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

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

Items covered:

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

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

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

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

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

  84. How far does Earth's Gravity Extend?
  85. How far is the Moon?
  86. Twinkle, twinkle little star
    How I wonder, what you are.

  87. Teaching about seasons
  88. Space Launches by Cannon--A
  89. Space Launches by Cannon--B
  90. The Southern Pole of the Sky
  91. Do Astrologers use Wrong Positions for Planets?
  92. Why does the Moon have bigger craters?
  93. Why does Gravity Exist?
  94. Atmospheric "Thermals"--Triggered by Electric Forces?
  95. What would happen if Earth rotated faster?
  96. Where do gravity of Earth and Sun balance?
  97. The Ultimate Astronomy Tool
  98. High Temperature in Cold Outer Space

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

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

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

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

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

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

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

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

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

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

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

  1. 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  1.  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. "

  2.   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.


  3.   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

  4.   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.


  5.   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


  6.   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

  7.   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://



  8.   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.

  1. 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.


  1. 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.




  2.   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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Last updated 9-17-04