Sunlight

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(S-4) The Many Colors of Sunlight and (S-5) Waves and Photons From here one can continue with sections on the Sun, its energy and related sections on nuclear energy. All these provide a reasonably comprehensive, non-mathematical introduction to solar-related physics, at the level of high school or beginning college. However... whereas rest of "From Stargazers to Starships" revolves around Newton's laws of motion and their applications, Sun-related physics is frequently concerned with the physics of the atom and nucleus, where those laws are greatly modified. Like Newtonian mechanics, "quantum mechanics" is a mathematical field, but even its basic applications demand much more mathematics. Its coverage here is therefore very much simplified. Do not expect "Stargazers" to teach you quantum physics: the most these web pages can do is give a quick survey of its origins, and sketch out some basic ideas. You will learn what the main components are, and a bit about the way they evolved, but to do more, you will have to fill those empty boxes with solid, mathematical knowledge. This is optional material: if you skip it and continue, you may still get a fairly coherent picture. Planck's Constant Einstein's 1905 formula E = hν was an early indication that on the atomic scale the laws of physics were quite different. Furthermore, it suggested that the new laws were intimately connected with a new physical constant, now known (for reasons discussed below) as "Planck's constant" h. To an accuracy of 6 decimals h = 6.626068 10–34 joule-sec; Much of the physics of the 20th century involves atomic phenomena, an area rarely covered in high school or beginning college, where studies stress "classical physics" based on the laws of Newton and Maxwell; Indeed, with the limited time available in physics classes, even that part of physics is only partially covered. Classical physics is indeed more relevant to most engineering applications. Furthermore, the mathematical tools involved in the "new physics," the mathematics of wave functions, are only taught in classes on advanced calculus. Thus a gap is almost inevitable. This overview will not close that gap: at most, it can present some of the framework of the physics of the atomic level. As in other parts of "From Stargazers to Starships," here too the pattern is outlined by a history of its discoveries. :

1. "1" The View Through the Telescope Index 2a. The Sundial 3. The Seasons 3a. Angle of sunlight 4. The Moon (1) 4a. The Moon (2) 4b. Moon Libration 5.Latitude and Longitude 5a. Navigation 5b. Cross-Staff 5c. Coordinates 6. The Calendar 6a. Jewish Calendar 7.Precession 8. The Round Earth When Galileo became the first human to view the Moon through a telescope, our understanding of the Moon changed forever. No longer a mysterious object in the sky, but a sister-world full of ring-shaped mountains and other formations! Giovanni Riccioli in 1651 named the more prominent features after famous astronomers, while the large dark and smooth areas he called "seas" or "maria" (singular "mare," mah-reh). Some of the names he used for the Moon's crater are of persons discussed in "Stargazers"--Tycho (distinguished by bright streaks that radiate from it), Ptolemy ("Ptolemaeus"), Copernicus, Kepler, Aristarchus, Hipparchus, Erathosthenes; Meton and Pythagoras are on the edge, near the northern pole. Late-comers who lived after the 17th century had to make do with left-overs: the craters Newton and Cavendish are at the southern edge of the visible disk, Goddard and Lagrange too are near the edge. Also, "Galilaei" is a small undistinguished crater (because of Galileo's banishment?). However, since the Russians were the first to observe the rear side of the Moon, a prominent crater there bears the name of Tsiolkovsky, who at the end of the 19th century promoted the idea of spaceflight. Note: Strictly for moon junkies--all you ever wanted to know, perhaps even more. From Cambridge University Press (1999), Mapping and naming the moon: A history of lunar cartography and nomenclature by Ewen A. Whitaker, xix+242 pp., $59.95. The Craters What had created those strange round "craters"? ("Krater" is Greek for a bowl or wide-mouthed goblet.) They reminded some observers of volcanic craters on Earth, or better, of the large "calderas" (cauldrons) formed by the internal collapse of volcanos, e.g. Crater Lake in Oregon. Others suggested that they were formed by the impact of large meteorites, but this was countered by the argument that most meteorites probably arrived at a slanting angle, and were expected to leave not a round ring but an elongated gouge.
   1. "1.1" "[He] who made for you the earth a bed [spread out] and the sky a ceiling…" (Quran 2:22) In the first verse God swears by the sky[1] and its function of ‘returning’ without specifying what it ‘returns.’ In Islamic doctrine, a divine oath signifies the magnitude of importance of a special relation to the Creator, and manifests His majesty and the supreme Truth in a special way. The second verse describes the Divine Act that made the sky a ‘ceiling’ for the dwellers of earth. Let us see what modern atmospheric science has to say about the role and function of the sky. The atmosphere is a word which denotes all the air surrounding the earth, from the ground all the way up to the edge from which space starts. The atmosphere is composed of several layers, each defined because of the various phenomena which occur within the layer. This image shows the average temperature profile through the Earth’s atmosphere. Temperatures in the thermosphere are very sensitive to solar activity and can vary from 500°C to 1500°C. Source: Windows to the Universe, (http://www.windows.ucar.edu), the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-04 University Corporation for Atmospheric Research. Rain, for one, is ‘returned’ to Earth by the clouds in the atmosphere. Explaining the hydrologic cycle, Encyclopedia Britannica writes: "Water evaporates from both the aquatic and terrestrial environments as it is heated by the Sun’s energy. The rates of evaporation and precipitation depend on solar energy, as do the patterns of circulation of moisture in the air and currents in the ocean. Evaporation exceeds precipitation over the oceans, and this water vapor is transported by the wind over land, where it returns to the land through precipitation."[2] Not only does the atmosphere return what was on the surface back to the surface, but it reflects back into space that which might damage the flora and fauna the earth sustains, such as excessive radiant heat. In the 1990’s, collaborations between NASA, the European Space Agency (ESA), and the Institute of Space and Astronautical Science (ISAS) of Japan resulted in the International Solar-Terrestrial Physics (ISTP) Science Initiative. Polar, Wind and Geotail are a part of this initiative, combining resources and scientific communities to obtain coordinated, simultaneous investigations of the Sun-Earth space environment over an extended period of time. They have an excellent explanation of how the atmosphere returns solar heat to space.[3] Besides ‘returning’ rain, heat and radio waves, the atmosphere protects us like a ceiling above our heads by filtering out deadly cosmic rays, powerful ultraviolet (UV) radiation from the Sun, and even meteorites on collision course with Earth.[4] Pennsylvania State Public Broadcasting tells us: "The sunlight that we can see represents one group of wavelengths, visible light. Other wavelengths emitted by the sun include x-rays and ultraviolet radiation. X-rays and some ultraviolet light waves are absorbed high in Earth’s atmosphere. They heat the thin layer of gas there to very high temperatures. Ultraviolet light waves are the rays that can cause sunburn. Most ultraviolet light waves are absorbed by a thicker layer of gas closer to Earth called the ozone layer. By soaking up the deadly ultraviolet and x-rays, the atmosphere acts as a protective shield around the planet. Like a giant thermal blanket, the atmosphere also keeps temperatures from getting too hot or too cold. In addition, the atmosphere also protects us from constant bombardment by meteoroids, bits of rock and dust that travel at high speeds throughout the solar system. The falling stars we see at night are not stars at all; they are actually meteoroids burning up in our atmosphere due to the extreme heating they undergo."[5] This is an image of Earth’s polar stratospheric clouds. These clouds are involved in the creation of Earth’s ozone hole. Source: Windows to the Universe, (http://www.windows.ucar.edu/) at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-04 University Corporation for Atmospheric Research. Encyclopedia Britannica, describing the role of Stratosphere, tells us about its protective role in absorbing dangerous ultraviolet radiation: "In the upper stratospheric regions, absorption of ultraviolet light from the Sun breaks down oxygen molecules; recombination of oxygen atoms with O2 molecules into ozone (O3) creates the ozone layer, which shields the lower ecosphere from harmful short-wavelength radiation…More disturbing, however, is the discovery of a growing depletion of ozone over temperate latitudes, where a large percentage of the world’s population resides, since the ozone layer serves as a shield against ultraviolet radiation, which has been found to cause skin cancer."[6] The mesosphere is the layer in which many meteors burn up while entering the Earth’s atmosphere. Imagine a baseball zipping along at 30,000 miles per hour. That’s how big and fast many meteors are. When they plow through the atmosphere, meteors are heated to more than 3000 degrees Fahrenheit, and they glow. A meteor compresses air in front of it. The air heats up, in turn heating the meteor.[7] This is an image which shows the Earth and its atmosphere. The mesosphere would be the dark blue edge located on the far top of the image underneath the back. (Image courtesy of NASA) Earth is surrounded by a magnetic force field - a bubble in space called "the magnetosphere" tens of thousands of miles wide. The magnetosphere acts as a shield that protects us from solar storms. However, according to new observations from NASA’s IMAGE spacecraft and the joint NASA/European Space Agency Cluster satellites, immense cracks sometimes develop in Earth’s magnetosphere and remain open for hours. This allows the solar wind to gush through and power stormy space weather. Fortunately, these cracks do not expose Earth’s surface to the solar wind. Our atmosphere protects us, even when our magnetic field does not.[8] An artist’s rendition of NASA’s IMAGE satellite flying through a ‘crack’ in Earth’s magnetic field. How would it be possible for a fourteenth century desert dweller to describe the sky in a manner so precise that only recent scientific discoveries have confirmed it? The only way is if he received revelation from the Creator of the sky.
   2. "1.2" This verse mentions the darkness found in deep seas and oceans, where if a man stretches out his hand, he cannot see it. The darkness in deep seas and oceans is found around a depth of 200 meters and below. At this depth, there is almost no light (see figure 1). Below a depth of 1000 meters there is no light at all.[1] Human beings are not able to dive more than forty meters without the aid of submarines or special equipment. Human beings cannot survive unaided in the deep dark part of the oceans, such as at a depth of 200 meters. Figure 1: Between 3 and 30 percent of the sunlight is reflected at the sea surface. Then almost all of the seven colors of the light spectrum are absorbed one after another in the first 200 meters, except the blue light. (Oceans, Elder and Pernetta, p. 27.) Scientists have recently discovered this darkness by means of special equipment and submarines that have enabled them to dive into the depths of the oceans. We can also understand from the following sentences in the previous verse, “...in a deep sea. It is covered by waves, above which are waves, above which are clouds....”, that the deep waters of seas and oceans are covered by waves, and above these waves are other waves. It is clear that the second set of waves are the surface waves that we see, because the verse mentions that above the second waves there are clouds. But what about the first waves? Scientists have recently discovered that there are internal waves which “occur on density interfaces between layers of different densities.”[2] (see figure 2). Figure 2: Internal waves at interface between two layers of water of different densities. One is dense (the lower one), the other one is less dense (the upper one). (Oceanography, Gross, p. 204.) The internal waves cover the deep waters of seas and oceans because the deep waters have a higher density than the waters above them. Internal waves act like surface waves. They can also break, just like surface waves. Internal waves cannot be seen by the human eye, but they can be detected by studying temperature or salinity changes at a given location.[3]
2. "2" 23b) Loop-the-Loop Index 22c. Flight (1) 22d. Flight (2) 23. Inertial Forces 23a. The Centrifugal Force 23b. Loop-the-Loop 24a.The Rotating Earth 24b. Rotating Frames The Sun S-1. Sunlight & Earth S-1A. Weather S-1B. Global Climate S-2.Solar Layers S-3.The Magnetic Sun (Optional Addition) In the preceding section the motion of the "loop the loop" roller coaster was handled using the centrifugal force. You can also view this problem from the point of view of the outside world, using the centripetal force, but it is not as easy. At point A, on the top of the loop, both gravity and the centripetal force point downwards. So what is there that can keep riders in their seats? Let us try solve that motion, using the concept of the centripetal force. A car going around a loop, with radius R and velocity V, is accelerating at a rate of V2/R towards the center (as long as it stays on the rails), and is therefore subject to a centripetal force mV2/R, also directed to the center. When the car is at point A, that force points downwards. Let "down" be now be taken as the positive direction along the vertical axis. The centripetal force is provided by two sources: the weight mg of the car, directed downwards, and the reaction FR of the rails. We have at point A mg + FR = + mV2/R Hence FR = + mV2/R – mg where a positive FR pushes the car down, a negative one pulls it up Now the car rides on rails. At point A the rails are above the car and therefore it can only push up against them. The rails then, reacting to the force, must push it down, somewhat similar to the situation in "Objects at Rest", in section #18 on Newton's second law. Thus FR must be positive: if it were negative it would mean that the rails were pulling the car upwards, which they cannot do. We thus require FR > 0, that is mV 2/R – mg > 0 or, after adding mg to both sides mV 2/R > mg This is the same result as was obtained using the centrifugal force: the problem can be solved in the outside frame of reference--but the process is a bit more complicated. The intuitive meaning is shown in the drawing. If all forces on the car ceased at point A, it would continue along a straight line to point B, in accordance with Newton's first law. If only gravity acted, it would follow a parabola to point C. For the rails to exert a positive pressure, they must constrain the car to a tighter curvature than gravity alone, forcing it to move to point D. Exploring further Actual looping roller coasters do not follow a circular path, but a "clothoid curve" (look up the term!!) which curves more tightly at the top and more gently at the bottom (you can see this in the illustration on the previous page). The tight curvature on top helps keep the passengers pushed outwards, and the gentle one at the bottom reduces the downward force on them, in sections where the centrifugal force and gravity add up in approximately the same direction. A short article on the Clothoid curve, linked to the Italian version of this web collection, was translated to English by Dr. Giuliano Pinto and after some further editing was incorporated into this collections: click here. Next Stop: The Rotating Earth Next Stop: #24b Rotating Frames of Reference Timeline Glossary Back to the Master List Author and Curator: Dr. David P. Stern Mail to Dr.Stern: stargaze("at" symbol)phy6.org . Last updated: 9-22-2004 Reformatted 25 March 2006
3. "3" Weightlessness An astronaut in low Earth orbit moves in (approximately) a big circle extending around the Earth. The acceleration required for such motion is provided by gravity mg(RE/r)2= mv2/r where the astronaut's weight mg on the Earth's surface at r = RE is adjusted on the left side for the greater distance. That is of course the same equation as the one used to demonstrate Newton's study of gravity. However, it can also be written mg (r/RE)2 – mv2/r = 0 That can be interpreted as stating that in the astronaut's frame of reference, all bodies are subject to two forces, gravity and the centrifugal force, and the two are in perfect balance, adding up to zero. It is sometimes claimed that astronauts in space are in a "zero gravity" environment, but actually they are still very much under the influence of the Earth's gravity. True, the astronaut observes no tendency at all to fall towards Earth, but the reason is different and can be stated in one of two ways: Gravity is already kept fully occupied by supplying the ongoing acceleration (the first of the above equations); or The force of gravity is perfectly balanced by the centrifugal force (second equation). Take your choice! Weightlessness Simulation in an Airplane What if the spaceship's orbit is not circular but (say) elliptic? It makes no difference. If the force of gravity at distance r is F = mg(RE/r)2 Then the equation of motion of an object subject to F alone is ma = mg(RE/r)2 or a = g(RE/r)2 The acceleration a is what a spacecraft in orbit experiences, viewed from the fixed frame of the Earth. In a circular orbit of radius r it equals v2/r, while in an elliptic orbit it may have a different magnitude and different direction, which could also be calculated. The important thing to note here is that an astronaut inside that spacecraft is subject to the same gravity and therefore undergoes the same acceleration as the spacecraft itself. Viewing the astronaut's motion in the frame of the moving spacecraft, the astronaut is not pulled towards the floor of the cabin or in any other direction, and therefore has the impression that gravity has been eliminated. Suppose that instead, the astronaut rode inside a freely falling cabin, near the surface of the Earth. There, too ma = mg(RE/r)2 but since r is very close to 1 RE, we may set that ratio equal to 1 and get simply a = g The cabin falls with acceleration g, but the passenger also falls with the same acceleration, so again, no force exists that pushes the passenger towards the floor of the cabin. Acting on cues from the surrounding cabin, the passenger will again get the illusion that gravity did not exist. It makes no difference if the cabin started with a constant velocity--e.g. tossed upwards with an initial velocity u and with an initial horizontal velocity w--because neither of these affects the forces and accelerations. Both cabin and passenger would still be accelerating downwards at a=g, creating an illusion of zero gravity. If this experiment were actually conducted, that illusion and also the cabin would all too soon be shattered by contact with the hard ground below. Furthermore, air resistance would soon reduce the cabin's acceleration below g. The passenger inside, still subject to a=g, would then overtake the cabin, a process which would appear in the frame of the cabin like a partial return of gravity. However, the same experiment can be safely performed aboard a high-flying aircraft, which could match any air resistance by the thrust of its engines. By following a programmed parabolic path similar to that of a projectile subject to gravity alone, such an aircraft can create--for a limited time--a zero-gravity environment inside its cabin. NASA has done so with a KC-135 aircraft (reported to be now retired), a 4-engine jet nicknamed "The Vomit Comet" because its sudden transition to zero-g made some passengers quite airsick. The airplane could produce a temporary zero-g environment in its cabin, and was used for training astronauts and for short experiments on zero-gravity phenomena. The cargo space inside it was completely covered with padding, and a "zero gravity" illusion could be maintained for about 20-30 seconds. But what does it feel like? NASA now uses a smaller jet for such exercises, and in 2006 invited some high school teachers and students to experience weightlessness in it. Read here one report of such a flight. The Coriolis Force Wheel-shaped space station with visiting winged spaceship (Von Braun's, from the early 1950's) The science-fiction film "2001: A Space Odyssey" featured spinning space station, whose rotation provided the crew with "artificial gravity." It was a wheel-shaped structure, with hollow spokes connecting the wheel to a cabin in the center (drawing). The cabin in the middle was where transfers between the station and visiting spacecraft took place. Click here for more on that design. Given such a rotation, something like gravity would indeed be produced, with "down" being towards the outside (Larry Niven expanded that notion into the fanciful science-fiction novel "Ringworld" and its sequels). When calculating this effect it is simplest to use the station's frame of reference and add a centrifugal force to all other forces there. However, when one moves in this rotating environment, especially motion up and down the spokes, an additional force is encountered, named for the Frenchman Gaspard Gustave de Coriolis (1792-1843). Imagine an astronaut moving along one of the spokes, say from point A in the drawing to point B--most likely, climbing a ladder, since such motion goes against the station's "artificial gravity." At any point, as viewed in the frame of the outside world, the astronaut is also rotating around the station's axis. At both point A and B, the rotation is in the same direction, but at B it is slower, because that point is nearer to the axis of rotation and therefore describes a smaller circle. What happens at B to the extra speed the astronaut had at A? According to Newton's first law, loosely applied, the astronaut would tend to keep that extra speed and would therefore be pushed agains the side of the spoke (direction of the arrows). That push is the Coriolis force. When the motion is in the opposite direction, from B to A, the direction of the force is. . . the same or reversed? Work it out yourself! Swirling Water in a Bathroom Sink From time to time the claim is made that water draining from bathroom sinks swirls in opposite directions north and south of the equator. The physical principle is sound, but the actual effect is so microscopic that it is unlikely to be observed in the draining of bathroom sinks. On the other hand, the same effect is very important in large-scale swirling of the atmosphere, in hurricanes and typhoons as well as in ordinary weather patterns. The Earth, when viewed from above the north pole, spins counter-clockwise. Imagine 3 points in the northern hemisphere, at the same geographical longitude (drawing)--A is closest to the equator, B is somewhat poleward and C further poleward still. Each of these points covers in one day a full circle around the Earth's axis: A has the biggest circle, goes the greatest distance and therefore moves the fastest, B with a small circle moves more slowly, and C is even slower. The points are redrawn at the bottom on a magnified scale, with dashed arrows indicating the directions and magnitude of the velocity of the surface of the Earth at each point. Next consider the air above those points. If no wind is blowing, the air moves together with the surface. Its velocity viewed from the outside ("Frame of the Universe") is given by the dashed arrows, but its velocity relative to the surface of the Earth is everywhere zero. In the frame of the rotating Earth it stays at the points (A,B,C) and does not leave them. Suppose now that for some meteorological reason, a low atmospheric pressure develops at B. Air from A and C will flow towards it, heading (in the frame of the Earth) north and south, respectively. Newton's laws, however, apply without modification only in the outside frame, and there every mass of air tends to keep its eastward velocity. The air from C will therefore lag behind the ground below it, whose eastward motion is faster. The air from A, on the other hand, will move faster and overtake the ground. As a result, the flows of air relative to the Earth (solid arrows) are not simply southward and northward, but will bend as shown, creating a counterclockwise swirl around B. By similar arguments you can convince yourself that south of the equator the swirl is clockwise. Note that these arguments are based on viewing the motion from the outside. If we wish to solve the motion strictly in the frame of the rotating Earth (as atmospheric scientists do), it turns out that we need add two additional terms to Newton's equations. One is the centrifugal force, acting on all objects. The other is the Coriolis force, acting only on moving objects (or fluids) and responsible for the swirling effect described here. A hurricane viewed from space. Big storms in the atmosphere are usually centered on low-pressure areas and conform to those rules. This was first observed in weather patterns in 1857 by Christophorus Buys Ballot in Holland, though William Ferrel in the US had predicted the phenomenon using arguments like the ones given here. But don't expect to observe the effect in bathroom sinks. Water draining from a sink will usually swirl, because any rotation it has is greatly speeded up as it is drawn to the center of the sink. A slow circulation near the edges of the sink, e.g. because the sink itself is not completely symmetrical, becomes a fast vortex in the middle. The rotation of the Earth, however, is a much smaller factor than an uneven shape or heating of the sink, or a slow motion left from the time the sink was filled. If all 3 points A,B,C are inside the sink, with B at the drain, the difference in rotation speed (around the Earth's axis) between point B and either of the other two is typically only about 0.001 millimeter per second or about 1/7 of an inch per hour. The scale of the motion is what makes the difference. Hurricanes obey the "law of Buys Ballot", but the swirling of water in sinks is primarily due to subtle asymmetries and "remembered motions," too slow for the eye to detect. Even tornadoes are not large enough--according to reports, they are equally likely to swirl in either direction. Questions from Users: If no stars were seen--could Earth's orbital motion be discovered? Next Stop: Your Choice! #25 The Principle of the Rocket continues to the story of spaceflight. The Sun: Introduction opens a group of sections studying our Sun, from many different angles (spaceflight comes later). Timeline Glossary Back to the Master List Author and Curator: Dr. David P. Stern Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
   1. "3.1" Sunspots Around 1610, soon after the telescope first became available, three independent observers--Galileo, Galilei, Johann Fabricius and Christopher Scheiner--used it to observe dark spots on the face of the Sun. From their motion they deduced that the Sun rotated, with a period of 27 days close to the equator, relative to the moving Earth (25 days, relative to the stars). The period increased to about 29.5 days at higher latitudes, showing the Sun's surface was not solid. (For a bmore detailed view of a sunspot, see here.) The sunspots, Galileo guessed guessed, were clouds floating above the surface, blocking some of the sunlight from reaching us. We now know that sunspots are darker than their surroundings because they are moderately cooler, since their intense magnetic fields somehow slow down the local flow of heat from the Sun's interior. The process which causes this is still unclear. What is a "magnetic field," anyway? What follows below is a brief summary of magnetism; more details can be found on the files linked below, all of them parts of the web site The Exploration of the Earth's Magnetosphere. You may look them up--but be prepared to spend extra time! Magnetism Magnetism is familiar to most of us through specially treated iron or some related materials, found in compass needles and used for sticking messages to refrigerator doors, and also used for coating tapes and disks on which music and computer data are recorded. Actually, such "permanent magnets" are a fortunate accident of nature: most magnetism in the universe is not produced in this manner, but by electric currents. The magnetism of rare natural "lodestones" was known in ancient Greece--supposedly first noted in the town of Magnesia, from which comes the name. The magnetic compass (a Chinese discovery) was used by Columbus and other early navigators, but it was not until 1820 that a Danish professor, Hans Christian Oersted (pictured on the left), found by accident that an electric current in a wire could deflect a nearby compass needle (click here for the full story). A Frenchman, André-Marie Ampere, showed soon afterwards that the basic magnetic phenomenon was the force between two electric currents in parallel wires; they attracted each other when they flowed in the same direction, and repelled when they were opposed (click here for a more detailed discussion). Just as lines of latitude and longitude help us visualize positions on the Earth's globe, so magnetic field lines (originally named by Michael Faraday lines of force) help visualize the distribution of magnetic forces in 3-dimensional space. Imagine a compass needle which can freely turn in space to wherever the magnetic force tries to point it (such needles exist--see bottom of this web page). Magnetic field lines are then imaginary lines which mark the direction in which such a needle would point. A compass needle, for instance, has two magnetic poles at its ends, of equal strength, the north-seeking (N) pole and the south-seeking (S) pole, named for the directions on Earth to which they tend to point. Suppose the needle is free to point anywhere in 3 dimensions. If placed near the north pole, it would everywhere point towards the pole, and field lines therefore converge there (see drawing). If placed near the south pole, it would point away from it in all directions, and therefore field lines would diverge there, coming out of the Earth in a pattern that is a mirror-image of the pattern at the north pole. In between the lines form big arches above the Earth's equator, with their ends anchored in opposite hemispheres. Any bar magnet has a pattern of field lines like that of the Earth, suggesting that the Earth acted as if a short but very powerful bar magnet was inside it. Actually such a magnet does not exist, and the pattern comes from electric currents in the Earth core, and slowly changes, year by year; still, the "terrestrial bar magnet" remains a useful visualization aid. When two bar magnets are brought together, their (N,S) poles attract each other, their (S,S) and (N,N) poles repel: thus if a bar magnet were hidden inside the Earth, its S pole would be the one that pointed northwards, attracting the N pole of the compass needle. This strange mix-up of terminologies often confuses students: it is best to recognize the mix-up exists and then to ignore it. Michael Faraday who in the early 1800s introduced the concept of magnetic field lines, believed that space in which magnetic forces could be observed was somehow modified. His was a somewhat mystical view, but later mathematical developments found it quite useful, and today we refer to such a region of space as a magnetic field. The Sunspot Cycle Sunspots were studied by Scheiner and Galileo in the early 1600s (for a detailed but long account, see here), and then a strange thing happened: for about 70 years (1645-1715) they became a rarity. Some speculate that the unusually cold weather during those years was related to their disappearance, but in any case, by the time they returned, the attention of astronomers had moved elsewhere. It was only in 1843 that a German amateur astronomer, a pharmacist named Heinrich Schwabe (Shwah-bay), noted their most famous feature: their numbers grew and shrank, in a somewhat irregular cycle, lasting about 11 years. For the fuller account of Schwabe's discovery,see here. Ever since then solar observers have carefully followed sunspot cycles, and have also reconstructed earlier cycles from available observations, defining a suitable "sunspot number" index to gauge the level of sunspot activity. The nature of sunspots remained unclear until 1908, when George Ellery Hale, using an instrument that observed the Sun in narrow ranges of color emitted by selected substances, reported that the light from sunspots was modified in ways that indicated it was produced in intense magnetic fields. Sunspot fields turned out to be as intense as the ones we find near the poles of iron magnets--but extending across regions many thousands of kilometers wide. In conventional units, the magnetic intensity intensity in them reached about 1500 gauss (0.15 Tesla), while the field near the surface of Earth is typically 0.3-0.5 gauss, depending on location. In interplanetary space at the orbit of Earth, the magnetic field (carried from the Sun by the solar wind) is much weaker, typically 0.00006 gauss, while at the orbits of the outer planets, it is 20 times weaker still. Yet even there the instruments of spacecraft such as Voyager 2 still measure it reliably. Sunspots display many interesting features. Generally (though not always) they appear in pairs, with opposite magnetic polarities. In half the solar cycles, the "leading" spot (in the direction of the Sun's rotation) will always have an N polarity, and the "following" spot an S polarity; then in the following cycle, polarities are always reversed. The general magnetic field, producing the Sun's north and south magnetic poles, also reverses polarity at each cycle, the reversal typically occuring 3 years after sunspot minimum. All these suggest that the 11-year cycle is a magnetic phenomenon. Astronomers believe that the electric currents which flow in the solar plasma and create those fields get their energy from the unequal rotation of the Sun--faster at the equator--which in its turn is driven by large-scale flows of the solar gas. Variation of the solar constant, as observed by various spacecraft. Image provided by Davos Observatory, courtesy of Claus Fröhlich Sunspots and the Sun's Energy Output Ever since the solar cycle was first discovered, people had tried to associate it with other periodic observations. The orbital period of Jupiter is close to 11 years, but unlike the sunspot cycle it has an exact value, and on the long run does not fit. Large magnetic storms do tend to occur near sunspot maximum, and so do energetic solar disturbances, which are being constantly monitored. Alas, we can only vaguely guess what goes on beneath the visible surface of the Sun. For a century and a half people looked for a correlation between sunspots and weather (also using tree-rings as evidence for the distant past) and failed to find any. However, sunlight energy-flow is notoriously hard to measure accurately. Even on clear mountain tops the observer is below the ozone layer which absorbs some, and the blue color of the sky shows some light is scattered, and of that, not all ends up going downwards. In the infra-red region, it is hard to separate radiation from the cooling Earth (scattered by greenhouse gases) from the one coming from the Sun. As example of this problem, have you ever noticed that we are 3% closer to the Sun in January than in July, because of the eccentricity of the Earth's orbit? The difference has been invoked in a theory of ice ages, and observations of the solar constant" in clear air have detected it. However, its variation during the solar cycle amounts to only 0.2%, and to observe it (see graph above) scientists needed observations from spacecraft. Such observations have now been conducted for about 30 years. Naively, one might expect less sunlight when some of the solar surface is darkened by sunspots. Actually, years of the most sunspots are also peaks of the solar energy output. Perhaps this is caused by increased output in ultra-violet and x-rays in such years, or perhaps heat from the regions beneath sunspots is diverted to neighboring areas of the Sun and radiated from them. Alas, we can only vaguely guess what goes on beneath the visible surface of the Sun. Note on Solar Magnetic Fields The fact that sunspots were intensely magnetic was evidence that motions in conducting fluids (like the Sun's plasma) could generate electric currents, whose magnetic field helped maintain the same currents. This made Earth scientists realize that perhaps a similar "fluid dynamo" operated in the Earth's liquid core, and was the cause of the Earth's magnetic field (rather than some strange sort of permanent magnetism). Today "dynamo theory" is well developed, for both Earth and Sun; for more see "The Great Magnet, the Earth," home page http://www.phy6.org/earthmag/demagint.htm. Solar Activity Hale's "spectroheliograph, " invented in 1892 and viewing the Sun in narrow color bands, allowed a completely new range of phenomena to be observed. Many were associated with sunspots, e.g. bright clouds or "plages" (plah-jes, "beaches" in French) in the chromosphere, seen in the light emitted by glowing hydrogen. Such methods also made possible limited observations of the inner corona, outside times of total solar eclipses. And they revealed changes much faster than those previously noted in sunspots, some of which cause interesting magnetic effects at the Earth. The fastest and most significant among these was the solar flare--a brightening of the chromosphere near a sunspot group, rising within minutes and typically lasting 10-30 minutes. Flares are usually observed in the red light emitted by hot hydrogen (Hα or "H-alpha line"), but it so happened that the first observation in 1859 was of a rare "white light flare" observable with an ordinary telescope (see here and here for the full story). This was followed 17 hours later by a huge magnetic storm, a world-wide disturbance of the Earth's magnetic field: something apparently was ejected from the Sun, and took that long to reach Earth. We now know that "something" was probably a fast-moving plasma cloud, plowing through the ordinary solar wind, which takes 4-5 days to cover the same distance. The arrival at Earth of such clouds, with the shock wave that forms ahead of them, can be quite dramatic (see here for one story). The most remarkable aspect of such activity is the speed with which it takes place. If a typical big flare spreads over 10,000 km in 10 minutes, it must propagate quite rapidly. Some of its features begin much more abruptly, e.g. the associated x-rays (observable from space) can rise in just a few seconds. All this suggests that the energy source is not the heat of the Sun, which spreads and changes rather gradually, but the intense magnetic fields of sunspots. (For more on these matters, see here.) Exploring further A small bar magnet, on gimbals that allow it to point in any direction in space, can be procured from its manufacturer, Cochranes of Oxford, Ltd., Leafield, Oxford OX8 5NT, England. Two types are available, Mark 1 with jewelled bearings for $36.60, Mark 2 with simple bearings for $12.65. For details see their web site: http://www.cochranes.co.uk/secondary.html (scroll down to "Magnaprobe"). Some spectacular sunspots are shown here. Questions from Users: Should we fear big solar outbursts? Also: Is the solar cycle caused by the lining up of planets? . (And is this connected to reversals of the Earth's magnetic polarity?) *** The 2011-2 Sunspot Maximum. see more at :https://www.britannica.com/list/9-britannica-articles-that-explain-the-meaning-of-life