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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

43.   Stability of Lagrangian Points

    I am currently studying astronomy, and found your article on Lagrangian points thoughtful and very useful in helping me understand. I do have one question for you if you don't mind, however. You mention that were it not for other influences, the Lagrangian points would be stable. How can this be? If would appear to me that as an object starts to move away from one of the points, the change in the gravitational pull from the sun would cause its orbital velocity to change, which in turn would cause it to move farther away from the L point. A closely related question is this: how can an object orbit a L point without having some mass to which the object is attracted to?
    I am sure the answer is simple, but my brain is hurting trying to figure this out. Your answer will be most appreciated.
    Regards, Larry


  I wrote in "Stargazers" that if it were not for other attractions, L4 and L5 would be stable--but one should add that L1 and L2 are unstable. (Still, I am not sure about some "halo orbits" near them--see "The Art of the Orbit" by Gary Taubes, p. 620-622 Science, vol 283. 29 January 1999, section after the subhead "Three-body perfection.").

  If you are studying astronomy at the college level, you might find a relevant derivation in Symon's text "Mechanics." For objects that keep fixed positions in a ROTATING frame, the equilibrium can be studied in that frame by adding a centrifugal force, and then you can obtain a potential function and draw its contours. The problem then resembles that of a small ball rolling with no friction on a curved surface: if the L4 point is the center of a pit, small displacements would cause the ball to roll back, so the equilibrium is stable. Or else it could circle the pit, like a marble in a bowl: it needs no attraction from the middle.

  If instead it is on top of a dimple, a small displacement will cause the ball to roll even further away, never to return, which signifies an unstable equilibrium.

44. Can an Asteroid Impact Change the Earth's Orbit?

    I am 14 years old and enjoy doing physics a lot. I have read books on mechanics and quantum mechanics .etc. I have also been onto many physics websites. Yours is a very good one. I have a question for you. Do not laugh at it for I am only 14.

    If a meteor of significant mass hit the earth wouldn't this cause the earth in turn to move. Would its orbit be disrupted?


Dear Edward

    To give a short answer to your long question--not likely. Asteroids are far too small. An asteroid with a 10 km radius would have a volume less than one part in 200 million of the Earth, and if its mass were similarly scaled, the impact on the Earth would negligibly affect its orbit. Anything large enough to shift our orbit would have to be larger than any known asteroid, and the collision would be violent enough to wipe out all life.

    However.... you know that the Moon always presents Earth with the same face. If you read my section
"The Moon--the distant view" you know that the reason is a slight elongation along the Earth-Moon line, and that the Moon's long axis slowly swings back-and-forth around the direction of Earth, like a pendulum ("libration"). I do not know the theory of those swings--they may be linked to the equatorial bulge of the Earth--but I vaguely recall an article in "Science," maybe 20-30 years ago, claiming that an asteroid impact started them, even identifying the crater which that impact produced. The rotation of the Moon or the Earth contains much less energy than the orbital motion, it can be affected by a slanting blow, and the Moon is so much smaller than Earth, so THAT is possible.

    Enjoy your physics, as well as other things that interest 14 year olds, and don't let your grades in other subjects slip!

45. Can Gravity Increase with Depth?

    A debate is raging in our office regarding the change in gravity on an object as it moves from the surface of the earth to its center. We hope you can help us resolve this life and death issue. Given that the earth's mass is NOT uniformly distributed, is it possible that the gravitational force can actually increase as a body moves just below the earth's surface before it starts to diminish as it approaches the center?

    Personally, I would think so. The logic being that if I assumed that the mass of the upper crust were zero, the closer the object moves towards the core the greater the gravitational pull (till the object penetrates the core).



Dear Ron

    That is some neat question you have asked, and your qualitative argument is absolutely right. A short calculation (using some elementary calculus) makes it more precise.

    Suppose we are at a distance R from the center, the local density is D(R), and we move a test mass m downward by a small distance dR. If G is the constant of gravity and M the attracting mass, does gravitational attraction increase or decrease?

    In a spherically symmetric mass, any mass closer to the center than the attracted one acts as if they were concentrated at the center, while any which is more distant has no effect. That result is part of the theory of the potential, although Newton cleverly derived it from elementary considerations even before such a theory existed.

Therefore, as our test mass advances a distance dR towards the center, the mass that is attracting it diminishes by dM = 4 πR2 D(R) dR, and the attracting force decreases by

Gm dM/ R2 = Gm (4 πR2 D(R) dR)/ R2 = K D(R) dR

where K = 4 π Gm. On the other hand, the closer approach to the center adds to the force

GmM d(1/ R2) = GmM (2 dR/ R3)

    Let us ignore signs and just recognize the contributions are in opposite directions (the fact R is positive upwards while the force of gravity points downwards can confuse). If the average density of the mass M below is , then

M = (4π/3) R3

Substituting in the equation, canceling the cube power and introducing K gives

K (2/3) dR

Thus if D(R) is smaller than (2/3), gravity increases, if larger it decreases, which includes the case of constant density, D(R) = . A nice problem!             David


46.   Lightspeed, Hyperspace and Wormholes

    My name is Yoga, I live in Indonesia and am 12 years old. I am interested by science fiction movies, especially about star travel, such Star Trek, Babylon V, and so.

    When I saw those movies, there was always something that confused me so much. What's the differences between LIGHTSPEED, HYPERSPACE, and WORMHOLE?

I can understand about lightspeed, but I don't know if a wormhole could be used in space travel. As far as I know, quantum theory was just used to prove other dimensions of our world (parallel worlds), so is there any connections here between this wormhole and space travelling?

Well, Mr. Stern, I think these are the questions to which I'd like to know the answers. Can you please help me?


Dear Yoga

    The stories of science fiction movies come from professional writers, not from scientists. About 100 years ago Einstein found (something confirmed since then in many ways) that no material object can move faster than light, 300,000 kilometers per second. (If YOU moved that fast, time would pass at a different rate, so TO YOU the speed might seem greater--but not to someone in the outside world).

    Writers of fantasy stories, and later of fantasy movies, felt restricted by that fact, which suggested that back-and-forth travel or communication with civilizations on planets outside the solar system was impossible on the short time scale of travel and communications between countries on Earth. As seen now, a projected trip to another world (even using technology we do not have yet!) might take many thousands of years.

    So writers picked up some scientific terms, suggesting some day in the future the limitation of light speed may be overcome, by using hyperspace or wormholes. However, these are just ways for literature and films to imagine things which physics says (at least right now) cannot be done. I am not sure about wormholes, which have to do with general relativity: the added dimensions proposed by some theories extend only a very short distance into our universe, and are not likely to help us navigate the three principal dimensions of our universe (or 4--though time is a different kind of dimension)

    If you like science fiction, you might look up "Flight of the Dragonfly" by Robert Forward for a physically acceptable way (though one technologically extremely difficult) of flying to a nearby star.

47.   Why do Rockets Spin?

    I was recently watching a rocket launch down south and I was wondering why the rockets tend to spin upon take-off?? I know somewhat about rocket stability but this doesn't seem to apply, Is it something that can be controlled (automechanical) or is it an outside force?? I would greatly appreciate any info you could send me...


Dear Dave

    The spin-up is deliberate. Any spinning object resists having its spin axis changed. You may know that rifle bullets are made to spin by the grooves in the barrel of the rifle, in order to stabilize them. It is the same way in some rockets, especially solid fueled ones. Manned spacecraft obviously do not spin.

48.   Around what does the Sun revolve?


My almost 8 year young son Adam and I have a question about the revolution of the sun. We know that the planets revolve around the sun, and all have rotational periods also. We see that the sun aside from having a rotational period, also has a revolution of some 250 million years. We are curious what it is that the sun is revolving around?


    I can only guess that your son came across a reference to the rotation of the galaxy. Many galaxies are round and rotate around their center, and presumably ours does too, and so the Sun and the solar system share that motion.

    What do they rotate around? Good question. There is SOMETHING at the center of the galaxy, and radio astronomers have determined it is very compact--I read somewhere, smaller than the orbit of Saturn, or maybe Jupiter. It also seems massive, but does not shine brightly, and most astronomers favor a humongous black hole, created in the early years of the universe (yes, Adam, we are safe from it).

    Still, what holds galaxies together is a bit of a mystery. If it were just the gravity of something pulling it towards the middle, a galaxy would rotate like the solar system--fast motion near the middle, slower and slower as one gets away. Vera Rubin has examined the light of galaxies and has determined (by the Doppler effect) that many of them, apart perhaps for the outer edges, rotate together, like a spinning dish, which is SLOWEST near the middle.

So, Adam, maybe the correct answer is: we do not know.

49.   Why are planets in nearly the same plane?

    Is there an explanation as to WHY all of the planets orbit around the sun in a plane, the ecliptic? I understand why they orbit but not why the orbits are all restricted to one plane. In other words, why can't Earth rotate at, say 10 degrees, and Jupiter at, say 40 degrees?

Love your pages; they're very useful and educational.             Michael


Hello, Michael

    The fact the orbital planes of all planets and of most of their moons are so close to each other (though not exactly the same) suggests that they all were created from the same swirling cloud of dust, gas and flying rocks of assorted sizes. Different theories exist about how it happened, but I believe astronomers have observed such clouds, which one day may become planetary systems.

    The fact the Earth, and you, and I, contain fairly heavy atoms (oxygen, chlorine, even iron) suggest that at least some of the material of that cloud was previously part of another star, which "burned up" its hydrogen fuel and then exploded. See .

50.   The Shapes of Rockets and Spacecraft

    Hi, I am Alan from North Carolina. I am a junior at high school and I have just been assigned a project on the Physics of Aerodynamics of Rockets and Spaceships. I am trying to concentrate on the aerodynamics and why the spaceships are shaped the way they are. I have to admit that I do not know much about this subject matter, but I am extremely interested in learning more about it. Have a nice day.


    I do not know who assigned the project to you, because the aerodynamics rockets is not such a wide subject at your level.

    Space rockets are narrow and long to reduce air resistance. They are inherently supersonic--orbital velocity is 24-25 times the speed of sound. That means they do not use wings during ascent, wings only help at low speeds, and just create more air resistance later on (though the first stage of the Pegasus launcher does have short wings). Also, they have sharp noses, to create the weakest shocks in front--again, shocks create resistance.

    Out in space, more variety exists: spacecraft can spin or not, some are drum-shaped (those usually spin), some have solar panels that stick out. But all that does not involve aerodynamics.

    If the spacecraft is to reenter the atmosphere safely, a lot of energy must be dissipated. A blunt front creates a strong shock wave, and much of the energy goes to the heated air in the shock wave, it does not heat up the spacecraft. Still, the heating of the front of the spacecraft is strong enough to require protection, by ceramic tiles in the shuttle and by material that ablates (wears away) on the re-entry capsules of Apollo, Mercury and Gemini.


51.   Space Debris

Dear David,

    First of all please accept my thanks and regards as you have clarified many astronomical puzzle for which I was searching the correct answer. I am writing to you after a long time.

    As I know from Internet web site that plenty of space debris is revolving around the Earth at various altitude and definitely at different speeds.

    Frequently NASA or ESA etc sends an artificial satellite or space shuttle around Earth's orbit at a distance of more than 200 KM to 40000 KM. Even in 1994 astronaut Mark Lee was found flying over earth's surface as satellite.

    How do they avoid collision and monitor the movement of such unwanted space debris as the danger appears due very high speed?


Dear Bishnu

    Space debris is gradually being recognized as a serious problem, and at least one collision has already been reported, involving a French satellite. The density of spacecraft is still low, so the risk is small, but it is not zero. The US Navy is monitoring such objects by radar and yes, the number is increasing.

    The solution is uncertain. Low altitude orbits and highly elliptical ones reenter the atmosphere after a while, but communication satellites in synchronous orbit, of which hundreds now exist, will stay around for millions of years unless picked up.

    The danger also exists on the space station, even though at its low altitude debris does not last as long as at higher ones. One helpful fact is that most satellites are launched towards the east, so when they overtake their mutual velocity is only part of their total velocity. Still, collisions between satellites whose orbits have different inclinations to the equator can be very damaging.

52.   Teaching Nuclear Fusion

    As a 7th grade science teacher, I have been looking through many websites, to find activities to teach sun's fusion reaction "in a nutshell." That is how I came across yours sections S-7 and S-7A.

    Actually, I have been looking for a more kinesthetic "hands on" approach but hopefully I can take your material and "soften the edges" to make it more middle school friendly (although our population of students tends to be academically inclined and I hope that I won't have to take off too many edges). I hope to be able some way to come up with something like M&M's for them to experience fusion tastefully!

    Thanks for the info.


    Dear Sharon

    You have my respect for teaching nuclear fusion in middle school! However, the only hands on demonstration I can think of is to use a bunch of those small cylindrical magnets used for pinning messages to a steel partition (or refrigerator). They will all stick together, but the forces are short range--once you pry a magnet a short distance off the bunch, you have no problem pulling it all the way.

    Nuclear forces are like that too, their range is short, each nucleon (like each magnet in the analogy) attracts mainly the ones right next to it. That is why 4 nucleons in helium form a very strong combination and release a lot of energy. (Draw for your kid a pyramid of 4 balls--each one touches the other three. Use four M&Ms in a model?)

53.   Contribution of different elements to Sunlight


    Your website stargaze/Sun4spec shows and explains the visible spectrum.

    At present we are looking into the 1,400 Watts-sec/square metre constant and we would very much like to know the make-up of the light received on planet earth. [over full spectrum not only visible.]

    Ideally we would like to know what percentage in Watts of the 1,400 Watts is attributable to which of the elements in the periodic table 1] Helium 2] Hydrogen 3] Carbon 4] Oxygen etc.

    Only a very rough answer say within 10 % is necessary. If this information is already on an alternative website please point us in the right direction.

    Many thanks


    Dear Clive
    Visible sunlight, coming from the photosphere, may have started as a specific emission of single atoms, probably of hydrogen or helium, in a deeper layer of the Sun. However as this light works its way to the surface it undergoes absorption and re-emission many times. The final spectrum reflects not the original emission and its energy levels, but the way energy is shared among many interacting atoms. A similar situation exists in a hot glowing solid, and in both cases the spectrum is smooth and depends only on temperature. I seem to recall the Sun's color distribution fits about 5800 degrees and that about 1% is in the ultra-violet range.

    On top of this are emissions of individual atoms in the atmosphere of the Sun, coming from higher layers)--of helium (which was first discovered through its yellow emission), of hydrogen (the red line of hydrogen is used to study the chromosphere) and so forth. I do not know how much of the sunligh energy comes in these forms, but I suspect it is much less than 1%. In addition to light emitted by atoms in the higher layers, light is also absorbed, creating the famous dark "Fraunhofer lines" in the Sun's spectrum.

54.   Jewish Calendar

    Could you tell me how the Jewish calender originated ?


    Dear Robert

    As I wrote in the "Stargazers" unit, the Jewish calendar is very similar to the Babylonian one--in using the Metonic cycle, in its names of months and the ambiguity of the new year (all of which I found in my 1967 edition of Encyclopaedia Britannica). The similarity is understandable, because according to the biblical scriptures (which are quite consistent on this point), Jews lived in exile in Babylonia for 70 years in the 6th century BC.

    They came to Babylonia speaking Hebrew and returned speaking (except in religious usage) the language of Babylonia, Aramaic, which is somewhat similar to Hebrew and which prevailed over 1000 years, up to the Arab conquest. They came to Babylonia with their own alphabet, angular like the Greek one, and returned using Babylonian letters (although again, Maccabean kings for instance continued using the old script on their coins). The script known today as "Hebrew" and used in Israel is, in fact, Babylonian. And most probably the calendar, too--the bible still mentions some old names of months (Ziv, Bul, Eytanim, perhaps Aviv) which did not persist. The bible itself uses primarily numbers ("second month") but the names we have today are very close to the Babylonian ones.

    Still, there are signs that it took a long time before the new system was completely accepted. For many centuries a new month was supposed to begin, not on a pre-calculated date as in the Metonic cycle, but only after reliable witnesses had seen a "new moon" (supposedly--it is unclear what was done during prolonged cloudy weather!). According to Jewish tradition, the final form of the calendar was introduced in 358/9 AD by the patriarch Hillel the 2nd (Encyclopaedia Judaica).

    The reckoning by which this is year 5762 to the creation of the world is even more recent, derived by bridging between the historical record and biblical chronology. A similar calculation was performed by the Christian Bishop Ussher.

    I hope this satisfactorily answers your question!

    David P. Stern

55.   Spaceflight Without Escape Velocity?

    I will argue that it is unnecessary for an object to achieve 8km/hr to leave the Earth's gravity, as long as it has a continued thrust which is greater than the pull of gravity. With such thrust a rocket could literally crawl from the Earth at one mile per hour. Obviously each rocket has this thrust or it would not leave the surface of the planet, where gravity is at its strongest. Escape velocity pertains only to objects without any additional thrust available.


    Hello, J.D.

    Your argument is correct, but the conclusion you draw is not. Suppose you have a rocket of mass M accelerating from the pad with an acceleration a=g, which we will round off to 10 meter/second squared. That means its rocket must provide a thrust of 2Mg--Mg to support the weight of the rocket and Mg to accelerate it. To reach orbital velocity of 8000m/sec will take 800 seconds (8000/a = 8000/g). During that time the launch vehicle has to use half its thrust just to keep itself from falling--only half the thrust goes to accelerating.

    Actually the mass M of a rocket decreases as fuel is burned off, so the acceleration increases, making the time shorter (the space shuttle achieves orbit in about 6 minutes, less than half the above time). One reason stages are dropped in manned missions is to limit the acceleration to about 2-3g ; more than that is hard on the astronauts. See example of the V-2 rocket in "Newton's 2nd law", section 18 of "From Stargazers to Starships" at

    A launch vehicle crawling upwards at 1 mph would be wasting an enormous amount of thrust just to keep itself from falling! And even if you raise the space vehicle slowly to (say) 1000 miles, to keep it there from falling you still need give it an orbital velocity--less than 8000 m/s because of the greater distance, but not that much less.

    You might think that wings would be a more efficient way of keeping the vehicle in the air--after all, the thrust of an airplane engine (in cruising flight) may be only 5-10% of its weight. Unfortunately, this efficiency drops very quickly above the speed of sound, and 8000 m/s is about 24 times that velocity. Above a speed of several times the speed of sound, the extra air resistance of the wings outweighs any advantage they provide; it is better for the vehicle to quickly rise above the dense atmosphere and avoid air resistance altogether.

    In a stable orbit, with orbital velocity, gravity no longer threatens to bring down the vehicle, it just determines its orbit. From that point on, one can apply thrust at any rate. There is a story of a communication satellite (I think of NASA's TDRSS system) which made Earth orbit safely, but the engine which was to take it to its final orbit at 42000 kilometers (6.6 Earth radii) failed. However the spacecraft had plenty of on-board fuel, and a small motor meant to adjust its orientation, which was able to tap that fuel supply. So over the months that followed, guided by NASA controllers (the motor had to be switched on and off to prevent overheating), it slowly limped to its final station, reaching it safely.

    That is probably as close as we have come to your "one mph" motion. "Deep Space 1" with its ion engine is another such slowly accelerating spacecraft; see So are solar sails

56.   Who first proposed a round Earth?

    I'm an interested science teacher at St. Catherine's British Embassy School in Athens Greece. Is there any reliable information relating to when and by whom it was first proposed that the earth is a sphere?


    Funny that YOU should ask--living in Athens, Greece, you may well have the best experts on the subject within walking distance. From what I can quickly find on the web (I am at home, away from any library), the first solid arguments were made by Aristotle. The idea itself was raised earlier by Plato and Pythagoras:

    Presumably, you have read all I have on this in "From Stargazers to Starships."

57.   Does precession change the Length of the year?

    I've enjoyed your page on the precession of the equinoxes at

    I understand that a year is the time between two successive vernal equinoxes. In a year the earth will have orbited around the sun and the earth's axis will have precessed a very little bit so that both the orbit around the sun and the precession of the earth's axis go together to make up the length of time between two successive vernal equinoxes. Now suppose the earth's axis were not precessing. How long would a year be? How much does the precession of the earth's axis affect the length of a year?

    Thank you for your attention and any information will be greatly appreciated.


    Dear Gary

    Let's first try a simple minded approach. The phenomenon is called PREcession, so the spring equinox moves to a point a little EARLIER in the Sun's journey around the zodiac. The location of the spring equinox makes one circuit of the zodiac in 26000 years. Therefor, if the spring equinox did NOT move to intercept the Sun on its trip around the sky, the year (equinox to equinox, say) would be about (365 x 86400)/26000 seconds longer, or about 20 minutes.

    But it's more complicated. What year do you have in mind? A CALENDAR year extends from equinox to equinox, or from solstice to solstice. Most people want holidays to stay with the right seasons, not migrate between summer and winter (as Moslem ones do). If the precession were to stop, the year in which holidays kept a fixed position that would be 20 minutes longer.

    On the other hand, if your field is celestial mechanics or astronautics, "a year" is presumably the EARTH'S ORBITAL PERIOD around the Sun. The orbital period does not depend on which way the Earth's axis points in the sky--it is always the same, precession or no precession (and it hardly varies over millions of years). So it is always the longer of the two preceding ones.

    A similar analogy holds for the day. Is it NOON TO NOON (24 hours average) or is it the ROTATION PERIOD of the Earth around its axis? The latter is 4 minutes shorter, because "noon to noon" includes a small contribution from the shift of the Sun's position in the sky, about one degree per day.

    Ask a simple question... sorry about the complicated answer!


58.   The Analemma

    Hello Mr. Stern!                 (Received 21 December 2001)

    Excellent website. Here's my question:

    What is the name of the figure-eight traced upon the earth by the combination of axis tilt and orbit in a year; where the figure represents the shortest distance between the earth and sun?


    Dear Neil You are probably thinking about a figure known as the analemma. You can read all about it at

    It is related to "the equation of time," a correction to sundial time which must be applied (in addition to others) because the Earth orbits the Sun in an ellipse, not a circle, and its speed in that orbit varies around the year. The equation of time is mentioned in my site on the sundial


    Happy solstice day to you!  


59.   Changes of the Polar Axis of Earth

    Hello David,

    I was hoping perhaps you might be able to point me to a reference [if it exists]. I am doing research with a professor and looking for a site or reference that would state the coordinates of the polar axis location against year (ie 100 BC, 1000BC, 10,000 BC, etc.) (not the magnetic axis) Thank you so much!


    Dear Eve

    Your question is not completely clear: what do you mean by "location"? The DIRECTION of the spin axis, determined by the angular momentum of the Earth, is almost constant. The Moon's pull (and maybe the Sun's too) on equatorial bulge of the Earth, created by the Earth's rotation, causes a 26,000 motion of the axis around a cone, expressed in the precession of the equinoxes. See

    Smaller wobbles and motions of the axis exist, on shorter time scals.

    The OPENING ANGLE of the cone itself--the obliquity of the axis to the ecliptic--changes very slowly. See middle graph on

    The POSITIONS of the north-south poles on the surface of the Earth may of course change if the entire crust of the Earth somehow slides around the interior, staying intact in the process. The theory of such "polar wandering" was briefly fashionable 50 years ago, but it no longer is. It is hard to observe any such motion if it is slow enough, but the magnetic signatures of lavas suggests that if the effect exists, is negligibly small.

    To give you the argument, suppose you have samples of volcanic lavas (which record the direction of the magnetic force as they harden) at a location at latitude 40 North, from different eras. Then ancient magnetizations will cluster either around the directions observed today, or around directions opposed by 180 degrees. There are always some deviations, ascribed to variations of the global field, but they are the exceptions, not the rule. So, to misquote Kipling, "North is north and south is south" even though MAGNETIC north and south may reverse by 180 degrees.

60.   Van Allen Belt and Spaceflight

    Dear Sir:

    Would you please explain how the Van Allen Belt effected the first manned space flights. How were they protected?

    Thank you,


    Dear Belinda

    All manned flights (except those of Apollo) have stayed below the radiation belt: the Space Shuttle, for instance, orbits at about 215 miles. The atmosphere is very rarefied there, and radiation belt particles descending to that level may well come back without encountering anything. However, such particles have thousands of Earthward excursions each day, so the only ones which are likely to survive long are those that are always confined to higher levels.

    A more subtle effect is also at work. The equations governing the motion of trapped particle indicate that each has a characteristic value of magnetic intensity, below which is cannot penetrate. Suppose a particle is reflected by the intensity existing at 215 miles. As it happens, the Earth's magnetic field--its region of magnetic forces--has some irregularities, so in some regions that intensity is only reached at 100 miles. Now and then the particle's orbit will happen to descend in that region, where it penetrates to much deeper (and denser) layers of the atmosphere, and may be quickly lost, even if elsewhere it stays at safe heights. One such notorious region exists above the southern Atlantic Ocean.

    So the radiation belt does not reach the levels where Mercury, Gemini, Soyuz and Mir used to orbit and where the Shuttle and Space Station do so now. The early Russian Sputniks failed to discover the radiation belt because they too stayed in such low orbits and Explorers 1 and 3 only detected it because they were rather poorly controlled and rose above 1500 miles.

    You will find more on my web sites, e.g.

61.   Nearest Star Outside Our Galaxy

    Dear expert,                 (received 21 December 2001)

    Please answer this question which has been set at our school in East Sussex,UK. What is the nearest star outside our galaxy? I am a year 5, age 10. Thanks for your help and time


    Dear Oscar

    Nice of you to call me "expert." Actually, I am a space physicist, not astronomer, but will try to answer you anyway. All stars observed from Earth are in galaxies, so the nearest one outside our galaxy should be in the galaxy closest to us. I would guess that would be the Large Magellanic Cloud, so called because it was observed by Ferdinand Magellan after he crossed the equator (opening to his view stars never seen from Europe) as one of two fuzzy glows in the night sky.

    The LMC became famous in 1987, when a supernova exploded in it, allowing interesting phenomena to be observed. Of course, technically the explosion happened 164,000 years ago, because the LMC is 164,000 light years from us.

    The trouble is that stars in the LMC do not have names. Astronomers presumably have their designations (possibly, numbers in a catalog of stars), but they are not publicized in star atlases etc., because you only see such stars with powerful telescopes and perhaps time exposure photographs.

    To see some of these stars, go on the web to "Astronomy Picture of the Day", appropriately titled "Pick a Star." The address (one of several) is

    Merry Christmas, Joyous New Year--and oh yes, Happy Solstice Day.  

62.(a)     Why are Satellites Launched Eastward?
         What is a "Sun Synchronous" orbit?

1. I believe earth-orbit satellites are launched in either polar or basically west-to-east orbits. Why do we not launch in a westerly direction?

2. What is meant by "sun-synchronous" orbits?


    Concerning your first question, the velocity needed for a stable orbit around the center of the Earth above the atmosphere is about 8000 meter/second. An object on the surface of the Earth already has an eastward velocity, because of the Earth's rotation, but it is much too small: 409 meter/sec on the equator, and 409 times cosL at latitude L. That is much too small to fling you or me into orbit (for which we ought to be grateful), but it's still something, and satellite launchers, eager to make use of the smallest advantage, fire their rockets eastward. At Cape Canaveral you get a bonus of about 360 m/s.

    Israel has launched two satellites so far (maybe more). Lacking the choice, it must launch westward over the Mediterranean, and those 360 (or so) meters/sec hinder rather than help its rockets, reducing the available payload. Yes, it can be done, but when a choice exists, eastward is preferable.

    To answer your second question, let me paste from the glossary of "Exploration of the Earth's Magnetosphere":
        Sun-synchronous orbit--a near-Earth orbit resembling that of a polar satellite, but inclined to it by a small angle. With the proper inclination angle, the equatorial bulge causes the orbit to rotate during the year once around the polar axis. Such a satellite then maintains a fixed position relative to the Sun and can, for instance, avoid entering the Earth's shadow.

    More about it at

62. (b)     Why are satellites launched from near the equator?

    When you calculate the centripetal force at the equator it is 0.033 m/s2 (m v2/R) This means that there is approximately 0.3 % variation in the force that attracts a body towards the center of the earth between the equator and the north pole (where without precession no rotation would be felt). This would mean that it would be attractive to send rockets from the equator because it would save 0.3% in fuel. Is this true?


    Your argument is well known, but it is usually phrased differently. The centrifugal force acts only in a rotating system. Once an object is detached from that system, putting the centrifugal force into a calculation may lead to incorrect result. Example: your bicycle wheel picks up a piece of mud from the road. As long as the mud is attached, it feels an outward force, a centrifugal force (in the frame of reference of the rotating wheel). Once it works loose, however, it does not fly in the direction of that force, but tangentially to the wheel!

    However... launching from the equator is still advantageous. A spacecraft located on the equator is carried by the Earth at about 400 meters/second, or about 5% of the orbital velocity. Even at Cape Canaveral, one still has a large fraction of that velocity, about 4/5. That is a significant advantage, and the reason all rocket launches from Florida are eastward. Polar satellites are usually launched from Vandenberg in California, and obviously cannot use that advantage.

    Launch sites, of course, are subject to political limitations. Cape Canaveral is as close as one can get to the equator from the continental US. The European Space Agency has its launch base at Kourou in French Guiana, about 5 degrees north of the equator. Israel, on the other hand, had to launch its satellites westwards, over the Mediterranean, requiring extra velocity to overcome the rotation.

    The only launch I am aware of that was conducted on the equator itself was of an Italian "San Marco" satellite, launched from an off-shore platform near Somalia. Today, with the "Sea-Launch" ship available for launches (a commercial partnership of Boeing and Russia), such launches are again practical.

63.   How Tall Can People Get?

Dear Dr. Stern,

    My name is Jason and I am currently in the 11th grade. My brother and I were wondering whether it was possible for people to grow infinitely tall. My brother believes there is a limit to height because muscle strength would be insufficient to support a person that tall.

    I contend that there should be no limit to height if they live on an extremely small planet with huge mass.

Please let us know who is right!


Dear Jason

    Presumably, you meant "arbitrarily tall." Even with that change, I do not know what the proper answer would be, because the rules of your disagreement with your brother were not spelled out. On Earth, the answer is no: even trees do not grow arbitrarily tall, because of the difficulty of pumping sap very, very high. Pumping blood to comparable height would be very difficult (giraffes seem to have attained the limit) and even before that, the weight of an individual would make it hard to move or even stand.

    On another planet... if gravitation is very weak, the limits change. Of course a planet with such weak gravitation would probably not hold an atmosphere. A small planet with huge mass won't do, and in any case, we know no such planets.

    But... I have homework for you both. First of all, read "About Being the Right Size" by J.B.S. Haldane, written in 1928, on the web at

    And secondly, you might enjoy "Food of the Gods" by G.H. Wells, a science-fiction novel about people growing to gigantic size.

64.   Gunpowder and Rockets

    I'm a high school sophomore on Hawaii and I am doing a History Day project on the Gunpowder Revolution.

Some questions about the article you had written at this site:

    Your work stated that rockets were a spin-off from the invention of gunpowder. Can I ask you to elaborate on that a little, as I am intrigued about this. For instance, what is the connection between rockets and gunpowder that would make rockets a 'spin-off'? On a much wider scale, how did this affect battles and how they were fought?


    The Chinese made rockets by stuffing gunpowder into a hollow tube with one end plugged, aiming it skyward (with the open end towards the ground), lighting the gunpowder with a fuse or long match--and then standing safely aside and watching it rise (or explode, or do other unpredictable things). They used rockets for fireworks, and Europe and the rest of the world learned from them.

    Interestingly, the Chinese never invented guns, as the Europeans (prodded by an era of widespread warfare) did. One reason was that the Chinese did not at first get a very good gunpowder. You read that gunpowder consists of charcoal and sulfur, which provide the fuel, and salpeter, which provides the oxygen. True: but the salpeter needs to be refined, it starts out as a rather dirty mix. The Chinese only gradually learned to refine it, and one may guess that their earlier gunpowders were good for rockets and fireworks but not good enough to explode.

65.   Precession

    Is it true that, as a result of the precession of the equinoxes, and because the earth's spinning axis remains roughly constant at 23.5 degrees, in about 13,000 years the northern hemisphere will experience the summer solstice in December?


    One should not expect summer solstice to be in December in 13,000 years, because of the way the length of the year is defined, as the time between one solstice and the next, or one spring equinox and the next. In 13,000 the stars behind the Sun's position at the spring equinox will be quite different, but assuming the same calendar will still be current, the date will still be reckoned as 21 March, or near it.

66.   Solar Sails


    I was visiting your site ( and I must admit, it is a great site. My question is: When designing solar sails and computing their top speed, is the resistance provided by the interstellar gas particles taken into consideration? Also, at such high speeds, how likely is it that the sail might rupture because of the impacts?

    Thanks for your time


    I do not know your answer--you can calculate it. My guess is no, the flux is too small, and the extra push of the solar wind more than outweighs it.

    Take a solar sail moving at 10 km/sec. It intercepts about 1 interstellar atom per cc (depending on direction, since the solar system itself moves at about 20 km/s) or a million per second per cm-squared. From the other side, it is overtaken by solar wind ions at 400 km/s, density near the Earth orbit about 6 per cc, or about 240 million per sec cm2. So the solar wind pressure is larger, and that of sunlight larger still.

    Fast particles of course can cross the sail, but I don't think such "radiation damage" is serious. I would worry more about tearing due to the degradation of the material from short-wave sunlight and the space environment.


67.  (a)       Distance to the Big Dipper

Dear Sir,

    My son is doing a science project on the Big Dipper. One of the questions is "How far is the Big Dipper from the Sun". We have checked every site and cannot find this answer. Please help us.



    Dear Chick

    I do not know what answer the teacher expects to get to "How far is the Big Dipper from the Sun." The Big Dipper (or to astronomers, "Ursa Major," the big bear") is a group of stars which, when viewed from Earth, forms a striking pattern, but that by no means assures that they are all close to each other and have the same distance. It is quite possible that some are close to us, others distant, and only by chance are they found together in the same part of the sky. In fact, some of the stars do seem close to each other (in the real sense of the word), but others are just accidentally grouped together. They move differently, and if you could wait a few hundred thousands of years, those stars would be much further apart.

    Anyway: the numbers. Main stars are named in order of brightness in their constellation, according to the Greek alphabet, and those forming the "Big Dipper" are alpha, beta, gamma, delta, epsilon, eta and zeta of Ursa Major ("Alpha Ursa Majoris" etc.). A labeled map can be found at

    Their approximate distances from the Sun in light years, according to
alpha--86 ; beta, gamma, delta--100 ; epsilon--64 ; eta--95 zeta--78

    Unlike you, I do not mind signing my name. ....

67.(b)     Big Dipper star names


    I'd be grateful if you could tell me the name of the brightest star in the Big Dipper, and the name of the brightest star in the Little Dipper. If you cannot, then please pass this on to someone who might know.


    According to my copy of "Naked Eye Astronomy" by Patrick Moore (W.W. Norton, 1965), starting to count stars from the front of the Big Dipper and ending at the tip of the handle, they are named Dubhe, Marak, Phad (aka Phekda), Megrez, Alioth, Mizar and Alkaid (aka Benetnash). The names are Arabic, and if "Alkaid" reminds you of "Al Qaeda" that is no accident--"Alkaid" means the commander and "Al Qaeda" means the command.

    Dubhe is the"alpha star," but it's practically the same brightness as Alioth, and just a tad less bright than Alkaid. Except for Megrez, the others are not far behind. Mizar is a double star and can be resolved with binoculars.

    The "alpha star" in the Little Dipper (Ursa Minor) is none other but Polaris, the pole star. See

    The other stars are fairly dim, except for the two "guardians of the pole" at the front of the dipper. The one closer to Dubhe is Kocab, again very close in magnitude to Polaris.    

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