Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.
For a complete list, including earlier questions not listed below, click here.
You may also link from here to a listing of questions arranged by topic.
396A Posssibility of Asteroid Hitting Earth (1)
- 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
395. Newton's 3rd law
Dear Dr. P. Stern
My name is Andrew a sixth grader in Monroe, WA. I am writing to you to ask you some questions about a science project I'm doing on Newton's 3rd law. The project is due Jan 10th and I will be having a learning center Jan 22nd of the things I have learned to show my classmates
So far I have learned from Newton's 3rd law that every action has an equal and opposite action. So for example if you throw a football up in the air it must come down.
David P. Stern I was just wondering it there was anything else about Newton's 3rd law that you could share with me. Also I'm doing an experiment with three balls ties to a string and putting the left ball up to different degree and seeing if the right ball will go that same degree. Just asking if that would be a good experiment and if you have any more ideas.
You do not seem to understand the 3rd law. The example of the balls is not relevant: action and reaction are two opposite forces acting at the same time, not one first, then the other.
Go to my web site "From Stargazers to Starships" http://www,phy6.org/stargaze/Sintro.htm and read section #16 on Newton and his laws. Then you can go read other sections, e.g.#18A on Newton's 3rd law, #18b on momentum and the recoil of a cannon when fired, and #25 on the principle of the rocket. Recoil and rocket actions are two good examples of Newton's 3rd law; the examples in #18b of the rotary sprinkler, of the boat, and of falling off a log can help, too.
Your experiment with balls does not seem to relate to Newton's law. In any experiment it is important, not just that it works, but that you can explain what is happening. Separating two balls with a spring, held compressed by a string, and then cutting the string, may demonstrate the 3rd law, as in #25. You might find other examples.
396A. Posssibility of Asteroid Hitting Earth (1)
Hello, I'm Jake and I have a simple question.
Lately I've been concerned about something.
People around on the Internet and in my town say some asteroid is supposed to hit the Earth and blow us all away.
I was just wondering if maybe you could shed some light on this, because it is kind of worrying me
Will an asteroid hit the Earth sometime in the next billion years? Very likely. The image of Lake Manicougan (http://www.phy6.org/stargaze/Smoon2.htm ) shows the traces of such an impact, some 214 million years ago.
What is the likelihood of it happening in our lifetime?
Very small. All recorded history has no mention of such an event.
How big an asteroid?
Asteroids which have been tracked may be 1-5 kilometers across (the bigger, the fewer). Such an asteroid can cause great damage but will not destroy Earth or its life. Just today or yesterday the press reported a network of 4 large telescopes to survey the skies for objects down to 140 meters wide. Of course, even if one is seen to be heading for a collision with Earth, we still lack the technology to prevent such an encounter.
Smaller chunks of rock, gravel, iron and ice keep hitting Earth. During the daytime on August 10, 1972, a 200-ton meteorite skimmed the upper atmosphere above Utah, leaving a visible trail. It just missed Earth. In 1994 comet Shoemaker-Levy was observed to hit Jupiter. However, such events are rare enough to be ignored.
396B. Posssibility of Asteroid Hitting Earth (2)
Dear Dr. Stern,
Given that NEO's (Near Earth Objects) pose one of the earth's largest potential threats, I find myself constantly obsessed by the thought that one of catastrophic size will impact us at any time. I have tried to do as much research as possible to somewhat ease my mind; however, the more I find out, the more uneasy I become.
I'm aware that the funding for the search to locate as many NEO's as possible has well increased since the 1990's and even since the early 2000's. Though this bit of information is comforting, I was wondering if you had an idea of just how close we are to completing any type of test-worthy projects capable of deflecting a potential hazard away from the earth?
Also, I have read that there is no such thing as being "due" for an impact, and regardless of the dates of previous impacts, the chances of a PHA 300m+ striking the earth is about .02 percent. Is this true?
I read somewhere that the new telescope system will map the orbits of all near-Earth objects down to a size of 140 meters. Even for such objects, I doubt if we can do anything if we see such a rock (or bank of gravel, according to some) heading for Earth. The thing to do is of course shift its target from Earth to Moon; just diverting it from a close collision probably means it will menace Earth again at some later date.
Still, the chance of any of us being affected is too small to worry about. I recommend to you the book "First Light" by Richard Preston (now badly out of print--you need contact a seller of used books, such as http://www.abebooks.com/ and get it there). It tells in four parts about a year spent by the author at the Mt. Palomar observatory, full of stories about the place, its work, its people and the strange history behind it.
One part out of the four stands apart, titled "The Shoemaker Comets," about the annual pilgrimage by Caroline and Gene Shoemaker, using the 18" Schmidt telescope at Palomar to hunt for near-Earth objects. You will find there all you might want about the subject. Unfortunately, Gene is no longer with us, killed in a freak collision in the Australian outback. His memory however deserves to last throughout history.
397. Author's IQ
I was just browsing around on the whole 2012 theory when I found your site. and man I must say you're a genius...
My question has nothing to do with physics or calculation of some sort, i was just wondering what's your IQ...
Strange question I know, but I'm 20 and have this huge interest in physics but my current job has nothing to do with it, which is a shame...
Oh and and I apologize for the spelling and grammar I'm from South Africa so English isn't my language.
But it's nice meeting you sir.
I can't recall anyone measuring my IQ ("intelligence quotient"), and anyway, one's innate intelligence is only one side of personality. Persistence, ability to forge friendship, skill with your hands, ability to express yourself in words (written and spoken), capacity of true friendship, capacity to observe the world around you (which you have) and so many other talents.... They all count!
I hope you find my web sites useful and interesting, that you have a worth-while goal in life, and that you achieve it. I also hope you have a habit of reading books, and through them discovering the rest of the world.
Good luck in 2009
398. Global Warming caused by Sun getting nearer?
The world must listen to me! I'm an expert on the subject of global warming. The American governmental scientists are knowingly lying to the world about global warming. Telling the world green house gases are causing global warming, so the oil companies can continue to pump crude oil. The truth will cause a world wide economic collapse, but the alternative is far worse. The real reason for global warming is the earth's orbit around the sun is decaying, in other words the earth is moving closer to the sun. I've studied this phenomenon since July 1983, warning people of the coming destruction, and death. People called me crazy at first. I understand the weather was normal at that time, now the weather is beginning to support my findings. December 2007: Houston, Texas is experiencing warm sunny winters, a record high of 81 degrees. Eventually Houston's winters will completely disappear, as time goes on.
(The beginning of a much longer letter, pointing out melting polar icecaps etc.)
I read your concerned letter and believe me, it is wrong in more ways than one. I strongly urge you to learn more science before venturing out with catastrophic pronouncements--else you will continue to go astray.
Earth is not getting closer to the Sun: if it did, the year would become much shorter, and astronomers would notice the change long before our eyes (or Earth climate) will respond to it.
By pure chance, the visual sizes of Moon and Sun in the sky are almost the same. Actually, both vary, since the orbits of both Earth and Moon are slightly elliptic. Usually the Moon appears slightly bigger, so when it passes between us and the Sun we can get (in select areas) total eclipses of the Sun, lasting a few minutes. But sometimes it is smaller, and then we only get annular (=ring shaped) eclipses of the Sun, in which the Moon, even when crossing our line of view right in front of the center of the Sun, only leaves a bright ring all around it. If Earth were getting closer to the Sun, such eclipses should become the prevalent kind, but they don't do so.
The heat received from sunlight must equal the heat radiated back to space--otherwise the Earth might warm up to where oceans boil away. As explained elsewhere, greenhouse gases impede the flow of heat from the ground to the high atmosphere (from where it is radiated to space), and as a result, climate gets a bit warmer. But in addition, the area on the Earth involved in returning heat to space expands poleward, and that's why icecaps tend to melt.
399. The year 2012
My name is Lauren, I am 14 years old and I suffer from an anxiety disorder, so when one of my classmates declared that the 'world will end in 2012' I became extremely concerned about this, and it has turned into an increasingly obsessive topic, as it was when the LHC (Large Hadron Collider) was due to be turned on.
A few friends of mine explained that the Mayan Calendar resets on this date, a 'bot' has predicted that there will be a global catastrophe will occur (i.e. flooding, radiation) and so on.
I have researched this topic on Google, which only turned up thousands of "end of the world" theories, all of which have increased my worries and have led me to panic attacks.
Thank you for taking the time to read my email and I hope you can reassure me!
Take the advice of Douglas Adams' "The Hitchhiker's Guide to the Galaxy" and DO NOT PANIC. Yes, the Maya "long count" calendar will reset in 2012, but other than that, it promises to be a normal day--at least, no different from those preceding it or following it.
I know nothing about your "bot"-- presumably it is a web program sampling opinions--but I do not think it has any information which our instruments have missed.
I am a retired physicist, trying to enlighten the public about physics, astronomy, space and related topics. With 4 years to go, already a FLOOD of messages about 2012 is arriving at my web address. They get answered, and some of the more interesting ones (many are pretty dumb!) are answered on the web. Please go to
and you will find at the end there links to a selection of relevant questions-and-answers.
Read it, and tell your friends and teachers about it. Then go sleep soundly
400. Empty space behind North Star?
Is there an empty space behind the north star? Or surrounding it?
What do you mean by "behind"? The space around the north star--both in 2 dimension as seen through a telescope, and in 3-dimensional space--has no unusual properties. As you find out in
the Earth's rotational axis migrates around a big cone. It happens to be pointing now just half a degree off the pole star, but 2000 years ago or 2000 years from now will have a different direction, and the pole star will be "demoted" to just one of the many stars around the north pole of the celestial sphere. Space around it is practically empty, but if you go a few light years away, you will probably find some other star.
See also question #401 below.
401. The year 2012 and the 26,000 year cycle
Hello Mr. Stern,
This is a 2012 question, but I'm not asking here if the world will end, it is not something I have time to worry about. What I would like to know though, is if it is possible that the Milky Way, Sun, and Earth will line up, in what is proclaimed to be an event that happens only once every 26,000 years.
I have read some of your answers to other similar questions posted on the NASA website, but could not see that this question was answered. Just want to know if a magnificent lineup such as this is due to happen in 2012, and if pictures of it happening could be taken by standard digital cameras. Also, in the event that this did happen, would it be safe to look at? How long would the lineup last for?
As the poet Pope wrote,
"A little knowledge is a dangerous thing
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again."
There exists a 26,000 year cycle, but it has nothing to do with the alignment of the Milky Way galaxy. Because the Earth rotates, it has a small bulge all around its equator (see Srotfram1.htm,
and because the Moon's orbit is tilted slightly to the Earth's equator, the Moon pulls that bulge and causes the Earth's rotation axis to wobble, or more accurately, to precess, to slowly migrate around a cone. You will find more about this in
The point in the sky around which the celestial sphere seems to rotate (actually, the rotation is the Earth's) thus slowly migrates along a large circle in the sky. Right now it is near a star we call Polaris or the pole star, but it was in a different place 2000 years ago, when Greek astronomers discovered the process.
A circle has no beginning or end. Thus the point to which the axis points in 2012 is just one of its many points, nothing special about it.
402. "Quarter Squares"
You write "...the Romans also had a multiplication method of their own...", and ten you describe something I have heard called Russian multiplication.
(Note: this is in connection with http://www.phy6.org/outreach/edu/roman.htm )
However, I have somewhere heard that the Romans used a different method, using look-up tables for the function f(x) = int((x**2)/4), defined on the integers. Clearly f(a + b) – f(a – b) = ab, and the tables could be built up using additions in a way that was computationally cheap per entry.
I have not heard about Romans using the method you described. The tables would be enormous--the product in the example is 4071, which is big. By contrast, knowing how to halve or double numbers up to (say) 50 or 100 is easier, and is done relatively easily in a system where (say) X = V + V .
Your method resembles the "quarter squares" method
When I attended high school I owned an old British table of logs, sines etc., which also had a table of quarter squares. I never needed them.
403. Do Rockets need "something to push against"?
Hi Dr. Stern,
I have a question that I have wanted answered for some time now.
I know that when satellites or spacecraft are flying through space and they need to change direction or spin or speed up, they fire rockets to do that. I know that for something to move or stop moving it needs to apply force to some other object, so that object would apply force back to it, as required by Newton's third law. So for those rockets to move a satellite, wouldn't the exhaust from the rocket need to push against something else, such as an atmosphere or solid surface? I know there is no atmosphere in space, so how do the rockets change the direction of the ship's motion? Does the vacuum in space give some sort of support? I thought just firing a rocket in vacuum would produce no reaction to push it... but obviously there is also something else involved.
Your argument reminds me of some objections supposedly made by the press to Robert Goddard's idea of spaceflight, that he "needed the atmosphere" for a rocket to work.
No, he did not. The fallacy is in regarding the jet created by the rocket as a link, between a rocket pushed forward and something else that's pushed back, in accordance to Newton's 3rd law. In fact, it is not a link: the gas itself, in the fast jet, is what is pushed back!
Of course, there is a limited amount of rocket thrust available, because the mass of burnable fuel is itself limited. Therefore scientists use any other means, if available. For rotating the spacecraft, you can use flywheels ("inertia wheels"): you rotate them (or change their rotation) in one direction, and the spacecraft rotates in the opposite direction: that's how the Hubble telescope is turned around.
A spacecraft rising from the ground has no alternative to rocket thrust. However if it is already in orbit, it can use the gravity of the Moon or of a planet to help it change course--see http://www.phy6.org/stargaze/Stostars.htm . The Voyager spacecraft would never have made it to the edge of the solar system without its "gravity assist" from Jupiter and Saturn. It is even harder to reduce velocity--the "Messenger" artificial satellite of Mercury plans to use multiple encounters with Venus and Earth to slow it down enough for capture by Mercury's gravity.
404. Sunrise-Sunset asymmetry
I've been going out shortly before dawn lately to look for vagrants amongst the Ring-billed Gulls on a spillway, as a result of which I've noticed that the timing of sunrise is changing more slowly than the timing of sunset. For Dallas/For Worth, Texas, for example, the changes between the winter solstice and today are:
Dec 21: rise: 7:27 set: 5:26 http://tinyurl.com/8dh8yp
Jan 13: rise: 7:31 set: 5:43 http://tinyurl.com/79rec3
When I put this query into NOAA's search engine, your website was one of the ones that came up (and I'm delighted to find such an informative website!), but so far I haven't found an answer to this question there. If it is there, would you please be good enough to point me to it -- if it isn't, I'd be most grateful for an explanation of the reason for this puzzling (to me) difference.
If we ignore the slow shift of the Sun's position along the ecliptic, sunrise and sunset are expected to be symmetric with respect to noon. If all days are exactly 24 hours long (noon to noon), the latest sunrise should match the earliest sunset, and so forth.
Your information suggests such symmetry exists only approximately, not exactly. The probable reason is that the time of noon slowly shifts--noon-to-noon days are not exactly 24 hours long, but vary. If we had an exact 24-hour clock, the variation of its indicated noon from actual noon (Sun passing the south and reaching its greatest separation from the horizon) can accumulate at certain times of the year, reaching up to 15 minutes. That variation is called "the equation of time" and is briefly mentioned in
and in some questions linked from the end of that web page. It is that shift which caused your discrepancy.
405. Re-entry from orbit
I assume/know that a space craft needs a heat shield to prevent it from the heat generated while re-entering earth's atmosphere. As standard procedure all space crafts re-enter with a typical angle and at high velocity.
With latest space shuttles rather than spacecraft used earlier, why can't people try re-enter earth's atmosphere with a very low speed?? A speed which can't generate enough friction. Please try and answer, sorry if it sounds stupid. But I think it is worth investing in fuel/technology which can reduce the re-enter speed than depending on heat shields.
Any object orbiting Earth needs high speed to maintain its orbit, at least 24 times the speed of sound (sound at atmospheric conditions). With less speed--say, half as much--it will ultimately lose altitude and hit the ground, even though its forward velocity may carry it part of the way around Earth before that happens.
Velocity means kinetic energy, and at the velocity of the shuttle, that energy is sufficient to melt metal (as unfortunately happened in the "Columbia" disaster). To get rid of its orbital velocity, the shuttle has to give up energy, and the only practical way is by converting it to heat and letting the heat radiate away into space (to reduce speed with rocket thrust, you need a rocket nearly as big as the one used in the launch). Its forward-facing surface therefore must get very hot, which is why a heat-resistant ceramic is used, or a composition that chars away. Furthermore, by putting a large heat shield in front, the spacecraft creates a shock front which is even hotter, from which most of the heat is actually radiated.
Note also that this happens at high altitude, where air is very rarefied: at a lower altitudes--say, that of a jetliner--even if you could tolerate the heating, the force of air resistance may still be sufficient to break the spacecraft. Only after most of the energy is gone can a spacecraft re-enter the atmosphere with its occupants or payload intact. Spacecraft which enter too fast or too steeply are usually destroyed.
406. Earth Axis and Gravity
Here's my question. If the Earth's axis was horizontal, would our gravity remain the same?
Thank you for your time.
The direction "horizontal" is not definable away from Earth. With Earth beneath us, "vertical" is the direction of a falling stone, more or less towards the center of Earth, and "horizontal" is any direction perpendicular to it.
To define the direction of the Earth axis in space, we need a reference which is independent of our gravity--for instance, the plane of the ecliptic, the flat plane defined by the Earth's orbit, or the direction perpendicular to that plane. Currently the Earth's axis makes an angle 23.5 degrees with that perpendicular direction. If the axis were to shift, gravity would remain the same, but the apparent motion of stars across the sky would change. In fact the axis DOES slowly shift--see
407. The year 2012
Hello Dr. Stern,
With the coming of 2012, there is a lot of hoopla and stuff on the internet--I say "stuff" because it seems like there are people with facts on both sides. My question is this; what did Albert Einstein say about the subject, and given the earthquakes and tsunamies of recent years are we still safe?
I don't know what you mean by "both sides", I know of no evidence that 2012 will be astronomically different from, say, 1912, nor that the Maya (no telescopes, not metal) knew things about the universe not observed by later astronomers. As far as I know, Einstein had nothing to say, and as for earthquakes and tsunamis, they will probably occur sporadically in the future as they have in the past. Are you still safe? From natural disasters, probably. From cars on the road, mad leaders with nuclear bombs, a world wide economic upheaval--your guess is as good as mine.
408. Choosing an Aerospace Career
I am Charles, a grade 10 student. I wonder what do I need to prepare if I want to become a Scientist on Aviation when I grow up. I was crazy about the "airplane" when I was a 3-year-old boy. Can you give me any suggestions?Thanks a lot!
There exist very few "scientists on aviation" these days. Most airplane designers are engineers, and there are very few job openings (most of their work is for the military). You need a degree in aerospace engineering--e.g. from the University of Southern California in Los Angeles.
Work as a pilot is also hard to get--I know one young man, in love with flying, who ended up in Homer, Alaska, flying tourists to see bears catching salmon (as shown on the most recent 25-cent Alaska coin). I don't know if he has a job for the winter--for a while he was flying supplies and a few passengers to the Inuit village Bethel near Bering straits. Too many people love flying (or have acquired it while flying for the military), so few jobs go unfilled!
If you are in the 10th grade, you may find a model airplane club near you, where members build and fly models. Such models--with cameras--may, by the way, be the foundation to the next big thing in military aviation, namely unmanned drones. The US uses them in a big way (e.g. big drones above Afghanistan controlled by pilots in a closed room in Las Vegas), as does Israel (small drones checking on activities in Gaza). Club members may give you good advice.
You may also become an airplane mechanic, as Benny Benson was (http://www.phy6.org/stargaze/Spolaris.htm).
409. Advice for "new" physics teacher
Hello. My name is Jonathan, and I am a relatively new Physics teacher. At my school, Physics is only offered every other year. (Hard to believe, I know. I've been trying to change that...) Currently, I teach Biology and AP Biology, but I am working on my masters in Physics at the University of Virginia.
My question for you guys (this was addressed to a listserve on physics education) is this: How much time during your class do you spend on Modern Physics? When I taught the course last year, I spent the majority of time focusing on the Classical - velocity, acceleration, force, etc. Because time was short, I had to quickly go over the electricity and magnetism portion, and I barely got to touch on Modern Physics. So, in a common high school physics class is this enough? Should I spread my time out more and "cover all of the bases" or is classical physics enough to prepare a student for college level physics?
Any help or guidance would be appreciated. All of the high schools in my county are in the same boat as mine, and the other teachers that teach physics are not as well versed in it as I am
The question you ask is very fundamental and has never been properly answered.
The 3 comments in my e-mail may not satisfy you, either, but that's the best I can do.
(1) Your state of Virginia has acknowledged the problem and started an effort to compile a supplemental "Flexbook" where students can learn the physics of the last 50 years (100 years may be more like it), due to be freely available on the web as of 2-27-2009.
Read about it at
You will find me listed among the authors, and my chapter is an overview of nuclear energy. It is expanded from my web site
and from adjoining sections in its collection (it is also copied there--see index). I can send you a copy if you wish; finding the time teach it (or any other part of the "Flexbook") remains a problem.
At the same time, from what I have seen so far, the chapters of the "Flexbook" are rather disjoint and do not really cover most of "modern physics", certainly not in any systematic way.
(2) The proper answer to teaching modern physics in one year of high school is probably "it cannot be done." Our syllabus is hardly different from that of the 100-year-old text of Millikan and Gale, yet enormous strides in physics were made since then. I suspect physics will not be properly taught in the US until the course is broadened to several years (including middle school), as in Europe. In 1998 I tried to outline the coverage needed, and you will find that outline (submitted to the state of Maryland but not adopted) in
Your coverage of physics seems typical of public schools and ends with a hurried coverage of magnetism. Since time is short, the graduating class may be left with the impression that magnetism is a mysterious property associated with iron, missing the fundamental role of electromagnetism altogether. I tried to introduce magnetism more broadly in "The Great Magnet, the Earth", home page
It is too detailed (and too historical!) for anything but individual study, but you might at least tell about Oersted and present the vu-graph demonstrations of
(3) I tried to cover modern physics and astronomy in "From Stargazers to Starships"
and in related web collections (see http://www.phy6.org/readfirst.htm) but again,there is much more material than can be available in the skimpy time allocated. Let those few of your students interested in science explore those web sites on their own, and of course, you yourself can pick and choose whatever you fancy.
But as for covering all of physics meaningfully in one year, it can't be done. The most you can do is present a selection, something like a chocolate sampler, and those who want to go further can find material on the web.
Be very cautious with the use of Wikipedia. It often describes what the claimed facts are, but stops short of explaining how and why they were reached, which is really the soul of science!
Good luck. You may correspond with me if you wish.
410. Paradox of Time Travel
Hello Dr. Stern, I am just a normal guy that is fascinated by science. I have a decent understanding of conceptual physics and I am trying to get an answer to a question that I have.
Supposing time travel is possible, wouldn't you be hopelessly deserted in space if you traveled just 1 second back in time? The Earth would already be 30 km away from you. That is not even taking into account that the solar system itself is moving. It just seems to me that the location you started from is will be long gone when you arrive at another time.
Thanks for any light you can shed on this!
Time travel would create many paradoxes, of which the one which worries you is just a mild case!
The physical universe depends on 4 coordinates: 3 give position in space, and one is time. Time is a different kind of dimension--in the basic equations, we can put it in a role analogous to coordinates (x,y,z), but only if we use a wider class of numbers which include the square root of –1. Using that broader class, everything happens in a 4-dimensional universe of space-time--3 dimensions of space and one of time (there may be more dimensions, but if so they are represented by very small quantities).
Your imagined time travel moves the traveler in time, but not in space relative (say) to our galaxy. In the actual universe, you cannot move in space and time independently!
Imagine moving instantly to Bermuda, changing position but not time. By the laws of physics, you cannot do so! "Instantly" would mean traveling faster than light, and the laws of physics demand you arrive in Bermuda with at least a minimum delay, the time it takes for light to travel from where you are to Bermuda.
And just as you cannot move in space without changing your position in time, you cannot move in time without changing your position in space. You ARE moving in time just now--but moving in space too. Relative to distant galaxies, you may be moving at almost the speed of light.
411 Is 7th grade Earth science boring?
Hello, I am a grade 7 junior high student, and i am really interested in science, but my grades are really low in that class. Maybe it is just the unit we are on. We are learning about the folding of mountains, but the past week has been really slow. Our teacher took the week off because his 6 or 7 year old son was having surgery, so we had a substitute, and she clearly had no idea what she was doing. Do you suggest anything to get me more interested in this unit?
I don't know your level! Look up "The Great Magnet, the Earth"
or the book (your library may have it) reviewed at
And find a friend to study together, it's more fun! Also read the letters linked at the end of sections of "The Great Magnet", some are fun too.
412. Air Resistance
I hope that you are still answering questions, as I think that I have one that many people are interested in, or at least have seen before.
First of all, thank you for spending what appears to be an enormous amount of time and effort on your web site, and offering it as a free resource for educators, students, and those interested in Science. I found your site by accident, and must say that I spent hours here looking at what you have compiled and accomplished (when I should have been doing about 20 other more critical projects...).
My question: I am a new physics teacher, and am researching the correct description and explanation for Air Resistance. I have seen and heard of two different explanations, and wanted to know which one is more correct.
- Air Resistance is resistance to motion through the air mainly due to friction of the air moving along the surface of the material being pushed through it.
- Air Resistance is resistance to motion through the air "piling up" in front of the object, and collisions with individual molecules of gas with the object, in accordance with Newton's Laws and Conservation of Momentum.
- Or maybe Air Resistance has several components, which when summed together equal the total amount of force that resists something moving through a fluid.
The first explanation appears to be more common, even to the point where the Conceptual Physics book that I am holding now suggests that friction is what keeps planes flying in the air, opposing the force of gravity! I'm sure that this is incorrect, where the four principal forces that act on an airplane are thrust (from engines) and drag (air resistance), lift (due to Bernoulli's Principle from wings or rotors) and gravity (pulls object down).
With that in mind, what actually causes an object to 'heat up' when moving through the air? Is it the collisions of the air molecules with the object, or is it actually the air 'rubbing' along the outside of the object?
Thank you in advance, this question came up in class and I was not comfortable providing the answer that was supplied in the book.
There exist three main causes of air resistance.
First of all, there is friction. When air flows slowly over a wing, it keeps parallel, orderly lines of flow ("laminar flow") and resistance is relatively low. Friction is in a way what keeps the airplane flying, by producing a thin boundary layer attached to the surface. That makes possible patterns of flow which create lift, by modifying the pressure distribution over the wing. But that is not related to air resistance, which is relatively small.
In laminar flow (as above) the air layer next to the surface moves (relative to it) slowly, the next layer out a bit faster, and so on throughout the boundary layer, until the speed matches that of the surrounding air. The thickness of this boundary layer is determined by density and viscosity, and is usually small. The frictional force is caused by viscosity and by the velocity shear between any two neighboring layers moving with slightly different speed (the lowest layer is of course the object moving through the air).
Secondly there is turbulent drag. At very slow speeds, each layer tends to drag along the next layer above it. But is can also do something else: instead of dragging the entire next-higher layer, it can grab a part of it and rotate it as an eddy. At certain speeds (depending on a criterion called the Reynolds number, depending on speed, dimensions and fluid properties) flow near the surface becomes unstable and creates eddies, and these create eddies above them, and so on until the fluid slows down to where it is no longer unstable to eddy creation.
The important thing about turbulent flow is that it reduces the air pressure. Thus if you descend by parachute, the half-spherical shape of the top of the parachute curves too much for laminar flow to follow its surface and instead, its wake consists of eddies. This reduces the pressure on top of the parachute, and the fall is slowed down. The concave opening at the bottom just serves to keep the canopy inflated and though it produces the pressure on the bottom, it is the fact that the pressure on the top side is reduced that brakes the fall.
Kites also work by creating turbulent drag behind them. They pull quite strongly, and the Japanese have built some big kites which require many people to handle. Because the drag is perpendicular to the inclined kite, part of it keeps the kite up in the air. The flow on the bottom of the kite tends to be laminar.
Finally, if the speed of sound is exceeded, you get sonic (or supersonic) drag. This is what you mentioned in item (2) on your list, when air can no longer flow out of the way but piles up.
The work expended by air resistance ultimately becomes heat. Work by surface friction heats the skin of the plane, and I understand that skins of supersonic jets can become notably warm. The energy of turbulent eddies is ultimately lost to viscosity--like that of eddies in a stirred cup of coffee, or in a stirred can of paint. The energy of supersonic pile-up is largely converted to heat at the surface where the pileup occurs--at the shock front where the flow abruptly converts from supersonic to subsonic (following the so-called Rankine-Hugoniot equations). The shock front is usually somewhat ahead of the object causing it. The reentering space shuttle creates a very strong shock front, hot enough to radiate its energy as infra-red light in all directions. This is good, because the shuttle itself absorbs only a small part of the heating.
See what you have done? Ask a simple question, and you never know where it leads! As for the Conceptual Physics Book... maybe send a copy of this correspondence to its authors.
413. Experimenting with Microwave Oven
Is it safe to use magnets in an microwave oven? My name is Bhuvanya from India, doing my Bachelor of Technology. I also wish to know which material (preferably plastic)is best suited to be used in a microwave oven, even if it is in contact with the food.
I would advise against it. Most magnets conduct electricity, and the microwaves create fairly large electric fields. I have seen (though the window) metal objects inside an oven create large sparks.
Whatever you do inside a microwave oven, put in with it a cup of water. The water will always absorb some energy. If only the magnet is there, it might heat up very quickly, maybe enough to lose its magnetism, possibly enough to cause damage.
414. Is the Sun losing mass?
The Sun's gravity, like that of any object, is due to its mass. The Sun is constantly losing mass due to nuclear reactions happening at its core. Does that mean that the Sun's gravity would be decreasing?
If Yes, would it impact the planetary orbits?
If No, why wouldn't the Sun's lose gravity due to its reducing mass?
The Sun is indeed constantly losing mass due to nuclear reactions in its core, and also loses mass of comparable magnitude in form of the solar wind. For details, look up
That is a collection of problems and answers for a short course on nuclear energy, written as a chapter in a free supplementary physics text for schools in the state of Virginia (and anywhere else). Go there to the answers for section S-8A-2 on nuclear binding energy, questions 5 and 6.
In question 5, I ask how much energy is released by fusion of deuterium, and the result corresponds to about 0.5% loss of mass. That energy was largely contained in the neutrons, which are heavier than protons, with which fusion on the Sun starts. The Sun would lose that much of its mass over its hydrogen-burning period of about 10 billion years, through which we are about halfway. The effect on planetary orbits over (say) historical times is thus negligible.
Question 6 compares the mass loss from fusion to mass loss due to ejection of the solar wind. The two numbers are surprisingly close.
415. Sun's position at noon, south of equator
I live in Italy. What is the direction to the sun when we watch it at noon from a point in the southern hemisphere?
In Italy, the Sun always passes in the south at (astronomical) noon
In the southern hemisphere, at the same latitude (e.g. Capetown) the Sun passes to the north. It is a symmetric.process.
However, at latitudes between 23.5 N and 23.5 S, the Sun can pass noon either to the north or the south of the zenith, the point in the sky directly overhead--depending on the date. If you are at such a location, The noon Sun will seem to be overhead--but actually, when it is winter in Italy, it passes a little to the north of the zenith, and in Italian summer a little south of it. Twice a year, the longest days of the year, it is directly overhead, in the process of replacing one side with the other. Again, "summer" and "winter" do not have the same climate meaning as in middle latitudes.
You may also look up http://www.phy6.org/stargaze/StarFAQ11.htm#q175
416. Fred Hoyle's theory of the Sun's Corona
(From New Zealand--British spelling!)
You must be aware of Fred Hoyle and his book "Frontiers of Astronomy". In chapter 7, "The Mystery off the Solar Atmosphere", beginning page 106, he talks about hydrogen from the Sun and how recently (1950s) it was discovered that the sun has a bloated atmosphere that "might even extend out to interstellar space". He talks about the 3 competing theories (at the time), the Magnetic Origin Theory, the Sound-Wave Theory, and the Infall Theory. He favours this last theory based on "new evidence that has come to light in the last 2 or 3 years". He gives observations of the number of hydrogen atoms per cubic centimeter at various points from the Sun; from the splash corona all the way out to the interstellar areas at 2000 photospheric units. He talks about the ingenious method of Hewish, Machin and Smith whereby they used radio waves from Taurus passing the Sun to check the intermediate distances levels of ionised hydrogen.
So my question is: how does Hoyle's observations on this topic jive with the eventual discovery of the solar wind, which in fact followed soon after he wrote his book?
Oh ... and there's another part to this question: he says in the same chapter, page 118, "The Sun drills out a tube of material as it goes along, the material that was initially present in the tube being captured by the Sun.... A typical speed for the Sun would be 10 kilometers per second, and at this speed the diameter of the tube would be about the same as the diameter of the orbit of Saturn ... the Sun's power of sweeping up interstellar gas is therefore very considerable".
So again, how does this jive with modern-day solar wind theory?
About Sir Fred Hoyle... interesting that you should write about it! The book, written in 1955, seems to have been long out of print, but I own a copy, carefully preserved just because of his assertion on the solar wind, which is wrong. Hoyle had the talent of being spectacularly right and spectacularly wrong. He confidently predicted an unknown resonant interaction of helium needed to produce carbon (and heavier elements, and life--and since we are here, he thought, it must exist), also named the Big Bang though he did not believe in it, and claimed life propagated through the universe on tiny spores.
It has been firmly established that the solar wind is not caused by infall of small meteorites--though Lou Frank of Iowa still believes in a steady influx of icy "microcomets" bombarding the solar system. No good explanation coronal heating exists, but the infall theory has been disproved--none of the expected craters were found on the Moon, for instance. Also, the US navy radar which tracks objects around Earth larger than about 10-20 cm reports that none have speed in excess of escape velocity, suggesting they are satellite debris.
About the Sun sweeping up interstellar material.... Much of it is vaporized and ionized before it gets too close, after which it is swept out again by light pressure and by the interplanetary magnetic field, riding with the solar wind. And while the Sun could affect trajectories of objects out to the orbit of Saturn, it would also accelerate them into trajectories which swing by the Sun without hitting it (because of conservation of angular momentum, leading to Kepler's 2nd law). Until Sun-observing satellites were orbited, I believe no comet ever was seen to hit the Sun, though many "sun grazers" came quite close; now we have a few recorded hits.
One last little question: I was under the impression that what Hoyle meant by Infalling Material, was not incoming comets so much, but some kind of steady infalling material such interstellar gas or ions such as cosmic rays. Correct me if I am wrong
I looked up Hoyle--yes, he predicted infalling gas. The numbers he predicts are however much larger than observed--at Earth, 1 AU= 400 solar radii, the interplanetary gas in the solar wind has typically 6 ions/cc (it can rise to 20 or more, and lately has been quite low, because of a very deep sunspot minimum). I have also dim memories that the density of infalling gas does not rise near the Sun (or rises only moderately?) because of something known as Liouville's theorem. Ask at the Uni!
Somewhere I read, in any case, that infalling neutral hydrogen gets ionized by sunlight at about 1 AU, and then the solar wind outflow grabs it. Comet dust also gets blown away by light pressure.
417. Why don't Protons and Electrons combine?
Thanks for your great and very informative web site. The science teacher at my children's school has asked me to talk about black holes, etc., to the 5th-8th graders. It will be a fun challenge since astronomy is my hobby, not profession.
Why don't protons capture their surrounding electrons, and turn into neutrons in ordinary matter? The electrical attraction between p and e- must be very large at atomic distances, so why don't they merge? To put it another way: what forbids p + e- => n + electron neutrino?
I'm asking cuz I'm wondering what makes atomic matter stable, and what
prevents the Sun from turning into a neutron star.
Processes in nature do generally go in the direction which reduces the stored energy of the system. A neutron's mass is more than that of a proton + electron combined, so n => p + e happens (with half life of about 10 minutes) but the opposite process does not.
In calculating the energy of the above process with a single neutron, gravity is neglected. In a greater mass, the additional gravitational energy can be appreciable and can reverse the direction to equilibrium.
Just combining protons and electrons would create chemical elements (and some protons and electrons then combine to neutrons due to the weak nuclear force). However, chemical elements cannot get compacted enough to release enough gravitational energy. However if the total mass is several times the Sun's, enough energy is jointly released by the reverse process and by gravitational compaction of the resulting neutrons, to drive the process you suggested. This is how neutron stars can exist. They do not form, however, until the star uses up other sources of energy (e.g. nuclear, in the Sun) which keep it
bloated up and make it shine.
418. Days in a Year
I found "Stargazers" when I was googleing a question and was wondering if you would help me find the answer. If a revolution of the earth around the sun forms a circle, how many days are there in one year ?
The Earth goes around the Sun in one year (not in an exact circle but in a slightly deformed one), while the Earth turns around its axis in 1 day. Actually, the period is about 4 minutes short of 24 hours; 24 hours is the average time noon to noon, and during this interval the Sun's position in the sky--which determines noon--also shifts a little, because of the Earth's motion around the Sun.
The two motions are not related to each other, so the number of days in a year is not a whole number but has a fractional part--365.2422 days, if I recall. If we counted 365 days per year, seasons would slowly shift on the calendar in one direction, if 366 days, in the other direction. Counting 365.25 is almost correct--that's the Julian year, introduced by Julius Ceasar in 46 BC, with a cycle of one year of 366 days followed by 3 years of 365. Still, by the year 1600 the seasons had slipped 11 days of so, and another correction was added, dropping 3 leap years every 4 centuries.
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419. Position of the Moon in the sky
I'm an Spanish architect living in Mallorca (Spain).
I'm working on an experimental project for which I ask your help. Let me explain myself:
I need to know if there is the possibility to predict the exact position of the moon in the sky for an specific day and hour, and the way to calculate it. I spent some time surfing the web, but I didn't find this information.
I would appreciate if you were so kind to inform me where could I be able to find this
Try the US Naval Observatory Almanac on-line:
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