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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. "

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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.

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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.

Why is there such a difference between Southern and Northern Hemisphere? Is it because of the 23 1/2 degree tilt?

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At night, standing on the ground, you only see HALF the sphere. If you stood at the NORTH pole, half the sphere would be all you ever saw, appearing to spin around the point right overhead, the zenith. Standing at the SOUTH pole, you would see the other half, spinning around the point overhead, which is on the OPPOSITE end of the sphere from the overhead point at the north pole.

If you lived on the equator, the two poles of the sky would be on opposite sides of the horizon, and as the sphere of the heavens rotated around them, you would in principle see ALL the stars, sooner or later. In practice, those close to the poles will be near the horizon and not easy to see.

Maryland, where I live, is somewhere between the north pole and the equator, so the stars near the north pole are easily seen, and we get to see some stars of the southern hemisphere as well, though not those near the southern pole of the heavens (like the Southern Cross and Alpha Centauri), and many southern stars are only seen here near the horizon.

Similarly, from Chile you won't see the Pole Star, the Big Dipper or Cassiopeia, but the bright stars near the southern pole more than make up for them, and yes, the brightest part of the Milky way is there, too. North of the equator, the best view of the Milky Way is in mid-summer.

The 23 1/2 degree tilt has to do with the way the Sun, Moon and planets appear to move--not with the apparent motion of the distant stars.

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.

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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.

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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

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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?.

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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.

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?

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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

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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.

Sincerely

David

In http://www-istp.gsfc.nasa.gov/stargaze/Sorbit.htm (at the end) you state:

"The hardest object to reach would be the Sun itself. Our imaginary spacecraft, freed from the Earth, would be moving like the Earth around the Sun at about 30 km/sec. The only way for it to reach the Sun is to somehow kill that velocity--for instance, by a rocket imparting 30 km/s in the opposite direction; if that were done, the spacecraft would be pulled in by the Sun. The people who propose sending nuclear waste by rocket into the Sun do not seem to know much about orbits! "

It would occur to me that the challenge of getting to the sun would consist mostly of getting out of Earth's gravitational influence. Ignoring that step (or starting from a point in earth's solar orbit which is NOT on the earth's surface), I would think that just about any deceleration would allow you to reach the Sun. While a deceleration of 30 km/sec would allow an object to "fall like a stone" into the Sun, an orbital velocity of, say, 29.5km/sec, instead of 30 km/sec, would result in a very slow, long, death spiral, but still one which still eventually results in a solar plunge (making the assumption of no external interference, such as another orbiting body didn't snag you along the way or provide a velocity boost).

What am I missing? Or am I?

Jim

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.

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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.

Sincerely

David