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.
396A Posssibility of Asteroid Hitting Earth (1)
- Tapping Atmospheric Electricity
- Global Disaster in 2012?
- What's the difference between speed and velocity?
- Effect of Gravity on Electromagnetic Waves
- Why is North the reference, not South?
- The lowest 700 km of our Atmosphere
- Doomsday 2012?
- Where does a Flying Bird get its Support?
- Why does Sun seem to move?
- Why don't waves disturb each other?
- Does the Moon's motion Change?
- Big Dipper and Weather
- What IS the Ecliptic?
- Precession, Greenhouse and more...
- Latest Sunrise, Earliest Sunset
- Falling off the Earth's Bottom?
- Rolling down a slope
- Pelton Wheel Efficiency
- Energy loss rate of our Sun
- The Sun's distance
- Why does sunlight have a continuous spectrum?
- Harry Paul Sprain's perpetual Motion Device
- Can the plasma that fills space help spaceflight?
- Spiral arms of our galaxy
- What powers a glider?
- Maximum speed for propeller-driven airplane?
- The speed at which gravity spreads
- Layers of the Earth
- Why doesn't the sky fall on us?
- Imagine a non-rotating Earth
- Flying east with a 1° error
- US Flag on the Moon
- Rope stretched across a long lake...
- About studying electronics in the USA
- Publish Stargazers as a book?
- Charging of Earth by lightning: + or – ?
- If no stars were seen--could Earth's orbital motion be discovered?
- Local Solar Time in Iceland
- Is orbital motion same as free fall?
- Build a straight tower in warped space?
- The 2011-2 sunspot maximum
- Is Earth adding mass?
- Shape of a "bottle rocket"
- Fantasy spaceflight vs. reality
- Telling a 7-year old about stars "dying"
- (1) Why is lightning jagged? (2) What did Tesla do?
- How can the north wall of my house be in sunlight?
- Heating the inside of Earth
- Dawning of the Age of Aquarius
- Why is hydrogen the fuel of choice?
- Gamma ray bursts
- Counter-clockwise swirling motion in the atmosphere
- Leap years on the Jewish calendar
- Earth axis tilt and climate (1)
- Earth axis tilt and climate (2)
- Does the Sun move?
- Can the Sun interfere with the visibility of the Big Dipper?
- Why can't we feel the Earth's rotation?
- Is our galaxy held together by a central black hole?
- Why don't gas planets just evaporate?
- Source of the Sun's energy
- Point of gravity equilibrium
- Solar system motion through the Galaxy
- Spaceflight by waiting for the Earth to turn?
- Does Earth rotation affect size of creatures?
- The year 2012
- Are our galactic arms winding up or opening up?
- Advice to a home-schooling mother
- Are all stars we see suns?
- Newton's 3rd law
396B Posssibility of Asteroid Hitting Earth (2)
- Author's IQ
- Global Warming caused by Sun getting nearer?
- The year 2012
- Empty space behind North Star?
- The year 2012 and the 26,000 year cycle
- "Quarter Squares"
- Do Rockets need "something to push against"?
- Sunrise-Sunset asymmetry
- Re-entry from orbit
- Earth Axis and Gravity
- The year 2012
- Choosing an Aerospace Career
- Advice for "new" physics teacher
- Paradox of Time Travel
- Is 7th grade Earth science boring?
- Air Resistance
- Experimenting with Microwave Oven
- Is the Sun losing mass?
- Sun's position at noon, south of equator
- Fred Hoyle's theory of the Sun's Corona
- Why don't Protons and Electrons combine?
- Days in a Year
- Position of the Moon in the sky
- Earth crossing Galactic Equator?
- Is the geocentric theory ruled out?
- Does the Southern Sky have a Pole Star?
- What is the fate of starlight energy?
- Double-slit diffraction of particles
- Does the Sun overhead reduce effective weight?
- Evidence for Global Warming ?
- Distances to the Equator
- Weight on the Equator and at the Pole
- Why doesn't gravity overcome buoyancy?
- The Prime Meridian
- Is L2 in the Earth's Shadow?
- In what direction is Israel from NY?
- Is the World Overpopulated?
- Magnetic Carnot Cycle
- Defining the Equator
- Second Moon for Earth?
- The year 2012 and a distant companion of our Sun
- Distance between two points on a sphere
- Getting sucked I by Gravity
- The work of Nikola Tesla
- Firing a cannon straight up
- Does one see half the sky--or more, or less?
- Why are nights dark?
- The origin of the Solar System
- Flying to other planets
- The heat of the Sun's corona
- Crossing our galaxy's equator
- Newton's 3rd law--in statics and dynamics
- Barycenter of Earth-Moon system
- What if Earth rotated in only 10 hours>
- What Book covers material of "Stargazers"?
- Climate Change
- A classroom Demonstration
- Why doesn't the Solar System Collapse?
- A Diamond Core for Jupiter?
- Why no "artificial gravity" on the space station?
- Precession of the axis of Mars
- Does the axis of the Sun precess?
- Can a microwave oven produce 900 deg Celsius?
- Beaming energy through space
- The year 2012
- The Earth's Precession Cycle
- The empty space inside an atom
366. The 2011-2 sunspot maximum
I watched this show on the Discovery network about sun storms and it left me a little scared. It was about the effects it could have on earth and all the damages it can cause. The picture they painted was apocalyptic. Should we be worried about the 24th solar cycle when it reaches its maximum around 2011 or 2012? What could happen to the earth and to us? I would appreciate your answer.
The sunspot cycle has marginal effects on life on Earth. One important effect is higher flux of soft X-rays, which heats the upper atmosphere and expands it, causing faster decay in the orbits of low-altitude satellites. The effect on cosmic rays amounts to a few percent (depending on what is measured) and big magnetic storms are few, random and if they prefer any time, it is a few years past the maximum
I have not seen the "Discovery" program, but anything beyond the above is probably unscientific. The next cycle is predicted to have a high peak
but not as high as the peak observed during the international geophysical year (IGY) in 1957-8.
There have been some wild rumors (in the public and in some TV programs) predicting a catastrophe in the year 2012, at the end of the large cycle of the Maya calendar. I have received many inquiries about that, and some are linked from the end of
For effects of magnetic storms, see
Other pages in that web collection discuss sunspots and their cycle, as does the collection with home page
367. Is Earth adding mass?
Is the mass of the Earth increasing? I wonder when I read about ancient cities being excavated under tons of earth. Is this top layer accumulated from space debris or simply existing matter displaced from other areas of the Earth? The Grand Canyon is another example. The different layers of the canyon walls relate to different time periods, was the circumference of the Earth smaller at these earlier geological periods?
If the Earth is accumulating more mass how will this effect our orbit around the Sun?
I doubt the mass is changing significantly--if it did, that might show up in the rotation period of Earth, which is monitored very closely (enough to track changes due to tides caused by the Moon's and Sun's gravity).
Buried cities are often covered by wind-blown soil (and sometimes by their own debris, as earthquakes, wars and storms destroy buildings and people later build on top of the rubble).
The Grand Canyon eroded gradually, and the debris was swept downstream. Right now its path is blocked by big dams, and it is estimated these will silt up in a couple of centuries (as would other dams world-wide, e.g. the "High Dam" in Aswan, on the Nile in Egypt).
The San Juan River in Utah flows into the Grand Canyon and has carved an impressive canyon of its own (search "Goosenecks of the San Juan"). The interesting feature is that this canyon twists like a snake. Such twisting is often seen in shallow flood plains (e.g. of the Mississippi) and is caused by the curvature eroding its outer curve and depositing silt on the inside. The San Juan must have been a shallow river when those "goosenecks" were first created, then the land gradually lifted and the river bed dug in. The Grand Canyon is similarly believed to have arisen by the gradual lifting of the surrounding land. Other land of course may sink (e.g. Central Valley in California), though overall, the earth's mass stays more or less the same (but yes, there are meteorites arriving).
368. Shape of a "bottle rocket"
It is hard to reach my question without putting in my theory. I believe that a bottle rocket has a pointed tip for many reasons. I think that a rocket with a pointed tip goes easier through the air than one that is blunt. . . . . I've put into play Newton's laws but this is what I want to know: are there any other things that effect why a pointed tip is better than a blunt tip on a bottle rocket? I would like to include that the rocket is stabilized with wings "similar" to a trapezoid shape (as an example trapezoid is usually used in textbooks. Here is a similarity to what I mean: /=\ ).
Shock waves are not involved in a bottle rocket, but its motion follows simple conservation of momentum ("Stargazers" section 18b), which itself is a consequence of Newton's laws of motion. In general, I would advise you to study those sections (from 13 to 21, at least)--though if you are a high school senior, you probably will have other things to worry about.
The shape of the bottle rocket and the nose cone and fins are mostly designed to give it stability, so that it does not turn sideways. Its speed is so slow that air resistance is not much affected by the sharpness of its front end. Even airliners that fly at 600 mph have rounded fronts, allowing the air to divide smoothly. At low speed, air resistance actually depends more on the shape of the rear. If the shape is fishlike, closing smoothly in the rear, the air flow also closes smoothly and its pressure on the rear balances the pressure on the front. If the rear is suddenly cut off like a box (as in big trucks), air swirls around and those swirls have less pressure, so extra energy is needed to overcome the difference in pressure between front and back.
Everything changes when one passes the speed of sound--about 1000 feet per second in air. At such speeds air cannot move fast enough to get out of the way and is compressed in a way that heats is. Such heating creates a serious energy loss which cannot be recovered by anything happening in the rear, so resistance becomes much higher. Space rockets and bullets have sharp tips to reduce the region where air is compressed, and that reduces the loss of energy to shock formation. On the other hand, the reentry capsule of Apollo astronauts was wide in front to create a big shock and help the capsule start losing energy and speed even high up where the air is still very rarefied. It all depends on what you want!
369. Fantasy spaceflight vs. reality
A question that keeps burning in my head is: why don't we have interplanetary/stellar ships yet? I watch TV shows like Stargate SG1/Atlantis and Babylon 5 and ask myself why have'nt we got there yet? I mean on "Stargate" their first ship was during 2002-2004 and looks fairly easy to construct using resources we have here already. So I wonder, why don't we have star ships, are we capable of doing so?
Video shows may present beautiful illusions--people flying, reading thoughts, sensing the future... but that does not mean they are possible. Sending people to Mars takes awesome rocket power, much beyond what we have (even if we had a good reason to do so), and I calculated such a flight on my web site.
Even the Apollo landing on the Moon was a near-miracle. Have you watched on TV pole vaulting (as you may do at the coming Olympic games)? An athlete run towards a hurdle holding a long pole ahead, sticks it in the ground at the proper spot and flies high in the air. If he is lucky and fast enough, he skims half an inch above the hurdle without touching it, before falling down onto a big cushion. His jump is just barely possible--like the Apollo landing--and even then, it may fail, like the flight of Apollo 13. All spaceflight is barely possible! Hollywood stimulates our imagination by staging feats which are clearly beyond what we are capable of now (or sometimes, ever): enjoy those shows, but don't confuse them with reality!
370. Telling a 7-year old about stars "dying"
... few days ago, after having fun looking at stars in the night over our homeplace in Casalecchio di Reno (Bologna - Italy), I tried to explain to my 7 years old daughter the clear idea (... clear to me), that the star we're looking at, just now, ... maybe is already dead, since a long time ago.
She didn't really understand very well the fact, ... so since then she asks me every day to explain again and again, why stars die, why I can see light, ... from something which is already switched off, ...??
Can you help me, also with suggestion of book about Astronomy for children?
Maybe the word "dead" confuses your daughter. The star is still there, but its "fire" has gone out, after using up all its fuel, in the time taken for its light to reach us. Similarly, when the NASA "Phoenix" landed on Mars, yesterday or the day before, it took 15 minutes before Earth received the information.
Any book for your daughter should be in Italian (although she can read an Italian translation of my web pages). I suggest you contact an amateur astronomy club near you--there exist 178 in Italy. See
371. (1) Why is lightning jagged? (2) What did Tesla do?
How are you and thanks for the information, I signed as James Joule but my real name is (given) . I'm an electrician from Sheffield England, I've always called myself James on the internet because they say not to put your real name out there, and I admire the chap so I borrowed his name. I spent 4 years at college and did just enough to pass (I had other things on my mind in those days), we studied Maxwell's equations very briefly but Tesla was never mentioned. I have 3 sons who are all now qualified electricians, I asked them if they had ever heard of him and none of them had, I couldn't understand why after all he accomplished and that's why I started to research his work and anything to do with electricity no matter how obscure. I am now out of control, I can't stop learning, there are too many unanswered questions; I'm not trying to solve the energy crisis, I just don't like things that I can't understand.
Another question: if electricity takes the easiest path, why doesn't a bolt of lightning strike in a dead straight line to earth?
Take a newspaper page which has not been folded and rip it apart by pulling the two sides. It almost always tears along an irregular line, not a straight one. Why? When your pull gets strong enough (especially if the pulls of the two hands are not exactly opposed), somewhere the edge of the paper will tear, and the tear will then advance across the page. However, the advance of the tear does not depend on the shape of the part of the tear already existing, only on the forces at the tip of the tear. These generally advance across the page, but may slant this way or that, depending on the way tension is distributed across the page, and maybe also on the alignment of the fibers.
It is similar in lightning. The top of a thunderstorm cloud is usually charged positive, and the bottom (and ground) are negative in comparison to it--creating a certain electric tension, a voltage drop. In an electric conductor, a voltage drop causes a current to flow. Air, however is an insulator, allowing the voltage to build up, to millions of volts.
However, as you surely know, even an insulator can "break down" when the voltage drop is large enough to tear some electrons off their atoms. Strictly speaking, it is the electric field E which does it, a vector with direction, because the force on an electric charge q is Eq; E itself is the rate at which the voltage V drops off, its magnitude is dV/dx with x in the direction in which V varies most, and its full value (you studied Maxwell's equations once, no?) is –gradV (the minus because the force is from high voltage to low).
If the voltage drop across the cloud is uniform, the air insulation will contain it. If however some of it is concentrated somewhere (influenced by the distribution of updraft winds which create the voltage, and who knows, maybe other factors), a local breakdown will develop, creating a short channel of hot plasma--the leader of the stroke, like the first tear in the newspaper page. The plasma conducts electricity, so the high voltage moves to its tip, where a second tear opens up, microseconds later, and then a third, all the way to the ground. Each tear may have a slightly different direction, hence this is called the "stepped leader", a term you can look up on the web.
The stepped leader defines the path of the lightning, and is usually jagged. High-speed cameras can capture it--it is quite dim and carries relatively little current. However, once it has established a path of electric conduction, the main stroke follows--usually from the ground to the cloud--and it may repeat several times over the same path, which is now very hot and therefore very bright, and able to conduct a large electric current. That is why lightning at times may seem to last a good fraction of a second.
I have written about lightning in "Stargazers" and you might look it up again. As for Tesla... look up "Who invented AC?" at
including the questioner's response. I suspect he knows more about this matter than me!
372. How can the north wall of my house be in sunlight?
I read an article you wrote on NASA's site entitled The Angle of the Sun's Rays. In it, as expected, you explained that the sun is always in the southern sky in North America. Since I live in Colorado (and specifically one block south of the 40th parallel and therefore well north of the Tropic of Cancer) I can't figure out why the sun hits the north side of my house? To be sure, my compass shows my house facing due west. What am I missing?
In Colorado the Sun always crosses the southern sky, and achieves its greatest distance from the horizon due south. However, its path during the day takes it to other directions as well.
As the drawing on section (3a) of "Stargazers" shows, and as is also explained in section 1, the Sun rises exactly in the East and sets exactly in the west at equinox (March 21 or September 22, typically). In the summer or spring, between those dates, it rises to the north of east and sets to the north of west, and therefore, in the morning or late afternoon, you can have sunlight on the north wall of your house (though never at noon).
By the way, though I am retired from NASA, this is not a NASA site.
373. Heating the inside of Earth
Reviewing your article on the earth's magnetic field (http://www.phy6.org/earthmag/dynamos2.htm), how much heat can be attributed to gravitational and frictional forces at work in the earth's (or other planets') core?
Heat is a form of energy. Frictional forces generate heat at the expense of motion, and the motion itself (creating the magnetic dynamo in the Earth's core) is the result of heat-generated circulation, so it is probably a small factor (motions in the Earth's core are exceedingly slow, anyway).
Gravitational forces create heat only if the Earth shrinks (=everything is falling down to lower altitude). Stars can release a lot of energy that way, but I think the Earth is done with shrinking. Gravitational squeezing heats the interior of the satellite Io of Jupiter
but the effect here seems small.
So what remains? Mainly radioactivity: it is a very diffuse source of energy, it occurs mostly in the top hundred miles of the crust (if it were to occur at deeper layers, the interior could heat past its boiling point!), and it is effective only because the Earth is so large and therefore heat has difficulty escaping. See
It has been suggested that condensation of molten iron onto the solid inner core also contributes, but it's a secondary factor.
374. Dawning of the Age of Aquarius
I have a question about the age of Aquarius, the precession explanation does not say when the Age of Aquarius is supposed to begin exactly?
As for the age of Aquarius, I don't know. The ancients defined constellations by their brightest stars, and drew no exact borders. Modern star charts have exact boundaries, usually straight lines like the boundaries of states in the western US, but I don't know when the spring equinox crossed the modern Pisces/Aquarius boundary. Astrologers, anyway, ignore precession: see
375. Why is hydrogen the fuel of choice?
I was looking over your question and reply segment of your web site and a physics question came to mind. Why is hydrogen the most selected medium for fuel cells. I am probably missing something but if the objective is to strip electrons from the hydrogen atoms, would a more electron-rich element provide more energy in a smaller package, for example calcium.
Your question involves chemistry, which is not my field, but please realize that the binding energy of all electrons is not equal.
Looking just at the stripping of electrons, hydrogen certainly has two big advantages over calcium. First, it is lighter--most of the mass of the hydrogen atom is that of its proton, while calcium contains about an equal number of neutrons, which bind no electrons and just add weight. Second (and probably much more important), only the outermost electrons play a role in chemical reactions. Electrons closer to the nucleus are too tightly bound to do that, though the protons that bind them still add mass.
376. Gamma ray bursts
I was wondering if you could explain to me what gamma ray bursts are? I've read various things about them but I can't really get the gist of it...I understand that some think such a burst may have affected Earth in the past...could this happen again?
If gamma rays had not been observed, I doubt anyone would have predicted them, and even now a lot of them is not understood. I wrote about them in
and as explained there, at least two kinds exist--those in our own galaxy, thought to come from the supernova collapse of strongly magnetized stars ("magnetars"), and those coming from distant galaxies--obviously, involving much greater energy. Possibly, the latter class was emitted when large black holes formed at the center of those galaxies--see
They may be very old, relics of the earlier universe.
A new satellite, GLAST, is just starting to study gamma ray bursts
with higher resolution and larger field of view than earlier ones (it was since then renamed the Fermi observatory). Astronomers hope it will provide us with new clues.
377. Counter-clockwise swirling motion in the atmosphere
(From a physics teachers' listserve)
Pressure is a scalar [not a vector--it has magnitude but no direction] I don't see how a
scalar can produce any chiral [swirling] effect (clockwise OR counterclockwise).
The force on the air comes from a pressure gradient which is a vector,
but it is a curl-free vector field (funny thing about gradients) and I don't see how such a field could produce a chiral effect either.
Some of what appears below contains guesswork. If there is any error, I would like to be at least the second person to know, so please tell if you spot one.
Fluid motions can be divided into two classes: solenoidal (swirling) and irrotational (pressure related, associated with sources and sinks). There exist some ambiguous flows which can be assigned to either class, but they are a special group (source-free flows) and I suspect they can only exist in limited regions of space.
The classification is explained in high school terms in
in the last part of that web page titled "Background Concept: Vector Fields"
I suspect that atmospheric flows are basically solenoidal--swirling--and that areas of higher and lower pressure--so popular among weather forecasters--are but a secondary manifestation. They are simple indications of what the large scale flow is doing--and useful, too, since barometers are cheap, and they make local observations of pressure easy. However, local pressure is just a simple way of guessing the large-scale flow pattern, which is the real origin of weather phenomena.
The large-scale pattern is solenoidal, but it is NOT just swirling around low pressure areas. Instead, it has a global scale, going around the globe--from west to east in middle latitudes, from east to west near the equator. What creates the low pressure areas at middle latitudes are those big waves in the global flow (the weatherman says "in the jetstream" but that is because the jetstream is the fast-moving core of the global wind and thus a good indication of the location of its center). They are known as Rossby waves, and they help spread heat from equatorial latitudes to middle latitudes, involving a larger area of Earth in the return of solar heat to space.
I wrote about that in http://www.phy6.org/stargaze/Sweather2.htm
378. Leap years on the Jewish calendar
Please inform me if you can direct my search for an index with the Jewish leap years for the 20th century and the start of this one.
For some reason, my internet searches are not proving successful.
The Jewish calendar goes by 19-year cycles--the calendar year 2008 is numbered 5768 and at the end of September 5769 begins. The current cycle is #303 and began in 5757 (5757:19=303 with no remainder).
Each cycle contains 7 leap years--numbers 3, 6, 8,11, 14, 17 and 19 of the cycle (19 at the start of the next cycle). The number 5768 leaves a remainder 11 when divided by 19 and is therefore a leap year. The year 5769 leaves a remainder 12 and isn't. The calendar has additional adjustments to prevent, say, the Day of Atonement (Yom Kippur) from falling on Friday or Sunday, creating too much strain over a two-day period.
379. Earth axis tilt and climate (1)
What would happen if the earth's axis was changed, for instance, if it went from being tilted to being straight up [perpendicular to the plane of the ecliptic]. What types of things would happen to the earth aside from the days being the same length as the night and the seasons pretty much disappearing? Do we know what might happen as far as land mass, flooding, new land being exposed, anything like that?
Also, do we know if this could ever happen, and if so, what would cause this?
No, it is extremely unlikely. Any event energetic enough to cause it (e.g. collision with a planetary object encountering the solar system--though no such object was ever observed) would probably smash the Earth. The rotation of the Earth is governed by the conservation of angular momentum, a consequence of Newton's laws of motion, and to change it appreciably would take enormous force.
Because the Earth's rotation, its equator bulges a few extra miles, and the pull of the Moon on that bulge causes the axis to wobble in an approximately periodic fashion, but only by a tiny amount. Given a good telescope, that wobble is just barely detectable. The pull of the sun on the bulge should have a somewhat similar effect.
380. Earth axis tilt and climate (2)
Is the Earth's Tilt increasing?
I recently learned (from my mother - and I ALWAYS believe my mom) that the
Navajo tribe (in northern Arizona) believes in global warming. The medicine
men state that the Earth's tilt has increased. They say that the sun is
rising further north than it has traditionally and the star alignment has
changed. Thus, the Navajo believe that this is causing greater solar
exposure at the poles, resulting in the rising temperatures.
The observed shifts in tilt angle are of order 0.1 degree or less, due to disturbance of Earth's motion by gravity of the Moon, etc. That gravity does cause the pole of the heavens to rotate slowly around a large circle, as someone has already written to you, and it causes slow changes in the stars seen in the night sky, on any given date. A good simple discussion is at
It does not change the apparent motion of the Sun itself, and no such change is known.
The Sun heats the surface of the Earth, and in an equlibrium, an equal amount of heat must be radiated back to space--if it were retained, the oceans would gradually boil away. Global warming seems to originate in processes which interfere with the return of heat, due to the presence of complex molecules. It occurs naturally due to water vapor (a good thing--otherwise, much of the Earth would be too cold for us!), but carbon dioxide created by human activity adds to this and creates a gradual warming.
381. Does the Sun move?
I am working as a high school teacher in Mysore, South India .
I got a glimpse of your site StarFAQs. It was very useful. Thanks, but
I have a question not answered there.
Does the Sun move ? If so what is its path ?
I would be much obliged to have a reply .
Any motion is relative to something. You walk inside an airliner: what are you moving relative to? Relative to your seat, you may be moving at 1 meter/second. Relative to the ground, you maybe moving at 250 meter/second. Relative to the Moon it is again different, because you are attached to a rotating Earth (and the Moon also moves). And so on
In "classical" mechanics, you can always add a CONSTANT velocity (constant in direction and speed) to any motion and the result will still be correct--any velocity is just derived relative to a different point, one which movies with a different constant velocity. It is a fact of the universe ("the principle of relativity") that no matter where the point is chosen, you get the same physics.
On the other hand, classical mechanics gives a definite velocity to motion with acceleration (to which any constant velocity can be added). In a car moving around a curve or coming to a sudden stop, acceleration occurs, as determined by the motion, related to the forces which control the motion.
The Sun definitely accelerates, but again we can ask, relative to what should we calculate its motion? Inside the solar system, it is not true that Earth orbits the center of the Sun; it orbits the center of gravity (CG) of the Sun-Earth system, which is close to the Sun's center (because the Sun is much heavier). See
No, that is wrong, too: other planets must also be taken into account, and it is more correct to say that Earth (and Jupiter, etc.) all orbit the CG of the solar system, which changes as the planets move, but is always close to the Sun's center.
You may also ask--does the CG of the solar system itself move, relative to stars of our galaxy? It does, orbiting the center of our galaxy, though (on the scale of the galaxy) very slowly See
The motion is at 15-20 km/sec towards a point known as the Solar Apex. Look up that term on a search engine. Of course, our galaxy itself (the Milky Way galaxy) moves on the average away from all others, due to the expansion of the universe.
382. Can the Sun interfere with the visibility of the Great Dipper??
If the earth rotates around the sun as suggested, how is it possible that we can see the same star constellations in the sky all year round? It does not sound possible. If I can see the Big Dipper during the summer months, then it should be hidden by the sun during the winter months, right?
Depends on the location of the constellation! As explained in section #2 of "Stargazers," as the Earth circles the Sun once a year, the position of the Sun in the sky, on the background of distant stars, traces a circular path, covering--depending on date--all directions in the orbital plane of the Earth (see second image in that section). Any constellation located on that path will occupy the same direction as the Sun at some time of the year, and at that time cannot be easily observed. There exist 12 constellations like this, a group also known as the zodiac. Thus Scorpio is high in the sky in mid-summer but may be near the position of the Sun sometime in winter.
On the other hand, constellations located FAR from the ecliptic on the celestial map will never be in the direction of the Sun. The north pole of the ecliptic is in the constellation Draco, only 23.5 degrees from the celestial pole (the direction of the Earth axis). The big Dipper is in the same neighborhood, and its direction is therefore NEVER visited by the Sun. Sunlight may make it invisible to the eye in the daytime, but even then it is above the horizon enough hours at night.
383. Why can't we feel the Earth's rotation?
Why does a human feel stable. although Earth rotates??
I mean, how come we rotate with the Earth but feel no motion??
Forces are associated with acceleration. If you ride a car on a smooth road at constant speed and fixed direction, your motion exerts no force on you and you will feel as if you were not moving at all. The car's engine is needed to overcome friction of the road and air resistance, but inside the closed car you feel force-free.
Motion in a circle is accelerated (section #19 in "stargazers"), and an object will NOT move in a circle unless some force maintains the acceleration. But how big a force? The formula in section #19 tells you how big: the faster the rotation (turns per second, say) the greater it is, and the radius of the circle also figures
The Earth's rotation definitely exerts a force on you, but you don't feel it because it is small, a fraction of 1% compared to your weight (sect #24a). So the force responsible for your motion is hidden by the much greater force of gravity, making gravity appear a little stronger in some places and a little weaker in others. It can be observed by comparing the speed of pendulum clocks, which respond to the total downward force at their locations, and this was already done In the 1600s.
Some hotels have fancy restaurants sitting on turntables on top of the roof, with windows all around (entrances are all in a small tower in the middle). They take something like half an hour to go around, and because the rotation is so slow and smooth, you don't feel it at all. I once ate at such a restaurant in Montreal, Canada, and all I felt was that the view from the window gradually changed.
384. Is our galaxy held together by a central black hole?
I would like to know if it's true to say that the black-hole at the center of the galaxy maintains the galaxy in its cohesion.
Actually, this is an area of astronomy poorly understood. There certainly exists a concentration of mass near the center of the galaxy, and we now know that some of that mass is concentrated in a giant black hole--see
Fifty years ago astronomers believed that the gravity of all this mass is what holds the galaxy together, the way the gravity of the Sun holds the solar system together, and that it is disk-shaped because it rotates--again, like the solar system.
The trouble is, the solar system obeys Kepler's 3rd law (see "Stargazers"), as expected from Newton's law of gravity, and galaxies (including our own) DO NOT. For details, see
The conclusion follows from monitoring the rotation speed of the galaxy at various distances from its center (using the Doppler effect, described in the two sections preceding the one linked above). It follows that while the attraction of the central black hole--and the dense collection of stars near it--helps hold the galaxy together, it is not the only factor. Astronomers are still debating what the other factor may be--unseen "dark mass" or perhaps corrections to Newton's law of gravity, evident only at great distances.
385. Why don't gas planets just evaporate?
A student of mine wanted to know why a gas giant planet didn't just evaporate off into space - I wasn't sure ! Can you help?
Because the planet's gravity holds the gas down! The student may just as well ask why our own atmosphere does not escape into space!
Of course, that gravity better be strong enough, because if gas molecules (energized by heat of sunlight) have enough speed, a weak gravity may not be able to hold onto them and they fly off anyway. As is the case with the Moon, Mercury and asteroids.
386. Source of the Sun's energy
I need help explaining to a 7th grade student where the sun gets its energy. I have read your website, and although I understand what you are saying, I am having trouble trying to explain it in terms a pre-teen would understand. I thought maybe you could condense what you wrote and give a few ideas on how to do this.
The question arose from our conversation surrounding The Law of Conservation of Energy. As always, my students try to find exceptions to the law. I love that they do this; however, they do ask questions that I find hard to explain. This is one of them. In the past, I have just given a general answer and moved on. However, teens are very perceptive and often know if someone is glossing over an answer or doesn't really know the answer. I know its okay to say "I don't know" to them, but I'd like to be a model of someone who seeks out things they don't understand.
Let me try, but we must start with some assumptions. It would take too much time and space to justify them.
The assumptions are that matter is made up of atoms, and each atom contains a nucleus with a positive electric charge, where most of the mass is concentrated, and lightweight negative electrons (thousands of times lighter), bound to them by electric attraction.
Also, that nuclei consist of two kinds of particles, very similar in mass and other properties--protons and neutrons. The difference is that protons are positively charged and neutrons are uncharged, and at least the smaller nuclei tend to have equal numbers of each, e.g. oxygen has 8 and 8.
Now if oxygen has 8 protons bound together in a tiny space, there must exist some specific nuclear force attracting them together--without it, the positive charges repel each other and the whole thing falls apart. (Again, the force between two protons or neutrons--or in a pair with one of each-- is about the same, while the "equal numbers" property is enforced by the "weak nuclear force" which need not bother us here).
Such facts were found by early nuclear accelerators, examining collisions between fast nuclei. The important thing is that the nuclear force (unlike the electric force) falls off rapidly with distance, so small nuclei (like helium, 2 + 2) are strongly bound and big ones like iron, much less strongly--protons and neutrons ("nucleons") of opposite sides of the nucleus are already distant enough to weaken the force.
In fact, iron is also less stable because all its positive protons repel each other electrically, and the electric force falls off more slowly with distance. If you could add nucleons to nuclei past iron, you would have to invest more energy to overcome electric repulsion than the energy added by nuclear attraction. By the time you reach the vicinity of uranium, nuclei tend to be unstable--that's why no heavier nuclei exist in nature, and even those close to uranium tend to break up gradually by radioactivity.
But with light nuclei like protons (=hydrogen nuclei), the nuclear attraction pulls them together powerfully. A lot of energy is released by combining protons to form helium, a "nuclear fusion" process (and to keep charge balance, some protons in this process change into neutrons plus positive electrons, which crash into negative ones and "annihilate"). That's apparently what happens in the core of the Sun, where the weight of all the mass piles on top is strong enough to hold the fusing particles together, at the high temperatures required by the process. Laboratory experiments have tried to keep fusion going in hot gas held together by strong magnets, but so far not enough energy is released to provide help ease our energy shortage. For more, see
You may also look up a question similar to yours at
387. Point of gravity equilibrium
I am taking a Conceptual Integrated Science class (1 quarter) at ITT Tech. and we don't go very deep into any one branch of science, we just touch base in many, very quickly.
I am just curious if a spacecraft traveling from the Earth to the Moon can ever be pulled equally by the Earth and the Moon or if the gravity of the Earth will always remain stronger. I believe that at some point the gravitational pull must be equal but some of my classmates think the Earth's gravity will always be stronger. Please help us!
If you take the positions of the Earth and Moon at some moment and assume they are at rest at those points--then yes, there will exist some point in-between where their pulls are equal and opposite, canceling each other, and if you place a stone at that point, it will not fall in either direction.
In fact, however, everything moves. To simplify matters, let us ignore the Sun around which the system orbits, but assume the Earth is at rest, and the Moon orbits it in a near-circle. That stone will only be momentarily in equilibrium--as soon as the Moon moves away, the gravity of Earth predominates, though the pull of the departing Moon will pull it sideways to some extent. Because of the added sideways motion, the stone will not follow a straight line, but will end up in an elliptical orbit (perturbed by the Moon's gravity).
At whatever distance you place the stone, it will have its own "natural" orbital velocity around the Earth. Unfortunately, if the distance is less than that of the Moon, it will have a shorter period, move faster and again, pull away from equilibrium. There exists however one distance where the period is the SAME as that of the Moon, because the pull of the Moon subtracts from the pull of the Earth, and with less gravity, the stone moves more slowly.
The stone will then keep station with the Moon and stay in its place--if the Moon were small enough. That would be the Earth-Moon Lagrangian L1 point. I think it is not a practical orbital location, though certain orbits around it (halo orbits) could be.
There exists however an L1 point in the Sun Earth system, with similar properties, and it is in use. See
388. Solar system motion through the Galaxy
Hi, I'm from Mexico city and have several questions about climate changes.
Like the planets within a solar system, does our galaxy (or group of galaxies) turn around a bigger sun?
If so, could that be considered as a cause for the weather changes the earth is experiencing?
I appreciate your answer. Thanks! Have a nice day
In one word, no. The sun with its system, with other stars of the Milky Way galaxy, slowly rotate around the center of the galaxy (for more about that, see here), but on the scale of the galaxy that motion is slow . More important, it is hard to see how the motion of the entire solar system could affect weather changes on Earth. Such changes depend only on what happens between Earth and Sun: the rate at which the Sun supplies us with heat (which varies only very little, by 0.1% perhaps) and the rate at which the Earth loses heat to space, which DOES depend sensitively on the atmospheric content of gases such as carbon dioxide and methane--see
It is hard to imagine how our motion among distant stars may have any effect.
Our galaxy itself moves among other galaxies, but on that scale the most significant motion is probably that of all galaxies away from each other, as the universe expands because space itself grows bigger.
389. Spaceflight by waiting for the Earth to turn?
I have what seems like a simple question but I can't get a simple answer from anyone. Yours was one of the many websites I checked and it appeared to be the most thorough. I know you're busy, so I'll be brief.
Instead of flying within the earth's atmosphere, would it be possible to exit the earth's atmosphere, wait for the planet to rotate and then come back down when your destination reaches the point below you?
Example, I leave from a point on the equator. In space, I wait for the earth to rotate at 1400 mph and I return three hours later to a place 4200 miles away from my point of origin
No, this would not work. Going up and waiting is not a good way of traveling from one point on Earth to another.
Suppose you shoot a rocket vertically from the equator to an altitude of 1000 miles. It will fall right back! Not just that, but the velocity and momentum it has from rotating with the Earth is preserved. Because a point 1000 miles up is further away from the rotation axis, you need there a bit more speed to keep up with the rotation, so strictly speaking, it will lag somewhat behind the rotation and fall at a point some distance to the west.
And by the way, the velocity needed to reach 1000 miles may be enough (if you aim at angle) to reach 4200 miles away. I haven't calculated, but that is of the order of the range.
In any case, a spacecraft cannot just rise outside the atmosphere and wait. If it is not to fall back, it needs a velocity tangential to the surface of the Earth. If it is to stay up indefinitely, it needs an orbital velocity, circling the Earth in about 90 minutes (or more, for bigger orbits). That is about 24 times the speed of sound, so getting there requires an tremendous amount of fuel, and getting back an enormous amount of energy needs to be removed as heat, otherwise it will burn up in the atmosphere like "Columbia."
390. Does Earth rotation affect size of creatures?
My name is Ravi, I graduated in Canada. I read mostly all the questions and answers posted on your site. In fact, I was surfing to get answer to a question that popped up in my mind and I came across the site. The question is:
"Does the rotating speed of earth has any effect on the size of the living creatures?"
When dinosaurs walked on earth about 65 million years ago, I heard that they enjoyed 20 hours of day.
The size of creatures on Earth depends on effective gravity, including the centrifugal force associated with rotation. If gravity were stronger, creatures would be lower--assuming life were still possible.
The centrifugal force reduces effective gravity at the equator, but Earth rotates rather slowly and the effect is less than 1%. If Earth rotated faster, the effect would grow and the equator's radius would increase (see
http://www.phy6.org/stargaze/Srotfram1.htm); points on the equator would be more distant from the center, also reducing gravity. However, the effect cannot go too far before the centrifugal stress breaks up Earth.
Fifty-odd years ago the science fiction author Hal Clement (pseudonym of a physics teacher, I was told) imagined a planet of great mass and very rapid rotation, strong enough to resist break-up (no known material would do, but the idea is tempting). It is inhabited by intelligent centipede-like creatures, who sail the ocean to meet with terrestrial travelers, who can barely stand even the reduced the gravity at the equator. The sailors promise to retrieve a probe which landed near the planet's pole, where gravity is much stronger, barely tolerable even to the planet's natives. Even though the story is scientifically flawed, it is fun to read. I hope you are able to get a copy of "Mission of Gravity"!
BTW, earth rotation 65 MY ago was not much different from today's.
391. The Year 2012
The end of the Mayan calendar, et al, notwithstanding, is the earth facing a potentially devastating phenomenon on the above captioned date, when the sun is aligned with the astronomical center of our galaxy?
I've heard everything from potential pole shifts to concern about the possible impact of a huge black hole, also located near the center of our galaxy. Can you offer any scientific, empirically based comfort here?
I keep getting more and more questions about 2012, and as far as I can see, there is no scientific foundation to any of those worries. Links to some answered questions (not all, by any means!) are given at the end of
For fun, though, look up The Year of the Jackpot"
Maybe you even find the story somewhere
392. Are our galactic arms winding up or opening up?
Which way do spiral galaxies rotate, are they rotating inward (winding up) or outward (winding out)?
I am not sure, but I think they rotate in the sense of winding up.
However, even if spiral arms look the way they would if they were pieces of flexible rope (say, floating on water), the way they form is not the same. When I was active in science, the leading theory was that they were formed as "density waves." See
That was a long time ago. Please consult a working astronomer about more recent advances!
393. Advice to a home-schooling mother
I am a homeschooling mom developing a curriculum for my soon to be high schooler. What grades would you recommend this material to be used?
Homeschooling has great advantages and a possible disadvantage. The curriculum can be adapted to the interests of the student, and that is good (in moderation). However, the teacher may lack familiarity with some subjects, and teaching them then becomes a joint study, rather than guided expansion of knowledge with a knowledgeable guide.
About the first item--to what extent is your daughter or son interested in science and math? And related: (1) how fluent is that student in reading and in learning from a printed page? And (2) is the student also learning history, and liking it?
The related questions are relevant, because
The ideas are further discussed in
- (1) "Stargazers" is essentially an electronic book. No flashy pictures or videos, but a carefully worded exposition, leavened with tidbits of stories of technology and discovery.
- (2) History serves as a framework; see for instance the timeline linked from the end of every sections. You will find there, in black print events from the history of the world, in red those relevant to the text, and in the latter, links to appropriate passages.
also in "Using space to teach physics"
If your students has a keen interest in nature, and is not averse to investing effort, I would recommend that together you decide on a course of study. The first of the above web pages describes the material, and you also find descriptions in the central link page
which (at the end of the first yellow box) has links to descriptions of the contents. Copy the file on a disk as a zip archive
and make it (or a copy) freely available to the student.
Please also look up
which contains a concise course in the math used here, stopping short of calculus
Please note that not all physics is covered--measurement, levers and mechanics of solid object, heat, optics, electricity, these are sometimes included tangentially but not systematically. On the other hand it goes further into recent science that most courses--e.g. nuclear energy--and covers a lot of astronomy. An overview of what a science curriculum may contain is at
Some of this may have been covered in earlier courses. The first 10 sections are astronomy, and in particular 1 through 6 may have been covered at middle school level, at least qualitatively.
If your student does not like to work hard, and does not care about science, you have a problem. Ideally, home schooling should pool several students together, making it a social occasion as well. You may also look up
About the second angle--there are lesson plans attached to most sections, and I hope they help. Students may also enjoy reading questions and answers linked from the ends of the sections: they were selected from correspondence I receive, and give the material a certain degree of variety.
I can send you answers to the problems linked from the end of the index page. They are meant for teachers only, so see to it that students do not get the collection of answers.
394. Are all stars we see suns?
By definition, are stars that we see at night are in fact all suns? Is a glowing star a sun?
The answer is "practically, yes."
There exist a few nebulae which may appear to be stars, though the telescope shows them to be more extended--the Orion nebula in the "sword" of Orion, part of our galaxy, the great nebula of Andromeda which is a nearby galaxy, and the two Magellanic clouds, small galaxies near ours.
Also, if you view the sky away from the Milky way with a very sensitive instrument, most "stars" you see are distant galaxies. See
And finally, proper stars may differ greatly from our Sun in size, and may be double stars. But with these disclaimers, yes, you are right.
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