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Get a Straight Answer

Please note!

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

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

Items covered:

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

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

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

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

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

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

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

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

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

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

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

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

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

    (shortened message)

    Hi David

        I am quite interested in physics in general, mainly quantum physics and the likes. One thing that to this day baffles me about physics is the whole concept of energy.

        The thing I can't understand is that if you had a room temperature super-conductor (as I am aware, they haven't come up with one yet) you could have a device that produces either a repulsive or attractive force, just whilst it sits in your back garden, no extra energy required. So how is it able to produce this repulsive/attractive force without ever needing energy.

        And how can the universe be expanding on limited energy and why is it not slowing down there further out it gets (Surely the further out it goes the energy has a larger void to cover so should dissipate), How do black holes consume so much energy and release none (Where does the energy go to if energy can't be created or destroyed?)

        How does the Big Bang Theory work if with limited energy the universe started from something, that assumes that everything in the Universe was created from nothing, which could mean it is possible to create energy from nothing.

        Any thoughts on what I have dicsussed would be most appreciated.


    You seem baffled by some phenomena, e.g. asking why does the universe expand without slowing down? How can super-conductors keep currents forever, without energy input? And so forth.

        Nature owes us no explanations! WE are the ones who sometimes need change our thinking in light of observations. Nature's phenomena follow their own rules, not ours, and it's OUR job to understand them.

        Ancient Greeks believed (and Copernicus still subscribed to that belief) that heavenly bodies moved in circles, because the circle had perfect symmetry, and Nature could not be anything short of perfect. Then Kepler introduced ellipses, Newton proved orbits had to be elliptical, and astronomy adjusted to a new view.

        In the 19th century, science knew that material objects (sticks, stones, etc.) were localized, while waves filled space. Today we know electrons (say) can behave like waves or particles, depending on circumstances, and Schroedinger's equation bridges the gap between their extreme behaviors. Electromagnetic waves sometimes act as photons--particles of zero mass. Physicists had to adjust their views again.

       . Now about those superconductors. In everyday life, almost any physical process associated with energy transfer always suffers a loss. Motion loses energy to friction, current flow in a wire encounters ohmic resistance, and if you charge an electric battery, you can never recover all the energy invested. That happens because whenever an energy conversion process involves an large number of atoms, at least part of it that energy ends up shared by the atoms as heat. It's as if the energy had to pay a sales tax.

        Processes involving single atoms are often exempt from such "taxation." The Bernoullis (I think) were the first to explain the gas laws by assuming that a gas consists of zillions of molecules or atoms, little spheres which fly through space and collide with each other. Their collisions are perfectly elastic, never losing any energy. Some larger-scale quantum phenomena--e.g. currents in a superconductor--also behave like that, they may not involve a single particle but rather a single quantum state, which seems just as good. And a bar magnet that has been magnetized stays magnetized indefinitely, with no energy input.

        There is no contradiction: matter can sometimes maintain its state indefinitely. This isn't "classical" perpetual motion, as long as it cannot create more energy than it has started with.  

  2.   Shuttle orbit and Earth rotation.

    Hello Dr. Stern, I have been looking all over searching for the answer to this question....please help me.

        Since all things on the earth rotate with it at a constant speed, including the atmosphere (generally), how do spacecraft "recalibrate" themselves to the rotation of the earth (1000mph) upon reentry? When does it happen? What effect does it have on the passengers? I am assuming that it happens before they touch the earth since the atmosphere itself moves with the earth's rotation. Thanks for your help!


        The shuttle orbits a 5 miles per second or 18,000 mph, so 1000 mph is a relatively small part. Still, spaceflight is so difficult and expensive that any small advantage is precious.

        That is why the shuttle is launched eastwards from Cape Canaveral: its orbital velocity is measured relative to space and depends only on the Earth's gravity, which is the same whether the Earth rotates or not. Therefore, an eastward launch from Cape Canaveral already has an initial velocity in the right direction, and needs only add the remaining difference.

       . (More at http://www.phy6.org/stargaze/StarFAQ3.htm#q62)

        Same with reentry! It helps if the shuttle approaches landing from the east, because then the rotation of the ground (and of the atmosphere) is already eastward at about 1000 mph. To match velocities with the ground (and the atmosphere) the shuttle only needs to lose 17,000 mph or so, less than otherwise.

        If you remember the story of the break-up of the shuttle "Columbia," it planned to land at Cape Canaveral and it approached over California: the break-up was first seen above Arizona, and most of the debris came down in eastern Texas. It was approaching its landing from the east.

        Concerning re-entry, you might also look up a recent letter at http://www.phy6.org/stargaze/StarFAQ12.htm#q191  

  3.   Worrying about Wormholes and Black Holes

        I have a question about space. A few weeks ago, I asked a friend if she was afraid of aliens like those seen in "War of the Worlds" and she said no, we (have) more reason to be afraid of the giant black hole at the center of the Milky Way. I knew we didn't have to worry about that, as it is billions of light years away.

        But then I had a dream about it and awoke this morning with the theory that if a massive black hole opened a wormhole and sucked us right through the wormhole and into its center. Both teachers I told this to said I shouldn't worry about that (I never worry about drowning, but the supernatural is another story). But what do you think--is it at all possible? What would happen if we did get sucked in? Would it be, as my friend would call it, "an incredibly painful and excruciatingly long death" for everyone, or would it just me instant blackness? And are we at the mercy to this supposed black hole at the center of the galaxy? If so, how long does our planet have? How fast is it carrying everything in?

    Okay. I'll stop bugging you now.


        I am just a plain physicist and know nothing about wormholes. I know that astronomers have never seen anywhere a phenomenon which required wormholes for its explanation--and they do see some mighty weird stuff!

        I looked up wormholes on the web, on "Wikipedia." There exists a lot of theoretical speculation, but I found nothing positive. Maybe it should be filed for now under science fiction, in the realm of things we cannot prove to be impossible, even though they have yet to be observed.

        About the black hole at the center of the galaxy (whose distance is measured in thousands of light years, not billions), see my web page at
    There exist quite a few stars orbiting it, on what seem stable orbits. I would not like to live that close to the black hole--too many X-rays--but our solar system is at a safe distance.

        Neither do I worry about aliens. My greater worry is that actually, no aliens exist anywhere in the galaxy. If so, we have the only planet which harbors life--a planet which in recent years we have been treating rather shabbily. Until we know better, we ought to assume that we may be the only guardians of the flame of life, and if so, we ought to do a better job!  

  4.  What should I study?


        I would like to find new ways to produce energy, like solar panels do, by converting some natural form of energy into electrical energy. I am enrolled in my first semester of Mechanical Engineering and working as an RN in a local hospital.

        I am curious as to your advice on what I should be studying. My math is only at the level Of Algebra I (I haven't had any math really in 7-8 years). Should I stay in Engineering? If so what field? Mechanical or electrical? Or should I go for a physics degree.

        I would greatly appreciate your guidance, or else, ask others in your dept for their input and please write me back. I also thought about going back to school to get a degree in biology to do research in finding cures or vaccines for AIDS, Cancer, Herpes...Which I believe are all related because they have to do with changes in DNA/RNA.

        Can you please give me some assistance. The tests I have taken say Engineering, then Medicine as a second choice. My family has a heavy background in electronics. I like classes like Chemistry, Physics, and Biology.


        I hate to sound discouraging, especially in face of your enthusiasm, but there seems to exist a huge gap between where you are and what you are trying to reach.

        Maybe I share some of the blame: the web pages I have created--carefully avoiding complex math and theory--may have done too good a job, in making the complicated seem simple to you. Front-line science is not simple, the simple stuff was all solved long ago.

        This is particularly true in physics, which is very competitive, and where even reaching the starting line takes years of concentrated effort. My advice is--start with simple things. Improve your math: even the simple course in
    will be useful (although a regular class ultimately gives wider grounding, besides putting pressure on you to persist). Improve your physics. Again, "Stargazers" has some useful material, but it falls short of a complete course, and is rather space-oriented. You certainly need the simple foundations: density, liquid pressure, buoyancy, levers, balanced forces, centers of gravity, calorimetry, electric circuits, basic optics.

        And in any technical field these days, you may need to know about computers--also maybe to write computer code.

        Anything new you learn may qualify you to a higher level. If you have some physics, math and computers, you can take a technical job. With more, and with related skills, you might get into engineering: there exist many more jobs for technicians and engineers than for physicists, an important consideration for making a living. What kind of engineering is best for you--and when you should branch off from general topics (like those mentioned above) to some specialization, that is where your judgement comes in. Such decisions depend on you, on your opportunities, perhaps on finding a good mentor or good training (with your background, maybe it will be related to the health field). Keep improving your hold on generalities until you decide you have reached that point.

        If I can add something here, it is, technology and science are not everything: it also helps to get experienced in writing clearly and well. Reading books helps sharpen those skills, too.  

  5.  The greenhouse effect

    I'm 35 years old, married with one kid... and just want to help a cousin in an assignment in physics.

        When a photon from the sun hits the surface of the earth, it may be reflected (in which case its energy goes into making a new photon of the same wavelength) or it may be absorbed. The energy from photons that are absorbed goes into heating the earth. The earth takes some of the absorbed heat and generates new photons in the infrared part of the spectrum.

        Can those photons go through the atmosphere and into space, or are they trapped by the atmosphere? If they go into space, what happens to the temperature of the earth? If they're trapped, what happens to the temperature of the earth?


    Your letter sounds as if you read part of my web page
    but stopped in the middle. Actually, that web page answers your question. The infra-red photons are radiated upwards from the ground, and if they could just continue to space (as they can from the surface of the Moon), they would just go on and get lost in space.

        The atmosphere however absorbs those photons-- the molecules of "greenhouse gases" like water vapor, carbon dioxide and methane have "energy levels" in the infrared, able to absorb those photons. But what they absorb they can also emit, so that the infra-red bounces from one molecule to another, until it gets high enough to escape to space, around the altitude of jetliners. It's like rain hitting a forest--it does reach the ground (usually), but not before bouncing from branch to branch.

        The temperature of the Earth is balanced between what comes in and what goes out--"radiation balance" in scientific language. The process is complicated by humidity and by "convection"--hot air rising, radiating some of its heat higher up, then descending. That's why we need big computers to model the weather! And as any kid nowadays knows, adding "greenhouse gases" slows down the upwards transmission of heat (in the form of infra-red photons) and shifts the radiation balance towards a warmer Earth.

        Tell your cousin to read that section and maybe also the lesson plan linked from the top right corner.  

  6.  Separation between lines of latitude and longitude

    What is the distance in miles or kilometers between longitudes? Please and thank you!


        Depends where you are! To paraphrase Neil Armstrong, it's one small step when you are near the pole, one giant leap near the equator.

        Neglecting the ellipticity of the Earth, the length of the equator would be 40,000 kilometers (ellipticity adds about 74 km), making one degree of longitude 40,000/360 = 111.3 kilometers.

        The average radius of the Earth is R = 6371 km (a few less than the 40,000 km value gives). At latitude L, the length of a line of latitude is 6.2832 R cosL (the factor is 2π), so one degree is (6.2832 R/360) cos L or about 111 cosL kilometers. As the latitude L increases, cos L gets smaller and smaller, and for people walking around the south pole (as some do at the South Pole Scientific Station) each degree is but one small step.

    Follow up:

    How stupid can I be? Quite a lot evidently. I meant to type LATITUDE! Guess my brain was in a different place. Thank you.


        That's easier. Assuming the Earth is a sphere, a line from the north pole to the southern one and back is just as long as the equator, and contains 360 degrees of latitude. So one degree of latitude corresponds to about 111 kilometres.

        One dgree also contains 60 minutes of arc, making one minute of latitude equal to about 1.85 kilometers. That distance is defined as a "nautical mile", and a velocity of one nautical mile per hour is "one knot", the common unit for measuring speed at sea.


        Thank you so much. My nephew was interested in how much further north his home town of Red Deer, Alberta was from Kingston, Ontario. Now, with your help, it will be much easier to figure out. I know he is tricking me into doing his assignment for him, but when he calls me "his favourite aunt" what else can one do?
  7.  Motion of air: hot to cold, or high pressure to low?

    I'm sitting in chemistry class right now as I type this, and I was just told that air moves from hot to cold. I already knew that matter moves from high pressure to low, so this makes no sense; cold air is of higher pressure than high. Which is true? Does are move from hot to cold or from high pressure to low pressure, and why?


    Air (or any fluid, and matter too) moves in response to forces. If you want to understand motion, the question to ask is "what forces are involved?"

        Pressure is force per unit area (look up in the dictionary; example: pounds per square inch, the PSI of tire pressure). So given an area A and a pressure P, the force on it is AP (A times P).

        Suppose you have a cube with side equal to one unit (inch, or centimeter, or meter--whatever you work with; it helps to make a drawing here). The area A of each side is then one square unit. Suppose also that he pressure P1 in the surrounding fluid (air, water or whatever) on the cube's right side is higher than the pressure P2 on its left. So the force AP1 on the right side is greater the force AP2 on the left side. The greater force overcomes the weaker one and pushes the cube from right to left, from high pressure to lower one. That is a general rule.

        The force which moves air from hot to cold is BUOYANCY--the tendency for lighter fluids to float up when surrounded by heavier (denser) ones--for example, oil floats to the top of water. You can prove (look up in a textbook) that with a cube (say) of light fluid immersed in a denser one, the net pressure P1 on the bottom is larger than the pressure P2 on the top. So it rises.

        In the atmosphere, sunlight heats the ground, causing air there to heat up, too. Heated air expands and becomes less dense, and therefore rises (like a hot air balloon). As air rises, it expands and cools down. Such processes operate in the atmosphere all the time, so as you go higher (into high mountains, for instance), the air is cooler. As a result, "freshly heated" air rises only until its surroundings are just as cool.

        It's more complicated, but my space here is limited. For more, look up         http://www.phy6.org/stargaze/Sweather1.htm and the section which follows  

  8.  Removing "Killer Asteroids"

        I just discovered your site and fully enjoyed it. I was watching a program on Canada's Discovery channel about "Killer Asteroids" and how they might be pushed off course from a collision with the Earth. They first played with the idea of using Nuclear Weapons in a near explosion to push them to a new orbit but after the discovery of very porous meteorite material found in Northern British Columbia, it was determined that these type of Asteroids would absorb any energy from a near explosion thereby not changing their orbits.

        It was then suggested that a Solar Magnifier could be stationed close to the Asteroid and by focusing the sun's rays at the Asteroid, burn a hole which would release energy pushing the Asteroid in to a different orbit away from Earth.

        I then had the idea of an Ion Rocket landed on the surface of the Asteroid and used to gradually increase the push away from Earth's orbit, perhaps completely out of the Solar System. Could an Ion Rocket be used for such an application? Would it be to cost prohibitive? If needed, could not the Sun's rays be used to power the Ion Rocket?

        This is what brought me to your site for research on Ion Rockets. Your thoughts on this would be greatly appreciated as the program did not even mention it.


        I do not know who is responsible for the program on Canada's Discovery Channel, but your message suggests some rather loose thinking. The basis for ANY changes of asteroid orbit (as well for rocket design, and a lot more) is Newton's third law:
    "Forces always arise in pairs, equal in strength but opposite in direction. If object A exerts a force F on object B, object B always exerts a force -F (same strength, opposite direction) on object A."

        A nuclear explosion near an asteroid would vaporize the region closest to it, and the vaporized rock would stream away from the asteroid. By Newton's law, the force moving those vaporized molecules away from the asteroid would be exactly matched by a force pushing the asteroid in the opposite direction. Whether the asteroid is hard or soft makes no difference: the momentum transmitted to the asteroid is the same.

        If that would be enough to save the Earth depends on circumstances. Even small asteroids are very massive, and their orbit would probably be modified only slightly. However, if one can intercept an asteroid years before its expected impact on Earth, that may be enough. Of course, even then it would still be in an orbit which can some day again come close to Earth (unless one can make it hit the Moon).

        If the asteroid material is too loose, the vapors released (they blow in all directions) may also blow some of it apart. Indeed, a bomb dug into the asteroid may conceivably blast it apart completely, so instead of a single solid impact with Earth, we may end up with multiple impacts by many smaller objects. These may perhaps burn up in the atmosphere, but would still radiate a lot of heat. Whether that makes the impact less destructive or more so, I am not sure.

        A "solar magnifier" does not exist, and I cannot imagine it. A large light solar mirror will be quickly pushed away by sunlight pressure... and anyway what would it accomplish? If you want to equal the effect of a nuclear bomb by evaporating part of the surface, you need deliver a comparable amount of energy. Hard to imagine!

        Ion rockets are fragile devices whose thrust is measured in grams. Not enough to move an asteroid of billions of tons! Plus, they need a source of material to create the ions, and a source of energy to accelerate them. Solar energy, again, may suffice to accelerate a spacecraft (over months of time), but not a humongous hunk of rock.  

  9.  Strange light seen from Hawaii

        I live in Laie, HI. I looked out toward the east (the ocean) and noticed a bright red star. As I was looking at that star, I saw a meteor, or at least I think is a meteor. It was reddish orange in color and looked very close to the earth. It was much larger and broader looking than the meteors I'm accustomed to seeing, which are usually long, and thin. Any answers??


    It would be very hard to tell what you saw. Did it move? It could be an airplane or satellite.

        Meteorites arriving near the observer would not be reddish or orange--by the time they cool down to emit that color, they are already quite dim.

        If I were to guess, you may have seen a flare dropped by a military airplane or fired from a ship, as part of some training exercise or a rescue effort. Flares are very bright and are meant to illuminate large areas below, and they descend slowly by parachute. They burn with a bright, white light (produced by magnesium, or by some other burnable metal, as in a firebomb), but since the one you saw was close to the ground, it was probably far off--perhaps beyond the horizon, made visible only by the bending of its light in the atmosphere.

        As you know from the setting Sun, light near the horizon, which passes through a thick atmospheric layer, tends to take a reddish-orange color. Maybe that explains what you saw. You may ask someone in the coastguard about it.  

  10.  Is the Sun attached to another star?

    Good day Dr. Stern,
    Would you let me know if our Sun has a central Sun? Does our Sun have a planetary orbit around another star?
    Many thanks for your outstanding web page,


    As far as I know, the answer is no. If any ordinary star existed close enough to hold the Sun, it would be the brightest one in the sky, by far. It could of course be a burned-out star, or a black hole (in which case, the motion, strictly speaking would not be around that other object, but against the common center of gravity).

        The problem is that the Sun and the solar system are moving through space towards the "solar apex" (look it up) at about 20 km/sec. That is more than the orbital velocity around any distant object. Of course, that distant object could move at the same velocity, too.

        In my estimation, chances are very much against it. Another clue may come from tracking distant space probes: do they indicate an influence other than the gravity of the Sun and the known planets? It turns out that a small discrepancy exists with the Pioneer spacecraft, but no one has yet mentioned another star as its source.

        So take it from there. My guess is, unless clear proof arrives, we are alone in space.  

  11.  What if the Sun turned into a black hole?

    What would happen to the Earth if the Sun's mass suddenly collapsed to within its Schwarzschild radius and became a black hole? I know the Sun is not large enough to actually collapse to a blackhole, but it would be interesting to know what would happen to a planet orbiting a star that did become one. Would it spiral inward to be engulfed by the black hole or just move into a much smaller orbit? I'm very interested in the answer, especially after reading your articles on how hard it would be to actually reach the Sun from the Earth.


    I have no special expertise in those areas, but I believe the gravitational attraction would remain the same, and therefore, so would the Earth's orbit.

        Of course, life would quickly end. After being burned crisp by the energy released from the collapse, Earth would get very, very cold.

        Stars near a black hole do not spiral inwards. A huge black hole exists near the center of our galaxy, and quite a few stars orbit it (in Keplerian ellipses), and apparently have been doing so for a long time. In fact, I recall reading somewhere recently that new stars appear to be forming at a high rate in that vicinity. For more, see http://www.phy6.org/stargaze/Sblkhole.htm.  

  12.  Do absorption lines have a Doppler shift?

        We are studying atomic physics at school which involves the electromagnetic spectrum. We have been shown the absorption spectra for various substances. We have also been shown how if an object like a star is moving towards Earth the Fraunhofer lines will shift slightly to the blue end of the spectrum in a blue shift, and if the star is moving away the Fraunhofer lines will shift slightly to the red end of the spectrum in a red shift.

        My question is why is it the Fraunhofer lines which shift, as I would have thought the atoms in the atmosphere of both Earth and the star would absorb the same frequency of light regardless of the movement of the star or Earth? I asked my physics teacher and he did not know the answer.


        Imagine an absorption line in the spectrum of a star. To its right and left in the spectrum are bright emissions, which get shifted by the Doppler effect due to the relative motion between the star and us, in the same direction (to the red or the blue). Would the dark gap between them shift the same way?

        It would, if the dark absorption feature is caused by gas at the star. It would not, if it is caused by absorbing gas in the atmosphere of the Earth, which is one of the ways we can tell apart absorption that happens THERE from the one happening HERE.

        Again, physics adds some interesting details. You know that spectral frequencies are very, very narrowly defined. One Fraunhofer line due to calcium, for instance, has a wavelength 422.6742 nanometer, defined to 7 decimal figures. However, if you split sunlight in a spectroscope and look for spectral emission lines from various elements, you will find each covers a certain spread of wavelengths. One reason is the Doppler effect: the solar envelope is hot, which means its atoms have large random velocities. Some happen to be approaching us when they emit light, some are moving away, and the Doppler effect shifts their wavelengths in opposite directions. As a result, when the wavelength spread of a spectral line is examined (and instruments exist--interferometers--which do so very well) we get a sort of bell-shaped curve, peaking at the appropriate wavelength, but spread to both sides (a pressure effect can also contribute).

        However, there may be something interesting about this curve. Sometimes we see a dip right at the peak, because above that emitting region, there may be a cooler absorbing region (temperature may go down with height for a short distance before--on the Sun--it goes up again). The absorption line is narrower because, being cooler, it has less of a Doppler effect.

        You must have a good school, if it teaches atomic physics! You can find more of it in "Stargazers", the section on the Sun.  

  13.  What are "Electromagnetic Waves"?

        I just wanted to know how electromagnetic radiation is used to gain information about the universe. I can't seem to find what I am looking for anywhere.


    Scientists (unfortunately) use a somewhat technical language, and often do not realize that this creates problems for non-scientists.

        Electromagnetic radiation is another name for light, and for wave phenomena which are similar to light but with longer or shorter wavelengths: radio, microwave, infra-red, ultra-violet, X-rays and gamma rays are all electromagnetic waves.

        So electromagnetic waves are the ONLY source (with a few exotic exceptions) through which we get information about the universe outside Earth.

        The astronomer Martin Harwit used this fact to ask, what is the maximum information such waves CAN give us about the universe? He made a chart of all the wavelengths, and the resolution we can get in each (how small and faint are the details we can see), and then pointed out where unobserved "gaps" remained. Most were in wave lengths which the atmosphere does not let through, so to fill them meant setting up observatories in space. NASA's "great observatories" programs tries to fill those gaps--for instance, the "Chandra" X-ray telescope.

        What makes light etc. "electromagnetic"? The connection between light and electricity was only found by James Clerk Maxwell about 150 years ago. Look up
    and sites linked there, for the intuitive meaning of the connection. It is not a simple one, and to fully understand it you would need some rather complicated math.  

  14.  Why are the two daily tides unequal?

        Ocean tides on the west coast of the United States are mixed tides, usually two high tides and two low tides per day. The two highs and two lows are uneven. There is a higher high and a lower high, and a higher low and a lower low. Why do the lower low tides occur in the daytime during summer and in the night time during winter?


        I am just an ordinary retired physicist, specializing in magnetic fields and trapped particles in space... not in tides! Still, I tried to find out for you by asking Google about "Atlantic Tides." On the west coast, the situation is probably similar. I regret to report that the reason is rather complicated.

        A rather thorough description of tides exists on the web, starting from
    and continuing to a lot of detail (see "site map" at the bottom of the page there). Basically, the tide is a world-wide wave excited in the ocean by the gravity of the Moon and Sun, and it has two components:
            the M2 component with two peaks a day, and
            the K1 component with one such peak.
    If we just had the Moon circling the equator, that would (according to the above web page) excite the M2 wave alone, two equal peaks a day. However, the axis of the Earth is tilted, and that adds a K1 wave.

        You can see that the sum might very well give an asymmetric pair--once a day K1 adds to the M2 peak, 12 hours later it subtracts. The tilt of the northern hemisphere is opposite in the summer and winter (sunward at noon in the summer, away from the sun in winter), so maybe that explains what you claim happens.

        The exact addition of the waves depends on the relative timing of the peaks, which I believe is affected by shorelines and the shapes of the ocean bottom. These factors also add higher frequency, so the calculation of tides is a complicated art, based on large collections of tide data. I would guess you can find out a lot more from that web site.  

  15.   Why air gets cold higher up--a wrong explanation

    Today, I was researching why temperature drops as elevation increases and I came across an explanation on an ask-the-expert website that I just don't buy.

        It said that just as a baseball loses kinetic energy and gains potential energy as it rises in its trajectory, the air molecules lose kinetic energy as they rise.

        Here is the actual quote:

      "When the warm air rises, the speed of those air molecules slows down just like a ball that is thrown into the air slows down. The molecules convert their kinetic energy into potential energy when they rise into the air just like the ball did. And since temperature is a measure of kinetic energy, the lower kinetic energy means a lower temperature."


        You are right in questioning that "explanation." Actually, it IS correct above an altitude of about 100 kilometers, the approximate level of the last collision of air molecules. Since their thermal speed is much less than escape velocity, they rise in parabolas (more accurately, sections of ellipses) and ultimately fall back. That is the Earth's "exosphere."

        But where you and I live, temperature drops with altitude for a simpler reason. The Sun heats the ground, and unless this heating were to continue indefinitely (to where the oceans would boil, etc...) that heat must be returned to space. It is a complex process, involving convection and also radiation, which is emitted, absorbed and re-emitted by greenhouse gases as it works its way up, until at about 10-12 kilometers (typically) this infra-red radiation can continue to space.

        In all these processes, heat flows upwards, and as we all know, heat only flows (unaided) from higher temperature to a lower one.

        For more, see
    See also question #207 on this page.

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

Last updated 10 May 2005