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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 a complete list, including later questions not listed below, click here.
You may also link from here to a listing of questions arranged by topic.

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:
  154.                    (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   S t o p...     (1)
  165. If the Earth's Rotation would   C h a n g e...     (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?
  194. Astronomy and telescopes for ones' own children
  195. Does the solar wind have escape velocity
  196. Astronomy for cliff-dwellers of New York City
  197. Portable star finder
  198. What if the Moon was closer? (2 questions)
  199. Why doesn't the Moon have an atmosphere?
  200. Telling a 3-year old about the atmosphere (2 questions)
  201. Three-color vision

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

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

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

  255. Konstantin Tsiolkovsky
  256. The Color Indigo
  257. Electromagnetic Waves and Electromagnetic Induction
  258. Why do orbits curve?
  259. The "Sundial Bridge" in Redding
  260. (a) Temperature in space
      (b) Exposure to the space environment: Freeze or burn?
  261. Remote sensing of Thunderstorms
  262. What keeps Electrons away from the Atomic Nucleus?
  263. The Stars on the Winter Solstice of 2012
  264. Mars Sunset
  265. Gravity at the Center of Earth
  266. The Big Bang
  267. "How Often are Stars Born?"
  268. The Path of Lightning
  269. Launching a Rocket from an Airplane
  270. The Equation of Continuity
  271. The Four Corners Monument
  272. Which among the brightest stars is closest to N Pole?
  273. Gravity Assist
  274. Is the Centrifugal Force "Real"?
  275. How do we know Earth is bigger than the Moon?
  276. Sound Waves on the Sun?
  277. What if we had to give up use of Satellites?
  278. Rotation of Venus
  279. "Proper Motion" of Stars
  280. Doppler Shift from the Big Bang
  281. Science Fiction scenario for a Flight to Mars
  282. Motion of the Moon

  283. "Iridium" flares
  284. Discovering Planets outside the Solar System
  285. Landing speed of airplane
  286. The Hubble Constant
  287. Are Summer Nights Darker?
  288. "Space Elevator"
  289. Black Holes
  1. Don't use Diesel fuel in a gasoline car!
  2. How bright is our Sun when seen from space?
  3. What is a "field"?
  4. Outer limits of the Solar System
  5. Gravitational Energy
  6. Stresses on a Railroad Bridge
  7. The constellation of Cassiopeia
  8. Tracking of radioactivity carried by winds
  9. Why can't the space shuttle reenter "slowly"?
  10. "The Standard Model of the Universe"
  11. About the Maya Calendar
  12. Are light sabers possible?
  13. Can the heat of sunshine make the Earth expand?

  14. About Mountains
  15. "Will the World end in 2012?" (a,b)
  16. Advice to graduating High School Student
  17. Could a (heat resistant!) ship float on the Sun?
  18. Reducing the fuel weight of the Space Shuttle?
  19. How do Rockets Land?
  20. The Earth's Spin reduced by Global Warming
  21. Circumnavigation of the Sun
  22. Are nuclear forces merely gravity at very close distance?
  23. Changing the Earth's Rotation
  24. Why are planetary orbits eccentric?
  25. Forces on Comet-dwellers
  26. Nuclear reactors and bombs
  27. Why doesn't magnetism affect electro-magnetic waves?
  28. Is humanity changing the climate, or is it the Sun and the Earth's magnetism?
  29. Advice to home-schooling parent
  30. Science of Clothing
  31. Calculating a Collision
  32. The Coriolis force and more
  33. Why isn't the solar system stratified by density?

  34. Tapping Atmospheric Electricity
  35. Global Disaster in 2012?
  36. What's the difference between speed and velocity?
  37. Effect of Gravity on Electromagnetic Waves
  38. Why is North the reference, not South?
  39. The lowest 700 km of our Atmosphere
  40. Doomsday 2012?
  41. Where does a Flying Bird get its Support?
  42. Why does Sun seem to move?
  43. Why don't waves disturb each other?
  44. Does the Moon's motion Change?
  45. Big Dipper and Weather
  46. What IS the Ecliptic?
  47. Precession, Greenhouse and more...
  48. Latest Sunrise, Earliest Sunset
  49. Falling off the Earth's Bottom?
  50. Rolling down a slopea>
  51. Pelton Wheel Efficiency
  52. Energy loss rate of our Sun
  53. The Sun's distance
  54. Why does sunlight have a continuous spectrum?

  55. Harry Paul Sprain's perpetual Motion Device
  56. Can the plasma that fills space help spaceflight?
  57. Spiral arms of our galaxy
  58. What powers a glider?
  59. UFOs
  60. Maximum speed for propeller-driven airplane?
  61. The speed at which gravity spreads
  62. Layers of the Earth
  63. Why doesn't the sky fall on us?
  64. Imagine a non-rotating Earth
  65. Flying east with a 1° error
  66. US Flag on the Moon
  67. Rope stretched across a long lake...
  68. About studying electronics in the USA
  69. Publish Stargazers as a book?
  70. Charging of Earth by lightning:   +   or   –  ?
  71. If no stars were seen--could Earth's orbital motion be discovered?
  72. Local Solar Time in Iceland
  73. Is orbital motion same as free fall?
  74. Build a straight tower in warped space?

  75. The 2011-2 sunspot maximum
  76. Is Earth adding mass?
  77. Shape of a "bottle rocket"
  78. Fantasy spaceflight vs. reality
  79. Telling a 7-year old about stars "dying"
  80. (1) Why is lightning jagged?     (2) What did Tesla do?
  81. How can the north wall of my house be in sunlight?
  82. Heating the inside of Earth
  83. Dawning of the Age of Aquarius
  84. Why is hydrogen the fuel of choice?
  85. Gamma ray bursts
  86. Counter-clockwise swirling motion in the atmosphere
  87. Leap years on the Jewish calendar
  88. Earth axis tilt and climate (1)
  89. Earth axis tilt and climate (2)
  90. Does the Sun move?
  91. Can the Sun interfere with the visibility of the Big Dipper?
  92. Why can't we feel the Earth's rotation?
  93. Is our galaxy held together by a central black hole?
  94. Why don't gas planets just evaporate?
  95. Source of the Sun's energy
  96. Point of gravity equilibrium
  97. Solar system motion through the Galaxy
  98. Spaceflight by waiting for the Earth to turn?
  99. Does Earth rotation affect size of creatures?
  100. The year 2012
  101. Are our galactic arms winding up or opening up?
  102. Advice to a home-schooling mother
  103. Are all stars we see suns?
  104. Newton's 3rd law
  396A Posssibility of Asteroid Hitting Earth (1)
  396B Posssibility of Asteroid Hitting Earth (2)
  1. Author's IQ
  2. Global Warming caused by Sun getting nearer?
  3. The year 2012
  4. Empty space behind North Star?
  5. The year 2012 and the 26,000 year cycle
  6. "Quarter Squares"
  7. Do Rockets need "something to push against"?
  8. Sunrise-Sunset asymmetry
  9. Re-entry from orbit
  10. Earth Axis and Gravity
  11. The year 2012
  12. Choosing an Aerospace Career
  13. Advice for "new" physics teacher
  14. Paradox of Time Travel
  15. Is 7th grade Earth science boring?
  16. Air Resistance
  17. Experimenting with Microwave Oven
  18. Is the Sun losing mass?
  19. Sun's position at noon, south of equator
  20. Fred Hoyle's theory of the Sun's Corona
  21. Why don't Protons and Electrons combine?
  22. Days in a Year
  23. Position of the Moon in the sky

  24. Earth crossing Galactic Equator?
  25. Is the geocentric theory ruled out?
  26. Does the Southern Sky have a Pole Star?
  27. What is the fate of starlight energy?
  28. Double-slit diffraction of particles
  29. Does the Sun overhead reduce effective weight?
  30. Evidence for Global Warming ?
  31. Distances to the Equator
  32. Weight on the Equator and at the Pole
  33. Why doesn't gravity overcome buoyancy?
  34. The Prime Meridian
  35. Is L2 in the Earth's Shadow?
  36. In what direction is Israel from NY?
  37. Is the World Overpopulated?
    Magnetic Energy
  38. Magnetic Carnot Cycle
  39. Defining the Equator

  40. Second Moon for Earth?
  41. The year 2012 and a distant companion of our Sun
  42. Distance between two points on a sphere
  43. Getting sucked I by Gravity
  44. The work of Nikola Tesla
  45. Firing a cannon straight up
  46. Does one see half the sky--or more, or less?
  47. Why are nights dark?
  48. The origin of the Solar System
  49. Flying to other planets
  50. The heat of the Sun's corona
  51. Crossing our galaxy's equator
  52. Newton's 3rd law--in statics and dynamics
  53. Barycenter of Earth-Moon system
  54. What if Earth rotated in only 10 hours>

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.   About asteroids hitting Earth

    A friend asked if I could find anything on an asteroid heading for earth & a laser that supposedly is in space that will eliminate the asteroid before it hits earth. Is there any such thing, or is he reading too many sci-fi books?


    About asteroids heading for Earth: the best account I know is a section "The Shoemaker Comets" in the book "First Light" by Richard Preston. As for lasers capable of destroying one, they are (at least right now) pure sci-fi.

    The above book makes an interesting point: it would be very hard to spot an asteroid heading for Earth. Astreroids are usually detected in photographs of the sky (via a telescope) by the fact they move across the line of sight, leaving a streak rather than a spot. If they move across the line of sight, they are not going to hit Earth; if they are heading straight for Earth, they leave no streak and attract no notice.

    2.    The swirling of water in a draining tub

    I am a grandmother, soon to be 55 years old, who often gets into heated arguments in her maillists! My question, to avoid the argument by having facts, is: The direction of draining water in the tub, sink and toilet is said to be the opposite in Australia. Someone said the Coriolis Effect governs this, and it is a myth. Someone said the CE has nothing to do with it, and it is a myth. Then again someone said it is NOT a myth. I am on the fence with this one, as I cannot argue something I do not have one idea about!! Can you help me? Thanks in advance!


    Your first respondent was right: the Coriolis effect governs it, and it is a myth. The Coriolis effect can govern the swirling of fluid flows, and where it does, the swirling is opposite in opposite hemispheres. However, it is only appreciable on a very large scale. Hurricanes obey it: tornadoes, which are much smallers, do not, and neither do kitchen sinks, which are much smaller still.

    For details and explanations, look up on the world-wide web

    3.    Dispensing water at zero-g.

    I am a student currently studying for a degree in engineering. As part of this degree we have been given the task of designing a water heater and dispenser for use in zero gravity. It has been suggested to use a bladder in a pressurized container heated using microwave or Radio Frequency technology. The heater must heat approximately 100 ml to 80 degrees celsius, and the entire system can use no more than 12 volts. How would you suggest a layout for this type of system might look? Any information that you could provide would be most appreciated.


    Our research group is concerned with plasmas and magnetic fields in the rarefied medium between here and the Sun. We have no expertise at all in zero-g and space-station hardware.

    This does not stop me from speculating about your request, of course. The key word is "dispensing": what do you mean by that? You cannot just have a tap and let out the hot water--it will form globs that drift away in zero-g and ultimately contaminate your circuitry or mess up your living quarters.

    So you need three elements--a container where the water is heated, a tank from which the water is obtained and a container for the hot tea or whatever you want to make with the water.

    The first and third may well be plastic bladders, whose volume can adjust. The reservoir would have an outer bladder with water only and and inner bladder filled with air, and as the astronaut with a squeeze-bulb pumps air into the inner bladder, water is squeezed out.

    The heating vessel--you could use RF heating, but I suspect it will be somewhat heavy, will need stepping up the voltage from 12 volt and also will have to be shielded from radiating. In an environment where every superfluous gram costs a great deal, wouldn't a simple resistive heating element--with a thermostat, of course--be simpler? It could be in a cylindrical container with a spring loaded piston which is initially at the bottom. When refilling it from the reservoir the water enters from below and pushes the piston up, against a ratchet, and when the astronaut wants a drink, he or she releases the ratchet and the spring loaded piston pushes the water out, into the third container. The trick is to never mix any air with the water--once they are together, they are hard to separate.

    This opinion comes to you with no warranty by NASA or anyone. Have fun! Now, let me go back to serious work...

    4.    Robert Goddard and World War II.

    I just came upon your website "Stargazers to Starships". Great site! I am researching a school project on Goddard and found the information here useful. I have a few questions, perhaps you can help me.

    When the U.S. was spending money on the a-bomb during WWII, do you think this prevented the government from providing Goddard with money to research rockets for the military?

    The German's developed the V-2 rocket, and Goddard believed they copied his design. I read that Germany sent spies to observe Goddard. Did Goddard know he was being watched?


    To answer your questions:

    5.    Asymmetry of the Moon's orbit.

    Subj: Moon's perigee/apogee

    I have just read your article in "Stargazers", but what I am trying to dertermine is the observed variations in the time between events of perigee and apogee. For example it may be 14 days from perigee to apogee and say 12 days from apogee to perigee. Then at a further time the periods can be reversed. I am seeking an explanation of this dynamic variation.


    I looked up the ephemeris tables of the Moon, and you are right: counting only the times between minimum and maximum distance from Earth, those distances ARE variable, more than one would expect for, say, an Earth satellite in a long elliptical orbit.

    All I can give you now is a guess. The motion of the Moon is really a 3-body process, influenced by the Sun as well, with further perturbations perhaps due to Jupiter etc. The orbit is close to a circle, which means that a pull of a few 1000 km this way or that can shift the time of largest and smallest distance by a great amount, in contrast to what it would do to a high-eccentricity orbit.

    The literature comments "The orbit of the Moon is complicated" and I think your question illustrates that complexity. If you look at page D-46 of the US Astronomical Almanac, for instance, you will see that even the "low precision formulae" for lunar motion are alarmingly long, and better approximations (found for instance in "Astronomical Algorithms" by Jean Meeus) are even longer.

    So the bottom line (as they say) is that Kepler's laws still hold, but actual motions may be complicated by additional factors.

    6.    Measuring distance from the Sun.

    I hope this isn't too dumb a question, but when a planet's distance from the sun is given, should that be assumed to be from the center of the sun to the center of the planet, or is it a measure of the surface of the sun to the surface of the planet?

    -- and if it's surface to surface, then what is considered the 'surface' of a gas giant?


    Your question isn't dumb, and it has a simple answer: from the center of the Sun. A spherical mass--Sun, Earth, red giant or whatever-- pulls objects outside it with the same force as it would, if all its mass were concentrated in its middle. As far as gravity is concerned, the position of the surface makes no difference.

    By the way--the Earth does not orbit the center of the Sun. If the solar system contained only it and the Sun, the two would orbit their common center of gravity. Of course, the Sun being much more massive, that point is very close to the center of the Sun.

    With more planets, the system orbits around the common center of gravity, which I suspect is close to the center of gravity of its heavyweights--Sun, Jupiter and Saturn. Viewed from some other solar system, far away, this would make the Sun's position wobble a bit, in response to the motions of the planets. In recent years, astronomers have observed such subtle wobbles in the motions of quite a few nearby stars, and concluded that like the Sun, they had planets, too--big planets, like Jupiter. It is still too hard to detect the effects of lightweights such as Earth, but progress is being made.

    Keep up your interest!

    7.    Who owns the Moon?

    Dear Gentlemen,

    If you be so kind as to reply, please tell me, is it true that the Moon has a formal proprietor and who is this man?

    Thank you in advance for your kindness.


    I do not know who told you differently, but the moon belongs to all of us together, even you, even I. When Neil Armstrong stepped onto the moon he said "We came in peace in the name of all of mankind" and that still holds true.

    8.   Acceleration of a Rocket

    I've looked your site and have taken some information but I need more for my project. In my project I want to search on the G force on the rockets at launch.


    I really do not know. The g-forces on a rocket vary with the design. Manned rockets stay under about 5g, unmanned scientific satellites may be launched at up to 10-12g, small sounding rockets with strongly built instruments sometimes reach 30g, and missiles can also accelerate very rapidly. The greatest acceleration is usually not at launch but just before burn-out, because the thrust of the motor changes little (or not at all), while the mass goes down as fuel is burned.

    9.   Rebounding Ping-Pong Balls (re. section #35)

    Can you send or suggest any more references to support the 20 miles ping pong ball in and the 60 miles back out? I am having trouble with my colleagues who say it should be 40 miles back.


    I hope you have a nice bet riding on this matter, because in that case you win. The correct velocity is indeed 60 mph. The way I gave it in "Stargazers" was meant to make it intuitively easy, but a rigorous calculation gives the same result.

    In what follows we agree that velocities from right to left are positive, from left to right are negative.

Initially you have
    A ball of mass M1 moving with velocity – V1, against... A paddle of mass M2, moving with velocity V2
Afterwards we have
    A ball of mass M1 moving at velocity +W1 A paddle of mass M2 moving with velocity W2
We assume the paddle is much more massive--M2 >> M1 (actually, it is mostly the mass of the hand behind the paddle), so that V2 and W2 are almost the same (=the impact does not slow the paddle by any great amount).

Conservation of momentum:

M2V2 – M1V1   =   M2W2 + M1W1         (1)

Conservation of energy (we assume the encounter is perfectly elastic-- approximate for the ping-pong ball, very well observed by gravity-assist maneuvers of spacecraft around planets):

M2V22/2 + M1V12/2   =   M2W22/2 + M1W12/2

multiply by 2:

M2V22 + M1V12   =   M2W22 + M1W12         (2)

In both numbered equations we collect all M2 terms on the left and all M1 terms on the right:

M2(V2 - W2)   =   M1(W1 + V1)         (3)

M2[V22 – W22]   =   M1[W12 – V12]         (4)

By a well known factoring identity, for any two numbers A and B

A2 – B2   =   (A + B)(A – B)

so (4) becomes

M2(V2 – W2)(V2 + W2)   =   M1(W1 – V1)(W1 + V1)         (5)

If we divide equals by equals, what remains is still a valid equality. So let the left side of (5) be divided by the left of (3), the the right side of (5) by the right of (3):

V2 + W2   =   W1 – V1         (6)

Add V1 to both sides

V1 + V2 + W2   =   W1

V1, V2 and W2 are each 20 mph. Therefore, the rebound velocity W1 equals 60 mph. QED

10.    Rebounding ping-pong balls and gravity assist


I've enjoyed browsing your fine web site, From Stargazers to Starships, and I figured I would take a moment to let you know that. I was particularly intrigued by the chapter "Project HARP and the Martlet. " There is one possible error I found in the site. In Sections 35 and 35a, on planetary swing-bys and the so-called "slingshot effect, " you state that the maximum velocity increase is imparted to a spacecraft when it approaches a planet head-on, or retrograde to its orbit. My reading indicates that the opposite is true. My sources are the following web sites:

JPL's Basics of Space Flight

Scientific American

I expect the discrepancy stems from the fact that the model used in From Stargazers to Starships is based on the ping-pong paddle example. The key difference is that the force exerted by a ping-pong paddle on a ball is repulsive, whereas gravity is attractive. Thus the numbers are the same but the sign is reversed.

By the way, I did find the analogy to the Pelton turbine very interesting. Thank you again for a very informative web site!


I believe that the ping-pong analogy is still valid, because it can be reduced to simple arguments of the conservation of momentum and energy, which should hold equally in a planetary-assist maneuver. Some other correspondent questioned this result, and as a result, you can find that calculation in item #9 of the question-and-answer section of "Stargazers," linked at the end of the home page [the item preceding this one].

What seems to confuse the issue is the following. A spacecraft would get its biggest boost if it approached head-on, made a hairpin turn around the rear of the moving planet and returned along a path 180 degrees from its first approach (that would be the ping-pong analogy). Viewing the encounter from far north, if we put the moving planet at the center of a clock dial with the SunĂ­s direction at 12 o'clock, we would see the planet moving towards 3 o'clock, so our satellite has to approach from that direction and return to it again.

When the Voyager and Pioneer spacecraft approached Jupiter and Saturn, however, they were coming from the Earth, which is roughly in the same direction as the Sun; in any case, their initial orbital velocity, which was essentially that of the Earth, which moves in the same direction as other planets', did not allow a head-on approach. Instead, they entered around 12 o'clock on the dial. They still rounded the night side and exited around 3 o'clock, which gave them an apprecible boost, though perhaps not the biggest one possible.

I have some old issues of "Science" on these events and in the one of the Pioneer 10 fly by, for instance (page 304, 25 January 1974), the satellite enters at 1 o'clock and leaves a bit after 3 o'clock. For the Voyager 1 fly-by of Saturn (p. 160, April 10, 1981), entrance is around 11:30 and exit around 4:30 on the same dial.

You are right, of course, in that the force on the ping-pong ball is repulsive while the planet's gravity attracts the spacecraft. However, the strongest attraction occurs when the spacecraft is at its closest approach, on the night side, and its direction then is along the velocity of the planet, the same direction as the force exerted in the ping-pong analogy.

With all this, I am grateful for your message. It again shows that at least some users go into the details of "Stargazers". Quite a few errors were caught only thanks to people like yourself who checked out such details.

11.    Why don't we feel the Sun's gravity pull?

Dear Dr. Stern:

I have asked several teachers and many other people the following question but have not received any respectable answer:

The Earth is 93 million miles from the sun. Other planets, and even much denser planets I might add, are much further yet from the sun. The obviously strong gravitational attraction of the sun holds all of these planets in orbits around the sun. If gravity could be simply defined as a force that attracts matter, and the sun's gravitational pull is sufficient to hold the Earth in orbit, what keeps it from pulling me off the Earth? In fact, the gravitational pull of the sun is so weak at this distance that It can't even produce enough pull to raise a hair on my head. So how can it hold the Earth and several even denser planets (even further out) in orbit?

So--if the gravitational force of the sun is powerful enough to hold the Earth in orbit, then how could the Earth's gravitational force be powerful enough to hold me down, counter-acting the gravitational force of the sun? Please unconfuse me!


Dear student

    Two effects are at work, each of which would be quite sufficient:

(1) The force of gravity goes down with distance squared. For example, since the Moon is about 60 times further from the center of the Earth that you or anyone who is standing on the surface, the pull of the Earth on each pound or kilogram of the Moon is 60 x 60 = 3600 times weaker than the pull on the same mass on the surface.

So: the Sun is indeed more massive, but also much more distant. As a result, its pull on each kilogram or pound at the Earth's distance is only about 0.06% of the Earth's pull near the surface.

(2) Being on the orbiting Earth, your body already responds to the Sun's gravity, by sharing the Earth's velocity of 30 km/s around the Sun. Therefore there is nothing left over from the Sun's pull to make you move any more.

In a similar way, an astronaut in orbit feels weightless, because the Earth's gravity is already fully employed in keeping up the orbital motion. The astronaut is not beyond the reach of Earth's gravity: if it were so, the spacecraft would fly away never to return, rather than stay in orbit. It is just that--like a stone in free fall--gravity is already doing to the astronaut all it can. It also does so on the spaceship the astronaut rides in, leaving no extra force pulling the astronaut down to the floor, or in any direction.

12.    How hot are red, white and blue (etc.) stars?

Hi, My name is Donny and I have a question that I cannot seem to find an answer to.... How hot, exactly, is a blue star, a red star, a white star, and other color of star?


Your question has an answer, but you have also to learn a bit about what color is. Look at the following web site

The stars for which statements about temperature are made are glowing dense bodies of gas, so for them the "black body spectrum" is relevant. In that spectrum, the curve of intensity against wavelength (color) typically rises to a peak and then drops.

The total area under the curve tells how bright the light is: the hotter the emitter, the higher the curve and the brighter the light. You know this from experience: a flashlight with a weak battery glows weakly in orange, a flashlight with a good battery glows bright yellow, and if you connect a 3-volt battery to a 1.5 volt lightbulb, you get a very bright, very white flash, and then darkness, because you have heated the wire inside the lightbulb so much that it melted, and you have just lost your lightbulb.

And in that sequence, you also see the color move along the rainbow: orange with a little heating (a feeble red when the battery is almost dead) yellow under normal operation, white when it's too hot. The color you see is where the peak is--and if it is blue, you see white, because all other colors are also emitted, and white light is what the eye then sees.

Stars are like that too. We can say the Sun's photosphere radiates pretty much like a black body at 5780 degrees absolute or about 5500 degrees centigrade, by the way its colors are distributed. The color tells how hot it is, and I think "blue" here means white-blue; such a star would be at about 10,000 degrees.


13.    How does the solar wind move?

I am confused about the solar wind and don't want to mislead my students. On a web site about magnetic storms I read the following:
    "This storm affects the earth when it is on the western half of the sun, not when it is dead center. This is because the solar wind follows a curved path between the sun and the earth not a straight-line path."

Is the solar wind influenced by the magnetic field of the sun so it has a curved path to the earth? Or is this too much of a simplification? What really happens?


Physics and astronomy get complicated at times. Will the interplanetary magnetic field curve the path of the solar wind? Without peeking at the observations, one can only say "it depends," and what it depends on is the ratio between the density of particle energy (density n times average of 0.5 mv^2) and the magnetic field energy density (B^2/2 mu-zero). This ratio is often called "beta" in plasma physics, and it's an important quantity in experiments aimed at confining a plasma for nuclear fusion. If beta is much less than 1, the magnetic field is the dominant factor and particles meekly follow its field lines, making containment easy. Practical fusion however requires a greater beta, and if beta exceeds 1, the plasma starts pushing the magnetic field around. The way it does so is by subtly segregating its charges, to create a charge density and hence an electric field, and electric fields can allow a plasma to move whichever way it wants.

Suppose the magnetic field is constant and equals B0 in the z direction, and the plasma is moving along the x axis. Then an electric field E0= -vB0 in the y direction will allow it to do so, canceling the magnetic force on any electric charge q, equal to qvB0 along -y. (It also works out with spiraling particles).

The same happens with the solar wind, where beta may be 5 or more. As a result, the solar wind moves radially out, though it gets buffetted a bit, and it's not clear by what.

Now what about the MAGNETIC field? There is a rule (for plasmas with high beta, satisfying the "MHD condition"), that "particles that initially share a field line, continue doing so indefinitely" (there exist some extra "fine print" conditions, but we ignore them here).

    What follows below is the original answer sent to the questioner. Later this was converted to a graphical exercise,
Section S-6a   Interplanetary Magnetic Field Lines, linked to section S-6. You can either link there or continue below (or both), as you choose.

Take a sheet of paper, put on it a small circle--that is the Sun viewed from far north of it, or rather, it is a circle in the corona, some level above the Sun, where the solar wind begins. On this scale, let's say the solar wind moves one inch (1") per day (or if you wish, 2 cm). Draw from the center 6 or 7 radial rays 13.3 degrees apart. Mark as "P" the point where the first ray--the one furthest clockwise--cuts the circle. We look at 6 ions located at P, and therefore presumably on the same field line--let's number them 1, 2...6. We have advance information that 1 will be released into the solar wind today, 2, tomorrow, 3 the day after, and so on. Mark P with 1--that is where ion no. 1 is today.

Next day, P is on the second ray. Point 1 has moved 1" outward, radially, and Point 2 is at the base of the new ray, ready to go. Next day: Point 1 is now 2" out on the first ray, point 2 is 1" out on the 2nd, point 3 at the base of the 3rd, ready to move. And so on.

Five days later, 1 is 5" out on the first ray, 2 is 4" out on the 2nd 3 is 3" out on the 3rd, etc., and 6 is at the base of the 6th ray. However, all these points started on the same field line, so they are still strung out along one line. CONNECT THE DOTS marking the outermost ions on the 6th day and you have a spiral line of the interplanetary field: if the ions started on the same line, they must still be on one.

The solar wind in all this has moved radially. But now and then the sun releases bursts of high energy particles, say from flares. The energy of these particles may be high enough to endanger astronauts in interplanetary space--but their density is very low, so their beta is also low. THEY therefore are guided by the magnetic field lines (rather than deforming them to their own flow), and therefore they move spirally.

The solar wind takes about 5 days to cover 1 AU. Therefore, if the Earth is to receive particles guided by an interplanetary field line when it is on the first ray, the emission has to be at the base of ray 6--that is, near the western limb. The high-energy particles take only an hour or so to arrive, depending on their energy of course.

14.    The shape of the orbit of Mars

Dear NASA,

I have read your articles about stargazers and I believed this is one of the most interesting subjects in astronomy. Here is a question which came up when I was reading 'Planetary evolution', could you please help, thanks. Mars moves in an elliptical orbit around the Sun, what is the relative distance of the Sun to this ellipse? Would it be at one end of the major axis of the ellipse?


The eccentricity of the Mars orbit is 0.09337, the semi-major axis of the orbit is A = 1.524 AU (1 AU is the mean Sun-Earth distance, about 150,000,000 km; AU stands for astronomical unit) and distances of perigee (closest approach) and apogee (most distant) are B = 1.381 AU and C = 1.666 AU (letters are just notation for here).

These are the numbers. What do they mean? I remember seeing long ago a German physics text from the 1920s drawing the orbit of Mars. One side of the line was a circle, one side was the orbit, and the varying thickness of the line showed the difference between the two. It was hard to see that difference!

Let us calculate the length and width of the ellipse. The length (through the two foci--the line on which the Sun is located) is 2A = B + C = 3.048 or 3.047 AU. The displacement of the center from either focus is D = (C-B)/2 = 0.1425 AU and the width is 2G where

G2 = A2 – D2                 G = 1.51732 AU         2G = 3.035 AU
I do not think you or I would be able to distinguish an oval with dimensions (3.048, 3.035) from a true circle! The position of the Sun at one focus is however notably asymmetrical, about 10% of the distance from the center to the edge.


15.     What if the Earth's axis were tilted 90° to the ecliptic?

I was recently looking at the webpage
"Seasons of the Year" and I read about what would happen if the earth's axis were perpendicular to the ecliptic. I was just wondering if you could give me some insight on what would happen if the ecliptic was inclined at a 90-degree angle with respect to the celestial equator? Would this mean that earth's orbit would travel along this "new ecliptic" while the north and south poles are travelling along this "new ecliptic"?


The hypothetical case you describe does in fact exist: for some unknown reason, the spin axis of the planet Uranus is almost exactly in the ecliptic.

That means that at some time one pole (let me call it the north pole, even though that "north" direction is almost perpendicular to the northward direction from Earth) points at the Sun. Then the northern hemisphere is in constant light and the other one in constant darkness. Half an orbit later--42 years or so--the roles are reversed. And halfway between those times, the planetary rotation axis is perpendicular to the Sun's direction, making day and night alternate in a way similar to what the Earth experiences at equinox.

I leave it as an exercise to you to figure out whether Uranus ever receives sunlight the way Earth does at solstice.

16.   Mars and Venus

This is a query on Mars. The ferric oxide on the Martian surface contains a lot of oxygen and if heated sufficiently would yield free oxygen. Does the presence of this substance on Mars indicate that at one time there must have been a lot of atmospheric or dissolved (in water?) oxygen available on Mars?

These are my questions about Venus:

1.     Why is it that Venus is depicted by Magellan photos as being Red/Orange in colour? If these are radar images how is the "..actual colour of sunlight that reaches the Venusian surface through the thick cloud layer .. " derived? The Magellan images of the surface of Venus are quite bright. Would it really be that bright under such heavy cloud cover?

2.     What generates the high velocity (up to 400 km/hr) winds which move from east to west in Venus's upper atmosphere, given that the surface of Venus rotates at a leisurely 6.5 km/hr?

3.     What conditions did astronomers expect to find on the surface of Venus before the first probes landed? Thank you for your effort in advance.


    I am not a planetary scientist! My speciality is magnetic fields and plasmas near Earth. I will give you the best I can remember here:

--- Oxygen is not a rare element, Mars, the Moon, etc. have a lot of oxygen, always combined with other elements into stony stuff. The element that is really important to life is hydrogen. The recent signs that the Moon may have hidden water were exciting not because it is hard to find oxygen on the Moon (it must be separated, of course) but because hydrogen is rare. I am not sure about hydrogen on Mars. Jupiter and other cold big planets of course have plenty, and so do their moons, which may be more accessible. Still, they are pretty far away.

--- The colors of radar maps on Venus are probably false colors. The color and intensity of sunlight on the surface should be known, because the Russian landers took photos. However, I don't know it.

--- What drives winds on Venus is not the rotation but the same cause for winds as on Earth--the heat of the Sun. Venus is closer to the Sun, so maybe its atmosphere is more agitated. See the sections in "Stargazers" on
sunlight and on the way it creates the Earth's weather

What one observes, in any case, is the wind at the top of the clouds, which might be analogous to the jetstream above Earth, not a good measure of winds at the surface.

I don't know--you must find out by yourself. I know Venus was expected to be "as hot as hell"--but the features of the surface could not be guessed.

17.   Where is the boundary between summer and winter?

This may seem like a silly question but is there a line on the earth where on one side is summer and the other winter? Sort of in the same way that you can go back and forth between two days at the International Date Line.


Yes, there exists such a line and it is called the equator, but the boundary between summer and winter is not as sharp as the one of (say) the international date line.

Right now it is fall here, but spring in Chile and Argentina, and 2-3 months from now it will be mid-winter here and mid-summer in those countries. At our latitudes, these seasons are well defined. However, a broad belt centered on the equator does not have well-defined summer or winter. The Sun's heat fluctuates somewhat as its noontime passage moves north and south. On the equator, for instance, it passes right overhead around the 21st of March or September, 23.5 degrees off to the north on June 21 and the same amount south of "overhead" (zenith) on December 21.

These small changes do not make much of a change in solar heating. The big difference is made by local climate patterns, e.g. seasonal rains like the monsoon. These countries do not have summer and winter the way we do: for instance, when my daughter visited Darwin, Australia, some years ago, she was told that loacally the year had only two seasons, "the wet" and "the dry. "

18.   The Ozone Hole

I am a auto mechanic and I have one simple question for you. Scientists say there are 2 holes in the atmosphere, ironically they are around the north and south pole, and they blame these holes on chlorine monoxide or refrigerants i.e. fluorocarbons (CFC) escaping into the ozone. Wouldn't the more likely cause of the holes be the magnetic lines of flux? One more quick question: could there be a way of tapping into that magnetic field as an energy source?


There do indeed exist two "holes" in the Earth's magnetic field, around the MAGNETIC poles, whose magnetic field lines go very far from Earth and afford an easy connection to the solar wind and to interplanetary plasma phenomena. On those lines we do observe "polar rain", a drizzle of fairly energetic electrons (more energy than those of the ionosphere, less than those of the usual polar aurora) which seem to come from the Sun. Also, when solar activity floods interplanetary space with energetic ions and electrons, that is where they are most likely to come down to Earth.

However, the creation and destruction of the ozone layer does not involve the magnetic field. Instead, its factors are chemistry and sunlight, and the "ozone hole" is around the geographic pole, not the magnetic one.

The ozone layer is maintained as an equilibrium between creation of ozone by ultra-violet sunlight, and its destruction by various natural processes (this is not my field, and I do not know details). During polar winter, the polar cap is dark and ozone is not created, just destroyed (a bit further from the pole, with just a few hours of sunlight and the sun shining at a shallow angle, ozone creation is also reduced). The observation of an "ozone hole" in recent years suggest accelerated destruction, as predicted by Rowland and Molina to come from chlorine in man-made substances.

As for tapping electric currents from space, I don't think it will work, because (1) they are very spread-out, by our standards--how can you tap a current sheet 100-1000 miles wide?; and (2) between us and them lies the atmosphere, a very effective electric insulator--as is well known to power companies, which string their high-voltage cables through air without worrying about the power leaking away.


19.   What keeps the Sun from blowing up?

Dear NASA, I have a question abour the sun. We all know that the sun is powered by thermonuclear fusion reactions,so why doesn't it explode like an H-bomb?


What keeps the Sun from blowing up? Gravity.

The Sun is not exactly like an H-bomb--the bomb has fuel that is easier to "burn, " while to Sun "burns" ordinary hydrogen--but there does exist a similarity. Why does a hydrogen bomb explode? Because an enormous amount of energy is released in a short instant and inside a small volume, heating the material to extreme temperature. The material expands forcibly, creating a powerful shock, and that is the explosion.

The Sun releases much more energy every second in its central regions, but those materials are under great pressure, from the weight of all the matter piled up on top of them--the thick outer layers of the Sun, pulled down by a gravity much stronger than anything on Earth. That pressure confines the extremely hot gas in the Sun's core. The heat gradually works its way to the surface, but the Sun does not blow up.

If somehow it could yield to the pressure and expand, the central core would cool down and the nuclear energy release would drop. Then the pressure would decrease again and gravity would reassert itself. Some stars do in fact oscillate, but we should be grateful that the Sun does not belong to that class.

See more in

20.   Those glorious Southern Skies!

Not long ago I visited Chile for the first time and observed the night sky there. At that latitude, the centre of the Milky Way passes overhead, where it makes a grand show...

Why is there such a difference between Southern and Northern Hemisphere? Is it because of the 23 1/2 degree tilt?


Why such a difference between the southern and northern hemisphere? Because from the point of view of the Earth, all stars are so distant that they appear as if they were attached to a tremendously large sphere, with us in the middle.

At night, standing on the ground, you only see HALF the sphere. If you stood at the NORTH pole, half the sphere would be all you ever saw, appearing to spin around the point right overhead, the zenith. Standing at the SOUTH pole, you would see the other half, spinning around the point overhead, which is on the OPPOSITE end of the sphere from the overhead point at the north pole.

If you lived on the equator, the two poles of the sky would be on opposite sides of the horizon, and as the sphere of the heavens rotated around them, you would in principle see ALL the stars, sooner or later. In practice, those close to the poles will be near the horizon and not easy to see.

Maryland, where I live, is somewhere between the north pole and the equator, so the stars near the north pole are easily seen, and we get to see some stars of the southern hemisphere as well, though not those near the southern pole of the heavens (like the Southern Cross and Alpha Centauri), and many southern stars are only seen here near the horizon.

Similarly, from Chile you won't see the Pole Star, the Big Dipper or Cassiopeia, but the bright stars near the southern pole more than make up for them, and yes, the brightest part of the Milky way is there, too. North of the equator, the best view of the Milky Way is in mid-summer.

The 23 1/2 degree tilt has to do with the way the Sun, Moon and planets appear to move--not with the apparent motion of the distant stars.


21.   Should we fear big solar outbursts?

I have a question pertaining to your studies in the upcoming year, particularly surrounding the forecast Solar Maximum.

I have heard about the upcoming Solar Maximum starting soon ( article, Nov. 11, 1999). I have also heard (unofficially), that there could be a very large solar storm near the end of April.

Finally, it is relatively commonly known that there is going to be an unusual alignment of the planets in our solar system at the beginning of May, 2000. Has there been an in-depth study to determine effects of the combination of these phenomena, and the potential impacts on both our solar system, and our planet?

The real question; could this combination of phenomena:

1.) Promote a solar flare, or SME, significantly larger than previously experienced in recorded history?
  • I have heard of Super flares emitting from G Class Stars, and the theory describes large planets in a close orbit (Jan. 8th Article, Sun-like stars said to emit super flares, CNN). Now, I don't expect this size of phenomena to happen here, but with the unusual planetary alignment, I do believe that this could create larger effects than normal, like a significant Solar-Magnetic Ejection, especially with the excitation of the Solar Phenomena. I'm just curious as to how much larger.

2.) Disrupt the crust of our planet, creating a significant amount of tectonic activity, and if so, by how much?
  • Now, I know our planets are very far apart, but if the magnetic attractions are larger than normal, and these magnetic attractions promote significant SME activity, this could promote some strange tectonic happenstance, especially with the fragility of our planet and its crust.

3.) Potentially disrupt our magnetic field severely with the combination of solar magnetic and gravitational forces?
  • I am aware of changes in our earth's history of the magnetic poles, could this happen here with the combination of a large SME and gravitational forces?

No calculations, or in-depth study has occurred, but I have a hunch this should be looked at more closely, and by qualified people.


Your message made me once more appreciate the amount of misleading and loose information circulating on the web. I have spent a great deal of time and thought on creating a web site describing what is known about the magnetic field of the Earth and the Sun's effects on it, and for a real understanding, you better look there:

To answer your questions in brief: The solar maximum is already here [December 1999]. It is not an abrupt event you can date, but the crest of a wave whose width is at least several years. From what I have heard, the current peak is lower than expected.

No one can predict a large solar storm months ahead of time--the best we can say is that they are more frequent near the peak of the sunspot cycle. Some big ones cause little disturbance near Earth--depends on factors like the precise orientation of the interplanetary magnetic field. Planetary alignments have no effect whatsoever. [See also next item.]

The large planets you read about are unlike anything in the solar system --usually Jupiter-size or bigger, and very close to the star (this has to do with the method of detection--it's hard to detect long-period planets).

No solar eruption has ever been found to affect the solid Earth. Their energy is too small, and almost all of it is dissipated outside the breathable atmosphere. No earthquakes follow CMEs.

I have no control over CNN. But if you seek to understand nature, look up my site and sources linked or cited there.

Happy new century


22.   Planetary line-up and the sunspot cycle

Enjoyed browsing through some of your efforts on the Web. I am hoping you could help settle some of my thoughts before I make a fool of myself.

In your experience, has anyone tried to correlate lineups of the sun, earth and major planets' magnetospheres with the sunspot cycles? My spare-time effort found some correlation between lineups and cycles in a number of years. My wonderment centers around the possibility that some forces of the planets when lined up, possibly relating to their magnetospheres, impact the suns magnetosphere causing a solar max. I've also considered the possibility that related magnetosphere effects could be the cause of previous polar reversals on the earth. Additionally, ringing of our magnetosphere might impact charged tectonic plates...but that is again another direction. Only if you have time, please comment.


There exists a tempting closeness between the length of the solar cycle and the orbital period of Jupiter, but I don't think the two are related. I cannot imagine any mechanism coupling the two-- especially since the Sun rotates in about 27 days, so the relative period of Jupiter going around the Sun is of that order. Furthermore, the solar wind moves with supersonic speed, which means that solar disturbances can (and do) travel downstream with it, but disturbances from a planetary magnetosphere (whatever they might be) don't easily propagate sunward.

Above and beyond all these, there is always the question of energy-- the currency in which the price of any physical process must be paid. The energy required in the solar cycle is much bigger than anything planetary magnetospheres can supply.

So what causes the cycle? The Sun rotates unevenly, slower near the poles, faster near the equator, probably because of the way gas flows in it (Jupiter also has such a difference). In a magnetized hot gas, this difference deforms and amplifies the magnetic field, and there exist some general theories of the sunspot cycle based on this, although many details remain unclear. The general idea is that as the magnetic field gets amplified, it forms concentrated "ropes" which push out the hot gas, and when they reach a certain strength, enough gas is displaced that the ropes are light enough to float to the surface, where they are seen as sunspots.

Again, the magnetosphere is a relatively weak influence on the Earth's internal magnetism--even a big magnetic storm only reduces the surface equatorial field by 1%. Furthermore, the time scale differs--reversals happen on time scales of 0.5-1 million years, while magnetic storms have a 1-day scale or faster.

What seems to be involved are the currents which circulate in the Earth's core, presumably driven by flows there, which (like flows on the Sun) get their energy from heat. The magnetic field is fairly complicated--the 2-pole structure we see (north-south) is dominant, but not by as much as it seems, because more complicated modes get filtered away faster by distance. Right now the 2-pole field is declining at about 5-7% per century, but the late Ed Benton has shown that the more complex parts are gaining energy, and the total sum is fairly constant. Maybe, when a reversal occurs, for a while the 2-pole part gets small and the total field is rather complex (4, 8 poles..), and when the simple pattern re-emerges, it is reversed.

Anyway--keep studying, keep up your sense of wonderment

23.    What are comet tails made of?

Are comet tails the reult of melting and evaporation of ices from the comet core or are they dust collected by the comet as it moves in its orbit?


Comet tails contain both evaporated ices and dust, as explained in the section "Comet Tails and the Solar Probe" near the end of the file

The dust however is not collected by the comet in its orbit, but is part of its make up, probably dating back to the beginning of the solar system. Comets may have two tails, and sometimes these are well separated, as in the recent Comet Hale Bopp: they differ in color, composition, and direction, and are pushed away from the Sun by different forces.

Dust tails are pushed by light pressure, and their colors are those of sunlight, scattered by them the way clouds on Earth also scatter sunlight. The other tails contain plasma--free electrons and ions, that is, atoms from which sunlight has removed one or more electrons, leaving a positive charge. They glow in the colors characteristic of their material (a bit like the way streetlights produce the characteristic glow of sodium), and are pushed back by the solar wind. As explained on the above web page, the velocity of the solar wind is not too many times larger than that of the comet, and that causes them to point not straight away from the Sun but at a small angle to that direction.

See also htpp://



24.    If light speed sets the limit, why fly into space?

Dear sir

I have a question as to space travel. What is the point in exploring space? Is it just for achievment purposes or in the future will man discover LIGHTSPEED? Because otherwise, the whole thing seem rather pointless. It is rather like asking a garden snail to tour America in it`s own lifetime. Can you put any light on this question?.


From all we know, achieving lightspeed or anything close to it is well beyond today's technology, and I suspect, tomorrow's as well. The purpose of exploring space is different--to expand humanity's reach, and to understand the universe in which we live. Ancient humans may have been content to see the sun, moon and stars rise and set without caring what they were, or how distant. We have come a long way from then--to electricity, cars, airplanes and the internet--essentially, because humans want to understand more. Most Americans would probably feel rather stale if no progress happened over their lifetimes--just different teams making it to the superbowl, different wars being fought overseas. I for one felt excited by the landing on the moon, by the first pictures from Jupiter and Neptune, and of the sun in X-rays (quite different from the bland disk we see). I also feel excited by evidence of distant planets and giant black holes at the center of galaxies. No, I don't think we'll get there in my lifetime or in the next 1000 years--but humanity has a longer timetable, much longer than that of any individual.


25.   Does precession mis-align ancient monuments?

Dear Mr Stern,

I live in Ireland. There is an ancient monument at Newgrange in this country which was constructed some 5000 years ago. The particular alignment of the monument to the sun allows an inner chamber to be lit by the sunrise at the winter solstice. The construction of such a building so long ago with such accuracy seems almost incredible to me.

I have a query however. Is the tomb doomed to a long darkness in the future due to precession?


How the Sun's Position in the Sky Changes during a Year

Look at the above figure, taken from the web page about the seasons of the year, in section #3 of ""Stargazers." It shows the relation between the Sun and the Earth with its tilted rotation axis, throughout the year (north is up). That relation is what varies the length of the day and the apparent motion of the Sun across the sky, from season to season.

Imagine you were able to rotate this arrangement by some angle--by 10°, 30°, 90° or whatever--around an axis perpendicular to the plane of the ecliptic. You rotate just the Earth and its orbit (and perhaps the Sun), while the rest of the universe stays as it is.

The relation between the Sun and the Earth then remains exactly as before--the only difference is that you are looking at it from a different direction. Seasons and the apparent motion of the Sun across the sky are still the same as they were.

What our imaginary rotation has done is exactly the same as what the precession of the equinoxes does to the Earth's axis. So if an ancient monument is lined up to point at the Sun during solstice, it will continue doing so. Our calendar is also adjusted, so it would also remain the same day of the year.

On the other hand, an ancient monument aimed at a certain passage of a star would no longer fulfil its function, because now the axis of the Earth points towards a different part of the celestial sphere.



26.   Why does the Earth rotate? Why is it a sphere?


I have been unable to get answers to the following questions concerning the earth, can you help?

Why does the earth rotate - what are the forces causing the rotation?
Why did it start rotating in the first place?
Why are planets round?

Any help to get these answers is most appreciated


Hi, Mike

You have asked some very fundamental questions. Why does the Earth rotate? Because whatever it arose from--probably a cloud of gas and dust--was rotating to begin with.

The thing to keep in mind that even a very slow rotation of a cloud of objects gets greatly speeded up as it condenses at the center. The reason is a basic law (a consequence of Newton's laws of motion) by which a quantity known as ANGULAR MOMENTUM (or rotational momentum) is conserved. The angular momentum can be defined as the average radial distance, TIMES the average velocity of motion, TIMES the mass.

The mass does not change when matter collects near the axis of rotation, so we neglect the last part. Then, if the material collectes in the middle, where its average radius of rotation is 10, 100 or 1000 times smaller, the average velocity of its particles increases by the same factor.

You see this happen every day. When water drains from a filled bathroom sink, even if the water in the sink is rotating so slowly that you do not notice, by the time it reaches the drain it is spinning rapidly enough to form a funnel. (It does not spin in the opposite direction in Australia: the effect on which this claim is based is far too weak to have much effect. See "Stargazers," section 24). You also see this in hurricanes and tornadoes.

And you see it happen when a large object in space collapses. In 1054 a star "went supernova" in the constellation of the Crab, a process in which the top layers blow off and the core collapses to a tiny "neutron star," perhaps 15 km across and as massive as the Sun. The collapsed core of the Crab Nebula apparently rotates 30 times a second, because that is the frequency at which it blinks in x-rays and radio (and I believe in its light, too).

In the solar system all planets orbit in the same direction and nearly in the same plane, and they and the Sun rotate in the same direction too (= counterclockwise, viewed from north). This suggests that they all condensed from the same cloud.

Now that other question: why are planets round? Because of their gravity. On the surface of the Earth, solid material--say, rock cliffs --can easily stand the pull of gravity without deforming. But go just a few hundred kilometers inside the Earth, and you find everything under enormous pressure, from the weight of the layers heaped up on top. Under such pressure (and helped by the heat down there!), even solid rock deforms like putty.

If the Earth were all fluid, gravity would pull it into a symmetric sphere--the same way as it shapes the oceans. The Earth is not fluid, but as mentioned above, it makes no great difference. Actually, a ROTATING fluid Earth would be deformed by the centrifugal force, with the equator bulging out slightly. Gravity is weakened there, by the centrifugal force and by the greater distance from the center. That was observed in Newton's time, and Newton explained it by essentially using a fluid analogy.

Jupiter is much bigger than Earth and rotates much faster: its equator bulges out so much that pictures taken through a telescope suggest a definite ellipticity.

Voyager 2 and other space probes have by now cruised through most of the solar system and have imaged its planets and their moons. The rule seems to be that objects with a radius above 150 km are spherical; smaller ones do not have a strong-enough gravity and may be potato-shaped, e.g. the moons of Mars.

    (A different slant, from later correspondence) Planets are pulled into their round shape by gravity, which evens them out all around. Anything sticking out is pulled down! For example, the highest mountain on Mars, Olympus Mons, is about 2 1/2 times the height of Everest. How come? Because smaller Mars has at its surface only 1/3 the pull of gravity we feel on Earth. On Earth, the greater weight of a mountain that high would make it sink into the surface.




27.    What's so hard about reaching the Sun?

Hi there. hope you don't mind a question.

In (at the end) you state:

    "The hardest object to reach would be the Sun itself. Our imaginary spacecraft, freed from the Earth, would be moving like the Earth around the Sun at about 30 km/sec. The only way for it to reach the Sun is to somehow kill that velocity--for instance, by a rocket imparting 30 km/s in the opposite direction; if that were done, the spacecraft would be pulled in by the Sun. The people who propose sending nuclear waste by rocket into the Sun do not seem to know much about orbits! "

It would occur to me that the challenge of getting to the sun would consist mostly of getting out of Earth's gravitational influence. Ignoring that step (or starting from a point in earth's solar orbit which is NOT on the earth's surface), I would think that just about any deceleration would allow you to reach the Sun. While a deceleration of 30 km/sec would allow an object to "fall like a stone" into the Sun, an orbital velocity of, say, 29.5km/sec, instead of 30 km/sec, would result in a very slow, long, death spiral, but still one which still eventually results in a solar plunge (making the assumption of no external interference, such as another orbiting body didn't snag you along the way or provide a velocity boost).

What am I missing? Or am I?


P.S. No, I'm not a "nuclear waste into the sun" kinda guy. A perfect place for disposing the stuff, but not worth the risk of putting the waste on top of all that explosive energy and lighting the fuse.


Hello, Jim

No, I am afraid it won't work. All orbits in the Sun's gravity field are ellipses, or other conic sections: there are no spirals. If you place an object in Earth's orbit around the Sun but free of the Earth's own pull, and cut its velocity from 30 km to, say, 5 km. velocity, it would certainly fall sunward, but it would gain velocity doing so and would whip around the Sun in an extended ellipse. That ellipse would have its apogee (highest point) in the Earth's orbit.

If you cut down its velocity to near zero, it would again fall sunwards. It is still moving in an ellipse--a very long and skinny one--and if the Sun were just point-size, it would still miss and return to apogee in the Earth's orbit. However, the Sun does has an appreciable size, so that when when the ellipse is sufficiently narrow, the object hits the Sun, and then it never comes back.

Until recent decades comets were never seen to hit the Sun, because even a slight sideways velocity makes them miss. Since then, because of observations from space (which are able to see small comets close to the Sun) some such comets were observed. Still, it is not a common thing.



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Author and Curator:   Dr. David P. Stern
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Last updated 9-17-04