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August 12

Feynman Lectures. Exercises. Exercise 27-2 PNG

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27-2. A certain beam of light is converging toward a focus at a certain point P. It is desired to insert a single reflecting surface passing through a given axial point Q which will reimage the light to a new point focus at a given point P'. Find the shape of the required surface. Let the distance QP' = D and QP = d.

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

The Solutions give an answer that according the Fig. 26–11 the surface is an ellipse. But I think that a plane mirror between P and P' is also acceptable according to image . Is it correct? Username160611000000 (talk) 07:09, 12 August 2018 (UTC)[reply]

Only for specific values of P' and Q. Given the system in the image, for any arbitrary point Q to the right of P and P' to the left of Q, you can design an elliptical mirror containing the point Q and having P and P' as its foci. (That obviously goes tits-up if Q is to the left of P', and less obviously if Q is between P' and P.) --Link ^(t•c•m) 10:40, 12 August 2018 (UTC)[reply]

Aiming for planets

When Apollo craft were coming back from Moon, were the modules aimed akin to firing a bullet onto a moving target (i.e., slightly in front of the target's actual position in order to account for passage of time), or does the Earth never seem to move... which it does, some tens of thousands of kilometers per second on its yearly journey around the Sun? 46.188.151.206 (talk) 10:19, 12 August 2018 (UTC)[reply]

The Moon moves with the Earth, so aslong as you are only going to the Moon (and back to Earth), then a geocentric model should suffice. LongHairedFop (talk) 12:08, 12 August 2018 (UTC)[reply]

'Thus it was not surprising that one of the first contracts NASA let for Apollo was for the guidance and navigation system for the journey... The system was primarily inertial, but to correct for drift, it would carry “…a space sextant to make periodic navigation angle measurements between pairs of celestial objects: the sun, the near planets, and selected stars..”'. See US Institute of Navigation - Sextant, Apollo Guidance and Navigation System. Alansplodge (talk) 12:15, 12 August 2018 (UTC)[reply]

The Earth and Moon orbit around their common center of mass (which is close to the Earth's center) and the Earth+Moon combined mass is in orbit around the Sun. The Apollo capsule was always part of that combined mass. The article Trans-lunar injection describes the transfer of a mass such as the capsule with its contents from the Earth to the Moon. The velocity of orbiting the Sun does not enter the calculation because it is unchanged for Earth, Moon and capsule. The burn maneuver differs from that of a bullet which receives only a one-time explosive thrust. The Apollo missions (see Apollo 11) required separate stages of self-propelled controlled rocket propulsion to achieve these targets: inital low Earth orbit, initiate trans-lunar maneuver, enter lunar orbit and lunar descent. The initial rocket stage was discarded after burn and flew into orbit around the Sun. DroneB (talk) 12:38, 12 August 2018 (UTC)[reply]

Assuming you mean the third Saturn stage, which is what put the craft onto the trans-lunar injection trajectory, starting with Apollo 13 they crashed them into the Moon instead. The first and second stages fell back to Earth. --47.146.63.87 (talk) 08:47, 17 August 2018 (UTC)[reply]

Some of the Saturn V third stages went into orbit around the Sun and some were crashed into the Moon. But I think the OP is asking about the CSM. Bubba73 ^{You talkin' to me?} 02:25, 19 August 2018 (UTC)[reply]

Neither. The path was within the gravitational influence of the Earth-Moon system, and journeys in space are done by coasting to save fuel. So this path was (inevitably) going to be an orbit, or partial one. The idea of "aiming" is more like orbital injection into the correct (and in this case, trans-lunar) orbit, less like lead in aiming (where the aim has to be offset for a moving target, but the projectile isn't then influenced by the gravity of the target).

It's also difficult to calculate this. So it was done in advance on Earth (it couldn't even be done easily on demand on Earth, as computation wasn't fast enough). The "aiming" from the Apollo vehicle was a question of being tracked accurately by an Earth station, the necessary corrections worked out, then Apollo told to orient themselves on a particular bearing (using the on-board sextant) and firing the engine for an Earth-calculated period. Despite the difficulty of this, Apollo 11 was scheduled for something like four in-course correction burns, but its track was so accurate that they only needed to fire two of them. Andy Dingley (talk) 16:52, 12 August 2018 (UTC)[reply]

The general case of this: Three-body problem § Restricted three-body problem. Yes, essentially, the Earth "never seem[s] to move" in your frame of reference. You're launching from Earth. Earth is moving around the Sun, but so are you, on the same trajectory. Since you don't care about going to the Sun or some other planet, you only need to calculate your trajectory with respect to the Earth-Moon system. --47.146.63.87 (talk) 02:09, 13 August 2018 (UTC)[reply]

Black or white?

Given a house under the sun and you sprinkle water on the exterior walls.

Would it be cooler inside, if the walls are painted white or painted black?

I assume that dry white walls would keep the interior cooler. However, I wonder whether a wet black wall would evaporate more water, hence reducing more the temperature inside.--Doroletho (talk) 14:19, 12 August 2018 (UTC)[reply]

India decreased heat-related deaths by painting roofs white. Los Angeles has been painting some roads a light gray to decrease temperatures.

From my own experience, you're always better off drinking the water. Ian.thomson (talk) 14:25, 12 August 2018 (UTC)[reply]

Yes, I don't doubt that white or light grey decreases temperature. And I don't believe using water would be an efficient way of cooling down a house, especially not in India or California, which seem to have water shortage. I just want to know the physics behind it.--Doroletho (talk) 14:43, 12 August 2018 (UTC)[reply]

You might be surprised. Evaporative cooling systems are fairly widespread in areas meeting the right criteria: high air temperatures, low relative humidity, reasonable access to water. Our article has a number of details. (And if you're interested, it's pretty easy to find HVAC companies that will be happy to sell you an evaporative cooler in California.) Note that while evaporative cooling will increase an individual household's water usage, it also decreases that household's electricity usage—and since there is a remarkable amount of 'hidden' water usage involved in electricity generation, evaporative cooling can result in a net reduction in water usage overall. TenOfAllTrades(talk) 15:57, 12 August 2018 (UTC)[reply]

With a black wall, more water would evaporate, but only because the wall gets warmer. A warmer wall means a warmer interior. In other words, the black wall increases cooling by evaporation, but not enough to compensate for the increased heating by absorbsion of light. Now, you could design a clever cooling system that's powered by solar heat. PiusImpavidus (talk) 15:13, 12 August 2018 (UTC)[reply]

That exists. Cooling is a byproduct of water production. In hot/dry places, there is still water in the air - just not much. So, use a black box to absorb heat and make it very hot inside the box. Now, run coolant from outside box into coils inside the box. While the coolant is actually hot, the box is hotter. So, water will condense on the coils and drip off (filter it for drinking water). Then, run the coolant through the walls of a building to cool it down. Then, back up to the evaporator box. Use solar panels to run the pump. You can build one easily, but it won't be very efficient. Science fair models I've seen are lucky to get a gallon of water a day and cool by a few degrees. An industrial installation I inspected in New Mexico pulled out 8-15 gallons of water a day (depending on humidity and heat) and cooled a 4-bedroom house by up to 10 degrees. They were able to leave AC off all day while at work and school, having it turn on shortly before they returned home. 209.149.113.5 (talk) 13:12, 13 August 2018 (UTC)[reply]

Decreasing light pollution as night progresses

I'm thinking since more people turn their lights off and more places get closed as night progresses, logic tell us that resulting light pollution would decrease therefore darkening the skies as it gets later in the night. For example, there will be less light pollution at 3am than at 10pm albeit by a slight margin. Honoring peak Perseid meteor activity tonight, I'm asking you if my thought is true so it can show people when it is the better time to watch for meteors for those who live in light-polluted cities or towns. Planet Star 18:04, 12 August 2018 (UTC)[reply]

I could never tell in my city but people turning their lights off hardly matters when there's like 1500 streetlights per square mile and rent is so high compared to the cost of electricity that closed businesses leave a lot of brightly lit signs on. The higher the percent of light pollution not coming from light that turns of while it's still dark the lower the artificial light maximum vs minimum ratio. At any rate most meteor showers are best seen at the start of morning astronomical twilight, when you're looking through Earth's windshield instead of Earth's rear window but it's not dawn yet. In extreme light pollution the best time might be around to the start of morning nautical twilight. Cause in a not especially hazy or clear summer night without bright lights in my eyes I could barely see the twilight at the bright edge of astronomical twilight, the extreme light pollution overwhelms it. Sagittarian Milky Way (talk) 19:08, 12 August 2018 (UTC)[reply]

The University of Vienna monitors sky brightness, the measurements are here. Leave the location at "SQM-LE @ IFA" (that's within Vienna) and look at a few nights. "09.08.2018-10.08.2018" shows the behaviour proposed by PlanetStar; that was a clear, moonless nights (except, apparently, for a bunch of clouds moving through at around 1:30am). Other nights are less clear-cut. The moon clearly increases sky brightness, clouds do so much more, and also cause strong fluctuations in the measurements, which makes many nights harder to interpret). The city of Vienna switches off every other street lamp at 11pm (mainly to save energy and money rather than to fight light pollution); I think the effect can be seen here. --Wrongfilter (talk) 20:04, 12 August 2018 (UTC)[reply]

That's pretty neat. Unfortunately that number of magnitudes is not that noticeable to the naked eye (it might if you view the sky at shutoff making it somewhat like a blink comparator but the actual improvement isn't that great) Maybe too many of the other municipalities in the area don't join in? If half of every light within 200-300km of Vienna turned off at 11 you might get about 0.75 magnitudes drop right? Sagittarian Milky Way (talk) 20:25, 12 August 2018 (UTC)[reply]

I just talked to the local expert. It's less than a factor of 2 because of the significant fraction of light from private households (out of windows, etc.). Also, it's not exactly half the street lighting that's switched off, more like a third or so. In a paper, they published a number of 0.18 mag/arcsec² for the 11pm curfew, and another 0.09 mag/arcsec² at midnight, when facade lighting is reduced (also just visible in the graphic I linked to). --Wrongfilter (talk) 13:19, 13 August 2018 (UTC)[reply]

If only other cities would do such a thing. Why don't they just reduce the streetlight current or voltage instead of turning some off? Turning some off would increase the ratio between dim and bright patches of street (which is one of the reasons why blasting dozens of thousands of lumens per lamp doesn't help crime (the street is so bright the mugger or burglar covers himself in black, waits in a shadow and the elderly (maybe younger) don't see him)). Also, if they reduce brightness slowly over 10 minutes or 1 hour or something it'd look less obvious and maybe they can cut more than half before the public starts complaining (of course if big enough drop people would start noticing things about the light level itself instead of just the suddeness like the private lights looking much brighter late and maybe color vision weakening (mesopic) and I don't know the politics of Vienna enough to guess at what point the public would start complaining) Sagittarian Milky Way (talk) 20:39, 13 August 2018 (UTC)[reply]

Why don't they just reduce the streetlight current or voltage instead of turning some off? In at least some parts of the UK that's exactly what they do, but it's only been possible in my county (Hampshire), for example, since the most recent street lighting upgrade which installed LED streetlamps and road signage lighting), plus the necessary control system, between 2010 and 2015. {The poster formerly known as 87.81.230.195} 94.0.130.143 (talk) 22:01, 17 August 2018 (UTC)[reply]