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    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 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°?
  129. "Constellation" or "Asterism"?
  130. "Position of the Stars when I was Born"
  131. Rotation of the Earth's Core"
  132. How hot is the Sun?
  133. How much weaker is gravity higher up?
  134. Eclipse of Venus?
  135. The Big Bang

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

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

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

  187. No "Man in the Moon" from Australia?
  188. Picturing the Sun from a different distance
  189. What makes the sun shine so brightly?
  190. Re-entry from orbit
  191. Effects of weightlessness on one's body
  192. Blimps on Mars
  193. Planet Mars "huge" in the sky, in August 2005? Astronomy and telescopes for ones' own children
  194. Does the solar wind have escape velocity
  195. Astronomy for cliff-dwellers of New York City
  196. Portable star finder
  197. What if the Moon was closer? (2 questions)
  198. Why doesn't the Moon have an atmosphere?
  199. Telling a 3-year old about the atmosphere (2 questions)
  200. Three-color vision

  201. Superconductors work, universe expands--with no energy input. Why?
  202. Shuttle orbit and Earth rotation
  203. Worrying about Wormholes and Black Holes
  204. What should I study?
  205. The greenhouse effect
  206. Separation between lines of latitude and longitude
  207. Motion of air: hot to cold, or high pressure to low?
  208. Removing "Killer Asteroids"
  209. Strange light seen from Hawaii
  210. Is the Sun attached to another star?
  211. What if the Sun turned into a black hole?
  212. Do absorption lines have a Doppler shift?
  213. What are "Electromagnetic Waves"?
  214. Why are the two daily tides unequal?
  215. Why air gets cold higher up--a wrong explanation

  216. Any limits to Newton's 2nd Law
  217. Gravity at the Earth's center
  218. Does the Earth follow a "squiggly" orbit?
  219. Third grader asks: how far to zero gravity?
  220. "How does inertia affect a rolling ball"?
  221. What determines the quality of a telescope?
  222. Why design maps around curved lines?
  223. "Drag" by the Sun on the Earth's motion
  224. Does precession affect the time of summer? (2 questions)
  225. Newton's law or Bernoulli's?
  226. Does the universe have an axis?
  227. Frictional electricity
  228. Syllabus for catching up on physics
  229. Parabolic reflector
  230. At what distance does Earth start looking spherical?
  231. Is the Sun on fire?
  232. Confusion about the "Big Bang"
  233. How did Tycho calibrate his instruments?

  234. Gases that fill balloons
  235. Asian tradition on the start of winter
  236. Why our year starts at January 1
  237. Sticking a hand out of a window...
  238. One year of continuous sunlight?
  239. Shielding out radio waves
  240. The way gravity changes with depth
  241. The Sun's Axis
  242. "Gravity Particles"?
  243. A "short stay on Mars"
  244. Weight and mass
  245. "The Moon Hoax"
  246. Shuttle re-entry from space
  247. Energy levels: plus or minus?
  248. How can such small targets be accurately hit so far away?
  249. A teacher asks about compiling lesson plans
  250. Why the Moon has its phases
  251. How can a spacecraft self-rotate?
  252. Stability during a rocket launch
  253. Boiling point of water in space

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

  1. How far does Earth's Gravity Extend?

    I live in Guatemala, Central America, and I was wondering something.

        Do you know at which distance (miles)) and object in space gets attracted or caught by the Earth gravity, and brought down to Earth.


        Supposedly, a very similar question occurred to Isaac Newton in 1666, possibly when he saw an apple fall from a tree. The apple was attracted to the Earth, which made it fall. How far from Earth did this pull extend? In particular, the Moon must also be attracted to the Earth--otherwise it would wander off into space. Did this same force extend all the way to the moon, and keep it in its orbit?

        Newton's answer was "yes." Assuming that the force decreased with distance r like r-squared, he calculated the orbital period the Moon should have, and came out with the correct number. You will find all this, including the calculation, on

        Whether an object is brought down to Earth or enters an orbit around it depends on its velocity relative to the Earth. Objects at rest relative to the Earth are brought down, objects with appreciable velocity transverse to the line between them and the center of the Earth are not.


  2. How far is the Moon?


        How far away from the Earth is the moon?


        About 60 Earth radii, fluctuating a bit because the orbit is an ellipse. An earth radius is 6371 km, and you can do the multiplication yourself.

        A much more interesting question is "How do we know that"? A clever Greek astronomer, Aristarchus, figured that out more than 2000 years ago--without telescopes, using just a few simple observations and logic. See


  3. Twinkle, twinkle little star
    How I wonder, what you are.

        Help me please ! I am trying to help my young sister in-law with some research for a project and I cannot find the information I need on stars. I need to know their locations in the sky, their sizes, what they are made up of and what impact they have on the Earth. I need this information as soon as possible as she has an assignment to hand in Friday the 6th of September. If you can help me I know we would both be so grateful...

       Thank you

    [Received from Australia, 5 September]


    Dear Janet It's incredible, the way teachers nowadays give assignments for major projects, with only 2 days lead time! In this case, it's almost impossible to summarize concisely all the material. Still, let me try; using this, your sister-in-law better get busy in the library.

    (1)     Stars are seen all over the sky. Those are the closer ones. Our Sun belongs to a disk-shaped wheel of stars, the galaxy, and most of that wheel is so far that to the eye, its stars blend into a fuzzy glow. That is the Milky Way.

        Other galaxies can be seen, usually using time exposures on telescopes. The Hubble telescope photographed two small regions with 10 day exposures ("deep field") and saw a great number of faint distant galaxies.

    (2)     Sizes--mostly around the size of the star, say from 20 times as bright to 1/20th. Of course, there may exist faint "brown dwarves" too dim to see. Very bright stars burn away rapidly, so not many are around.

    (3)     What they are made of--mostly hydrogen (basic fuel) and helium (what is left from burning hydrogen. Heavier elements are rare--on the Sun, but not on Earth, of course. The heaviest are produced in an instant, when supernovas collapse.

    (4)     What impact? We exist because a star--the Sun--provides us energy, allows plants to live and thereby makes all other life possible. Our distance from the Sun is critical, it's just right for water to be liquid. A bit closer and the oceans would boil away, a bit further and they would freeze solid.


  4. Teaching about seasons


        I was reading your explanation of the seasons of the year. Are you aware of any "hands on solar system tool" that would assist me to explain your information to my daughter?

        As a student I recall a tool where the students could move the earth around the sun. That tool really helped me to understand day, night and the seasons.

        Thank you for any help.             :)


    Hello, Kelly Jo

        I am not aware of any such tool, although one could imagine it. The traditional way in class is have one student hold a big ball (or a small lamp) representing the Sun, and another holding a small ball (Earth) walk around the "Sun" A globe is also useful, but not many people have globes.

        Someone on the web has collected a big heap of reference links, to web sites explaining why seasons occur. See: I looked at some of them, and maybe I am prejudiced, but I feel that my contribution A HREF=""> (also listed there, close to the end) is shorter, yet covers all the essentials. Brevity is important when you want to reach kids. That file also has a lesson plan attached:

        One site I looked at proposed as "tools" a flashlight and a tennis ball with a knitting needle stuck through it: good idea, better than the apple suggested in the lesson plan (a toy ball, with the poles marked in masking tape also makes a good prop). You may try that

        A lot depends on the age of your daughter. My recommendation--use the tennis ball and flashlight, together with my files, and have a lot of scratch papers handy. Kids love it when their parents do the explaining, rather than relying on ready-made props. I have had the experience--see .


  5. Space Launches by Cannon--A

    Dear Sir

        I am doing some research into low cost satellite launch system, and came across this link ( ) and your contact details. Is there anything currently going on? Could you point me in the direction of any relevant experts.


    This is far from my expertise (though not from my interests). I would recommend that you look up on Google (or some other search engine) under "light gas gun," for links related to this topic. I did so just now and saw that quite a few such links do exist.

        My own guess is that a cannon probably is not a good low-cost system, for several reasons: high initial cost, unproven technology, intense aerodynamic heating after exit from the gun, need for additional propulsion and ferocious acceleration. It was included in "Stargazers" mainly to stimulate unconventional thinking.


  6. Space Launches by Cannon--B

    Dear David P. Stern

        I read your webpage about the 'Far-out Pathways to Space', where you state that a cannon could not be used to send humans into space.

        I have recently conducted an experiment that I think you will find interesting. I accelerated some shrimps and crabs up to 83 G with success. They were not harmed at all. I am quite certain that humans would also survive this if we could find a way for them to survive with water in the lungs for an extended period of time. If it turns out that humans could survive at 100G, then a 2km gun would enable a 2km/s start velocity. If the projectile also had a rocket engine....

        What I'm trying to ask here is, would an initial velocity of 2-3km/s be sufficient for this kind of launch to be economically feasible? Weighing the problems with the launch, the fuel saved, cost of launch system, etc., would it still be a 'Far-out Pathway to Space'?


        I suspect the method remains "far out." It is true that water-creatures can stand more acceleration (though you did not accelerate them for 2 full seconds!), but filling human lungs with water is a bit hard. There do exist experiments of filling lungs of small animals with oxygenated fluids, though it is very hard for humans to push it in and out. Also, the water would weigh a ton, and I wonder how much damage it could inflict in 2 seconds. Humans have stood up to 25 g in a compressed water suit, I believe.

        The rocket would also have to be rather massive, to stand an acceleration which briefly increases its weight 100-fold. It won't be a small rocket, either: from 2 km/sec to orbital velocity there is still a long way.

        A better way to reach an initial 2 km/sec velocity would be with a hydrogen ramjet. It is attractive from the point of view of fuel economy, because the greatest part (8/9) of the mass of the fuel would be drawn from the atmosphere, and ramjets do work up to about Mach 6. They do have to be first pushed past the speed of sound--perhaps by a very big airplane, burning some of the hydrogen as fuel. It seems more practical than a 2-km cannon, with an internal diameter matching that of the rocket.


  7. The Southern Pole of the Sky

    Hi David, I'm a third year university physics student and am currently working on a navigation project, explaining how sailors used the pole star to find latitude, the longitude problem and then onto modern techniques of GPS and space navigation. Using the pole star as a "motionless" object to use as a reference is easy to explain and there is lots of information on it. However, I would also like to explain how the southern hemisphere sky was used to make a determination of latitude and cannot find all that much information on it. As there isn't an equivalent of the pole star, what was used and how was latitude determined? If you could possibly give me a brief explanation or point me in the direction of a useful web site or book, it would be much appreciated.


        Your question seemed interesting, so rather than just tell YOU to go and search, I went to and searched " Finding southern celestial pole".

        One answer there (curator of a web site "Ask an Astronomer") stated (in brief) "I know of no bright star in that location, and if you find an answer, let me know." But other links gave usable methods, usually starting with "find the Southern Cross, then continue the long axis of the cross a distance of 4.5 times its length." One site is


    another, with a picture of the sky


        You realize of course the north pole star is not exactly at the northern pole--it's half a degree off now, and was much further off at the time of Columbus, say. Also, that sailors preferred to "shoot the Sun," much more visible than the pole star--find its maximum altitude above the horizon, usually by combining several observations spaced apart, using a sextant, or earlier a cross-staff. As additional bonus, with a chronometer (or a tolerable clock and radio signals), the time that happened also gave longitude.

        And how do you find north by the stars near the equator, when neither celestial pole is easily seen? Try problem #3 in

        Good luck with your project


  8. Do Astrologers use Wrong Positions for Planets?

        I write from Ljubljana, Slovenia. I appology for my bad english. If you don't have time to answer me, I will understand.

        About sixth month ago I came in contact with the astrology and the position of planets in certain constellation. This data are provided in ephemerides. Because I had studied physics and a little astronomy I noted that astrologers give an incorrect data about the planets and sun position in the constellations. I asked some of astrologers why they don't accept the real position or why the data in ephemerides are incorrect but they can't answer me. Did I something understand wrong or astrologers just don't want to accept that the time is going on?


        Congratulations for finding this inconsistency. You were absolutely right!

        "Classical" astrology does not take into account the precession of the equinoxes, caused by the motion of the Earth axis around a cone. I have written about that (including "the dawning of the age of Aquarius") in or

        The precession causes the constellations associated with positions of the Sun and the planets at a certain part of the year to slowly drift around the zodiac.

        You can find more about this by going to and asking it to search for "Precession in astrology." I did, and some of the addresses returned are listed below:


  9. Why does the Moon have bigger craters?

        I got your name off the net. I was wondering if you might be able to help me with a question.

        Why are the largest craters we find on the moon and mercury so much larger then the ones found on the earth?

        (a.) because the earth's magnetic fields protect us from impact

        (b.) because the largest craters were made early in each worlds history, and geological activity has erased all traces of this early period on the earth

        Is it "b"?


    Magnetic fields are certainly no obstacle, while item (b) does have some validity. Certainly, much early cratering evidence has been erased--by weather and by the reworking of the surface by earthquakes, faulting etc. But you may also keep in mind some other factors:

        (1) Gravity is weaker on Mercury and the Moon, so ejecta from impacts that produce craters fly to greater distances.

        (2) Most of the Earth is covered by oceans, and because of plate tectonics, the ocean floor is constantly being reworked (see and the book cited at the end there). I suspect none of the Atlantic seafloor is older than 100,000,000 years, around 2% of the age of the Earth.


  10. Why does Gravity Exist?

        Dear Dr. Stern:

        I'd like to ask you something that's been buzzing into my head for some months, but I can't explain because of my lack of knowledge about physics (I'm just an architecture student, not trained in physics like you).

        My wondering is the following:

        I mean: Sir Isaac Newton explained that two bodies attract each other with a force directly proportional to the mass of each one, and inversely proportional to the square of the distance between them. Any body, just by having mass, is able to attract any other, and that's why Earth (a big mass body) attracts our bodies (or any kind of object having mass) against the floor with a force we call Gravity. We already know all this matter, but my doubt is:

        Where does that energy emerge from? Why is ANY body able to attract another one just because it has mass? What is the mechanism of Gravity, how is it created? Could we reproduce or dominate this natural process artificially and produce an "anti-gravity" field (imagine somebody floating into the air over the surroundings of the Earth Gravity field, because he's able to dominate this force) Newton said: "The apple falls due to Gravity", and I wonder "where does this force appear from?".

        Some of my friends, when asked this question, answered that I was crazy, but I feel like a primitive man (thousands of years ago) wondering why lightning appeared in the sky. Thousands of years later we know about electrical energy, and are able to dominate electricity, even reproduce lightning artificially. Can we hope the same for gravity?

        Best Regards from Spain Dr. Stern, thanks for your patience in advance.



        Nothing wrong about asking questions. For the record, when Newton was asked that question, he answered "Hypotheses non fingo" (I hope that is the correct Latin) or "I don't frame hypotheses," that is, "I do not speculate."

        What might be a satisfactory answer to a question like "why does gravity exist"? If the question can be rephrased "can we deduce the existence of gravity from more fundamental laws?" the answer seems to be "no." Gravity seems to be one of 4 fundamental forces in nature, the other 3 being electromagnetism, the strong nuclear force (holding nuclei together) and the weak nuclear force (mediating between neutrons and protons). Each is independent.

        A more profound question is the following. Each type of force is proportional to some "charge" carried by elementary particles. For gravity, that charge is mass. Why then is the mass of an object also proportional to its inertia, to its resistance to changing its velocity?

        This brings in general relativity, which is not my field. However, I do not think Einstein "explained" this proportionality--he just asserted that it, too, is a fundamental law of nature, and then deduced from it certain interesting consequences, which experiments have confirmed.

        So, I do not know any better answer than "gravity is fundamental, and so is its connection to inertia." You are studying architecture, so you know that, if you dig down from where you stand, at first you usually only bring up loose soil. But after digging some distance, you hit bedrock, and that is where you put the foundation of your house. Gravity is on the bedrock of physics, on which all of our more complex science is built.


  11. Atmospheric "Thermals"--Triggered by Electric Forces?

        A debate is a raging in the community of foot-launched-aircraft--paragliders and hangliders--to better explain and understand formation of "thermals." Those are the rising air currents that hawks circle in, and that allow human pilots to soar high above the earth, using wings & harness weighing only 10kG with an area of about 25m^2. I enjoyed this sport myself just yesterday, as I have many hundreds of hours before, spending more than two hours in the air 1300 meters above where I launched.

        One view in the debate goes roughly like this:

    1. 1. Thermals have no cohesive quality or adhesive quality like an air bubble in water, because the surface tension or cohesive quality attributable to water molecule do not apply to the buoyant forces causing warmer less dense air to rise. These act continuously, are stronger than any other forces that might cause this warm air to linger near the ground. Less dense warmer air is ALWAYS rising relative to denser air.

      The other view:

    2. 2. Thermals are made of air, a mixture or gas, aerosols, dust, etc. This air can be ionized near the surface of the earth (say within the first meter) by natural radioactivity in surface rocks, radon or sunlight. This ionized air en masse can linger, in cloud-like pools, near the earth (again for the sake of argument say within the first meter), held by electrostatic forces that are many orders of magnitude stronger than the buoyancy due to the difference in density of the surrounding air until some "triggering" event takes place.

          Some claim this triggering could be a gradual (over a few seconds) cascading neutralization of the charge. Others feel it simply happens when the volume of heated air becomes large enough for the buoyancy forces to 'outweigh' the ion forces, because there's a real limit to the ion density. The overall mass of this less dense air then becomes large enough or deep enough to overcome the electrostatic clouds ability to keep it close to the earth.

          Which is correct?


        First of all, I am pretty sure ionization has no effect. Not only is it very weak, but it produces no net charge: if you tear an electron off an atom, the two are separated but not by much. Presumably, ions and electrons wander around the air until they manage to recombine, but unless you manage (say) to draw away all electrons to some distant place, the bulk of the air stays neutral.

        Thermal currents are part of atmospheric convection, something described on web page (S1-A) "Weather and the Atmosphere" at
        Thermal currents such as are used by gliders are briefly mentioned there at the very end. As you note, the currents are driven by the buoyancy of warmed air, the tendency of less dense air to float upwards. Air near the ground is heated, and that gives it a tendency to rise. The process is definitely not electric--buoyancy is all you need. However, the motion will organize itself into independently circulating loops, because air masses cannot rise without others coming down to take their place.

        Surface tension has no role, as you note, and mixing at the boundaries seems small, because the rising air masses are fairly big--hundreds of meters across, rising to about1000 meters.  

  12. What would happen if Earth rotated faster?

    Dr. Stern,

        I was wondering if you can point me in the right direction (books, web resources, articles, etc) to answer the following question:
    ---What happens to life on earth if the earth's revolution is faster or slower? That is, what are the effects of a higher gravitational pull (oceans no longer exists, the moon crashes into the earth, etc) or lower gravitational pull (ozone layer no long exists, UV radiation kills all life, all water evaporates and life is non-sustainable)?


        The Earth's rotation has only a small effect on gravity, about 0.5%. This effect was only observed after pendulum clocks were introduced their timekeeping was compared to the motion of stars across the sky (the period of a pendulum depends on gravity). Without sensitive instruments you won't notice a thing.

        And it only affects objects that rotate with the Earth. The Moon moves in an orbit which is essentially the same whether the Earth rotates or does not rotate.

        Some objects rotate faster, e.g. Jupiter rotates fast enough to cause it to appear visibly oblate on telescope photos (near 10%). Objects cannot rotate much faster than that, however, without breaking up, even a rocky planet like Earth. Yes, it looks pretty solid, but the solidity of rocks is no match to forces exerted by masses as great as that of a planet. If the Earth spun in (say) half an hour, it would break up, and whether it was all liquid or all solid rock would make no real difference. In fact, an early theory proposed the Moon was "spun off" the Earth: see George Gamov's book "Biography of Earth" (if you can still find it).

        Of course, the force of human imagination is even more powerful than the laws of physics. Many years ago Hal Clement (real name, Harry Stubbs; someone told me he used to teach physics) wrote a wonderful work of science fiction titled "Mission of Gravity", about a planet made of very dense and enormously strong material. The planet rotated extremely fast, and its equatorial radius was several times its polar radius. Gravity at the equator was weakened by the centrifugal force and by the greater distance from the center, to where a human could land there. However, regions closer to the pole were habitable only to local creatures, many-legged squat crawlers (highly intelligent, too). Even they had trouble reaching the pole, where the mission of the book takes them.

        Read it. It's pure fantasy, but has interesting twists.

    If the Earth stopped rotating altogether, the Coriolis effect would no longer exist
    Because that force is a major factor in global atmospheric circulation
    you would expect climate and weather to become very different. I would not hazard a guess, but suspect the folks at NCAR (National Center for Atmospheric Research) in Boulder, Colorado, may have simulated that on their computers.


  13. Where do gravity of Earth and Sun balance?

    Hello Sir,

        I am a grade eleven student and came across this site looking for help in something I'm trying to find out.

        I was asked to find out where in the distance between the Earth and the sun the gravity of both the sun and the Earth will cancel each other out. To put it more straight forward: IF a shuttle is between the Earth and the sun, at that distance the Earth's gravity acting on it and the sun's gravity acting on it are equal to each other. I've only been taught a few formulas so far such as Fg=GMm/ r^2 and stuff like it. Using those so far I cannot figure out this problem. If you could help me solve this it would be really appreciated. Thank you


        You cannot solve your problem without knowing the mass of the Sun. After all, the answer would be changed quite a lot if the Sun had only (say) 1/100 of its actual mass! So please be informed that the Sun is about 300,000 times heavier than Earth. First solve the problem measuring distance in "astronomical units" (AU), in which the average Earth-Sun distance is 1 AU. Then, knowing that 1 AU = 150,000,000 kilometers (approximately), multiply your result by that number to get the result in kilometers. That saves you the trouble of calculating with very big numbers.

        However ... if you place a spaceship at that distance, it will NOT be in equilibrium, because the Earth orbits around the Sun once a year, and (to maintain its position relative to Earth) the spaceship must orbit with the same period. In the rotating system, the centrifugal force must be taken into account, and the problem gets a bit longer. That problem is solved in section 34a of "From Stargazers to Starships," and the answer is given there. That however may not be the answer to what your teacher wants you to find!


  14. The Ultimate Astronomy Tool

    Congratulation on answering and compiling a huge & useful set of question. It has been extremely beneficial to me. I am a software professional in India with Engineering degree and decent interest in Astronomy. I have a 6" Meade telescope of my own and do occasional stargazing. Most of the times I look on web for answers to most of my questions.

        I have a somewhat fundamental question for you, please let me have your thoughts on the same.

        In order to explore the universe we employ optical telescopes and radio telescopes. along with observations in infra red, ultraviolet etc. As I understand these are altogether quite diverse fields, but looking at an object of interest in space requires all kind of data. Then possibly we compile data from all those sources before making a reasonable opinion about the object. This requires a lot of coordination among various organizations, maybe across the globe, collect the data, convert it into some common standard and then possibly make sense out of it. Data from various sources may be fairly accurate but time would be essentially different. That seems to me a serious constraint in exploration mission of mankind considering the fact that universe is infinite and we still have to explore a lot more objects, quickly. Won't it be nice to have an integrated instrument which studies a celestial object using all possible mechanisms and gives complete information? Is there some thought process or project in that direction? Where do we stand in terms of compiling all the data we capture and analyzing it fully.


        About the integrated instrument you envision... it does not exist, no more than a single instrument that can wash laundry, brew coffee and saw wood. The requirements are too diverse!

        Most astronomical observations do not depend strongly on time--but where they do, astronomers do combine observations. The most recent instance has been the tracking down of gamma-ray bursts, which flash into view, last a few seconds and fade again. It was suspected that they were enormously energetic, very distant events in the early universe, perhaps collapses leading to massive black holes in the centers of galaxies, but until recently, they happened too quickly, and their locations too poorly measured to allow optical telescopes to find the visible signature of the objects left behind. Now this has been done.

        One astronomer who has followed lines similar to yours has been Martin Harwit, whose other interest is the history of astronomy (he headed for a while the National Air and Space Museum of the Smithsonian, in Washington). In a book "Cosmic Discovery (Basic Books, 1981; you might find it in a university library, perhaps) he asked--what were the main discoveries in astronomy, who made them, and what made them possible? He listed 43 key discoveries, and noted that many of them introduced some new technology, also that often the discoverer was not an astronomer but a person knowledgeable in that technology.

        He also noted that practically all astronomical observations used electromagnetic waves, and that any such observation had two main properties: wavelength and esolution (angle). Drawing a plane diagram in which one axis was wavelength and another resolution, he plotted areas already observed and those still unknown, and claimed that opening to exploration new regions in that diagram (in the past and in the future) was the best way towards making new discoveries. NASA has adopted his ideas: its "Great Observatories"--Hubble, Chandra and the new infra-red NGST telescope at the Lagrangian L2 point are all parts of that effort--EGRET may be too, not sure.

        Final note: you wrote you were an amateur astronomer, with a 6" Meade instrument. If so, I highly recommend to you a recent book "Seeing in the Dark" by Timothy Ferris, an overview of astronomy from the point of view of a dedicated amateur astronomer. It is very well written (a few items may be questioned, but that is secondary) with interesting personal sketches of some remarkable people, most of them talented amateurs. You will love it.  

  15. High Temperature in Cold Outer Space

        How does one explain the concept of the temperature being perhaps 700 degrees C high up in the atmosphere, but that an person would freeze to death (if not succumb to some other fate) if exposed up there?

        And what would it mean to have a temperature of 700 degrees C in empty space?

        Thanks for any hints on this, I am sure it is a simple idea.


        Empty space is neither hot nor cold: these are qualities usually assigned to the matter which fills it. When this matter is dense, its temperature influences the temperature of anything else placed there. However, if that "matter" is just very rarefied gas, the temperature of whatever else occupies that space (astronaut, spacecraft etc.) may be determined by other factors.

        The upper atmosphere is very rarefied. As its atoms absorb short wave radiation from the Sun (extreme ultra violet etc.), they get very energetic, that is, very hot. However, their density is low, so the density of their heat energy is negligible, and anything that touches them, e.g. a satellite, can easily absorb this heat. Instead of melting, it cools down the atoms which hit it. The temperature of the satellite is determined by the balance of heat it absorbs from sunlight, against the heat it radiates to space in the infra-red, both much bigger items in the heat budget.

        Another example. Plasma containment experiments in the lab (conducted with an eye towards possibly extracting energy from the fusion of hydrogen nuclei) can involve rarefied plasmas at a temperature around a million degrees, held trapped by magnetic fields. These fields tend to develop instabilities, much to the annoyance of experimenters, who try to foil them. If such a plasma gets too unstable, it may touch the wall. But the result is not the wall evaporating, rather the plasma cools down and recombines.

        Yet another example, somewhat similar. You read about the electric field in the atmosphere, which gets stronger beneath thunderstorms. Supposedly, there exists a potential difference of 150 volts (this number from one report I saw) or more, between the level of our heads and the ground. Why don't we feel it? Because our body is a conductor of electricity, and easily shorts out the field. Very little electric charge is involved and we feel nothing.

        Bottom line: Quality (temperature, voltage...) is important, but without a large enough source of energy behind it, it alone will not accomplish much.

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Author and Curator:   Dr. David P. Stern
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Last updated 9-17-2004