability test. I suspect a liberal sprinkling of tags in that article would reveal that it is mostly factoids from on line forums. Greglocock (talk) 03:39, 26 June 2017 (UTC)

It depends on how far advanced in the power cycle the knocking or pre-ignition is taking place. The effect of a slightly advanced (a few degrees of rotation) spark can usually be heard when the accelerator is pressed hard under load. This causes engine wear, because the spark takes place while the piston is still rising on its compression stroke, and rings, pistons and big end bearings are subjected to loads they were not designed for. Obviously, the more advanced the spark, or the more advanced the pre-ignition due to other causes, the greater will be the damage. It's not unusual for piston rings to be cracked as a result. Sometimes, small broken pieces of the rings manage to fall out of their grooves and if they get sucked up by the oil pump, they may ruin the pump's gears very quickly or travel through the oil bores and score crankshaft or camshaft bearings. A kind of compounding of the problem may develop: the more advanced the spark, the weaker the performance of the engine, leading the driver to put his foot down harder on the accelerator, and causing more wear. Akld guy (talk) 05:42, 26 June 2017 (UTC)

• Knocking is after spark plug ignition, pre-ignition (also called rumble) is before spark. The basic reason for either is if the gas-air mixture stays too hot for too long it auto-ignites (depending on thermodynamical parameters), which is usually not desired as the spark allows combustion control. Knock autoignition is easier, because as the flame front develops around the spark plug gases outside the flame get compressed and heated a bit, plus there is more time for the auto-ignition delay to run out. Obviously, if you can have the hard thing (pre-ignition) you can have the easy thing (knock) unless pre-ignition is triggered by a solid surface that is a lot hotter than other places in the engine (e.g. the exhaust valve), so knock often follows pre-ignition.

The problem in both cases is that the pressure buildup in the cylinder takes place faster than intended, which can damage the engine. Pre-ignition is worse because it makes more gas burn in the uncontrolled mode and hence the pressure buildup is faster. Some modern engines are intended to run in permanent "preignition" mode but most are not (and consequently the dimensions, material strengths etc. cannot withstand rumble or prolonged knock). Tigraan^{Click here to contact me} 11:01, 26 June 2017 (UTC)

It's mostly a question of BMEP. The worse the engine is for predilection to knocking, the less damaging it can be. If the engine knocks at low loads, then the forces arre low and nothing gets damaged. If an engine is heavily loaded though, without knocking, and then suddenly begins to knock (such as a fault in the timing suddenly occurring, or failure in something like a water injection system), then it can indeed fail in a few strokes. Many engineers will have started out with a single cylinder variable compression research engine, built like a brick cowshed, in a test cell at university, happily listening to it knocking all day whilst demonstrating high compression and low fuel octane. They'll do it indefinitely, that's what they're for. But tune the 'nads off something highly strung, have some part of the control gear lose the plot momentarily, and it can be expensive.

That's a dreadful article though, hopelessly confused. To understand the fundamentals, there's still no substitute for sticking down quietly with the old editions of Ricardo and reading them cover to cover. Andy Dingley (talk) 11:18, 26 June 2017 (UTC)

Do you mean these books by Harry Ricardo 1885 - 1974 ? Blooteuth (talk) 15:05, 27 June 2017 (UTC)

Yes, particularly the 2nd, 3rd and 4th editions. They're not simply editions, they're pretty much different books (and a different author for the 3rd). The pre-war editions are expensive, late copies (and the modern paperback) are OK. Andy Dingley (talk) 15:28, 27 June 2017 (UTC)

Engine knocking on gas engines causes damages. Diesel engines rely on the self ignition. Diesel fuel burns slower. A cold diesel engine is hard and dirty to start. For this reason, have the glowplugs heat up, before turning the starter. Engine knocking on gas engines occurs form early ignition or wrong engine timing. Incomplete burned fuel or engine oil which came into the cylinder head by any other failure is still glowing from previous ignition. This ignites the new fuel earlier than the controlled ignition over the spark plug. This effect can be seen like stepping the pedal of a bike before it reaches the top position. As the engine has no free run, the pressure of the combustion forces a little reverse to the drive shaft but most against the curb shaft bearings and the cylinder head. Increasing the thermodynamic efficiency of the engine needs to use a more lean gas-air mixture, causing more heat drop on the cylinder and cylinder head. This is the reason for installing a knocking sensor into the engine. When knocking is detected, the mixture ratio will be changed by the engine controller unit. --Hans Haase (有问题吗) 17:54, 27 June 2017 (UTC)

Knocking is not the ignition process in a diesel engine. Nor is ignition in a diesel engine, or even a semi-diesel engine, the same process as knocking in a petrol engine. Andy Dingley (talk) 18:06, 27 June 2017 (UTC)

Yes. Likewise: The speed or propagation of the flame front is dependent on the hexane number. The fuel is injection starts just a few degrees BTDC and continues for a few degrees after. If it was injected in all at once, the engine would under go rapid unplanned disassembly -just like Rudolf's first engine did. Aspro (talk) 15:09, 29 June 2017 (UTC)

Come to that. Why do we now call compression-ignition engines 'diesel' engines? Rudolf patented his cycle to bypass the existing patents but the diesel-cycle was so inefficient that it was quickly abandoned. We don't refer to petrol/gasoline engines as Otto engines. No modern compression-ignition engines are 'diesel' engines. No jet engines are called Whittle engines. No steam engines are called Watt engines. So why? Aspro (talk) 15:36, 29 June 2017 (UTC)

What is a "hexane number"? Do you mean octane number (for a spark ignition petrol engine, and relevant to knocking) or cetane number for a compression-ignition diesel engine? Knocking in diesel engines is pretty much the opposite of that in petrol engines, nor is a single flame front relevant in anything like the same way.

Also to think diesel fuel injection only "continues for a few degrees after [TDC]" is to have missed the whole point of Diesel's cycle and engine.

Modern diesel engines are called that because they evolved from Diesel's work, even though they now use a mixed cycle rather than Diesel's cycle. Although pure Diesel engines, using the Diesel cycle, are still in use at large ship sizes and are the most efficient internal combustion engines yet built. Diesel invented this to use cheap fuels, not to dodge a patent. Not even Akroyd Stuart's. Diesel's innovations for the cycle, the working pressure, and the injection system were all his own. Whittle didn't invent the gas turbine, and we do still call that the Brayton cycle (although I've only ever seen one Brayton engine). Watt's engines are always described specifically as Watt's various engines and cycles (although there is no such specific thing as a Watt steam engine), because I hope you don't think that Watt invented the steam engine, nor that Watt's engines bear much relation to any steam engine after 1900 (and that was an oddity as the last Cornish engine). Andy Dingley (talk) 16:16, 29 June 2017 (UTC)

See THIS comment!

{{xt|Would not the brightness at higher latitudes and elevations be affected by snow and ice? In the Arctic the glare can be painful. 15:05, 26 June 2017 (UTC)

The discussion is at Wikipedia:Reference desk/Archives/Miscellaneous/2017 February 3#Daylight intensity by month?. 92.62.8.1 (talk) 15:05, 26 June 2017 (UTC)

See snow blindness Wymspen (talk) 15:17, 26 June 2017 (UTC)

Feynman Lectures. Exercises PDF. Exercise 7-11 JPG

. .

...

7-11. In making laboratory measurements of g, how precise does one have to be to detect diurnal variations in g due to the moon's gravitation? For simplicity, assume that you laboratory is so located that the moon passes through zenith and nadir. Also, neglect earth-tide effect.

—  R. B. Leighton , Feynman Lectures on Physics. Exercises

First, I can't understand how the laboratory can be just below and above moon (zenith and nadir). It is only possible if the north and south poles of the earth are equidistant from the moon (e.g. at vernal equinox moon is on the line sun-earth) and simultaneously the laboratory is situated on the earth equator. Does Feynman mean this?

To solve the problem we should say how many decimal digits we need. And it returns us to my unanswered question here.

Now I understood what you meant by the word "lag". There is a lag 6 hours between the bulge and the moon-earth line. But first I must understand the bulge appearance (and height) explanation and only then consider the lag. E.g. I don't understand why is the tides effect explained by the gap of (4.51 - 4.36)×10^-5 but much bigger gap of equator point is ignored. The point on equator moves through lunar orbit plane. When the point intersects the plane, acceleration = 4.36×10^-5 , but at leftmost and rightmost positions acceleration = 3.38 ×10^-5 [1]. So gap =(4.36 - 3.38)×10^-5 . Username160611000000 (talk) 20:21, 13 June 2017 (UTC)

Username160611000000 (talk) 16:47, 26 June 2017 (UTC)

The exercise is intended to measure the difference in gravity felt on Earth as the Moon gets closer and further away. The Moon is not always exactly the same distance from the Earth. It get closer and further away, which is the zenith and nadir. So, for simplicity, we assume that your lab is directly below the Moon when it is at the zenith and directly below the Moon when it is at the nadir. In other words, you will measure the absolute maximum difference in gravity change because you will be at the point where you have the maximum difference between close and far distance measurements. 209.149.113.5 (talk) 17:08, 26 June 2017 (UTC)

According to [2] the lunar tidal acceleration at the Earth's surface along the Moon-Earth axis is about 1.1 × 10⁻⁷ g. If we calculate g_max/g_min = (384 400 + 6400)² / (384 400 - 6400)², we get 1,068 or difference = 6.8× 10^-2g what is absolutely distinct from article value 1.1 × 10⁻⁷ g. Username160611000000 (talk) 17:26, 26 June 2017 (UTC)

Hehe, which g is that? You just calculated how much the gravity of the moon varies, but now you want to compare it to the gravity of the Earth. Though I'll admit I didn't look up what the ratio of those two is to make sure this works out right. Wnt (talk) 19:28, 26 June 2017 (UTC)

Thank you. It was mistake.
g_max = 6.67•10^-11 • 7.35• 10²² / ((384 400 - 6400)• 10³)² = 3.43• 10^-5 m/sec²
g_min = 6.67•10^-11 • 7.35• 10²² / ((384 400 + 6400)• 10³)² = 3.21 • 10^-5 m/sec².Username160611000000 (talk) 20:17, 26 June 2017 (UTC)

Difference = 3.43• 10^-5 - 3.21 • 10^-5 = 2.2 • 10^-6 m/sec² = 2.2 • 10^-7 • 9.8 m/sec² = 2.2 • 10^-7 • g. It seems, article uses this non-uniform field , generated by the moon. But it's wrong. On the surface of the earth there is a centrifugal acceleration due to revolution about the barycenter and a gravimeter counts it. Username160611000000 (talk) 12:17, 27 June 2017 (UTC)

There's no mistake. The centrifugal acceleration does not show the diurnal ("daily") variation that the question asks about. R. B. Leighton did not mispell "you[sic] laboratory". Blooteuth (talk) 14:52, 27 June 2017 (UTC)

The earth is rotating about its axis, so the centrifugal force applied to a piece of the earth (e.g. 1 kg of rock) will not change its direction and magnitude (period = 27,3 days) , but earth surface will move (period = 1 day). Therefore there are two fields : one due to gravitation, second due to centrifugal force. Both fields have a period of 27 days. There is no difference in the effects shown by these fields. Username160611000000 (talk) 16:22, 27 June 2017 (UTC)

Now you've got me confused. ;) An object orbiting about a barycenter generally does not experience centrifugal force (in its rotating frame of reference), because that force is counteracted by gravity. The whole point of an orbit is to keep the bodies apart by "centrifugal force", you might say. On the other hand, it does occur to me that a phase locked body like the Moon does move its far side further each revolution than its near side. So ... at least by the Kepler third law approximation ... the far side is in the "wrong orbit", whereas the center of the Moon is just right. Well, I think this greater "centrifugal force" is just the flip side of the tidal gradient seen in a usual calculation made in a non-rotating frame, but now I'm not entirely sure. Wnt (talk) 16:01, 27 June 2017 (UTC)

An object orbiting about a barycenter generally does not experience centrifugal force (in its rotating frame of reference), because that force is counteracted by gravity. It is correct for material point, and as Feynman mentions moon’s attraction just balances the “centrifugal force” at the center of the earth [3]. We should return to the previous discussion and consider a model in which the earth is made up of 1 kg bricks. The brick on the far side of the earth feels gravitation force almost the same as at the center (lower by 1.1 • 10^-6). To stay on the circular orbit the brick needs centripetal force ω²R , since R is bigger almost 2 times on the far side, ω must be lower 1.5 times. But the earth is a rigid body, so rotates about the barycenter with averaged ω (sum over all bricks of centripetal forces must be equal to the sum over all bricks of gravitational forces). Therefore the earth pulls the far-side brick. If the molecular or gravitational forces acting between the bricks had disappeared for a time, then the brick would have flown away upward. Username160611000000 (talk) 16:22, 27 June 2017 (UTC)

It seems centrifugal accelerations on this image https://s.sender.mobi/u/image/2017/6/11/1jJF_T7wN/-.PNG are calculated wrong. Centrifugal acceleration = const.Username160611000000 (talk) 19:28, 30 June 2017 (UTC)

Have there ever been lunar calendars where a solar eclipse makes the month start early?

Presumably they would have to not set before the moment of New Moon else that'd just be the old Moon. Sagittarian Milky Way (talk) 22:52, 26 June 2017 (UTC)

Have there ever been any calendars where eclipses have any function whatsoever? --jpgordon^{𝄢𝄆 𝄐𝄇} 02:33, 27 June 2017 (UTC)

I don't know but lunar calendars start when the Moon is seen or calculated to be seen. The world record for young crescent sighting is about 11.5 to 15.5 hours after New Moon (depending on "found with naked eye", "binoculars to find it", "binoculars to see it", "telescope", "high altitude, computer to aim telescope just to find it, have to hide under a sheet for x minutes first or you're not dark adapted enough", that kind of thing). However you can see a c. 0-1 hour old Moon during a solar eclipse. Sagittarian Milky Way (talk) 03:19, 27 June 2017 (UTC)

Solar eclipses always occur at new moon. In the Hebrew calendar, at least, new moon marks the beginning of the month (they no longer use the crescent moon method). ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:03, 27 June 2017 (UTC)

There are limitations on the length of the lunar month. It may not have fewer than 29 days or more than thirty. So I doubt that a solar eclipse would prompt anyone to declare a new month beginning at nightfall if it happened on the 28th day. There might be something in Babylonian records, but in our culture, when this happened the actual date of the eclipse was suppressed Computus#History. If the regulation is impeded by clouds the new month will start at the end of the thirtieth day. There could be a succession of months where the starting date was empirically determined in this way - if a new moon appeared at the close of the 28th day I wonder how the regulators would handle the situation. 94.195.147.35 (talk) 09:15, 27 June 2017 (UTC)

According to this a solar eclipse cannot occur at the beginning of Ramadan, because it takes a day or two for the crescent to become visible. So at least that lunar calendar rules it out, even though it relies on direct observation of a crescent. (Then again, there are often schisms in Islam, and I've noticed that sources about it seem uncommonly willing to omit the differing point of view). I haven't heard of another lunar calendar where people were actually watching to sight a fresh crescent, but I don't know them all! Wnt (talk) 16:06, 27 June 2017 (UTC)

That's how the lunar and lunisolar calendars all started in the past though, actual sighting. Though this issue wouldn't turn up before a culture realized a solar eclipse is the Moon and thought it was a dragon eating the Sun or something. Sagittarian Milky Way (talk) 19:37, 27 June 2017 (UTC)

If you were trying to ask whether all Muslims use direct observation, the answer is no. Our article isn't well sourced but does cover this Islamic calendar#Astronomical 12-moon calendars although as also mentioned there, not all of them use it for religious purposes. See also Tabular Islamic calendar and [4]. Both Islamic calendar#Astronomical considerations and Islamic calendar#Theological considerations also cover the various controversies and debates. As mentioned there and covered by this French source [5] some of the observations or decisions seem questionable based on what is known and what the country claims to do. However the majority of Muslims do theoretically follow the observation method at least for religious purposes. Nil Einne (talk) 11:15, 28 June 2017 (UTC)