Wikipedia:Reference desk/Archives/Science/2019 July 14

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July 14

Is my drawing of the phases of the moon from different latitudes correct?

Phases of the moon at selected latitudes with Mare Crisium highlighted

Hi,

I drew this image based on the diagrams at astronomy.stackexchange.com and commons:Category:Lunar_phases. To make the images match, I had to flip the orientation before full moon. File:Ficha_y_actividad_3.jpg also shows a flip around the full moon.

That doesn't seem quite right. Is it correct? If so, why is it so? Otherwise, how do I fix it?

Thanks,
cmɢʟee⎆τaʟκ 02:22, 14 July 2019 (UTC)

You've got the first quarter and last quarter on a slant. What time of night are you depicting? ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:05, 14 July 2019 (UTC)

Well the waxing moon will be more visible after sunset, so assume it is viewed just after sunset, and t waning moon, assume it is viewed just before sunrise. You have the crescents unequal, but at the same time they will appear to be about the same size at the different places around the world. Another point is that if the moon is entirely visible from the South Pole, it is below the horizon at the North Pole. Graeme Bartlett (talk) 08:17, 14 July 2019 (UTC)

Thanks, @Baseball Bugs: and @Graeme Bartlett:. I'd like to depict either midnight or when the moon is highest in the sky. I didn't know that the rotation changes over the night. Lunar_phase doesn't make that clear. Can you briefly explain how the rotation changes over the night?

Good point about not being visible from both poles. The diagrams do not necessarily represent the moon seen at the same time, but how a moon during a particular phase looks like from these places. The images in each column are exactly the same, just rotated to an angle corresponding to the latitude. I just found that the unequal size is a bug in thumbnail rendering; the SVG file shows it as intended. I'll try to fix it. Thanks, cmɢʟee⎆τaʟκ 08:32, 14 July 2019 (UTC)

You have fixed the thumbnail unequal rendering. To get an idea of the moon angle, imagine someone standing at the equator looking at the moonrise, and the moon is in the plane of the equator. Then at the same point in time imagine other observers in a circle around the earth looking at the same moonrise or moonset. If the person is at +X° latitude, the they will be standing at that angle to the person at the equator and the moon will have apparently rotated −X°. At the North Pole it is −90°, and past that in the moonrise side of the Earth the moon will appear at X-180°. The formula should also work if lattitude X is negative in the Southern Hemisphere. Graeme Bartlett (talk) 00:18, 15 July 2019 (UTC)

We've had some of this discussion before. How the full moon looks, for example, will change depending on whether you're facing north or south, regardless of which hemisphere you're in. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:51, 15 July 2019 (UTC)

You have the right idea but it varies by a lot. I can see the waxing crescent horns almost straight up at 40.75n when it's the right part of spring and ecliptic latitude is high. Also it has to be very thin for this to happen due to geometrical reasons and some years get closer than others (assuming you are not traveling to other longitudes of 40.75n to see the youngest moon of the month). If you could see the crescent past the point of naked eye visibility you could even see it the wrong way (the southern hemisphere side of straight up). Sagittarian Milky Way (talk) 01:05, 15 July 2019 (UTC)

The reason it doesn't seem right is because you are drawing it when it is setting or rising, whichever occurs at night and NOT at dusk or dawn, whichever has it above the horizon. Sagittarian Milky Way (talk) 01:10, 15 July 2019 (UTC)

Production of curium-250

I asked this on the talk page at Talk:Curium#So how is curium-250 produced? about a year and a half ago, but never got an answer: so how exactly is this isotope produced? The article notes that its production by neutron absorption in reactors is unlikely because ²⁴⁹Cm is likely to decay before it can absorb another neutron, so only nuclear explosions could create high enough neutron fluxes to make it. While ²⁴⁴Pu has the same problem, it can be produced through the alpha decay of ²⁴⁸Cm. Are these just extracted from what old nuclear explosions made (as the half-lives are long enough that they should still mostly be around)? Double sharp (talk) 07:40, 14 July 2019 (UTC)

"Curium is a byproduct of plutonium production activities and results from the successive capture of neutrons by plutonium and americium, generally in nuclear reactors."[1] Looks like ORNL has a production route.[2] And OSTI reports that thermonuclear explosions are indeed a good source of it.[3] DMacks (talk) 08:10, 14 July 2019 (UTC)

@DMacks: The ORNL production route doesn't pass through the specific isotope ²⁵⁰Cm, though, so it seems there's nothing else but nuclear explosions as in your third ref for that isotope. Which is rather a pity as it would provide a neat route to ²⁹⁴Lv and ²⁹⁵Lv in fusion-evaporation reactions with ⁴⁸Ca, as noted at livermorium. I suppose that since the half-life is so long, most of the ²⁵⁰Cm recovered from that test should still be around. Double sharp (talk) 07:39, 20 July 2019 (UTC)

@Double sharp: There's one very interesting report I found [4] describing in detail the hypothetical production of ²⁵⁰Cm and even more exotic nuclides (such as ²⁵⁴Cf, ²⁵⁵Es, ²⁵⁷Fm, and even ²⁵⁸Md!). And it indeed states that nuclear explosions "several orders of magnitude larger" are the most likely means to recover significant quantities of ²⁵⁰Cm. ComplexRational (talk) 15:17, 21 July 2019 (UTC)

Hot chrome

While living in Arizona in summer with a two-tone car, I noticed the following. The white metal top was much cooler to the touch than the dark (cobalt blue "sea blue") metal part, of course. However, the chrome door handle was much hotter than either painted metal. One would think that chrome, being highly reflective, would be cooler -- but its not; why is that? 2606:A000:1126:28D:E5B5:B088:3A46:1619 (talk) 17:33, 14 July 2019 (UTC)

The sheetmetal of the car is thin steel. The doorhandle is diecast zamak, much thicker. What you may be feeling here is not a temperature difference, but a heat difference. They may both have been at the same temperature, but when you touch the thin sheet, that temperature soon drops as heat is conducted into your hand. With the thicker and heavier casting though, the amount of heat energy stored is far greater, and your hand perceives this: it stays hot, even while you hold it.

An infrared thermometer (quite cheap these days) might reveal more. Andy Dingley (talk) 18:37, 14 July 2019 (UTC)

Thanks, that makes sense. I hadn't considered the thickness aspect -- Thermal mass might also come in play. Unfortunately, I no longer have that car (and modern cars, if they have any chrome at all, have chrome-like plastic). 2606:A000:1126:28D:E5B5:B088:3A46:1619 (talk) 18:58, 14 July 2019 (UTC)

Postscript: Agaaach!, I found one just like mine online; theoretically could be mine from Arizona -- sold for $110k! (albeit fully restored). 2606:A000:1126:28D:E5B5:B088:3A46:1619 (talk) 19:20, 14 July 2019 (UTC)

Another significant difference is the thermal conductivity. The metal will conduct the heat fast into your hand, unlike the cloth/plastic. You can notice the same effect with a blow torch. When it hits a brick, the surface glows red or orange immediately, but on metal it takes much longer to reach red heat.

Also, I don't know if it plays any role here, but "white" objects may or may not also be reflective to infrared and ultraviolet, which could affect the temperature. SinisterLefty (talk) 02:06, 15 July 2019 (UTC)

Yeah, any native Arizonan could tell you that (one can tell a newcomer: if they have a black car). 2606:A000:1126:28D:E5B5:B088:3A46:1619 (talk) 03:24, 15 July 2019 (UTC)

The vast majority of solar radiation is in the visible and near-IR range (arguably, sunlight defined the "visible" range via evolutionary pressure to have light receptors in the range there is signal to receive). UV are a very small part in energy (but cause cancer, unlike the rest), as are far-IR (λ>5μm). So color is a good indicator of the absorption of solar radiation. Tigraan^{Click here to contact me} 07:58, 16 July 2019 (UTC)

The conclusion I reach from viewing that chart is that, while there may be less energy at any particular frequency of IR or UV, there is a wider range of frequencies in the IR spectrum, so that if you integrate under the curve, the total energy from all IR and UV looks equal to or more than that from visible light. SinisterLefty (talk) 12:16, 16 July 2019 (UTC)

arguably, what defined visible is the inverse of water absorption spectrum ( electromagnetic_absorption_by_water#Visible_region ) --since eyes are surrounded by water, it would be the only thing seen; also apply to taste and smell --, intersected by absorption spectrum of organic pigments available to living beings. Sunlight is not that important, some animals have decent night vision. Gem fr (talk) 12:57, 16 July 2019 (UTC)

• Relevant article (for how "hot" an object feels to touch): thermal effusivity. Tigraan^{Click here to contact me} 07:58, 16 July 2019 (UTC)

The complement of centrifuging

If you separate a mixture of liquids by applying a centrifuge force to them, how would you call the complement of it, I mean, the force to obtain a homogeneous mixture of liquids? — Preceding unsigned comment added by C est moi anton (talk • contribs) 20:56, 14 July 2019 (UTC)

Homogenization? —2606:A000:1126:28D:E5B5:B088:3A46:1619 (talk) 21:01, 14 July 2019 (UTC)

You can't homogenize a mixture by applying a single force to it. Instead there needs to be some mixing of the fluid, which is done by applying a non-constant (i.e. inhomogeneous!) set of forces to it: some sort of shear force, where part of the mixture goes one way and an adjacent portion goes another. This can be done by turning the mixture over (concrete mixer), stirring it slowly (food mixer), quickly (blender), extremely fast (colloid mill), ultrasonically (homogenizer or sonication) or blowing it through a small nozzle (homogenizing valve). Andy Dingley (talk) 21:29, 14 July 2019 (UTC)

(edit conflict)Mixing (process engineering), Mixing (physics). Also, check emulsion Gem fr (talk) 21:32, 14 July 2019 (UTC)

In labs with a centrifuge you may well find a Shaker nearby; Vortex shakers are particularly fun. Klbrain (talk) 22:19, 14 July 2019 (UTC)

If you find a Quaker nearby, don't bother; they object to all forms of violence, and are not particularly fun. 107.15.157.44 (talk) 22:59, 14 July 2019 (UTC)

Often done to cans of paint. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:57, 14 July 2019 (UTC)