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    Having lived in the southern hemisphere for 2 years, the 'man in the moon' from my UK childhood is becoming a distant memory. In fact I can't find any reference to the face I was seeing. Would the moon surface be different in appearance in Australia and New Zealand than in Britain?

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    Where can we find some actual (normal visible light) photographs of the sun taken from outside the near-earth environment? There seem to be no photos of the sun available, taken, for example, from a point half-way between the earth and the sun, or taken from the vicinity of Saturn, Jupiter, Uranus, Neptune or Pluto. Voyager 1 and 2, Pioneer 10 and 11, and numerous other deep-space spacecraft, from the USA as well as from Russia and from the ESA, surely must have taken some photos of the sun. Where can we find them? Why has NASA never released them?

Thank you very much.

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    What makes the Sun hot? Energy is released in the core of the Sun by nuclear processes, which combine atoms of hydrogen into atoms of helium. Four hydrogen nuclei (aka protons) combine to form one of helium, with some energy left over, and that energy provides the heat.

    Many chemical reactions require a higher temperature (boiling an egg, for instance), and so do nuclear reactions. The temperature required for the solar energy release is enormous, and creates enormous pressure. Only near the center of the Sun can these conditions exist, with the weight of the higher layers of the Sun keeping the energy-release region confined. If that confinement did not exist and it had the chance to expand, it would cool down and all nuclear energy release would end.

1. What concept of physics causes the space shuttle to get so hot that it almost burns up when returning to earth? 2
2. What one word describes this concept?
3. What kind of energy is present during reentry?

Also how are these problems fixed if nessecary?

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    Many of these problems were addressed on the space station "Skylab". Astronauts tend to lose weight, muscle tone and bone--in a way which somewhat resembles problems of patients who are bedridden for a long time. Exercises help prevent these from occurring. I am not sure about more subtle effects--you will have to search on the web.

    Unfortunately, I'm getting hung up on that lifting gas. I suspect that Helium will be in short supply on Mars. On earth, it is a byproduct of natural gas, but on Mars, there should be little, or no natural gas, and helium would have to be mined by itself.

    My idea however, is that since the atmosphere is mostly CO2, blimps with much heavier gasses could be used. I would expect hydrogen, oxygen, and nitrogen blimps to be common, since these three gasses are highly useful commodities in their own right on a world with little water, and no breathable atmosphere.

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    About the lifting power--it equals to the weight of the displaced atmosphere, minus the weight of the gas inside the balloon, required to keep it inflated. Whether that is hydrogen or helium should make little difference, because either gas weighs much less than the air (or on Mars, carbon dioxide) which is displaces. Hydrogen should be available on Mars, if only a little water can be extracted there, which seems likely. Hydrogen was of course used in all the Zeppelins in WW-I and with proper caution is quite usable.

    Oxygen and nitrogen are rather heavy, cutting the lifting power by a factor of about 4: considering how marginal any balloon is on Mars, they are clearly unsuitable..

[Note: The above turned out to be a false rumor, which had quite wide circulation. The reply below was written before the news media reported on it.]

    Beats me. The average Mars-Earth separation is 0.5237 AU, and with 1 AU equal approximately to 150,000,000 km or 93,000,000 miles, this comes to 48.8 million miles. The actual closest approach may be larger or smaller, because of the ellipticity of the orbits (mainly the one of Mars),

    In any case, Mars seen by the unaided eye is never as big as the Moon. It is true, though, that viewed through a telescope, Mars appears to be about as big as the Moon seen by the unaided eye (although, unless seeing conditions are very good, the image may shimmer and distort).

    If they do continue to show interest more than a few days in the telescope project, then I hope to help them build (instead of buy) a set of telescope drive controls. So far their enthusiasm has not waned, and they are asking a lot of questions about how to find objects in the sky and point the scope. My experience doing some controls engineering will help, but I want to let them do some of the programming to make the telescope follow the right target. My challenge will be to keep their challenges at the right level.

    Your pages provide a nice balance. Appropriate examples, and reasonable approximations that make some of the fundamental calculations of motion of celestial objects trackable. I think if I can learn that art of finding and sharing simple examples, then the telescope project will generate several fun and useful lessons. Otherwise, it will become just another project.

Thanks for your contributions in making physics, astronomy and earth sciences fun for non-scientists.

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    Also, what are the leading "hot" topics on Solar Physics now?

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    The solar wind starts not from the Sun's surface, but from the corona, and is accelerated somewhat gradually. Obviously, it has to overcome solar gravity, which I suspect is one of the conditions needed for accelerating the solar wind--maybe like a lid on a pressure cooker, holding down the hot corona until it can just barely escape.

    'I've read, and taken advantage of places like the Hayden planetarium, and I have made (and continue to make) my own observations.

    I am blessed now to live in a high rise, with a clear south and west view.--more or less.. (I live close to Flushing Meadow park,)

    My observation is: at the winter solstices, the sun sets in Brooklyn.. and by now, its getting close to down town Brooklyn. soon it will set in Manhattan. and by summer, it sets in midtown.. (certain building of the NY skyline are my personal stonehenge!)

    But the moon, oh , the fickle moon.. there must be a pattern.. but in the 18 months I have lived here, I haven't worked it out..

    Sometimes it sets in downtown, some times in midtown.. but it doesn't seem to move in a smooth pattern, (a sine wave say) but its all over the map! I can't tell from one month to the next were the moon will set.

    I am sure information about the moons patterns and movements (or perceived movements) must be documented. I 1 read about the sun's movements and began as a child to note were it set (and more often where it rose, since my childhood home had clear eastly views.) observation and reading have brought me a clear sense of the movement.

    But the moon.. can you suggest some reading?

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    A useful trick which someone told me requires fast-drying "whiteout" paint used by typists to blot out mistaken letters or words, which are then overtyped correctly. Draw the stars in the constellations which you want to watch on a fairly large sheet of paper, put on top of it a sheet of saran wrap from the kitchen, and with the white-out brush mark on it the stars you want to identify--bigger marks for brighter stars.

    In fact, the Moon WAS closer to Earth in the past, and is even now gradually receding. This is a side-effect of the tides, which may be viewed as a world-wide system of waves, losing energy as it hits continents and islands. This takes away energy from the Earth's rotation, which slows down (days get longer, but at a very slow rate). Because of conservation of angular momentum in the Earth-Moon system, this causes the distance of the Moon to gradually increase.

    How close was the Moon originally? The answer depends on the way the Moon originated--was it formed with the Earth, captured by the Earth or somehow torn off the Earth? George Darwin, son of George Darwin

http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Darwin.html

who was a well-known scientist in his time, believed in the last possibility. He claimed that originally the Earth rotated too fast to be stable, a bulge formed, and it was torn off by the rotation, to form the Moon. In this view the Moon was originally "within touching distance", but the action of the tides (which initially must have been huge) caused it to recede to where it is now.

    More careful studies showed Darwin's theory cannot hold. I think most astronomers feel the Moon and Earth were created together, as a double planet, though other views persist--see "The Big Splat", a book by Dana MacKenzie. If you can find the old book "Biography of the Earth" by George Gamow (written about 50 years ago) you will find he still supported Darwin's theory. It's a delightful book, even though not everything in it has stood the test of time.

  What if the Moon was closer?     (B)

    This happens (and your teacher might have said so already) because of the tides in the oceans. The moon pulls the water into a wave which tries to remain pointing at the moon; the moon also pulls the entire Earth, leaving behind a second crest of the tide-wave on the opposite side of the Earth.

    The two crests try to line up with the Earth-Moon direction, but the rotation of the Earth under them causes them to collide with shorelines. Such collisions slow down the rotation of the Earth and give up energy, whose ultimate source of that energy is the rotation of the Earth and the motion of the Moon.

    How exactly are atmospheres held? By gravity of course, but note that the density of the atmosphere decreases with altitude. With Earth, it drops to one half about every 5 kilometers in altitude (temperature can modify this). If you rise 5 kilometers, only half the atmosphere is above you, with only half its weight compressing the air, so density and pressure are down to 50%. Rise to 10 km, and it's only 25%, and so on. At 100 kilometers you are down to about one part in million, and collisions between molecules become more rare, so that they rise and fall like stones tossed upwards.

    The "halving distance" could be more than 5 kilometers, or less. If gravity were smaller (as it is on the Moon) and temperature higher (as happens at high altitudes), it would be larger--and the atmosphere would rise higher. The somewhat early loss of the space station "Skylab" in 1977 was helped by higher sunspot activity, which caused the upper atmosphere to heat up and expand (from more solar X-rays), with more air resistance in low orbit.

    Also, above 100 kilometers, sunlight may break up molecules (e.g.) of water, and the atmosphere is no longer evenly mixed, in constant proportions. Lighter atoms (e.g. hydrogen) then rise higher than heavy ones (e.g. oxygen), because their velocity is larger, at the same temperature.

    The velocity is still much less than escape velocity--but in a hot gas, a spread of velocity exists (a "Maxwellian" spread, named for James Clerk Maxwell who first derived it) and a few atoms or molecules always move fast enough to escape. On Earth, over geological time spans, all the helium has escaped (the helium we get from natural gas was produced as alpha particles by radioactive elements such as uranium). As the attached graph shows, on the Moon all common gases are expected to be gone.

The graph is from
    http://abyss.uoregon.edu/~js/ast221/lectures/lec14.html

    First of all, it is a GOOD THING there is no air in outer space! Outer space is SO BIG, if our air could spread out over all of it, almost nothing would be left for us to breathe.

    Luckily, air has WEIGHT. (You don't feel it, but the air in a room weighs more than Sarah).

    Without weight, everything would FLOAT FREE, like astronauts in a spaceship (I hope Sarah has seen pictures). Weight is what keeps tables, chairs, people, cats, dogs, cars and so on standing on the ground, also keeps down the water in oceans and lakes. And it also keeps our air from floating away.

    Weight tries to keep everything as LOW DOWN as possible. Water flows downhill. In the winter, sleds slide down a hill by themselves, but need to be pulled back. Weight also keeps air attached to the ground.

    Weight is caused by GRAVITY--the pull which any piece of matter has on any other. The pull is very weak: any two stones pull each other, but so weakly, you can never feel it. But the EARTH is such a BIG piece of rock, that it has enough pull to keep all of us, and all things around us, safely on the ground.

    It even has enough pull to keep the MOON from running away. As well as all the satellites NASA and others have launched, which like little moons go around the Earth..

    Please let me know if the above is good enough for a 3-year old!

 Telling a 3-year old about the atmosphere     (b)

    (Adapted from question asked on Physhare, a list server of physics teachers)

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    I have looked up this question in the writings of the true master, Richard Feynman, who dealt with color vision in chapters 35 and 36 of "The Feynman Lectures", volume 1. It is remarkable how far his interests have ranged, how few were the topics he did NOT look into. My own field of magnetospheric physics is unfortunately one of those few, and that only because his sister Joan (who is still with us) worked in it. When her brother showed interest, she told him to butt out, leave something for her to do. He did, and now we are stuck with all sorts of unresolved problems.

    You should read Feynman's exposition, in particular chapter 35, which I tried to flesh out a bit here. Feynman is so much more articulate than I can ever be, and what I wrote below is anyway my own interpretation, not necessarily the same thing.

    The important thing to do is to pose the proper question: what exactly do we want to claim? There is no question that the normal eye has 3 kinds of receptors, which we may designate (R,G.B) for (red,green,blue), although each is sensitive to a wide spectral range beyond the "pure" rainbow colors of red, green and blue.

    When we look at some colored object, with some standard brightness (I choose here a standard brightness for all colors, so that only relative intensity matters), if the (R,G,B) sensors detect intensities (a,b,c), we will see SOME color, and that color will be characterized by the 3 numbers (a,b,c). The range of values for (a,b,c)--say, from 0 to 1 for each, after we have standardized intensities--covers ALL colors the eye can see. There exists nothing more.

    The real question to ask is--is it always possible to "fake out" the color (a,b,c) by presenting the eye with a superposition of 3 colored sources, of three standard colors (U,V,W). These can be pure spectral colors, or not. (Red, Green, Blue) are a good choice, though, because each stimulates primarily one kind of receptor. Therefore, by twiddling with their intensities, we can make each type of receptor receive an EQUAL intensity, and the result registers in our mind as white. Let this be an assumed restriction on the choice of "primary" colors, requiring that we CAN get white; three different hues of green may never be able to do so.

    Let the color U create responses (a₁, b₁, c₁) in the 3 receptors of the eye. Similarly:
        --the color V creates responses (a₂, b₂, c₂), and
        --the color W creates responses (a₃, b₃, c₃)

    These numbers (a₁ . . . c₃) are all COMPLETELY determined by our choice of reference colors (U,V,W).

    Suppose we want to illuminate a white surface with a combination (a', b', c') of the 3 sources, in such a way that the eye will be made believe that it sees a GIVEN color (a,b,c). Then

    a  =   a₁ a'  +   a₂ b'   +   a₃ c'
    b  =   b₁ a'  +   b₂ b'  +   b₃ c'
    c  =   c₁ a'   +   c₂ b'   +   c₃ c'

    It is now necessary to invert the equations: (a,b,c) are given, (a', b', c') are to be determined. (You can write it in matrix notation if you wish, and you then need invert some matrix). There always exists a solution (unless the equations are dependent--e.g. the 3rd is the sum of the first two), so it SHOULD be possible.

    Having reached that point, Feynman raises a warning. A solution exists, but do we have a guarantee that (a', b', c') are all positive? They have to be if we add light from three sources--we cannot subtract (except maybe by using a colored screen, something he briefly discusses).

    If we choose (U,V,W) wisely, the coefficients will be positive for a wide range of color combinations (a,b,c), all of which can be represented by combinations of our reference colors. But there may exist some oddball combinations (a,b,c) where the solution gives some negative coefficient. NO combination of (U,V,B) can then fool the eye to believe that it sees that color. Feynman claims that there will always exist such colors, and at this point I tend to believe him.

Unless there is some flaw here, you may pass all this to your class. Let them see Feynman's book, too!