"Get a Straight Answer" Site Map

Get a Straight Answer

Please note!

    Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.

For an index file listing questions by topic, click here.

Items covered:

  1. About asteroids hitting Earth.
  2. The swirling of water in a draining tub.
  3. Dispensing water at zero-g.
  4. Robert Goddard and World War II.
  5. Asymmetry of the Moon's orbit.
  6. Measuring distance from the Sun.
  7. Who owns the Moon?
  8. Acceleration of a rocket.
  9. Rebounding ping pong balls (re. #35)
  10. Rebounding ping pong balls and gravity-assist
  11. Why don't we feel the Sun's gravity pull?
  12. How hot are red, white and blue (etc.) stars?
  13. How does the solar wind move?
  14. The shape of the orbit of Mars
  15. What if the Earth's axis were tilted 90° to the ecliptic?

  16. Mars and Venus
  17. Where is the boundary between summer and winter?
  18. The Ozone Hole
  19. What keeps the Sun from blowing up?
  20. Those glorious Southern Skies!
  21. Should we fear big solar outbursts?
  22. Planetary line-up and the sunspot cycle
  23. What are comet tails made of?
  24. If light speed sets the limit, why fly into space?
  25. Does precession mis-align ancient monuments?
  26. Why does the Earth rotate? Why is it a sphere?
  27. What's so hard about reaching the Sun?

  28. Where does space begin?
  29. Gravity at the Earth's Center
  30. Radiation hazard in space (3 queries)
  31. "Danger, falling satellites"?
  32. The Lagrangian L3 point
  33. Distance to the Horizon on an Asteroid
  34. Overtaking Planets
  35. Falling Towards the Sun
  36. The Polar Bear
  37. Are the Sun's Rays Parallel?
  38. More thrust in reverse than going forward?
  39. The varying distance between Earth and Sun
  40. Mission to Mars
  41. Kepler's calculation
  42. The Appearance (Phase) of the Moon

  43. Stability of Lagrangian points
  44. Can an Asteroid Impact Change the Earth's Orbit?
  45. Can Gravity Increase with Depth?
  46. Lightspeed, Hyperspace and Wormholes
  47. Why do Rockets Spin?
  48. Around What does the Sun Revolve?
  49. Why are planets in nearly the same plane?
  50. The Shapes of Rockets and Spacecraft
  51. Space Debris
  52. Teaching Nuclear Fusion
  53. Contribution of different elements to Sunlight
  54. Jewish Calendar
  55. Spaceflight Without Escape Velocity?
  56. Who first proposed a round Earth?
  57. Does Precession change the Length of a Year?
  58. The Analemma
  59. Changes of the Polar Axis of Earth
  60. Van Allen Belt and Spaceflight
  61. Nearest Star Outside Our Galaxy
  62. (a) Why are Satellites Launched Eastward?
          What is a "Sun Synchronous" orbit?
     (b) Why are satellites launched from near the equator?
  63. How Tall Can People Get?
  64. Gunpowder and Rockets
  65. Precession
  66. Solar Sails
  67. (a) Distance to the Big Dipper
     (b) Big Dipper star names

  68. Was Moon landing a hoax?
  69. Clockwise or counter-clockwise?
  70. Isotopes in Center of Earth
  71. Density of the Sun's corona and the "Scale Height"
  72. Did Tesla extract free energy from thin air?
  73. What does "lapse rate" mean?
  74. Motion of the Sun through space
  75. Teaching about tides
  76. Distance to the Horizon
  77. Can geocentrist theory still be possible?
  78. Can Earth's rotation reverse, like its magnetic polarity?
  79. Why is the Earth round?
  80. The De Laval Nozzle
  81. Why 23.5 degrees?
  82. What is Gravitational Collapse?
  83. Can Earth capture a second moon?

  84. How far does the Earth's gravity extend?
  85. How far is the Moon?
  86. Twinkle, twinkle little star
    How I wonder, what you are.
  87. Teaching about seasons
  88. Space Launches by Cannon--A
  89. Space Launches by Cannon--B
  90. The Southern Pole of the Sky
  91. Do Astrologers use Wrong Positions for Planets?
  92. Why does the Moon have bigger craters?
  93. Why does Gravity Exist?
  94. Atmospheric "Thermals"--Triggered by Electric Forces?
  95. What would happen if Earth rotated faster?
  96. Where do gravity of Earth and Sun balance?
  97. The Ultimate Astronomy Tool
  98. High Temperature in Cold Outer Space

  99.   Refraction of sunlight and starlight by the atmosphere
  100.   Advice to a would-be astronomer
  101.   The effect of the Color of Light on the Output of Solar Cells
  102.   What is "radiation"?
  103.   Height of the Atmosphere
  104.   How does the upper atmosphere get so hot?
  105.   History of the use of De Laval's nozzle on rockets
  106.   Why don't Space Rockets use Wings?
  107. Distance of horizon on Mars
  108. Stopping the rotation of Earth?
  109. The equation of a parabola
  110. When does Jewish Sabbath start in the far north?
  111. Where is the center of the global landmass?
  112. What if our Sun was a much hotter star?
  113. Finding the north direction

  114. Why not use a heat shield going up?
  115. When and where can rainbows be seen?
  116. The unusual rotation of the planet Venus
  117. Why not use nuclear power for spaceflight?
  118. "Doesn't heat rise?"
  119. Have any changes been observed on the Moon?
  120. Why isn't our atmosphere flung off by the Earth's rotation?
  121. Can kinetic energy be reconverted to work?
  122. Does any location get the same number of sunshine hours per year?
  123. Speed of toy car rolling off an inclined ramp
  124. Acceleration due to gravity

  125. Re-entry from Space
  126. Balancing a Bicycle
  127. Is Absolute Zero reached on the Moon?
  128. Why isn't Longitude measured from 0° to 360°? "Constellation" or "Asterism"?
  129. "Position of the Stars when I was Born"
  130. Rotation of the Earth's Core"
  131. How hot is the Sun?
  132. How much weaker is gravity higher up?
  133. Eclipse of Venus?
  134. The Big Bang

  135. Thanks for the "Math Refresher" in Spanish
  136. The Pressure of Sunlight
  137. How is the instant the seasons change determined?
  138. Operation of Ion Rockets
  139. Physical Librations of the Moon
  140. The De-Laval Nozzle
  141. Why does the space shuttle rotate at take-off?
  142. Cold Fusion
  143. What if a Neutron Star hit the Sun?
    Why did the Moon appear Red?
  144. Centrifuge for Whirling Astronauts
  145. What Holds Galaxies Together?
  146. View of Earth and Moon from Mars
  147. Appearance of the Moon (1)
  148. Appearance of the Moon (2): Does it "roll around"?
  149. Altitude of the tail of the Big Dipper
  150. Sudden decompression, 5 miles up

  151. Do Negative Ions make you Feel Good?
  152. Shape of the Earth's Orbit
  153. Questions about the Solar Corona:
                       (1) Why don't its particles separate by weight?
                        (2) What accelerates the solar wind?
  154. Why does the rising Sun look so big?
  155. Drawing a Perpendicular Line in Rectangular Coordinates
  156. Unequal Seasons
  157. Is the Big Dipper visible from Viet Nam?
  158. Holes in a Solar Sail
  159. Consequences of no more solar X-rays
  160. Science Fair Project on the Size of the Earth
  161. Superposition of Waves
  162. The Sun and Seasons
  163. If the Earth's Rotation would   S t o p...     (1)
  164. If the Earth's Rotation would   C h a n g e...     (2)
  165. What if the Earth stopped in its orbit?
  166. Fast Trip to Mars     (1)
  167. Fast Trip to Mars     (2)

  168. Spacecraft Attitude
  169. What makes the Earth rotate?
  170. Energy from the Earth's Rotation?
  171. How were planets created?
  172. Does Precession of the Equinoxes shift our Seasons?
  173. "Zenial Days" on Hawaii
  174. Sun's Temperature and Energy Density of Sunlight
  175. Teaching about energy in 8th grade
  176. About the jetstream
  177. What would a breach in a space station do?
  178. Gravity at the Earth's center
  179. Freak waves on the ocean
  180. Citation on "Bad Greenhouse" web page
  181. How can radio waves carry sound?
  182. Do Cosmic Rays produce lightning?
  183. Star positions shifted by the atmosphere
  184. The equation of time
  185. Launch window of the Space Shuttle

  186. No "Man in the Moon" from Australia?
  187. Picturing the Sun from a different distance
  188. What makes the sun shine so brightly?
  189. Re-entry from orbit
  190. Effects of weightlessness on one's body
  191. Blimps on Mars
  192. Planet Mars "huge" in the sky, in August 2005?
  193. Astronomy and telescopes for ones' own children
  194. Does the solar wind have escape velocity

If you have a relevant question of your own, you can send it to
stargaze["at" symbol]phy6.org
Before you do, though, please read the instructions


  1. No "Man in the Moon" from Australia?
  2. Fascinating site. One quick question, if I may.

        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?


        You are absolutely right: the Moon does look different from Australia or New Zealand, and you may miss there the "Man in the Moon" arrangement of dark areas ("seas" or maria). That is not because the Moon moves, but because the "up" direction there may be almost the reverse of what it is in the northern hemisphere. Our mind always relates appearances to what we consider "up" or "down."

        Asian cultures, by the way, do not see "the man in the Moon" but rather "the rabbit in the Moon," and have a legend about that. Ask Google or some other search engine about that, if you wish.  

  3.   Picturing the Sun from a different distance
  4. Here's a question for you that I could not find already answered on your very impressive and very useful web site.

        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.


    There is no advantage in taking pictures of the Sun from a different distance. You get the same result from a telescope.

        There does exist a considerable advantage in taking pictures from a different angle. For instance, when a sunspot group disappears around the edge ("limb") of the Sun's disk, it takes two weeks before we get any information on what has happened to it. The capacity of observing the Sun from other angles does not exist yet (spacecraft designed for planetary observations are not designed to look at the bright Sun) but NASA is planning a "Stereo" mission to look at the Sun from two somewhat different directions and get something of a 3-D view.  

  5.   What makes the sun shine so brightly?
  6. What makes the sun shine so brightly?


    What makes the Sun shine? Its heat, because all hot objects shine--like filaments in lightbulbs.

        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.

    For more, see http://www.phy6.org/stargaze/Sun7enrg.htm

                    Response to reply:

    thanks but my assignment was due like three days ago and i kinda didnt get your answer in time so thanks anyway. love jane  

  7. Re-entry from orbit
  8. I am 17 years old. I go to school at Father Lopez Catholic High School where I run cross-country and track and play soccer. I have a few questions for you:
    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?


        You probably mean "process', not "concept" (concepts are in your mind, processes occur in nature). The process involved is a shock, a piling-up of air around the shuttle, since the shuttle moves too fast for air to flow out of its way. The shock heats the piled-up gas, but luckily, most of the heat is then radiated away, since anything hot glows and radiates light--e.g. a flashlight filament. Only a small fraction of the heat reaches the heat shield tiles, although, since radiation is beamed in all directions, some of it will hit the shuttle, too.

        The amount of kinetic energy which must be given up (as radiation) is tremendous. The shuttle starts at 24 times the speed of sound, while a rifle bullet may have twice that much, maybe 3 times. So pound for pound, the shuttle may have 100 times more energy. Lead bullets melt when they hit a sandbag, so getting rid of that energy is a tricky feat indeed.  

  9.   Effects of weightlessness on one's body
  10.     I was wondering how does going up into space have an imapct on your body if you are up there for a certain amount of time. How does it affect your health? what changes occur for males and what changes occur for females and both?

    Also how are these problems fixed if nessecary?


    Dear Australian friend

        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.

    About weight loss, see http://www.phy6.org/stargaze/Sskylab.htm

        The principles used in measuring the mass of astronauts in a "weightless" environment, see the end of the section preceding
    and perhaps the one following

  11.   Blimps on Mars
  12.     I'm trying to figure out lofting capacity for various gases on Mars, so I can more realistically visualize a lighter-than-martian-atmosphere-craft. What with the different gravity, different atmospheric pressure, and different atmospheric composition, I figure that the lofting capacity of various gasses would be different.

        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.


        Flying a balloon on Mars is not easy. The ground-level atmosphere is (I believe) 1/140 times as dense as ours, and even with 1/3 of the gravity, it may be equivalent to flying a balloon at 25,000 meters on Earth or at 80,000 feet. It can be done, but takes a big balloon. With the fierce storms of Mars, one also wonders how long such a balloon would last.

        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..

        Scientific high-altitude ballooners on Earth prefer helium because of the way its temperature changes as the balloon rises and the gas inside expands, but on Mars a balloon probably won't rise far. Rubber balloons launched to probe the weather are filled with hydrogen, which is much cheaper: they rise quickly and explode at their maximum altitude, but by then all required data have been transmitted by radio.  

  13.   Planet Mars "huge" in the sky, in August 2005?
  14. Is there a terrestrial phenomenon occurring this month with Mars? I've heard form a few "wannabe" astrologers that the planet Mars will be "huge" is the sky and look as big as the moon with the naked eye. This is an event that has happened every 5000 years or so. Mars is to come within 34,649,589 miles to Earth. I have not seen any television reports on this (maybe due to the recent NASA landing). Could you expound upon this for me ? Thanks,


      [Note: The above turned out to be a false rumor, which had quite wide circulation. The reply below was written before the new 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).

        After writing the above I looked up Google for some more precise facts--see
    The next closest approach is October 30, 2005, and will being us within 43,000,000 miles of Mars. The closest approach your friend mentions happened 2 years ago and was indeed unusually close.

  15.   Astronomy and telescopes for ones' own children
  16. Having looked at the elementary school texts my children have been assigned, I have been disappointed. Some of the most enthusiasm they have shown recently toward learning more about the physical world and doing a bit of thinking, is during a recent discussion of "how to point a telescope." But they do not get those kind of examples, or anything that seems remotely stimulating from their textbooks. Unfortunately, that kind of motivation seems only to come from strictly extracurricular activities.

        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.


    I do not know your children--what ages, what interests, what school. Nor do I know the limits of your budget.

    Textbooks are generally uninspiring--they are produced to sell, not to inspire, and are often the result of committee work. If your kids would like to know about the universe, here are two books they may like
              "Seeing in the Dark" by Timothy Ferris
              "A short History of Nearly Everything" by Bill Bryson.
    I reviewed the 2nd of these in an article later picked up by "Eos", on

        As for telescopes, I might have a different philosophy here--give the kids an instrument with minimal hassles, to let them go right away and observe. For our son's birthday (32nd, I think) we gave him a Meade 3.5" telescope. We scrimped and did not buy the computer controlled model, and that was a mistake--I think he would have used it more if he had the one with built-in star catalog. On advice we also bought for it a 90-degree elbow eyepiece (easier to look at objects high in the sky), a better eyepiece and a subscription for "Sky and Telescope" (he had a tripod already, and he crafted his own camera attachment).

        If your kids are not yet in that league, they have plenty to read in my web pages. I can also send you solutions to the problems, if you want to teach them that way. They should keep in mind, though, that no one will care as much about their own education as they themselves. If they want it, it's theirs for the taking.  

  17.   Does the solar wind have escape velocity?
  18. Your site claims the particles in the solar wind are leaving the Sun at about 400 km/s. This is less than the escape velocity which is about 600 km/s. Does it mean many of these particles will eventually fall back to the Sun? Is there any evidence of such as behaviour?

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


        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.

    You might want to look up http://www.phy6/org/ Education/FAQs6.html#q82

        Incidentally. NASA has been toying for years with the idea of a solar probe, approaching the Sun within 4 solar radii--following boost from a "hairpin" orbit around Jupiter (mentioned briefly here and in the page following it). It would be shielded from the Sun's intense heat by an "umbrella" of tungsten or similar material, and would study the solar wind in its source region. How can it do so with a metal barrier between it and the Sun? Simple: at closest approach is moves at about 300 km/s, perpendicular to the line to the Sun, so in its own frame of reference, solar wind particles (unlike sunlight) would seem to arrive from the side, at an angle. They would seem to have the vector sum of their own velocity and that of the corona relative to the fast-moving probe.

        "The leading hot topics"? I am only slightly involved in solar research, but the big question there seems to be what happens beneath the surface--where do sunspots originate and how, what are they, and what creates the uneven rotation of the Sun (which drives sunspots and magnetic fields). When I got into the space research, it was proposed all these were shallow phenomena, in the outermost layers of the Sun, but current study suggests they actually extend to appreciable depth. If you have a science library nearby, try to find

    Parker, E.N., The physics of the Sun and the gateway to the stars, Physics Today, 53, p. 26-31, June 2000

Go to main list of questions   (by topic)

Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated 10 May 2005