Wikipedia:Reference desk/Archives/Science/2011 January 24

= January 24 =

2012
will the earth end in 2012 — Preceding unsigned comment added by Tommy35750 (talk • contribs) 01:12, 24 January 2011 (UTC)


 * Highly unlikely if you ask me, but see 2012 phenomenon. PrimeHunter (talk) 01:18, 24 January 2011 (UTC)
 * (EC)We do not engage in speculation on the reference desks. However our article on the subject is at 2012 phenomenon. --Tagishsimon (talk) 01:19, 24 January 2011 (UTC)


 * There is no accepted scientific reason to think that the Earth might be significantly more likely to end in 2012 than in any other year. That is, there is no accepted scientific reason to think that the Earth has anything greater than almost no chance of ending in 2012.  For a scientific viewpoint of Earth ending, skip the 2012 phenomenon article, which is about a sociological phenomenon that has essentially nothing to do with science, and read Risks to civilization, humans and planet Earth.  Red Act (talk) 02:03, 24 January 2011 (UTC)


 * NASA says no at this link. Phew.  Comet Tuttle (talk) 18:30, 24 January 2011 (UTC)

Probably not.NASA--Cjc811 (talk) 19:17, 24 January 2011 (UTC)

Look at this site Maybe--Cjc811 (talk) 19:18, 24 January 2011 (UTC)
 * Don't look at that site, it's just an idiot's ramblings. --Tango (talk) 19:41, 24 January 2011 (UTC)


 * No reason why it shouldn't except applications of inductive logic as flawed as the optimism of someone who has jumped from a 100 story building and notes that nothing bad has happened after he has fallen 80 stories, 81 stories, 82 stories, ... and who concludes that therefore he might fall without limit, right? The Earth has hosted lifeforms for hundreds of millions of years, so it will continue for hundreds of million or billions more, right? Edison (talk) 02:52, 25 January 2011 (UTC)


 * This recent article predicting a supernova of Betelgeuse in 2012 is a clear example of the 2012 phenomenon, since those "predictions" of a star emerging from Orion would have been correct should this actually occur. However, even though the diameter of Betelgeuse has shrunk by 15% in the past 15 years, this estimate is highly uncertain due to blurry imaging and differences in development of the technique, though if the estimate is correct, this corresponds to nearly a 40% decrease in volume, although mass has also been decreasing, suggesting the star is shedding its outer layers (spectroscopy of the star may indicate changes); this may represent either an early or a late stage of pre-supernova changes in the star, though periodic changes similar in magnitude also occur in non-near-supernova variable stars, even though those stars too are often red supergiants that are nearing the supernova stage; according to the original article version, a supernova of Betelgeuse may not occur for another million years. Generally, many scientific (and pseudoscientific) predictions imply that "something" of significance may be at an increased likelihood of occurring in 2012, but that it will likely not be the end of the world and may simply represent a self-fulfilling prophecy. ~ A H  1 (TCU) 23:48, 26 January 2011 (UTC)


 * Of course if Betelgeuse does go supernova, it will do so some time in the late Middle Ages, and the world did not end then. As far as I know, I wasn't actually here at the time. 148.197.121.205 (talk) 14:30, 28 January 2011 (UTC)

the lower dendrites
in a neuron, there are the upper (receiving) dendrites. and there are the lower and smaller-in-amount (giving) dendrites.

i know this is not the acceptable names for this parts (upper, and lower), so what is the right way to call these so-called "lower dendrites"...?

thanks and blessings. —Preceding unsigned comment added by 109.65.14.202 (talk) 01:35, 24 January 2011 (UTC)


 * Have you read our Neuron article? Perhaps the word you're looking for is "axon". –Henning Makholm (talk) 01:53, 24 January 2011 (UTC)


 * Yes, almost certainly "axon". Looie496 (talk) 02:53, 24 January 2011 (UTC)

hey, i have read the article but i understand that the Axon is a fiber which kinda' connect the "upper" dendrite, to the "lower" one... what you guys say is that THE "LOWER" ONE IS THE AXON...

i i understand right,

thanks. —Preceding unsigned comment added by 109.65.14.202 (talk) 12:10, 24 January 2011 (UTC)


 * There are various neuron structure types, but the most common one taught is the pyramidal neuron: this has a single axon (the signal sender) projecting from one side of the soma (cell body) with a "tree" of many dendrites (signal receivers) projecting from the other side. The axon itself has a number of axon terminals that sort of resemble dendrites, but are structurally different.
 * So it looks like you are confusing the axon terminals (signal senders) with actual dendrites (receivers) that are on opposite ends of the (pyramidal) neuron. SamuelRiv (talk) 18:32, 24 January 2011 (UTC)

c-1 m/s
If I've understood things correctly, no massive object can be accelerated to the speed of light because, as you approach c, relativistic effects increase the energy required to accelerate ad infinitum.

So, what would happen exactly if I took a 1-kilogram dumbbell moving at c - 1 m/s and exerted a force of one newton on it? At what speed would it be travelling after this?

Thanks. Leptictidium (mt) 07:31, 24 January 2011 (UTC)


 * Your "1 kg" object has a relativistic mass of over 12,000 kilograms, and will accelerate at less than a tenth of a millimeter per second per second, and become heavier all the while. Someguy1221 (talk) 07:41, 24 January 2011 (UTC)


 * It's actually worse than that. Assuming that you measure the force in your restframe (i.e. in the frame where the dumbbell moves at (c -1)m/s, then the force relates to momentum and velocity as $$F = \frac{d}{dt} p = \frac{d}{dt}(\gamma m v)$$ (here "m" = 1 kg is the invariant or rest mass, and γ is the Lorentz factor). Doing the derivative gives $$F = \gamma^3 m a$$. If you insist on using relativistic mass (which is ugly because coordinate-dependent), then that is $$F=\gamma^2 m_\mathrm{rel} a$$. You forgot to take into account that relativistic mass also changes when the dumbbell is accelerated. At v- (c-1)m/s, $$\gamma = 12243$$, hence a= 5.45⋅10−13 m/s2.--Wrongfilter (talk) 11:10, 24 January 2011 (UTC)
 * So what if I applied that very same force to an object with relativistic mass = 1 kg? --Leptictidium (mt) 11:17, 24 January 2011 (UTC)
 * Well, I gave the equation. You've absorbed one factor of γ into your relativistic mass (rest mass is 1/γ kg), hence the acceleration is larger by that factor, i.e. 6.7⋅10−9. --Wrongfilter (talk) 11:24, 24 January 2011 (UTC)
 * Then you'd get closer to the speed of light a little quicker, but the energy (AKA weight/mass) you are expending will transfer to your object making it heavier, and the acceleration will slow down. Objects getting heavier as they reach the speed of light isn't from magic. The energy contained in their velocity has mass, and that is what makes them heavier, and since they are heavier, you need even more energy to speed them up, but that extra energy makes then even heavier than before, etc, etc. till you have a runaway effect. Ariel. (talk) 11:30, 24 January 2011 (UTC)
 * You mean "more massive" not heavier. Cuddlyable3 (talk) 12:03, 24 January 2011 (UTC)
 * I do, but I will often say heavier so people understand it's not some sort of weird relativity thing, but that it really does weigh more (if you were to bring it near a planet or something). Ariel. (talk) 12:10, 24 January 2011 (UTC)
 * It is a "weird" relativity thing anyway, isn't it? And even in that pseudo-Newtonian understanding of relativity, it's not weight that matters here, but inertia. --Wrongfilter (talk) 12:22, 24 January 2011 (UTC)
 * By weird I mean some kind of mathematical thing used for calculations but not "real". And no one has ever found a difference between inertia and weight (aka gravitational mass) - that equivalence is a cornerstone of relativity. Ariel. (talk) 12:32, 24 January 2011 (UTC)
 * Phenomenologically, they are different effects, even if in general relativity they can be unified. --Wrongfilter (talk) 12:55, 24 January 2011 (UTC)
 * It doesn't really make any difference if the mass is 1kg or 1/12g (which it would need to be to have a relativistic mass of 1kg), you just multiply or divide the final answer by 12,000. --Tango (talk) 19:46, 24 January 2011 (UTC)
 * Would it exert gravity as if it weighed 12,000 kg? —Preceding unsigned comment added by 205.193.96.10 (talk) 17:21, 24 January 2011 (UTC)


 * Tricky. That would seem to make gravity frame-dependent. In a comoving frame, the dumbbell clearly exerts gravity as fits a 1kg rest mass. With the dumbbell zooming past, you would feel a time-dependent gravitational field. Taking into account the required coordinate transformations gravity might be pretty strong for a short time. There certainly is no reference frame where gravity acts like that created by a stationary dumbbell of 12,000 kg. --Wrongfilter (talk) 17:56, 24 January 2011 (UTC)


 * In its own reference frame, the dumbbell still weighs only 1 kg and exerts only one kg-worth of gravity on its surrounding objects.  Also, it is still accelerating at 1 m/s by its own measurements.  Only the observer in a different reference frame sees the situation differently.    D b f i r s   19:32, 24 January 2011 (UTC)
 * The force is a vector, so you have to specify in which reference frame the (3-)force is given. That's why I wrote "Assuming that you measure the force in your restframe". If the force is measured in the rest frame of the dumbbell instead of that of the observer, then things are a little different and you get a couple of factors γ in different places. --Wrongfilter (talk) 19:53, 24 January 2011 (UTC)
 * Also, the 3-force components are three members of the 4-force which also include power as a fourth member which means that the force will have different magnitudes depending on the observer. Dauto (talk) 20:35, 24 January 2011 (UTC)
 * So, in its own reference frame, the dumbbell has accelerated to c, Dbfirs? Leptictidium (mt) 20:06, 24 January 2011 (UTC)


 * In its own reference frame the dumbbell is always at rest (duh). Dauto (talk) 20:23, 24 January 2011 (UTC)


 * Yes, the dumbbell can measure only its speed relative to something else, but in its own reference a measured force of one Newton will produce a continuous acceleration of 1 m/s/s using the length and time of that reference frame. Adding an increase of 1 m/s in speed to a speed of "c-1" in the other direction does not give a speed of "c" by the relativistic addition law of speeds.  As others explain, the interpretation in other frames depends on how the observations are taken.    D b f i r s   13:05, 25 January 2011 (UTC)

Even in Newtonian mechanics, you need to specify a force and a duration of time to estimate a velocity-change. Special relativity is very nice, because duration of time will also be Lorentz-contracted. So if you exert a 1 newton force for one second, you need to specify in which reference frame you have exerted that force. If you press on the object for 1 second, measured in the "stationary" (laboratory) coordinate frame, the effective force experienced by the moving object will be much more instantaneous; and the momentum transferred will be much smaller. The coordinates all work out using a lorentz transform and you just have to keep your coordinates straight. Convenient, eh? The paradoxes of paradoxical nature of measurements in different coordinate systems, with high velocities relative to each other, have been thoroughly explored: see Ladder paradox for a start. Nimur (talk) 21:10, 24 January 2011 (UTC)

Herpes virus
I was wondering how long the herpes virus stays around outside the body. I had a customer touch her lip and touch some of my book at work she was looking through. Now I am afraid to even touch the books. I can't really spray lysol on the books or clean them. So what else can I do? How long will the virus stay around? —Preceding unsigned comment added by 76.169.33.234 (talk) 08:12, 24 January 2011 (UTC)


 * Our article on Herpes simplex says "Herpes is contracted through direct contact with an active lesion or body fluid of an infected person." It seems you really need skin to skin contact for transmission. Get back to those books! HiLo48 (talk) 08:34, 24 January 2011 (UTC)


 * This says that under ideal conditions (temperature and humidity), it can survive a couple of hours. Ariel. (talk) 09:07, 24 January 2011 (UTC)

Inside a mirrored sphere
If you found yourself inside a mirrored sphere with a diameter of twice your eye level height from the ground and a source of light above your head, what would you see around you? How would the view change if you moved around? TheFutureAwaits (talk) 09:18, 24 January 2011 (UTC)
 * Mostly you would see yourself, but upside down. Your own body will block most of the reflections from other parts of the sphere. Your body will also absorb light, so it won't get constantly brighter and brighter (plus no mirror reflects all light, so some is lost). It looks like Japanese TV actually tried it but someone took down the video. Supposing you had a perfect point source light and a perfect mirror, and you placed the light in the exact center of the sphere, then you would see almost nothing because except for the light reflecting off of the top of your head, all of it would reflect right back into the light and none would reach you. Even if the light was not in the center, all of it would end up focused on one spot, lighting that spot but nothing around it. Just how perfect a mirror and light are we talking about here? Are you making use of reflected light from your clothing? It would be cool to model this with PovRay and it's pretty likely someone already did. Ariel. (talk) 09:47, 24 January 2011 (UTC)
 * We had a big parabolic mirror at school, over a meter in diameter, and I used to love starting into that thing. It had a tiny spot to mark the very center, if you walked towards the mirror with your eye right in line with the spot eventually, when your eye was right at the focal point, the whole mirror would be a reflection of just your pupil, it was really cool! Obviously even slight imperfections in the mirror make this effect not "flawless" but it was certainly impressive enough for me:) Vespine (talk) 00:52, 25 January 2011 (UTC)
 * Your eye was twice as far from the mirror as the focal point. 71.101.41.73 (talk) 09:11, 26 January 2011 (UTC)
 * Yeah good pick up, incorrect assumption on my part. i hadn't actually thought it through completely. Vespine (talk) 01:17, 27 January 2011 (UTC)
 * Actually, you will only appear upside down (and left/right reversed) if the center of the sphere is in front of your eye. If the center of the sphere is behind your head, you will appear right-side up.  And if your pupil is in the exact center of the sphere, all you will see is the blackness reflected from your pupil.  Red Act (talk) 00:49, 28 January 2011 (UTC)

Parts of a horses skull
I would like to know and cannot find out the name of the small holes on a horses skull on both sides just above the nasal bone and below the eye socket and what is it's use —Preceding unsigned comment added by 216.218.29.249 (talk) 16:31, 24 January 2011 (UTC)
 * The word for this type of hole in other animals seems to be " temporal fenestra", but our skull article also says "Mammals, which are synapsids, possess no fenestral openings in the skull". SemanticMantis (talk) 16:44, 24 January 2011 (UTC)


 * I believe these are some type of fenestra, See Skull. Mammals characteristicly seem to have these fenestra, located under the eye sockets. You can see small ones on the chimpanzee skull, and they really stand out on the bulldog skull. I am pretty sure that most of the fenestra act as conduits for nerves and blood vessels to get through the skull and other bones that have them, our article on fenestra is woefully weak in this regard, and I am remembering back to the last anatomy class I had, sometime in the fall of 1994. Perhaps someone with more anatomy knowledge can fill in the gaps. Post EC answer after SemanticMantis: I don't think these are temporal fenestra, the article synapsid has a picture which shows the temporal fenestra at the back of the skull, between the postorbital and squamous bones, no where near the nasal bone.  If you look at the other skulls of mammals, you can see a fenestra of various sizes in the location described by the OP.  This isn't the temporal fenestra.   -- Jayron  32  16:49, 24 January 2011 (UTC)


 * Correct myself. These are not fenestra, these are foramen, see File:Gray190.png where the human analog to these holes is called the "infraorbital foramen", and according to foramen, serve exactly the purpose I describe above.  I knew it was an "f" word, and got my foramens and fenestra confused.  -- Jayron  32  16:53, 24 January 2011 (UTC)


 * Addendum to the correction: We even have an article titled Infraorbital foramen which describes the exact purpose of those holes. -- Jayron  32  16:54, 24 January 2011 (UTC)
 * Also, fenestra is singular; the plural is fenestrae. Not a big deal, but just kind of obvious if you speak Italian, where fenestra is the everyday word for "window". --Trovatore (talk) 00:22, 25 January 2011 (UTC)
 * (Oh, actually I think it's finestra now that I think about it.) --Trovatore (talk) 00:23, 25 January 2011 (UTC)
 * Which is of course also the origin of one of my favorite words: Defenestration. Vespine (talk) 00:45, 25 January 2011 (UTC)

Eye bone
What is the name of the bone in the eye of some reptiles? I can't find the article anywhere. Here are some examples: It is a torus shape with radial lines. - Craig Pemberton 17:10, 24 January 2011 (UTC)

Sclerotic ring. L ANTZY T ALK 17:20, 24 January 2011 (UTC)
 * Thank you!- Craig Pemberton 17:34, 24 January 2011 (UTC)

Registering a new rock
My Dad found a rock which he sent to an organization (which I'm trying to find) and they told him it was indeed a newfound rock and should be registered with them. He has died since and I don't know if he registered it or not. What is the organization I go to, and how do I go about it (will ask them if necessary)? Thanks —Preceding unsigned comment added by 4.240.78.111 (talk) 18:22, 24 January 2011 (UTC)
 * Your IP address geolocates to New Mexico, so the organization could either be the Mineralogical Society of America or the CNMNC (part of the International Mineralogical Association). Physchim62 (talk) 19:08, 24 January 2011 (UTC)

Pure proteins
I'm looking for suggestions for proteins that can be obtained in a relatively pure form (maybe >95%). I'm look for things that I could purchase for research in quantities of about a gram for a reasonable price, say < US$200. I need to identify as many as possible. I'm not asking for specific commercial recommendations or prices, just names of proteins.

So far I've found:
 * BSA
 * beta-casein
 * Lysozyme
 * Hemoglobin
 * gamma-globulin

Thanks for the suggestions! ike9898 (talk) 20:42, 24 January 2011 (UTC)
 * Even suggestions on how to search for such things would be helpful. ike9898 (talk) 21:18, 24 January 2011 (UTC)
 * Pepsin is a cheap enzyme, cheap enough to be used in UK high schools 25 years ago to demonstrate enzyme kinetics. I just checked the price for pharmaceutical grade from a supplier who's known not to be the cheapest, and it's less than €1/g. Physchim62 (talk) 21:29, 24 January 2011 (UTC)
 * Do you have any requirements other than price, like size, function, species, etc.? I'm not a biochemist so I don't know the specialty-companies in this arena, but one way to search is to pick a chemical/biochemical company and search its catalog. For example, I just poked the Sigma-Aldrich online catalog and found a whole Proteins and Derivatives section. In it are several you did not list. DMacks (talk) 21:34, 24 January 2011 (UTC)
 * Actually, what I need is variety. I want to study the relationship between various protein characteristics (MW, amino acid composition, secondary structure, ...) and a their activity in a particular practical application I'm interested in. I need them to be soluble under mild, aqueous conditions (keratin and zein won't work).  I think ideally I want plain-vanilla proteins, rather than things like glycoproteins, but I'll take what I can get. Thanks! ike9898 (talk) 21:46, 24 January 2011 (UTC)
 * If you want a big enzyme, then I know you can buy rubisco from spinach. Might you be able to hint at the "particular practical application" that you're interested in? It might help us think of more suitable proteins. SmartSE (talk) 22:26, 24 January 2011 (UTC)
 * Check over the contact lens cleaning tablets - depending on the store you go to, you might find a variety. Pancreatin and subtilisin are the two most common, I think. Wnt (talk) 20:53, 25 January 2011 (UTC)

Hypothetical question
If all of the illnesses in the world were cure, would human beings still die naturally? --Thanks, Hadseys 22:09, 24 January 2011 (UTC)
 * It's kind of begging the question. If you define illness as "something that can go wrong with us" then the answer is obviously no. However, these days if you don't die of a specific thing like heat attack or cancer or stroke, then you will still eventually die from the very fact that your chromosomes don't copy perfectly when your cells split. You might find Telomeres interesting. However, you don't actually "die" of this, you die of liver failure (sure you can replace a liver, but not so easy to replace a spine or a brain) or something else which is caused by this, so eventually, excessive telomere shortening might be defined as an illness. Vespine (talk) 23:19, 24 January 2011 (UTC)
 * Actually our Death by natural causes article also seems to address your question. Vespine (talk) 23:24, 24 January 2011 (UTC)


 * Even if you somehow could evade death and live forever (assume that the universe will go on forever without a heat death), you would still effectively die, because there are only a finite number of states your brain can be in. You cannot go on to store more and more information in the brain, at some point you have to erase old information to make room for new information. Since you can only be in one of a finite number of states, you will eventually return to some old state you were previously in, meaning that you will have erased all of the information that you had accumulated since you were in that state the last time. Count Iblis (talk) 01:33, 25 January 2011 (UTC)
 * I'm struggling to reconcile your interpretation with the actual definition of death.. As far as I can tell death has nothing to do with what information is in your brain, i might have forgotten everything that happened to me before the age of 5 but by no rational definition am I "dead". Vespine (talk) 02:35, 25 January 2011 (UTC)
 * I suppose a philosopher might posit that your 4 year old self is now dead, but that's pretty tortured.
 * Also, I think the "finite number of states" is a red herring, I wouldn't be surprised if you could survive all the way to the heat death without duplicating states. APL (talk) 04:11, 25 January 2011 (UTC)
 * lol yes, by adding rational i was attempting to exclude the philosophical "out", maybe I should have said airy fairy ;) Vespine (talk) 05:42, 25 January 2011 (UTC)
 * Well, the heat death or the big rip would imply mortality, so you have to assume that this doesn't happen and that the universe goes on forever. Then we apply a similar reasoning as in the Poincare recurrence theorem. Count Iblis (talk) 13:44, 25 January 2011 (UTC)


 * The traditional Four Horsemen of the Apocalypse idea divides up the ways of dying into oppression, war, famine, and pestilence. (I suppose crime, terrorism, ignorance, and suicide or old age also fit roughly into these categories)  Curing all diseases (including old age and mental diseases) would cut off one of the exits, but the question is whether the others are always defined by the reader as "natural death". Wnt (talk) 05:14, 25 January 2011 (UTC)


 * How broadly do you define 'all illnesses' and 'natural causes'? Do accidental deaths count?  (Car accidents, falls, drownings, and the like?)  The all-ages all-causes rate of accidental deaths in the United States in 1996 was 35 per 100,000 per year: about 0.03%.  Assuming that value remains constant, one would have a half-life of about two thousand years.  In practice, that risk peaks twice &mdash; a bump in late adolescence (ages 15-24) and very sharply in old age (65 and up).  So the question is, will our otherwise-immortal humans have the reflexes and durability of early middle age, or will they be frail and fragile geriatric patients?  The over-75 risk is actually about four times the average, at 140 per 100,000.  (Those numbers also don't include homicides, ranging from the individual crime of passion to the global thermonuclear war.)  TenOfAllTrades(talk) 14:34, 25 January 2011 (UTC)


 * even without diseases, which I'll take as viral, bacterial and parasitic infections for the sake of simplicity, the human body has a certain lifespan. The article on Telomeres explains rather well, but to put it in layman's terms: every time DNA is replicated not all of it can be duplicated, because it is copied at an "angle" (a gross oversimplification of 5' to 3' copying for the sake of a simple explanation).  To prevent this there are protective "caps" on the end of chromosomes, these caps become shorter over time, and this is thought to be a reason for biological aging.  Lengthening these caps can result in cancer, and all known 'immortal' cell lines are derived from a cancerous growth of some form. 65.29.47.55 (talk) 03:45, 26 January 2011 (UTC)


 * See biological immortality. Humans do not seem to naturally posess this ability. ~ A H  1 (TCU) 23:19, 26 January 2011 (UTC)
 * It is very true that transgenic human beings might well be immortal, however eliminating diseases does not automatically confer biological immortality and experiments with telomere lengthening in animals have usually lead to cancer. It's uncertain if a complex mammalian organism could ever achieve biological immortality without tumor formation. 65.29.47.55 (talk) 09:07, 27 January 2011 (UTC)

Taraxacum
The Taraxacum article says people eat them, but does not say if preparation is required. Can people just eat them raw? 82.44.55.25 (talk) 23:20, 24 January 2011 (UTC)
 * In fact it does: under 'Classification: Cultivars' it mentions some varieties that are suitable for blanching, and others mild enough (in taste) not to require it, which implies that they can be eaten raw as salad leaves. Further on under 'Properties and Uses: Culinary use' it mentions boiling or eating raw the leaves and buds, and also preparation of the petals for dandelion wine and the roots for dandelion coffee (or tea), with links to more detailed articles on those topics. 87.81.230.195 (talk) 23:47, 24 January 2011 (UTC)
 * The leaves of the young (pre-flowering) plants are very widely used as a salad green. Once they get more mature, they are too bitter.  The flowers are also very bitter. Looie496 (talk) 02:45, 25 January 2011 (UTC)
 * I eat them every spring, in salad. You pick the leaves early on before the flower blooms or and just make salad with a vinegar and oil dressing, preferably with slices of warm cooked potatoes, because the leaves are somewhat hard and the heat from the potatoes softens them. TomorrowTime (talk) 20:34, 25 January 2011 (UTC)
 * Just a warning: the flower (yellow type) tastes awful and has no resemblence to the nectar taste. ~ A H  1 (TCU) 22:47, 26 January 2011 (UTC)

Moon
If you look at pictures such as this or this, you can still see some light on the back of the moon. This light is reflected from Earth, correct? And if not, where from? --  T H F S W  (T · C · E) 23:59, 24 January 2011 (UTC)
 * Yes, it's earthlight. HiLo48 (talk) 00:10, 25 January 2011 (UTC)
 * Starlight would also be a small component of any light reflected by the moon that does not originate from the Sun. WikiDao    &#9775;  00:15, 25 January 2011 (UTC)


 * As I have pointed out on this desk in another post a few weeks ago, the image File:Lunar libration with phase2.gif is a synthetic image stitched together from many thousand individual photographs from the Clementine (spacecraft). The photos were stitched into a moon image, and then accurately animated to mimic the time-lapse behavior as if viewed from Earth.  Any "earthshine" in those photos should be considered synthetic.  The image meta-data even explains: the moon was rendered with an ambient illumination of 1% intensity of the solar illumination - much brighter and more isotropic than actual earthshine.  Earthshine is a real phenomenon, though, and you can easily see it tonight, even with the naked eye, if you look beyond the sunlit portion toward the lunar limb.  This evening, the lunar sunset will be visible from the Apollo 17 landing site.  If you photograph the moon, you can compare it to the synthetic image and see for yourself - the luminosity contrast is much greater than 100 to 1.  Our article on Apparent Magnitude lists a full- to new-moon contrast ratio of 10 magnitudes - about 1000x - but apparent magnitude is measured in nonlinear units of luminous intensity, designed to match human perceptions of brightness.  If you can manually set exposure on your camera, you can measure the actual luminous intensity difference.  Have a look at the exposure chart in this guide to lunar photography.  Nimur (talk) 00:16, 25 January 2011 (UTC)
 * Let's note also that File:Luna Nuova.jpg is no better. Such a view of the moon (no sunlit portion, dark sky) is effectively impossible, as the new moon only occurs when the sun and moon are aligned from Earth.  We note that the image is an edit of this one, which displays no earthshine.  As such, it's just a darkening of a picture of a full moon. &mdash; Lomn 14:09, 25 January 2011 (UTC)


 * The ballad of Sir Patrick Spens contains the lines (with slight variants in different versions) "Late late yestreen I saw the new moone, / Wi the auld moone in hir arme, / And I feir, I feir, my deir master, / That we will cum to harm." Apparently, "the new moon with the old moon in her arm(s)" was a common idiom referring to a thin (waxing) crescent moon with the rest of the disk dimly visible by earthshine. I've seen it many times myself and only rarely come to harm thereafter. Deor (talk) 04:17, 25 January 2011 (UTC)
 * You might like to read planetshine which is a little more complete than earthlight, I fancy there may be a case to merge the articles . Richard Avery (talk) 07:23, 25 January 2011 (UTC)


 * Light also reaches the Moon from Earth during a lunar eclipse, but the light is refracted through Earth's atmosphere rather than reflected. ~ A H  1 (TCU) 22:44, 26 January 2011 (UTC)