10. Rebounding ping-pong balls and gravity assist
I've enjoyed browsing your fine web site, From Stargazers to Starships,
and I figured I would take a moment to let you know that. I was particularly
intrigued by the chapter "Project HARP and the Martlet. "
There is one possible error I found in the site. In Sections 35 and 35a, on
planetary swing-bys and the so-called "slingshot effect, " you state that the maximum velocity increase is imparted to a spacecraft when it approaches a planet head-on, or retrograde to its orbit. My reading indicates that the opposite is true. My sources are the following web sites:
JPL's Basics of Space Flight
I expect the discrepancy stems from the fact that the model used in
From Stargazers to Starships is based on the ping-pong paddle
example. The key difference is that the force exerted by a ping-pong
paddle on a ball is repulsive, whereas gravity is attractive. Thus the
numbers are the same but the sign is reversed.
By the way, I did find the analogy to the Pelton turbine very interesting.
Thank you again for a very informative web site!
I believe that the ping-pong analogy is still valid, because it can be
reduced to simple arguments of the conservation of momentum and energy,
which should hold equally in a planetary-assist maneuver. Some other
correspondent questioned this result, and as a result, you can find that
calculation in item #9 of the question-and-answer section of
"Stargazers," linked at the end of the home page [the item preceding this one].
What seems to confuse the issue is the following. A spacecraft would
get its biggest boost if it approached head-on, made a hairpin turn
around the rear of the moving planet and returned along a path 180
degrees from its first approach (that would be the ping-pong analogy).
Viewing the encounter from far north, if we put the moving planet at the
center of a clock dial with the Sunís direction at 12 o'clock, we would
see the planet moving towards 3 o'clock, so our satellite has to
approach from that direction and return to it again.
When the Voyager and Pioneer spacecraft approached Jupiter and Saturn,
however, they were coming from the Earth, which is roughly in the same
direction as the Sun; in any case, their initial orbital velocity, which was essentially that of the Earth, which moves in the same direction as other planets', did not allow a head-on approach. Instead, they entered around 12 o'clock on the dial. They still rounded the night side and exited around 3 o'clock, which gave them an apprecible boost, though perhaps not the biggest one possible.
I have some old issues of "Science" on these events and in the one of
the Pioneer 10 fly by, for instance (page 304, 25 January 1974), the
satellite enters at 1 o'clock and leaves a bit after 3 o'clock. For the
Voyager 1 fly-by of Saturn (p. 160, April 10, 1981), entrance is around
11:30 and exit around 4:30 on the same dial.
You are right, of course, in that the force on the ping-pong ball is
repulsive while the planet's gravity attracts the spacecraft. However,
the strongest attraction occurs when the spacecraft is at its closest
approach, on the night side, and its direction then is along the
velocity of the planet, the same direction as the force exerted in the
With all this, I am grateful for your message. It again shows that at
least some users go into the details of "Stargazers". Quite a few errors
were caught only thanks to people like yourself who checked out such
11. Why don't we feel the Sun's gravity pull?
Dear Dr. Stern:
I have asked several teachers and many other people
the following question but have not received any respectable answer:
The Earth is 93 million miles from the sun. Other planets, and even much
denser planets I might add, are much further yet from the sun. The
obviously strong gravitational attraction of the sun holds all of these
planets in orbits around the sun. If gravity could be simply defined as
a force that attracts matter, and the sun's gravitational pull is
sufficient to hold the Earth in orbit, what keeps it from pulling me off
the Earth? In fact, the gravitational pull of the sun is so weak at this
distance that It can't even produce enough pull to raise a hair on my
head. So how can it hold the Earth and several even denser planets (even
further out) in orbit?
So--if the gravitational force of the sun is powerful enough to
hold the Earth in orbit, then how could the Earth's gravitational force
be powerful enough to hold me down, counter-acting the gravitational
force of the sun? Please unconfuse me!
Two effects are at work, each of which would be quite sufficient:
(1) The force of gravity goes down with distance squared. For example,
since the Moon is about 60 times further from the center of the
Earth that you or anyone who is standing on the surface, the pull
of the Earth on each pound or kilogram of the Moon is 60 x 60 = 3600
times weaker than the pull on the same mass on the surface.
So: the Sun is indeed more massive, but also much more distant. As a
result, its pull on each kilogram or pound at the Earth's distance is
only about 0.06% of the Earth's pull near the surface.
(2) Being on the orbiting Earth, your body already responds to the Sun's
gravity, by sharing the Earth's velocity of 30 km/s around the Sun.
Therefore there is nothing left over from the Sun's pull to make you move
In a similar way, an astronaut in orbit feels weightless, because the
Earth's gravity is already fully employed in keeping up the orbital
motion. The astronaut is not beyond the reach of Earth's gravity: if
it were so, the spacecraft would fly away never to return, rather than
stay in orbit. It is just that--like a stone in free fall--gravity is
already doing to the astronaut all it can. It also does so on the
spaceship the astronaut rides in, leaving no extra force pulling the
astronaut down to the floor, or in any direction.
12. How hot are red, white and blue (etc.) stars?
Hi, My name is Donny and I have a question that I cannot seem to find an
answer to.... How hot, exactly, is a blue star, a red star, a white
star, and other color of star?
Your question has an answer, but you have also to learn a bit
about what color is. Look at the following web site
The stars for which statements about temperature are made are glowing
dense bodies of gas, so for them the "black body spectrum" is relevant.
In that spectrum, the curve of intensity against wavelength (color)
typically rises to a peak and then drops.
The total area under the curve tells how bright the light is: the hotter
the emitter, the higher the curve and the brighter the light. You know
this from experience: a flashlight with a weak battery glows weakly in orange,
a flashlight with a good battery glows bright yellow, and if you connect
a 3-volt battery to a 1.5 volt lightbulb, you get a very bright, very
white flash, and then darkness, because you have heated the wire inside
the lightbulb so much that it melted, and you have just lost your lightbulb.
And in that sequence, you also see the color move along the rainbow: orange
with a little heating (a feeble red when the battery is almost dead)
yellow under normal operation, white when it's too hot. The color you see
is where the peak is--and if it is blue, you see white, because all other
colors are also emitted, and white light is what the eye then sees.
Stars are like that too. We can say the Sun's photosphere radiates pretty
much like a black body at 5780 degrees absolute or about 5500 degrees
centigrade, by the way its colors are distributed. The color tells how
hot it is, and I think "blue" here means white-blue; such a star would be at about 10,000 degrees.
13. How does the solar wind move?
I am confused about the solar wind and don't want to
mislead my students. On a web site about magnetic storms I read the following:
"This storm affects
the earth when it is on the western half of the sun, not when it is dead
center. This is because the solar wind follows a curved path between the sun
and the earth not a straight-line path."
Is the solar wind influenced by the magnetic field of the sun so it has a curved path to the earth? Or is this too much of a simplification?
What really happens?
Physics and astronomy get complicated at times. Will the interplanetary
magnetic field curve the path of the solar wind? Without peeking at the
observations, one can only say "it depends," and what it depends on is
the ratio between the density of particle energy (density n times average of
0.5 mv^2) and the magnetic field energy density (B^2/2 mu-zero).
This ratio is often called "beta" in plasma physics, and it's an important
quantity in experiments aimed at confining a plasma for nuclear fusion.
If beta is much less than 1, the magnetic field is the dominant factor
and particles meekly follow its field lines, making containment easy.
Practical fusion however requires a greater beta, and if beta exceeds 1,
the plasma starts pushing the magnetic field around. The way it does so
is by subtly segregating its charges, to create a charge density and
hence an electric field, and electric fields can allow a plasma to move
whichever way it wants.
Suppose the magnetic field is constant and equals B0 in the z
direction, and the plasma is moving along the x axis. Then an electric
field E0= -vB0 in the y direction will allow it to do so, canceling the magnetic force on any electric charge q, equal to qvB0 along -y. (It also works out with spiraling particles).
The same happens with the solar wind, where beta may be 5 or more.
As a result, the solar wind moves radially out, though it gets buffetted
a bit, and it's not clear by what.
Now what about the MAGNETIC field? There is a rule (for plasmas with
high beta, satisfying the "MHD condition"), that "particles that initially share a field line, continue doing so indefinitely" (there exist some extra "fine print" conditions, but we ignore them here).
What follows below is the original answer sent to the questioner. Later this was converted to a graphical exercise, Section S-6a Interplanetary Magnetic Field Lines, linked to section S-6. You can either link there or continue below (or both), as you choose.
Take a sheet of paper, put on it a small circle--that is the Sun viewed from far north of it, or rather, it is a circle in the corona, some level above the Sun, where the solar wind begins. On this scale, let's say the solar wind moves one inch (1") per day (or if you wish, 2 cm). Draw from the center 6 or 7 radial rays 13.3 degrees apart. Mark as "P" the point where the first ray--the one furthest clockwise--cuts the circle. We look at 6 ions located at P, and
therefore presumably on the same field line--let's number them 1, 2...6.
We have advance information that 1 will be released into the solar wind
today, 2, tomorrow, 3 the day after, and so on. Mark P with 1--that is
where ion no. 1 is today.
Next day, P is on the second ray. Point 1 has moved 1" outward, radially, and Point 2 is at the base of the new ray, ready to go. Next day: Point 1
is now 2" out on the first ray, point 2 is 1" out on the 2nd, point 3 at the base of the 3rd, ready to move. And so on.
Five days later, 1 is 5" out on the first ray, 2 is 4" out on the 2nd
3 is 3" out on the 3rd, etc., and 6 is at the base of the 6th ray.
However, all these points started on the same field line, so they are
still strung out along one line. CONNECT THE DOTS marking the outermost ions on the 6th day and you have a spiral line of the interplanetary field: if the ions started on the same line, they must still be on one.
The solar wind in all this has moved radially. But now and then the
sun releases bursts of high energy particles, say from flares. The
energy of these particles may be high enough to endanger astronauts
in interplanetary space--but their density is very low, so their beta is
also low. THEY therefore are guided by the magnetic field lines (rather
than deforming them to their own flow), and therefore they move spirally.
The solar wind takes about 5 days to cover 1 AU. Therefore, if the Earth
is to receive particles guided by an interplanetary field line when it
is on the first ray, the emission has to be at the base of ray 6--that
is, near the western limb. The high-energy particles take only an hour or
so to arrive, depending on their energy of course.
14. The shape of the orbit of Mars
I have read your articles about stargazers and I believed this is one of
the most interesting subjects in astronomy. Here is a question which came
up when I was reading 'Planetary evolution', could you please help, thanks.
Mars moves in an elliptical orbit around the Sun, what is the relative distance
of the Sun to this ellipse? Would it be at one end of the major axis of the
The eccentricity of the Mars orbit is 0.09337, the semi-major axis
of the orbit is A = 1.524 AU (1 AU is the mean Sun-Earth distance, about
150,000,000 km; AU stands for astronomical unit) and distances of
perigee (closest approach) and apogee (most distant) are B = 1.381 AU
and C = 1.666 AU (letters are just notation for here).
These are the numbers. What do they mean? I remember seeing long ago a
German physics text from the 1920s drawing the orbit of Mars. One side
of the line was a circle, one side was the orbit, and the varying
thickness of the line showed the difference between the two. It was
hard to see that difference!
Let us calculate the length and width of the ellipse. The length (through
the two foci--the line on which the Sun is located) is 2A = B + C = 3.048
or 3.047 AU. The displacement of the center from either focus is D = (C-B)/2 =
0.1425 AU and the width is 2G where
G2 = A2 – D2 G = 1.51732 AU 2G = 3.035 AU
I do not think you or I would be able to distinguish an oval with dimensions
(3.048, 3.035) from a true circle! The position of the Sun at one focus
is however notably asymmetrical, about 10% of the distance from the center
to the edge.
15. What if the Earth's axis were tilted 90° to the ecliptic?
I was recently looking at the webpage "Seasons of the Year" and I read about what would happen if the earth's axis were perpendicular to the ecliptic. I was just wondering if you could give me some insight on what would happen if the ecliptic was inclined at a 90-degree angle with respect to the celestial equator? Would this mean that earth's orbit would
travel along this "new ecliptic" while the north and south poles are
travelling along this "new ecliptic"?
The hypothetical case you describe does in fact exist: for some
unknown reason, the spin axis of the planet Uranus is almost exactly
in the ecliptic.
That means that at some time one pole (let me call it the north
pole, even though that "north" direction is almost perpendicular
to the northward direction from Earth) points at the Sun. Then the
northern hemisphere is in constant light and the other one in
constant darkness. Half an orbit later--42 years or so--the roles
are reversed. And halfway between those times, the planetary rotation
axis is perpendicular to the Sun's direction, making day and night
alternate in a way similar to what the Earth experiences at equinox.
I leave it as an exercise to you to figure out whether Uranus ever
receives sunlight the way Earth does at solstice.
Mars and Venus
This is a query on Mars. The ferric oxide on the Martian surface
contains a lot of oxygen and if heated sufficiently would yield free
oxygen. Does the presence of this substance on Mars indicate that at
one time there must have been a lot of atmospheric or dissolved
(in water?) oxygen available on Mars?
These are my questions about Venus:
1. Why is it that Venus is depicted by Magellan photos as being Red/Orange in colour? If these are radar images how is the "..actual colour of sunlight that reaches the Venusian surface through the thick cloud
layer .. " derived? The Magellan images of the surface of Venus are quite bright. Would it really be that bright under such heavy cloud cover?
2. What generates the high velocity (up to 400 km/hr) winds which
move from east to west in Venus's upper atmosphere, given that the
surface of Venus rotates at a leisurely 6.5 km/hr?
3. What conditions did astronomers expect to find on the surface of Venus before the first probes landed? Thank you for your effort in
--- Oxygen is not a rare element, Mars, the Moon, etc. have a lot of
oxygen, always combined with other elements into stony stuff.
The element that is really important to life is hydrogen.
The recent signs that the Moon may have hidden water were
exciting not because it is hard to find oxygen on the Moon
(it must be separated, of course) but because hydrogen is
rare. I am not sure about hydrogen on Mars. Jupiter and other
cold big planets of course have plenty, and so do their moons,
which may be more accessible. Still, they are pretty far away.
I am not a planetary scientist! My speciality is magnetic fields and
plasmas near Earth. I will give you the best I can remember here:
--- The colors of radar maps on Venus are probably false colors. The
color and intensity of sunlight on the surface should be known,
because the Russian landers took photos. However, I don't know it.
--- What drives winds on Venus is not the rotation but the same cause
for winds as on Earth--the heat of the Sun. Venus is closer
to the Sun, so maybe its atmosphere is more agitated. See the sections in "Stargazers" on sunlight and on the way it creates the Earth's weather
What one observes, in any case, is the wind at the top of the
clouds, which might be analogous to the jetstream above Earth,
not a good measure of winds at the surface.
I don't know--you must find out by yourself. I know Venus was
expected to be "as hot as hell"--but the features of the
surface could not be guessed.
17. Where is the boundary between summer and winter?
This may seem like a silly question but is there a line on the earth where on
one side is summer and the other winter? Sort of in the same way that you can
go back and forth between two days at the International Date Line.
Yes, there exists such a line and it is called the equator, but the
boundary between summer and winter is not as sharp as the one of (say)
the international date line.
Right now it is fall here, but spring in Chile and Argentina, and
2-3 months from now it will be mid-winter here and mid-summer in those
countries. At our latitudes, these seasons are well defined. However,
a broad belt centered on the equator does not have well-defined summer
or winter. The Sun's heat fluctuates somewhat as its noontime passage
moves north and south. On the equator, for instance, it passes right overhead around the 21st of March or September, 23.5 degrees off to the north
on June 21 and the same amount south of "overhead" (zenith) on December 21.
These small changes do not make much of a change in solar heating. The
big difference is made by local climate patterns, e.g. seasonal rains
like the monsoon. These countries do not have summer and winter the way
we do: for instance, when my daughter visited Darwin, Australia, some years
ago, she was told that loacally the year had only two seasons, "the wet" and "the dry. "
18. The Ozone Hole
I am a auto mechanic and I have one simple question for you.
Scientists say there are 2 holes in the atmosphere, ironically they
are around the north and south pole, and they blame these holes
on chlorine monoxide or refrigerants i.e. fluorocarbons (CFC) escaping into
the ozone. Wouldn't the more likely cause of the holes be the magnetic
lines of flux? One more quick question: could there be a way
of tapping into that magnetic field as an energy source?
There do indeed exist two "holes" in the Earth's magnetic field,
around the MAGNETIC poles, whose magnetic field lines go very far
from Earth and afford an easy connection to the solar wind and
to interplanetary plasma phenomena. On those lines we do observe
"polar rain", a drizzle of fairly energetic electrons (more energy
than those of the ionosphere, less than those of the usual polar aurora)
which seem to come from the Sun. Also, when solar activity floods
interplanetary space with energetic ions and electrons, that is where
they are most likely to come down to Earth.
However, the creation and destruction of the ozone layer does not
involve the magnetic field. Instead, its factors are chemistry and
sunlight, and the "ozone hole" is around the geographic pole, not
the magnetic one.
The ozone layer is maintained as an equilibrium between creation of
ozone by ultra-violet sunlight, and its destruction by various natural
processes (this is not my field, and I do not know details). During polar
winter, the polar cap is dark and ozone is not created, just destroyed
(a bit further from the pole, with just a few hours of sunlight and the sun shining at a shallow angle, ozone creation is also reduced). The observation of
an "ozone hole" in recent years suggest accelerated destruction, as
predicted by Rowland and Molina to come from chlorine in man-made substances.
As for tapping electric currents from space, I don't think it will work,
because (1) they are very spread-out, by our standards--how can you tap
a current sheet 100-1000 miles wide?; and (2) between us and them lies
the atmosphere, a very effective electric insulator--as is well known
to power companies, which string their high-voltage cables through air
without worrying about the power leaking away.
19. What keeps the Sun from blowing up?
I have a question abour the sun. We all know that the sun is powered by
thermonuclear fusion reactions,so why doesn't it explode like an H-bomb?
What keeps the Sun from blowing up? Gravity.
The Sun is not exactly like an H-bomb--the bomb has fuel that is
easier to "burn, " while to Sun "burns" ordinary
hydrogen--but there does exist a similarity. Why does a hydrogen bomb explode? Because an enormous amount of energy is released in a short instant and inside a small volume, heating the material to extreme temperature. The material expands forcibly, creating a powerful shock, and that is the explosion.
The Sun releases much more energy every second in its central
regions, but those materials are under great pressure, from the weight
of all the matter piled up on top of them--the thick outer layers of the Sun, pulled down by a gravity much stronger than anything on Earth. That pressure confines the extremely hot gas in the Sun's core. The heat gradually works its way to the surface, but the Sun does not blow up.
If somehow it could yield to the pressure and expand, the central
core would cool down and the nuclear energy release would drop.
Then the pressure would decrease again and gravity would reassert
itself. Some stars do in fact oscillate, but we should be grateful that the Sun does not belong to that class.
See more in http://www.phy6.org/stargaze/Sun7enrg.htm
20. Those glorious Southern Skies!
Not long ago I visited Chile for the first
time and observed the night sky there. At that latitude, the centre of the Milky
Way passes overhead, where it makes a grand show...
Why is there such a difference between Southern and Northern Hemisphere?
Is it because of the 23 1/2 degree tilt?
Why such a difference between the southern and northern hemisphere?
Because from the point of view of the Earth, all stars are so distant
that they appear as if they were attached to a tremendously large
sphere, with us in the middle.
At night, standing on the ground, you only see HALF the sphere. If you stood
at the NORTH pole, half the sphere would be all you ever saw, appearing to spin
around the point right overhead, the zenith. Standing at the SOUTH pole,
you would see the other half, spinning around the point overhead, which
is on the OPPOSITE end of the sphere from the overhead point at the north
If you lived on the equator, the two poles of the sky would be on opposite
sides of the horizon, and as the sphere of the heavens rotated around
them, you would in principle see ALL the stars, sooner or later. In
practice, those close to the poles will be near the horizon and not easy
Maryland, where I live, is somewhere between the north pole and the
equator, so the stars near the north pole are easily seen, and we get to
see some stars of the southern hemisphere as well, though not those
near the southern pole of the heavens (like the Southern Cross and
Alpha Centauri), and many southern stars are only seen here near the horizon.
Similarly, from Chile you won't see the Pole Star, the Big Dipper
or Cassiopeia, but the bright stars near the southern pole more than
make up for them, and yes, the brightest part of the Milky way is there,
too. North of the equator, the best view of the Milky Way is in
The 23 1/2 degree tilt has to do with the way the Sun, Moon and planets appear to move--not with the apparent motion of the distant stars.
21. Should we fear big solar outbursts?
I have a question pertaining to your studies in the upcoming year,
particularly surrounding the forecast Solar Maximum.
I have heard about the upcoming Solar Maximum starting soon (CNN.com
article, Nov. 11, 1999). I have also heard (unofficially), that there could be a very large solar storm near the end of April.
Finally, it is relatively commonly known that there is going to be an
unusual alignment of the planets in our solar system at the beginning
of May, 2000. Has there been an in-depth study to determine effects of the
combination of these phenomena, and the potential impacts on both our
solar system, and our planet?
The real question; could this combination of phenomena:
1.) Promote a solar flare, or SME, significantly larger than
previously experienced in recorded history?
- I have heard of Super flares emitting from G Class
Stars, and the theory describes large planets in a close orbit (Jan.
8th Article, Sun-like stars said to emit super flares,
CNN). Now, I don't expect this size of phenomena to happen here, but
with the unusual planetary alignment, I do believe that this could
create larger effects than normal, like a significant Solar-Magnetic
Ejection, especially with the excitation of the Solar Phenomena. I'm
just curious as to how much larger.
2.) Disrupt the crust of our planet, creating a significant
amount of tectonic activity, and if so, by how much?
- Now, I know our planets are very far apart, but if
the magnetic attractions are larger than normal, and these magnetic
attractions promote significant SME activity, this could promote some
strange tectonic happenstance, especially with the fragility of our
planet and its crust.
3.) Potentially disrupt our magnetic field severely with the
combination of solar magnetic and gravitational forces?
- I am aware of changes in our earth's history of the
magnetic poles, could this happen here with the combination of a large
SME and gravitational forces?
No calculations, or in-depth study has occurred, but I have a hunch this should be looked at more closely, and by qualified people.
Your message made me once more appreciate
the amount of misleading and loose information circulating on the web.
I have spent a great deal of time and thought on creating a web site
describing what is known about the magnetic field of the Earth and
the Sun's effects on it, and for a real understanding, you better
To answer your questions in brief: The solar maximum is already here [December 1999]. It is not an abrupt event you can date, but the crest of a wave whose width is at least several years. From what I have heard, the current peak is lower than expected.
No one can predict a large solar storm months ahead of time--the best
we can say is that they are more frequent near the peak of the sunspot
cycle. Some big ones cause little disturbance near Earth--depends on
factors like the precise orientation of the interplanetary magnetic
field. Planetary alignments have no effect whatsoever. [See also next item.]
The large planets you read about are unlike anything in the solar system
--usually Jupiter-size or bigger, and very close to the star (this has to
do with the method of detection--it's hard to detect long-period planets).
No solar eruption has ever been found to affect the solid Earth. Their
energy is too small, and almost all of it is dissipated outside the
breathable atmosphere. No earthquakes follow CMEs.
I have no control over CNN. But if you seek to understand nature,
look up my site and sources linked or cited there.
Happy new century
22. Planetary line-up and the sunspot cycle
Enjoyed browsing through some of your efforts on the Web. I am hoping
you could help settle some of my thoughts before I make a fool of
In your experience, has anyone tried to correlate lineups of the sun,
earth and major planets' magnetospheres with the sunspot cycles? My
spare-time effort found some correlation between lineups and cycles in a
number of years. My wonderment centers around the possibility that
some forces of the planets when lined up, possibly relating to their
magnetospheres, impact the suns magnetosphere causing a solar max. I've
also considered the possibility that related magnetosphere effects could
be the cause of previous polar reversals on the earth. Additionally,
ringing of our magnetosphere might impact charged tectonic plates...but
that is again another direction. Only if you have time, please comment.
There exists a tempting closeness between the length of the solar
cycle and the orbital period of Jupiter, but I don't think the two
are related. I cannot imagine any mechanism coupling the two--
especially since the Sun rotates in about 27 days, so the relative
period of Jupiter going around the Sun is of that order. Furthermore,
the solar wind moves with supersonic speed, which means that solar
disturbances can (and do) travel downstream with it, but disturbances
from a planetary magnetosphere (whatever they might be) don't easily
Above and beyond all these, there is always the question of energy--
the currency in which the price of any physical process must be paid.
The energy required in the solar cycle is much bigger than anything
planetary magnetospheres can supply.
So what causes the cycle? The Sun rotates unevenly, slower near the
poles, faster near the equator, probably because of the way gas flows
in it (Jupiter also has such a difference). In a magnetized hot gas,
this difference deforms and amplifies the magnetic field, and there
exist some general theories of the sunspot cycle based on this, although
many details remain unclear. The general idea is that as the magnetic
field gets amplified, it forms concentrated "ropes" which push out
the hot gas, and when they reach a certain strength, enough gas is
displaced that the ropes are light enough to float to the surface,
where they are seen as sunspots.
Again, the magnetosphere is a relatively weak influence on the Earth's
internal magnetism--even a big magnetic storm only reduces the surface
equatorial field by 1%. Furthermore, the time scale differs--reversals
happen on time scales of 0.5-1 million years, while magnetic storms
have a 1-day scale or faster.
What seems to be involved are the currents which circulate in the Earth's
core, presumably driven by flows there, which (like flows on the Sun) get
their energy from heat. The magnetic field is fairly complicated--the
2-pole structure we see (north-south) is dominant, but not by as much as
it seems, because more complicated modes get filtered away faster by
distance. Right now the 2-pole field is declining at about 5-7% per
century, but the late Ed Benton has shown that the more complex parts
are gaining energy, and the total sum is fairly constant. Maybe, when a
reversal occurs, for a while the 2-pole part gets small and the total
field is rather complex (4, 8 poles..), and when the simple pattern
re-emerges, it is reversed.
Anyway--keep studying, keep up your sense of wonderment
23. What are comet tails made of?
Are comet tails the reult of melting and evaporation of ices from
the comet core or are they dust collected by the comet as it moves
in its orbit?
Comet tails contain both evaporated ices and dust, as explained in the
section "Comet Tails and the Solar Probe" near the end of the file
The dust however is not collected by the comet in its orbit, but is part of its make up, probably dating back to the beginning of the solar system. Comets
may have two tails, and sometimes these are well separated, as in the recent
Comet Hale Bopp: they differ in color, composition, and direction,
and are pushed away from the Sun by different forces.
Dust tails are pushed by light pressure, and their colors are those
of sunlight, scattered by them the way clouds on Earth also scatter sunlight.
The other tails contain plasma--free electrons and ions, that is, atoms
from which sunlight has removed one or more electrons, leaving a positive
charge. They glow in the colors characteristic of their material (a bit like the
way streetlights produce the characteristic glow of sodium), and are pushed back by the solar wind. As explained on the above web page, the velocity of the solar wind is not too many times larger than that of the comet, and that
causes them to point not straight away from the Sun but at a small
angle to that direction.
See also htpp://www.phy6.org/Education/wsolwind.html
24. If light speed sets the limit, why fly into space?
I have a question as to space travel. What is the point in
exploring space? Is it just for achievment purposes or in the future
will man discover LIGHTSPEED? Because otherwise, the whole thing seem
rather pointless. It is rather like asking a garden snail to tour America
in it`s own lifetime. Can you put any light on this question?.
From all we know, achieving lightspeed or anything close to it
is well beyond today's technology, and I suspect, tomorrow's as well.
The purpose of exploring space is different--to expand humanity's reach,
and to understand the universe in which we live. Ancient humans may
have been content to see the sun, moon and stars rise and set without
caring what they were, or how distant. We have come a long way from
then--to electricity, cars, airplanes and the internet--essentially,
because humans want to understand more. Most Americans would probably
feel rather stale if no progress happened over their lifetimes--just
different teams making it to the superbowl, different wars being fought
overseas. I for one felt excited by the landing on the moon, by the first
pictures from Jupiter and Neptune, and of the sun in X-rays (quite
different from the bland disk we see). I also feel excited by evidence
of distant planets and giant black holes at the center of galaxies.
No, I don't think we'll get there in my lifetime or in the next
1000 years--but humanity has a longer timetable, much longer than
that of any individual.
25. Does precession mis-align ancient monuments?
Dear Mr Stern,
I live in Ireland. There is an ancient monument at Newgrange in this
country which was constructed some 5000 years ago. The particular
alignment of the monument to the sun allows an inner chamber to be lit
by the sunrise at the winter solstice. The construction of such a
building so long ago with such accuracy seems almost incredible to me.
I have a query however. Is the tomb doomed to a long darkness in the future due to precession?
Look at the above figure, taken from the web page http://www.phy6.org.stargaze/Sseason.htm about the seasons of the year, in section #3 of ""Stargazers." It shows the relation between the Sun and the Earth with its tilted rotation axis, throughout the year (north is up). That relation is what varies the length of the day and the apparent motion of the Sun across the sky, from season to season.
Imagine you were able to rotate this arrangement by some angle--by 10°, 30°, 90° or whatever--around an axis perpendicular to the plane of the ecliptic. You rotate just the Earth and its orbit (and perhaps the Sun), while the rest of the universe stays as it is.
The relation between the Sun and the Earth then remains exactly as before--the only difference is that you are looking at it from a different
direction. Seasons and the apparent motion of the Sun across the sky
are still the same as they were.
What our imaginary rotation has done is exactly the same as what the
precession of the equinoxes does to the Earth's axis. So if an ancient monument is lined up to point at the Sun during solstice, it will continue doing so. Our calendar is also adjusted, so it would also remain the same day of the year.
On the other hand, an ancient monument aimed at a certain passage of a star would no longer fulfil its function, because now the axis of the Earth points towards a different part of the celestial sphere.
26. Why does the Earth rotate? Why is it a sphere?
I have been unable to get answers to the following questions concerning
the earth, can you help?
Why does the earth rotate - what are the forces causing the rotation?
Why did it start rotating in the first place?
Why are planets round?
Any help to get these answers is most appreciated
You have asked some very fundamental questions. Why does the Earth rotate? Because whatever it arose from--probably a cloud of gas and dust--was rotating to begin with.
The thing to keep in mind that even a very slow rotation of a cloud of
objects gets greatly speeded up as it condenses at the center. The reason
is a basic law (a consequence of Newton's laws of motion) by which a
quantity known as ANGULAR MOMENTUM (or rotational momentum) is conserved. The angular momentum can be defined as the average radial distance, TIMES the average velocity of motion, TIMES the mass.
The mass does not change when matter collects near the axis of rotation, so we neglect the last part. Then, if the material collectes in the middle, where its average radius of rotation is 10, 100 or 1000 times smaller, the average velocity of its particles increases by the same factor.
You see this happen every day. When water drains from a filled bathroom sink, even if the water in the sink is rotating so slowly that you do not notice, by the time it reaches the drain it is spinning rapidly enough
to form a funnel. (It does not spin in the opposite direction in Australia:
the effect on which this claim is based is far too weak to have much effect.
See "Stargazers," section 24). You also see this in hurricanes and tornadoes.
And you see it happen when a large object in space collapses. In 1054
a star "went supernova" in the constellation of the Crab, a process
in which the top layers blow off and the core collapses to a tiny
"neutron star," perhaps 15 km across and as massive as the Sun. The
collapsed core of the Crab Nebula apparently rotates 30 times a second,
because that is the frequency at which it blinks in x-rays and radio
(and I believe in its light, too).
In the solar system all planets orbit in the same direction and
nearly in the same plane, and they and the Sun rotate in the same
direction too (= counterclockwise, viewed from north). This suggests that they all condensed from the same cloud.
Now that other question: why are planets round? Because of their
gravity. On the surface of the Earth, solid material--say, rock cliffs
--can easily stand the pull of gravity without deforming. But go
just a few hundred kilometers inside the Earth, and you find everything
under enormous pressure, from the weight of the layers heaped up
on top. Under such pressure (and helped by the heat down there!), even
solid rock deforms like putty.
If the Earth were all fluid, gravity would pull it into a symmetric
sphere--the same way as it shapes the oceans. The Earth is not fluid, but as
mentioned above, it makes no great difference. Actually, a ROTATING
fluid Earth would be deformed by the centrifugal force, with the
equator bulging out slightly. Gravity is weakened there, by the centrifugal
force and by the greater distance from the center. That was observed
in Newton's time, and Newton explained it by essentially using
a fluid analogy.
Jupiter is much bigger than Earth and rotates much faster: its
equator bulges out so much that pictures taken through a telescope
suggest a definite ellipticity.
Voyager 2 and other space probes have by now cruised through most
of the solar system and have imaged its planets and their moons.
The rule seems to be that objects with a radius above 150 km are
spherical; smaller ones do not have a strong-enough gravity and may
be potato-shaped, e.g. the moons of Mars.
(A different slant, from later correspondence) Planets are pulled into their round shape by gravity, which evens them out all around. Anything sticking out is pulled down! For example, the highest mountain on Mars, Olympus Mons, is about 2 1/2 times the height of Everest. How come? Because smaller Mars has at its surface only 1/3 the pull of gravity we feel on Earth. On Earth, the greater weight of a mountain that high would make it sink into the surface.
27. What's so hard about reaching the Sun?
Hi there. hope you don't mind a question.
In http://www-istp.gsfc.nasa.gov/stargaze/Sorbit.htm (at the end) you
"The hardest object to reach would be the Sun itself. Our imaginary
spacecraft, freed from the Earth, would be moving like the Earth around
the Sun at about 30 km/sec. The only way for it to reach the Sun is to
somehow kill that velocity--for instance, by a rocket imparting 30 km/s
in the opposite direction; if that were done, the spacecraft would be
pulled in by the Sun. The people who propose sending nuclear waste by
rocket into the Sun do not seem to know much about orbits! "
It would occur to me that the challenge of getting to the sun would
consist mostly of getting out of Earth's gravitational influence.
Ignoring that step (or starting from a point in earth's solar orbit
which is NOT on the earth's surface), I would think that just about any
deceleration would allow you to reach the Sun. While a deceleration of
30 km/sec would allow an object to "fall like a stone" into the Sun, an
orbital velocity of, say, 29.5km/sec, instead of 30 km/sec, would result
in a very slow, long, death spiral, but still one which still eventually
results in a solar plunge (making the assumption of no external
interference, such as another orbiting body didn't snag you along the
way or provide a velocity boost).
What am I missing? Or am I?
P.S. No, I'm not a "nuclear waste into the sun" kinda guy. A perfect
place for disposing the stuff, but not worth the risk of putting the
waste on top of all that explosive energy and lighting the fuse.
No, I am afraid it won't work. All orbits in the Sun's gravity field
are ellipses, or other conic sections: there are no spirals. If you
place an object in Earth's orbit around the Sun but free of the Earth's
own pull, and cut its velocity from 30 km to, say, 5 km. velocity, it
would certainly fall sunward, but it would gain velocity doing so and
would whip around the Sun in an extended ellipse. That ellipse would
have its apogee (highest point) in the Earth's orbit.
If you cut down its velocity to near zero, it would again fall sunwards.
It is still moving in an ellipse--a very long and skinny one--and if the Sun were just point-size, it would still miss and return to apogee in the Earth's orbit. However, the Sun does has an appreciable size, so that when
when the ellipse is sufficiently narrow, the object hits the Sun, and then it never comes back.
Until recent decades comets were never seen to hit the Sun, because
even a slight sideways velocity makes them miss. Since then, because
of observations from space (which are able to see small comets close to the Sun) some such comets were observed. Still, it is not a common thing.
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